LAKE UPERIOR GEOLOGY LAKE SUPERIOR...

17TH ANNUAL

INSTITUTEON

LAKE SUPERIOR GEOLOGY

DULUTH, MINNESOTAMAY 5-8,1971

1

-4',

17TH A UA

INSTITUTE

ON

LAKE UPERIOR GEOLOGY

DULUTH, MINNESOTAMAY 5-8,1971

ThGL?J CAL SESSIONS

ABSTPACFS

and

FIELD GUIDES

for the

17th ANNUAl..

INSTITUTE ON LKE SUPERIOR GEOLOGY

Sponsored by

UNIVEPSIfl OF IINNESOTA, DULUTI I

held at

DULU'fl!, ?IINNESOTA

Jay 5 - 8, 1971

Edited by D. DavidsonD.G. DafoyJ.C. GreenJ.A. Grant

TEOINICAL SESSIONS

ABSTRACfS

and

FIELD GUIDES

for the

17th A\'NU.t\L

INSTITUTE ON L!\.KJ:: SUPERIOR GEOLOGY

Sponsored by

UNIVEPSI1Y OF ; IIr-,rNESOTA, DULUTH

held at

DULUTIl, i'UNNESOTA

..lay 5 - 8, 1971

Edited by D.ll. DavidsonD.G. DarbyJ.e. GreenJ.A. Grant

TABLE OF COYI'EN]7S

Page No.

INSTI'IlJTE DIRECRS AND LOCAL CCLt1IflEE 1

PIJGRAt4 (TflLE of ctpirgwrS pt*R 4S1RfrtCrS)2

ABSFRACI'S OF TECINICAL SESSIONS 7

FIELD TRIPS

A - North Shore Volcanic Group 73(Keweenawan)

B - Cross-Section, Precambrian Rocks, 97I'brtheastern Minnesota

C - Mesabi Range - Biwabik Taconite US

D - Vermilion District 141

TABLE OF CONTENTS

INSTITIJTE DIRECTORS A!\JD IDCAL CCl:IMITTEE

ABSTRACTS OF TECIWICAL SESSIONS

FIELD TRIPS

A - North Shore Volcanic Group(Keweenawan)

B - Cross-Section, Precambrian Rocks,Northeastern ~tinnesota

C - ~Iesabi Range - Biwabik Taconite

D - Vermilion District

Prtge ~~o.

1

2

7

73

97

128

141

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17th Annual

INSTITUTE ON LAKE SUPERIOR GEOLOGY

Sponsored by

University of Minnesota, [*iluth

at

Duluth, Minnesota

May S - 8, 1971

INSTITUTE BOARD OF DIRECTORS

J. IV. Avery (Treasurer), Jones Laughlin Steel Corp.,Negaunee, Michigan.

* R. C. Reed (Secretary), Michigan Geological Survey,Lansing, Michigan.

D. M. Davidson, University of Minnesota, [AiluthDuluth, Minnesota

W. J. Hinze, Michgian State University, East Lansing,Michigan.

A. B. Dickas, Wisconsin State University, Superior,Wisconsin.

G. U LaBerge, Wisconsin State University, Oshkosh,Wisconsin.

U. W. Bartley, Thunder Bay, Ontario

* Permanent members

LOCAL CCt1ITrEE

Coordinating Oiainiian: D. M. Davison

Arrangements Connittee: R. W. Marsden C. L. lttsthProgram Coninittee: D. C. Darby 3. C. Green

Field Trip Committee: J. A. Grant R. 14. Ojakangas

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17th Annual

INSTITIITE ON LAKE SUPERIOR GEOLOGY

Sponsored by

University of Minnesota, Duluth

at

Duluth, Minnesota

May 5 - 8, 1971

INSTITUTE BOARD OF DIRECTORS

* J. W. Avery (Treasurer), Jones &Laughlin Steel Corp.,Negaunee, ilichigan.

* R. C. Reed (Secretary), J'Hchigan Geological Survey,Lansing, ;"Iichigan.

D. M. Davidson, University of Minnesota, IW.uthDuluth, Minnesota

W. J. Hinze, Michgian State University, East Lansing,Michigan.

A. B. Dickas, Wisconsin State University, Superior,Wisconsin.

G. L. LaI3erge, Wisconsin State University, Oshkosh,Wisconsin.

[;1. ],V. Bartley, Thunder Bay, Ontario

* Permanent menvers

LOCAL C(J.·tUTTEE

Coordinating Chairman:

Arrangements Committee:

Program Comnittee:

Field Trip Conrrni ttee:

D. M. Davison

R. W. Marsden

D. G. Darby

J. A. Grant

C. L. Matsch

J. C. Green

R. W. Ojakangas

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PROGRAM

Tuesday, May 4, 1971

5:00 p.m. Field Trip A (North Shore Volcanics) leavesHotel liadisson Duluth.

Wednesday, May 5, 1971

6:00 p.m.to Field Trip A returns to Duluth

7:00 p.m.

7:00 p.m.to Institute Registration - Poolside, Hotel

9:30 p.m. Radisson Duluth

Thursday_, May 6, 1971

7:30 a.m.to Registration, Superior Street Foyer, Hotel Duluth

12:00 a.m.

5:00 p.m.

6:00 p.m.to

7:00 p.m.

7:00 p.m.to

9:30 p.m.

7:30 a.m.to

12:00 a.m.

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PROGRAM

Tuesday, ~~y 4, 1971

Field Trip A (North Shore Volcanics) leavesHotel Hadisson Duluth.

Wednesday, ~,~y 5, 1971

Field Trip A returns to Duluth

Institute Registration - Poolside, HotelRadisson Duluth

Tnursday, ~hy 6, 1971

Registration, Superior Street Foyer, Ibtel Duluth

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SESSION 1

;lorning

Thursday, 'ay 6, 1971

Symposium on Keweenawan Geology — Lake Superior Region

Co-chairmen: Jack Phillips and 1/alter White Page No.

8:45 J. C. Green Introductory Remarks

9:00 IV. White Keweenawan Stratigraphy of TYesternimost 71Others Michigan

9:20 N. K. Ih.xber The Keweenawan Geology of Isle Royale, 31Michigan

9:40 II. A. Hubbard Keweenawan Geology of the Porcupine 30tuntains, Western Upper PeninsulaUchigan

10:00 J. C. Green Stratigraphy of the North Shore Voltanic 20Group Northeast of Silver Bay, 1innesota

10:20 R. N. Annells Middle Keweenawan Volcanism of Eastern Lake 7

Superior

10:40 A. P. Jtotsala Characteristics of Some Alteration Minerals 59Portage Lake Lava Series, Michigan

11:00 IV. T. Jolly Zeolite and Prehnite - Puinpellyite Fades 34in the Keweenawan Basalts of NorthernMichigan II: The Role of Volatiles

11:20 T. A. Vogel Chemically Zoned Native Copper and 70R.J. Rohrbacher Chalcocite from White Pine Michigan

11:40 1!. C. halls The Isle Royale Fault 25

G. F. West

12:00 DON Mjourn for Lunch

There will be a lunch meeting of the Board of Directors in thehotel Radisson ililuth —. location to be announced.

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S E S S ION 1

i,lorning

l11Ursday, i.fay 6, 1971

Symposium on Keweenawan Geology - Lake Superior Region

Co-chainnen: Jack Phillips and Walter White Page No.

8: 45 J. C. Green Introductory Remarks

9:00

9:20

9:40

10:00

10:20

10:40

11 :00

11: 20

11 :40

Iv. White &Others

N. K. Huber

H. A. Hubbard

J. C. Green

R. N. Armells

A. P. Ruotsala

W. T. Jolly

T. A. Vogel &R.J. Rohrbacher

H. C. Halls &G. F. \\Test

Keweenawan Stratigraphy of lVesterrunostHichigan

The Keweenawan Geology of Isle Royale,;.1ichigan

Keweenawan Geology of the Porcupine;"buntains, Western Upper Peninsula~·,tichigan

Stratigraphy of the North Shore Vo1tanicGroup Northeast of Silver Bay, ;··linnesota

~·liddle Keweenawan Volcanism of Eastern LakeSuperior

Characteristics of Some Alteration ~tinerals

Portage Lake Lava Series, Michigan

Zeolite and Prehnite - Pumpellyite Faciesin the Keweenawan Basalts of Northern:.tichigan II: The Role of Volatiles

Chemically Zoned Native Copper andChalcocite from IVhite Pine Michigan

The Isle Royale Fault

71

31

30

20

7

59

34

70

25

12: 00 NOON Adjourn for Lunch

There 'vi1l be a lunch meeting of the Board of Directors in theHotel Radisson fuluth - location to be announced.

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SESSION 2

Afternoon

Thursday, May 6, 1971


Co-Chairmen: Raymond J. Leone and Paul K. Siiis Page No.

1:40 J. T. Mengel, Jr. Exploration Geology of Douglas County, 47

R. A. Hendrickson Wisconsin

2:00 D. vI. Davidson ,Jr. A New View of the Duluth Complex, 13

Minnesota

2:20 B. Bonnichsen Hornfelses in the Southern Part of the 11

Duluth Complex, ?•linnesota

2:40 M. G. i4idrey Reinvestigation of "Red Rocks" in the 53

P. W. Weiblen Pigeon Point Area, Minnesota

3:00to Coffee Break3:20

3:20 W. A. Ibertson The Great Logan Paleomagnetic Loop 58

W. F. Fahrig

3:40 A. Me.ttis Lower Keweenawan Sediments of the Lake 45

Superior Region

4:00 D. Myers The Sedimentology and Tectonic Significance 54

of the Bayfield Group, Wisconsin

4:20 C. B. Merey Revised Keweenawan Subsurface Stratigraphy, 50

Southeastern Minnesota

4:40 H. C. Halls Shallow Structure and Stratigraphy of the 23C. F. West Lake Superior Basin from Seisnic Refraction

Measurements

******* ** ***Evening

7:30 p.m. Banquet - Great Hall - Hotel Radisson Duluth

ADDRESS - Dr. Carl R. i\nnhausserEconomic Geology Research UnitUniversity of WitwatersrandJohannesburg, South Africa

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S E S S ION 2

Afternoon

Thursday, ~my 6, 1971


Co-01ainnen: Raymond J. Leone and Paul K. Sims

1:40 J. T. Mengel, Jr.& Exploration Geology of Douglas County,R. A. Hendrickson Wisconsin

Page No.

47

2:00

2:20

2:40

D. M. Davidson,Jr.

B. Bonnichsen

M. G. M.ldrey &P. W. Weiblen

A Ne,... View of the Duluth Complex,Minnesota

Hornfelses in the Southern Part of theIl1luth Complex, Minnesota

Reinvestigation of "Red Rocks" in thePigeon Point Area, Minnesota

13

11

53

3:00to Coffee Break

3:20

3:20 W. A. Robertson &W. F. Fahrig

3:40 A. 1'-attis

4:00 D. ;"fyers

4:20 G. B. ~brey

4:40 H. C. Halls &C. F. West

The Great Logan Paleomagnetic Loop

Lower Keweenawan Sediments of the LakeSuperior Region

The Sedimentology and Tectonic Significanceof the Bayfield Group, Wisconsin

I{evised Keweenawan Subsurface Stratigraphy,Southeastern Minnesota

Shallow Structure and Stratigraphy of theLake Superior Basin from Seismic RefractionMeasurements

* * * * * * * * * * * *Evening

58

45

54

50

23

7:30 p.m. Banquet - Great Hall - Hotel Radisson Duluth

ADDRESS - Dr. Carl R. ArrmlausserEconomic Geology Research UnitUniversity of WitwatersrandJohanneshurg, South Africa

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SESSION 3

!trning

Friday, May 7, 1971

General Session

Co-Chaitnen: Cedric. L. Iverson and J. KalliokoskiPage It.

8:40 F. C. Tan 4 Implications of Carbon Isotope Ratio 64

E. C. Perry, Jr. Variations in Carbonates from the BiwabikIron Formation, Minnesota

9:00 S. Viswanathan, Oxygen Isotopic Studies of Early Precambrian 66Ii. C. Perry, Jr. Granitic and Metamorphic Rocks from the

4 P. Sims Western Part of the Giants Range Batholith,Northeastern Minnesota

9:20 G. L. LaBerge Some Geology of the ?larathon County Volcanic 39Belt

9:40 P. 0. Banks 4 chronology of Precambrian Rocks of Iron 9

W. R. Van Schmus and Dickinson Counties, Michigan

10:00 L. A. Prince 4 Geochrono]ngy of the Giants Range Granite 57C. . Hanson

10:20 G. Klein Precambrian Clastic Paleotidal Sedimentation 36

10:40 Pt. i!inze, Continental Rifts 29I). H. iiavidson,Jr.4 R. Roy

11:00 It C. Malan Fj Distribution of Uranium and Thorium in Pre- 42

1]. A. Sterling cambrian Rocks of the Western Great LakesRegion

11:20 N. Pt. O'Hara 4 Lake Michigan Aerornagnetic Survey 56

iv. J. I:inze

11:40 . L. Kellogg 4 /u Aeromagnetic Survey of the SouthernPt. J. I-Jinze Peninsula of Michigan

12:00 NWN — Lunch

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S E S S ION 3

~'brning

Friday, May 7, 1971

General Session

Co-Chairmen: Cedric. L. Iverson and J. Ka11iokoski

8:40 F. C. Tan &E. C. Perry, Jr.

Implications of Carbon Isotope RatioVariations in Carbonates from the BiwabikIron Formation, Minnesota

Page No.

64

9:00

9:20

9:40

10:00

10:20

10:40

S. Viswanathan,E. C. Perry, J r.&P. Sims

G. 1. LaBerge

P. O. Banks ~

W. R. Van Sc!llTlUS

L. A. Prince &G. N. I·ranson

G. Klein

W. i Jinze,D. Iv!, Daviclson,Jr.& R. Roy

Oxygen Isotopic Studies of Early PrecambrianGranitic and Metamorphic Rocks from theWestern Part of the Giants Range Batholith,Northeastern Minnesota

Some Geology of the Harathon County VolcanicBelt

Chronology of Precambrian Rocks of Ironand Dickinson Counties, Michigan

Geochronology of the Giants P~ge Granite

Precambrian Clastic Paleotidal Sedimentation

Continental Rifts

66

39

9

57

36

29

11:00 R. C. 1\la1an & Distribution of Uranium and Thorium in Pre- 42D. A. Sterling cambrian Rocks of the Western Great Lakes

Region

11: 20 N. 11[. O'Hara & Lake nichigan Aeromagnetic Survey 56w. J. Hinze

11 :40 R. 1. Kellogg " An Aeromagnetic Survey of the Southern 35l]

W. J. Hinze Peninsula of ~lichigan

12:00 NOON - Lunch

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SESSION 4

Afternoon

Friday, lay 7, 1971

General Session

Co-Chairmen: Meredith F. Ostrom and Paul C. 1'chsen Page No.

1:20 &zsiness Meeting of the Institute

1:40 M. S. Lougheed Hematite Pseudomorphic After Biogenic 41

J. J. Mancuso Pyrite in the Negaunee Iron Formation

2:00 F. Dimroth E Textural Facies Analysis of Precambrian 15

.J. Chauvel Cherty Ironstones

2:20 W. ]Aihling Precambrian Iron Formation at Copper 18Nbuntain, Fremont County Wyoming

2:40 F. Frodeston Sonic Sedimentary Structures in the Lower 32

Cherty Member of the Biwabik Iron Formation:The Virginia Horn Area

3:00 G. Spencer Chert in Sediments 62

3:20 J. Mathersill Limnogeological Studies of Thunder Bay, 51Lake Superior, Ontario

3:40 R. Shegeiski The General Stratigraphy of Thunder Bay, 61Lake Superior

4:00 End of Technical Sessions

*** ** ** ** **

(Dinner and lodging are included in the field trip fee.)

5:00 Departure for Field Trips:

Field Trip A - North Shore Volcanic GroupField Trip B - Cross-section, Precambrian RocksField Trip C - Mesabi Range - Biwahik TaconiteField Trip U - Vemiilion District

Buses will depart from Hotel Radisson Duluth.

Saturday, May 8th, 1971

6:00to (approx.) REThRN OF ML FIELD TRIP BUSES.

7:00

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S E S S ION 4

Afternoon

Friday, I,1ay 7, 1971

General Session

Co-Chainnen: Meredith E. Ostrom and Paul C. Tychsen

1:20 Business Meeting of the Institute

Page No.

1:40

2:00

2:20

M. S. Lougheed &J. J. t,.1.ancuso

E. Dimroth &J. Chauvel

W. fuhling

Hematite Pseudomorphic After BiogenicPyrite in the Negaunee Iron Fonnation

Textural Facies Analysis of PrecambrianCherty Ironstones

Precambrian Iron Formation at Copper~·ibuntain, Fremont County 1Vyoming

41

15

18

2:40 E. Frodeston

3:00 G. Spencer

3:20 J. ~bthersill

3:40 R. Shegelski

Some Sedimentary Structures in the LowerCherty nember of the Bhvabik Iron Fonnation:The Virginia Horn Area

Chert in Sediments

Limnogeological Studies of Thunder Bay,Lake Superior, Ontario

TI1e General Stratigraphy of Thunder Bay,Lake Superior

32

62

51

61

4: 00 End of TecJmical Sessions

* * * * * * * * * * *

(Dinner and lodging are included in the field trip fee.)5: 00 Departure for Field Trips:

Field Trip A - North Shore Volcanic GroupField Trip B - Cross-section, Precambrian RocksField Trip C - j\!esabi Range - Biwabik TaconiteField Trip D - Vermilion District

Buses will depart from Hotel Radisson Duluth.

* * * * * * * * * * *Saturday, nay 8th, 1971

6:00to (approx . )

7:00RETIJRN OF ALL FIELD TRIP BUSES.

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MIDDLE KEWEENAWAN VOLCANISM OF EASTERN LAKE SUPERIOR.

R. N. ANNELLS

Geological Survey of Canada, Ottawa.

ABSTRACT

Following the study of the Michipicoten Island Keweenawan flows reportedelsewhere (Annells, 1970), the same type of detailed stratigraphic/petrographicstudy has been carried out on the Keweenawan volcanic rocks of the MamainsePoint, Alona Bay and Cape Gargantua sections on the east shore of Lake Superior.

These three east shore sections are mostly made up of inafic olivine—tholefitetypes of medium—coarse ophitic or diabasic texture, accompanied by a smaller numberof fine—grained olivine—poor tholeiite flows. Flows of intermediate compositionwere not found in these sections and the flows show little variation in mineralogy.Small volumes of both basic and acid pyroclastic rocks occur interbedded with theflows in the lower part of the 14,300 foot Mamainse Point section, and conglomer-ates with sandstone partings showing good cross—bedding outcrop in the upper halfof this section and in that at Cape Gargantua.

Evidence of the simultaneous availability of basaltic and rhyolitic materialis seen in the Matnainse Point section; plugs, dykes and sheets of fine—grained,often flow—laminated and autobrecciated leucorhyolite occur at all levels of thissection and one such intrusive sheet was found to he composite, having a 2—footbasal selvage of fine—grained basalt and a 50—foot acid upper part.

Fetrographic similarities found in flows of the Mamainse Point and CapeGargantua sequences provide a good basis for lateral correlation of these twosequences. Near the basal unconformity of theMamainse section there occurs a high-ly distinctive glomerophyric olivine—tholeiite flow crowded with large plagioclaselaths concentrated in spherulitic clusters up to two inches in diameter (Ciblin,1969); a flow of exactly similar type outcrops 40 miles NNW along strike near thebase of the Cape Gargantua section (Ayres, 1969). Both these occurrences are asso-ciated with a group of olivine—tholeiite flows rich in large pseudomorphs afterolivine phenocrysts which may make up to 35 per cent by volume of the rock in partsof such flows at Mamainse Point. This striking similarity of the flows in thelower levels of the Mamainse Point and Cape Gargantua piles suggests simultaneouseffusion of the same mafic magma supply over a wide area to preduce lava flows ofsii,!lar extent to that of the Greenstone Flow of the Michigan Keweenawan.

The Hamainse Point, Alona Bay and Cape Gargantua flo9s are also petrographical—ly similar to the olivine—tholeiite flows forming the basal part of the MichipicotenIsland section and it is possible that these latter flows, also cut by acid intru-sions, may represent the upper part of an extensive and largely uniform flood ba-salt pile whose earliest members rest directly on the Archaean on the east shore ofLake Superior. The more highly differentiated andesite and rhyolite flows ofMichipicoten Island are thus interpreted as being of much later extrusion than theMamainse Point, Alona Bay and Cape Gargantua flows. A general increase in the de-gree of alteration of lava flows is apparent as the Keweenawan lava pile is followeddownwards from the top of the Michipicoten Island section to the base of theMamainse Point section.

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MIDDLE KEWEENAWAN VOLCANISM OF EASTERN LAKE SUPERIOR.

R. N. ANNELLS

Geological Survey of Canada, Ottawa.

ABSTRACT

Following the study of the Michipicoten Island Keweenawan flows reportedelsewhere (Annells, 1970), the srone type of detailed stratigraphic/petrographicstudy has been carried out on the Keweenawan volcanic rocks of the MamainsePoint, Alona Bay and Cape Gargantua sections on the east shore of Lake Superior.

These three east shore sections are mostly made up of mafic olivine-tholeiitetypes of medium-coarse ophitic or diabasic texture, accompanied by a smaller numberof fine-grained olivine-poor tholeiite flows. Flows of intermediate compositionwere not found in these sections and the flows show little variation in mineralogy.Small volumes of both basic and acid pyroclastic rocks occur interbedded with theflo\vs in the lower part of the 14,300 foot Mamainse Point section, and conglomerates with sandstone partings showing good cross-bedding outcrop in the upper halfof this section and in that at Cape Gargantua.

Evidence of the simultaneous availability of basaltic and rhyolitic materialis seen in the Mamainse Point section; plugs, dykes and sheets of fine-grained,often flow-laminated and autobrecciated leucorhyolite occur at all levels of thissection and one such intrusive sheet was found to be composite, having a 2-footbasal selvage of fine-grained basalt and a 50-foot acid upper part.

Petrographic similarities found in flows of the Mamainse Point and CapeGargantua sequences provide a good basis for lateral correlation of these twosequences. Near the basal unconformity of therlamainse section there occurs a highly distinctive glomerophyric olivine-tholeiite flow crowded with large plagioclaselaths concentrated in spherulitic clusters up to two inches in diameter (Giblin,1969); a flow of exactly similar type outcrops 40 miles NNW along strike near thebase of the Cape Gargantua section (Ayres, 1969). Both these occurrences are associated with a group of olivine-tholeiite flows rich in large pseudomorphs afterolivine phenocrysts \vhich may make up to 35 per cent by volume of the rock in partsof such flows at Mamainse Point. This striking similarity of the flows in thelower levels of the Mamainse Point and Cape Gargantua piles suggests simultaneouseffusion of the S~le mafic magma supply over a wide area to pr~duce lava flows ofsiIIl~Lar extent to that of the Greenstone Flow of the Michigan Keweenawan.

The Mamainse Point, Alana Bay and Cape Gargantua f1o'to.{s are also petrographically similar to the olivine-tholeiite flows forming the basal part of the MichipicotenIsland section and it is possible that these latter flows, also cut by acid intrusions, may represent the upper part of an extensive and largely uniform flood basalt pile whose earliest members rest directly on the Archaean on the east shore ofLake Superior. The more highly differentiated andesite and rhyolite flows ofHichipicoten Island are thus interpreted as being of much later extrusion than theMamainse Point, Alana Bay and Cape Gargantua flows. A general increase in the degree of alteration of lava flows is apparent as the Keweenawan lava pile is followeddownwards from the top of the Michipicoten Island section to the base of theMamainse Point section.

—8—

Some differences in general chemistry exist between flows newly analysed fromthe Michipicoten Island and Maniainse Point sections; 36 analyses of the Island flowsfollow a tholeiltic trend of moderate iron—enrichment on an Nfl plot; a similar plotof 46 analyses of Mamainse Point flows gives a tholefitic trend of high iron—enrich-ment.

References;

Annells, It. N., 1970, Keweenawan volcanic geology of Michipicoten Island,Lake Superior; Program, 16th Ann. Inst. on. Lake SuperiorCeol., Thunder Bay, Ont., May 1970, 7.

Ayres, L. D., 1969, Geology of Townships 31 and 30, Ranges 20 and 19;Ontario Dept. Mines Ceol. Rept. 69,38.

Giblin, P. E., 1969, Ontario Dept. Mines, Prelim. Geol. Naps 553 and 555.

-8-

Some differences in general chemistry exist between flows newly analysed fromthe Michipicoten Island and Mamainse Point sections; 36 analyses of the Island flowsfollow a tholeiitic trend of moderate iron-enrichment on an MFA plot; a similar plotof 46 analyses of Mamainse Point flows gives a tholeiitic trend of high iron-enrichment.

References:

Annells, R. N.,

Ayres, L. D.,

Giblin, P. E.,

1970, Keweenawan volcanic geology of Michipicoten Island,Lake Superior; Program, 16th Ann. Inst. on. Lake SuperiorGeol., Thunder Bay, Ont., May 1970, 7.

1969, Geology of Townships 31 and 30, Ranges 20 and 19;Ontario Dept. Mines Geol. Rept. 69,38.

1969, Ontario Dept. Mines, Prelim. Geol. Maps 553 and 555.

—9—

CHRONOLOGY OF PRECAMBRIAN ROCKS OFIRON AND DICKINSON COUNTIES, MICHIGAN

P.O. BanksDepartment of Geology

Case Western Reserve University, Cleveland, Ohio, 44lO6and

W.R. Van SchmusDepartment of Geology

University of Kansas, Lawrence, Kansas, 6604k

U—Pb analyses of cogenetic zircon suites and Rb-Sr whole rock analysesfrom selected Frecambrian units in Iron and Dickinson Counties, Michigan,suffice to establish an internally consistent chronology for this classicarea. Stratigraphic nomenclature used herein is adopted from James et al.(1961).

Zircons from the Norway Lake Gneiss, which underlies the DickinsonGroup, yield a concordia intercept age of approximately 2375 m.y., butRb-Sr whole rock analyses from the Norway Lake Gneiss do not define anisochron, and the isotopic composition of Pb from separated feJ.dspars isabnormal. Thus, we conclude that the 2375 m.y. "age" obtained from thezircons is probably the result of complex metamorphism and deformation ofthe Norway Lake Gneiss and does not indicate its true absolute age. The

2375 m.y. event that affected the Norway Lake Gneiss may be pre—Dickinson,thus establishing a possible older limit for the age of the Dickinson Group.Earlier published zircon analyses suggesting a much greater antiquity forthe Dickinson Group (2700 m.y. or more: Aldrich etal., 1965) are interpretedto reflect detrital zircon components in the rnetasedimentary gneisses.

The pre—Animikie Porphyritic Red Granite, whose field relation to theDickinson Group is uncertain, gives a zircon conoordia intercept age ofapproximately 2100 m.y. , and whole rock Rb—Sr analyses are also consistentwith this age. We interpret this age to be post-Dickinson, thus bracketingthe age of the Dickinson Group between 2100 m.y. and, perhaps, 2400 m.y.

A single zircon analysis from the Hemlock Volcanios of the AnimikieSeries yields a Pb-Pb age of 1985 m.y., but the zircon is somewhat discordant,so that the concordia intercept age is expected to be slightly older atapproximately 2000 m.y.

Rb—Sr whole rock analyses on the post-Animikie Feavy Complex yieldan age of approximately 1700 m.y. However, previously published mineralage determinations on other post—Animikie units indicate that the AnimikieSeries is definitely older than 1900 m.y. (Aldrich etal., 1965). Thus,

deposition of the Animikie Series appears to be bracketed in the intervalbetween 2100 m.y. and 1900 m.y.

A maximum age of 2100 m.y. for the Animikie Series, in conjunction withpublished Rb-Sr data on the post—Huronian Nipissing Diabase (2160 rn.y.) andon 1-luronian sediments themselves (2285 m.y.) (Van Schmus, 1965; Fairbairnetal., 1969), severely restricts possible correlations between the AnimikieSeries and the Huronian formations of the North Shore of Lake Huron.

-9-

CHRONOLOGY OF PRECAMBRIAN ROCKS OFIRON AND DICKINSON COUNTIES, MICHIGAN

P.O. BanksDepartment of Geology

Case Western Reserve University, Cleveland, Ohio, 44106and

W.R. Van SchmusDepartment of Geology

University of Kansas, Lawrence, Kansas, 66044

U-Pb analyses of cogenetic zircon suites and Rb-Sr whole rock analysesfrom selected Precambrian units in Iron and Dickinson Counties, Michigan,suffice to establish an internally consistent chronology for this classicarea. Stratigraphic nomenclature used herein is adopted from James et al.(1961).

Zircons from the Norway Lake Gneiss, which underlies the DickinsonGroup, yield a concordia intercept age of approximately 2375 m.y., butRb-Sr whole rock analyses from the Norway Lake Gneiss do not define anisochron, and the isotopic composition of Pb from separated feldspars isabnormal. Thus, we conclude that the 2375 m.y. "age" obtained from thezircons is probably the result of complex metamorphism and deformation ofthe Norway Lake Gneiss and does not indicate its true absolute age. The2375 m.y. event that affected the Norway Lake Gneiss may be pre-Dickinson,thus establishing a possible older limit for the age of the Dickinson Group.Earlier published zircon analyses suggesting a much greater antiquity forthe Dickinson Group (2700 m.y. or more: Aldrich et al., 1965) are interpretedto reflect detrital zircon components in the metasedimentary gneisses.

The pre-Animikie Porphyritic Red Granite, whose field relation to theDickinson Group is uncertain, gives a zircon concordia intercept age ofapproximately 2100 m.y., and whole rock Rb-Sr analyses are also consistentwith this age. We interpret this age to be post-Dickinson, thus bracketingthe age of the Dickinson Group between 2100 m.y. and, perhaps, 2400 m.y.

A single zircon analysis from the Hemlock Volcanics of the AnimikieSeries yields a Pb-Pb age of 1985 m.y., but the zircon is somewhat discordant,so that the concordia intercept age is expected to be sligh~ly older atapproximately 2000 m.y.

Rb-Sr whole rock analyses on the post-Animikie Peavy Complex yieldan age of approximately 1700 m.y. However, previously published mineralage determinations on other post-Animikie units indicate that the AnimikieSeries is definitely older than 1900 m.y. (Aldrich et al., 1965). Thus,deposition of the Animikie Series appears to be bracketed in the intervalbetween 2100 m.y. and 1900 m.y.

A maximum age of 2100 m.y. for the Animikie Series, in conjunction withpublished Rb-Sr data on the post-Huronian Nipissing Diabase (2160 m.y.) andon Huronian sediments themselves (2285 m.y.) (Van Schmus, 1965; Fairbairnet al., 1969), severely restricts possible correlations between the AnimikieSeries and the Huronian formations of the North Shore of Lake Huron.

—10—

References

Aldrich, L.T., Davis, C.L., and James, FI.L., 1965, Ages of minerals frommetamorphic and igneous rocks near Iron Mountain, Michigan: Jour.

Petrology, v. 6, pp. +47-472.

Fairbairn, H.W., Hurley, P.M., Card, K.D., and Knight, C.J., 1969,CorrelaL on of radiometric ages of Nipissing diabase and 1-luronianmetasedimen-ts with Proterozoic orogenic events in Ontario: Can.

Jour. Earth Sd., i. 6, pp. Lt89_497.

James, ILL., Clark, L.D., Lamey, C.L., and Pettijohn, F.J., 1961, Geologyof central Dickinson County, Michigan: U.S. Ceol. Surv. Prof.

Paper 310, 176 p.

Van Schmus, R., 1965, The geochronology of the Blind River—Bruce Nines area,Ontario, Canada: Jour. Ceol., v. 73, pp. 755-780.

-10-

References

Aldrich, L.T., Davis, G.L., and James, H.L., 1965, Ages of minerals frommetamorphic and igneous rocks near Iron Mountain, Michigan: Jour.

~ Petrology, v. 6, pp. 447-472.

Fairbairn, H.W., Hurley, P.M., Card, K.D., and Knight, C.J., 1969,Correlation of radiometric ages of Nipissing diabase and Huronianmetasediments with Proterozoic orogenic events in Ontario: Can.Jour. Earth Sci., v. 6, pp. 489-497.

James, H.L., Clark, L.D., Lamey, C.L., and Pettijohn, F.J., 1961, Geologyof central Dickinson County, Michigan: U.S. Geol. Surv. Prof.Paper 310, 176 p.

Van Schmus, R., 1965, The geochronology of the Blind River-Bruce Mines area,Ontario, Canada: Jour. Geol., v. 73, pp. 755-780.

—11--

HORELSES IN THE SOUTHERN PART OF THE DULUTH COMPLEX, MIIESOTA

Bill Bormichsen

Cornell University

Hornfels bodies ranging from less than an inch to thousands of feet indimensions are abundant in the southern part of the Duluth Complex. Theywere derived from a wide variety of initial rock types, many of which occurin the complex footwall. The most abundant types were derived from argilla-ceous sediments of the Virginia Formation, mafic to intermediate volcanicrocks of Keweenawan age and various intrusive rocks indigenous to the complex.Others include iron formation, quartzite and d.ioritic rocks from the pre-Keweenawan basement and. elastic sediments of probable Keweenawan age.

Virginia inclusions are very similar to metamorphosed footwall Virginia;however, the grain size is larger arid the relative mineral proportions areslightly different. The silicate mineralogy of argillaceous inclusions in-cludes coexisting cordierite, orthopyroxene, biotite, plagioclase and potassiumfeldspar. Minor amounts of pyrrhotite, graphite, ilmenite and traces of chal-copyrite and pentlandite are common, but olivine and Ca pyroxene are invariablyabsent. Various types of relict se.imentary structures are distinguishable inall but the smallest of hornfelsed Virginia inclusions.

Volcanic hornfels inclusions are characterized by their lack of relictsedimentary structures, local round to elliptical plagioclase and Ca pyroxenesegregations that probably were amygdules and their mineralogy. They consistlargely of plagioclase and Ca pyroxene, and normally contain abundant niagne-

tite and orthopyroxene (commonly inverted pigeonite). Olivine, brown horn-blende, apatite, ilmenite, and traces of biotite occur in some bodies but noneare known to contain cordierite, graphite, or more than traces of potassiumfeldspar.

The proportions of weight percent 3102, and. FeO+MgO of varioustypes of hornfelses and equivalent rocks are ploted in figure 1 (Bonnichsen,in prep.). The A, B and C groups are materials from the Virginia Formation,whereas the D, E and F groups are volcanic hornfelses and. their equivalents.Note that, although both groups show considerable range in their proportionof SiOa, the Virginia materials have a higher proportion of A12O3.

The group A samples are unmetamorphosed Virginia and equivalent horn-felses from the footwall of the complex. The group B samples are from inclu-sions within the complex. Note their depletion in 5102 relative to the Agroup. This suggests that Si02 was lost from the inclusions; this loss isaccompanied by a similar depletion of K20 in several samples and suggests lossof a grantic partial melt during the hornfelsip.g process. The group C samplesare from the margins of hornfels bodies and thus were in direct contact withthe intrusive rocks. Their compositions deviate farther from the initialvalues than do those of group B; they are considered to be refractory resi-duals which had become nearly equilibrated with the adjoining magnias. Part

of the group B rocks contain texturally interstitial feldspars and cordierite

which are interpreted as recrystallized partial melts. During the hornfelsingprocess a melt with the composition of cordierite arid, alkali feldspar evi-dently developed after a granitic fraction had been lost. Hornfelses with thegroup C compositon are considered to have formed where this later type of melthad been lost. The formation of granitic dikes and the production of abundantbiotite and orthopyroxene at the expense of olivine arid C pyroxene are theprincipal effects on the enclosing intrusive rocks wrought by the assimilationof such partial melts.

-11-

HORNFELSES IN THE SOUTHERN PART OF THE DULUTH COMPLEX, MINNESOTA

Bill Bonnichsen

Cornell University

Hornfels bodies ranging from less than an inch to thousands of feet indimensions are abundant in the southern part of the Duluth Complex. Theywere derived from a wide variety of initial rock types, many of which occurin the complex footwall. The most abundant types were derived from argillaceous sediments of the Virginia Formation, mafic to intermediate volcanicrocks of Keweenawan age and various intrusive rocks indigenous to the complex.Others include iron formation, quartzite and dioritic rocks from the preKeweenawan basement and clastic sediments of probable Keweenawan age.

Virginia inclusions are very similar to metamorphosed footwall Virginia;hovrever, the grain size is larger and the relative mineral proportions areslightly different. The silicate mineralogy of argillaceous inclusions includes coexisting cordierite, orthopyroxene, biotite, plagioclase and potassiumfeldspar. Minor amounts of pyrrhotite, graphite, ilmenite and traces of chalcopyrite and pentlandite are common, but olivine and Ca pyroxene are invariablyabsent. Various types of relict sedimentary structures are distinguishable inall but the smallest of hornfelsed Virginia inclusions.

Volcanic hornfels inclusions are characterized by their lack of relictsedimentary structures, local round to elliptical plagioclase and Ca pyroxenesegregations that probably were amygdules and their mineralogy. They consistlargely of plagioclase and Ca pyroxene, and normally contain abundant magnetite and orthopyroxene (commonly inverted pigeonite). Olivine, brown hornblende, apatite, ilmenite, and traces of biotite occur in some bodies but noneare knovm to contain cordierite, graphite, or more than traces of potassiumfeldspar.

The proportions of weight percent Si02, Al20~ and FeO+MgO of varioustypes of hornfelses and equivalent rocks are plotted in figure 1 (Bonnichsen,in prep. ). The A, B and C groups are materials from the Virginia Formation,vlhereas the D, E and F groups are volcanic homfelses and their equivalents.Note that, although both groups show considerable range in their proportionof Si02, the Virginia materials have a higher proportion of Al203.

The group A samples are unmetamorphosed Virginia and equivalent homfelses from the footwall of the complex. The group B samples are from inclusions vlithin the complex. Note their depletion in 8i02 relative to the Agroup. This suggests that 8i02 was lost from the inclusions; this loss isaccompanied by a similar depletion of ~O in several samples and suggests lossof a grantic partial melt during the hornfelsipg process. The group C samplesare from the margins of hornfels bodies and thus were in direct contact withthe intrusive rocks. Their compositions deviate further from the initialvalues than do those of group B; they are considered to be refractory residuals which had become nearly equilibrated vlith the adjoining magmas. Partof the group B rocks contain texturally interstitial feldspars and cordieritewhich are interpreted as recrystallized partial melts. During the hornfelsingprocess a melt ,nth the composition of cordierite and alkali feldspar evidently developed after a granitic fraction had been lost. Hornfelses with thegroup C compositon are considered to have formed where this later type of melthad been lost. The formation of granitic dikes and the production of abundantbiotite and orthopyroxene at the expense of olivine and Ca pyroxene are theprincipal effects on the enclosing intrusive rocks \vrought by the assimilationof such partial melts.

A1203

—12—

to FeOtMgOV

Group E consists of hornfelsed. basalts; they have bulk compositions thatshow little deviation from the compositions of ordinary basalts. Group F rocks,however, have lower and Si02 and higher Tb2 and FeO contents than ordinarybasalts and have high Fe/Mg ratios. Two of these hornfelses may have been in-trusive residual liquids which separated from trocto].ite but the other one evi-dently is a hornfe].sed volcanic rock that contains probable relict axnygdules.Sample D is an unmetamorphosed magnetite-rich basaltic rock from along theeastern margin of the complex; its Fe/Mg ratio is like that of group F. Thevolcanic rock in group F may initially have had a composition similar to thatof sample D. If so, it lost a substantial amount of granitic material duringthe hornfelsing process.

The abundance and distribution of hornfelsed volcanic rocks in thesouthern part of the complex supports the concept that an extensive system ofKeweenawan flows was present throughout the region and had been laid down onan erosion surface cut in the Middle Precambrian rocks. The intrusion of theTroctolitic Series and accompanying crustal extension evidently broke up thevolcanic rocks. As a result the hornfelses are ddely distributed throughoutthe southern half of the complex. Many of the large bodies occur as septumbetween adjacent intrusive bodies. The great number of inclusions of varioustypes along the footwall of the complex indicated that the intruding trocto-litic magmas had considerable mechanical strength. Evidently, these magmaswere in the form crystal-rich mushes during emplacement.

Ref: Bonnichsen, B. (in prep.); The southern part of the Duluth Complex, St.Louis and Lake Counties, Minnesota; to be publ. in Geolor of Minnesota,G.M. Schwartz commemorative volume; P.K. Sims, ed.

:7'F

Si02

FeO*

Figure I.

A 00

E

F.

to 41203V V

50

V V

-12-

Figure I.

AI203v v v

c@

vto FeO+ MgO

v

Group E consists of hornfelsed basalts; they have bulk compositions thatshow little deviation from the compositions of ordinary basalts. Group Frocks,however, have lower K20 and Si02 and higher Ti02 and FeO contents than ordinarybasalts and have high Fe/Mg ratios. Two of these hornfelses may have been intrusive residual liquids which separated from troctolite but the other one evidently is a hornfelsed volcanic rock that contains probable relict amygdules.Sample D is an unmetamorphosed magnetite-rich basaltic rock from along theeastern margin of the complex; its Fe/lf~ ratio is like that of group F. Thevolcanic rock in group F may initially have had a composition similar to thatof sample D. If so , it lost a substantial amount of granitic material duringthe hornfelsing process.

The abundance and distribution of hornfelsed volcanic rocks in thesouthern part of the complex supports the concept that an extensive system ofKew"eenawan flOivS was present throughout the region and had been laid dOim onan erosion surface cut in the Middle Precambrian rocks. The intrusion of theTroctolitic Series and accompanying crustal extension evidently broke up thevolcanic rocks. As a result the hornfelses are widely distributed throughoutthe southern half of the complex. l~ of the large bodies occur as septumbetvreen adjacent intrusive bodies. The great number of inclusions of varioustypes along the footwall of the complex indicated that the intruding troctolitic magmas had considerable mechanical strength. Evidently, these magmaswere in the form crystal-rich mushes during emplacement.

Ref: Bonnichsen, B. (in prep.); The southern part of the Duluth Complex, st.Louis and Lake Counties, Minnesota; to be publ. in Geology of Minnesota,G.M. Schw"artz connnemorative volume; P.K. Sims, ed.

—13—

A NEW VIEW OF THE DULUTH COMPLEX, MINNESOTA

by

Donald H. Davidson, Jr.University of Minnesota, Duluth

and

Minnesota Geological Survey

ABSTRACT

The late Precambrian (Keweenawan) Duluth Complex is a sequenceof generally discordant intrusive rocks extending northeastward fromDuluth to 1-loyland, Minnesota in a roughly crescentic configuration.The outcrop pattern of the Complex is bifurcated on the east and southby rocks of the North Shore Volcanic Group.

As may be noted in Table 1, three rock series (anorthosite,troctolite—olivine gabbro and felsic) constitute the major petrologicunits of the Complex. Anorthosite series rocks were apparently emplacedas large, crystal—liquid masses in which cumulus plagioclase is the pre-dominant mineral. The anorthositic rocks, which form the central portionof the Complex, have been subsequently intruded by the troctolite seriesalong the northern and western footwall and by the olivine gabbro seriesto the southeast. The troctolites and gabbros are a sequence of gener-ally layered rocks containing cumulus plagioclase and olivine and poik—ilitic pyroxenes. The basal troctolite series dips gently (15°) towardsLake Superior, whereas the attitudes of the upper olivine gabbro seriesvary from northward dips of 40° (e.g. Sawbill Lake area) to southwardat low cLS°) angles (e.g. along the Brule River Prong). Felsic seriesrocks are predominantly granophyric granite which occur as subhorizontalsheet—like masses above both margins of the olivine gabbro series. Sep-arate minor intrusives such as the Endion Sill, the Beaver Bay Complexand the Hovland Complex appear to be younger than the Duluth Complex,whereas only Lie Gunflint Prong Layered Complex appears older. Petrologic

relationships suggest multiple magma sources derived from the mantle, toaccount for the petrogenetic sequence of the Duluth Complex.

The Duluth Complex is coincident with the axis of the Mid—ContinentGravity High. Footwall shearing and the asymmetric position of theComplex with respect to the Lake Superior Syncline suggest Complexemplacenent consanguinous with rifting. Gravity studies to date suggest

the Complex thins or is absent beneath Lake Superior. Dominant Complexfracture trends (N.-�. 90E. or W. , N.20E.—N.SOE.) are common throughoutthe Lake Superior area suggesting regional post—intrusive extension.

Significant base—metal mineralization occurs within selectedtroctolite series horizons with local ore controls afforded by hostrock lithology and structural settling shelves. Should the proposedfunnel configuration of the central complex be verified, subsurfaceexploration of the Southern Complex appears warranted.

-13-

A NEW VIEW OF THE DULUTH CO~PLEX, MINNESOTA

by

Donald M. Davidson, Jr.University of Minnesota, Duluth

andHinnesota Geological Survey

A B S T R ACT

The late Precambrian (Keweenawan) Duluth Complex is a sequenceof generally discordant intrusive rocks extending northeastward fromDuluth to Hovland, Minnesota in a roughly crescentic configuration.The outcrop pattern of the Complex is bifurcated on the east and southby rocks of the North Shore Volcanic Group.

As may be noted in Table 1, three rock series (anorthosite,troctolite-olivine gabbro and felsic) constitute the major petrologicunits of the Complex. Anorthosite series rocks were apparently emplacedas large, crystal-liquid masses in ~.,hich cumulus plagioclase is the predominant mineral. The anorthositic rocks, t.,hich form the central portionof the Complex, have been subsequently intruded by the troctolite seriesalong the northern and western footwall and by the olivine gabbro seriesto the southeast. The troctolites and gabbros are a sequence of generally layered rocks containing cumulus plagioclase and olivine and poikilitic pyroxenes. The basal troctolite series dips gently (15°) towardsLake Superior, whereas the attitudes of the upper olivine gabbro seriesvary from northward dips of 40° (e.g. Sawbill Lake area) to southwardat low Q5°) angles (e.g. along the Brule River Prong). Felsic seriesrocks are predominantly granophyric granite which occur as subhorizontalsheet-like masses above both margins of the olivine gabbro series. Separate minor intrusives such as the Endion Sill, the Beaver Bay Complexand the Hovland Complex appear to be younger than the Duluth Complex,whereas only the Gunflint Prong Layered Complex appears older. Petrologicrelationships suggest multiple magma sources derived from the mantle, toaccount for the petrogenetic sequence of the Duluth Complex.

The Duluth Complex is coincident with the axis of the Mid-ContinentGravity High. Footwall shearing and the asymmetric position of theComplex with respect to the Lake Superior Syncline suggest Complexemplacencnt consanguinous with riftine. Gravity studies to date suggestthe Complex thins or is absent beneath Lake Superior. Dominant Complexfracture trends (N.-~.90E. or W., ~.20E.-~.80E.) are common throughoutthe Lake Superior area suggesting regional post-intrusive extension.

Significant base-metal mineralization occurs within selectedtroctolite series horizons with local ore controls afforded by hostrock lithology and structural settling shelves. Should the proposedfunnel configuration of the central complex be verified, subsurfaceexploration of the Southern Complex appears ~varranted.

Table 1. Summary of Principal Petrologic Series and Units, Duluth Complex, Minnesota

Series Unit Principal References

Endion Sill Ernst, W. C., 1960, Jour. Pet., Vol. 1, p. 286—303

Late Minor Beaver Bay Gehman, H. M., Jr., 1957, Unpub. Ph.D. Thesis, Univ. of Minn.

Intrusive Complex Konda, T., 1970, Contr. Mm. Pet., Vol 29, p. 338—344Complexes

Hoviand Complex Jones, N., 1963 Unpub. M.S. Thesis, Univ. of Minnesota

Granophyric Davidson, D. M., Jr., 1969, Minn. Geol. Surv. Misc.Granite and Map Ser., M—7, M—8

FelsicMinor Felsic Grout, F. F., and others, 1959, Minn. Geol. Surv. Bull. 39.Intrusives

Upper Olivine Grout, F. F., and others, 1959, Minn. Geol. Surv. Bull. 39.

Troctolite— GabbroDuluth CompleX01i Gahbro

Bonnichsen, Bill, 1969,30th Annual Min.Sympos.,Univ.Minn.pp.8993

cambrian (l.10.2b.y.) Basal Phinney, W. C. ,1969, Minn. Geol. Surv. Rept. mv. 9 HTroctolite Taylor, R. B., 1964, Minn. Geol. Surv. Bull. 44

Central Anorthosite Bonnichsen, Bill,1969,3Oth Annual I'lin.Sympos. ,Univ.Mlnn.pp.89—93

and Davidson, D. M., Jr., 1969, Minn. Geol. Survey Misc. MapAnorthosite Anorthosite Ser. 1—7, M—8

Inclusions Phinney, W., 1966, N.Y. State, Mus. Sd. Mem. Vol.18, p.135—147Taylor, R. B., 1964, Minn. Geol. Survey Bull. 44

Early Minor Gunflint Prong Sill Babcock, R.C., 1959, Unpub. M.S. Thesis, Univ. Wisc.

IntrusiveComplexes Brule Lake Sills Grout and others, 1959, Minn. Geol.Survey Bull. 39.

North Shore Granoels Davidson, D.M., Jr., 1969, Ninn. Geol. Survey Map Ser. M—7, N—8

Volcanic Group

Volcanics—Undifferentiated

'Ii ddleLower

Duluth Complex Early Minor Gunf lint Prong Nathan, H., 1969, Unpub. Ph.D. Thesis, Univ. Minn.Intrusive Layered Complex

Complexes

II

I

Table 1. Summary of Principal Petrologic Series and Units, Duluth Complex, Minnesota

Hovland Complex Jones, N., 1963 Unpub. M.S. Thesis, Univ. of Minnesota

Series

Late HinorIntrusiveComplexes

Felsic

Troctolite-,e Duluth ComplexOl " G hb,cambrian (l.ltO.2b.y.) l.Vl.ne a ro

Anorthosite

Early NinorIntrusiveComplexes

~orth ShoreVolcanic Group

;-1iddleLower

Duluth Complex Early HinorIntrusiveComplexes

Unit

Endion Sill

Beaver BayComplex

GranophyricGranite andHinor FelsicIntrusives

Upper OlivineGabbro

BasalTroctolite

Central Anorthositeand

AnorthositeInclusions

Gunflint Prong Sill

Brule Lake Sills

Granofels

VolcanicsUndifferentiated

Gunflint ProngLayered Complex

Principal References

Er.nst~ W. G., 1960, Jour. Pet.~ Vol. 1, p. 286-303

Gehman, H. M., Jr., 1957~ Unpub. Ph.D. Thesis, Univ. of Minn.Konda, T., 1970, Contr. Min. Pet., Vol 29, p. 338-344

Davidson, D. M., Jr., 1969, ~linn. Geol. Surv. Misc.Map Ser.~ M-7, M-8Grout, F. F., and others~ 1959, Minn. Geol. Surv. Bull. 39.

Grout, F. F., and others, 1959, Minn. Geol. Surv. Bull. 39.

Bonnichsen, Bill, 1969,30th Annual ~lin.Sympos. ,Univ.Minn.pp.89-93

Phinney, W. C.,1969~ Minn. Geol. Surv. Rept. Inv. 9 I

Taylor, R. B., 1964~ Minn. Geol. Surv. Bull. 44 ~I

Bonnichsen, Bill~1969~30th Annual Hin.Sympos.,Univ.Minn.pp.B9-93Davidson, D. }[., Jr.~ 1969, Minn. Geol. Survey Misc. HapSer. H-7~ M-BPhinney, W., 1966, N.Y. State~ Mus. Sci. Hem. Vol.18, p.135-l47Taylor~ R. B., 1964, Minn. Geol. Survey Bull. 44

Babcock~ R.C., 1959, Unpub. M.S. Thesis, Univ. Wise.

Grout and others, 1959 ~ Hinn. GeoI. Survey Bull. 39.

Davidson, D.H., Jr., 1969~ lorinn. Geol. Survey Map Ser. M-7, H-B

Nathan~ H., 1969, Unpub. Ph.D. Thesis, Univ. Minn.

-15—

TEXTURAL FACIES ANALYSIS OF PRECAMBRIAN CHEItTY IRONSTONES*

by

Erich Dimroth, Service de P Exploration geologiqueMinistere des Richesses Naturelles, Quebec, P.Q.

and

Jean—Jacques Chauvel, Departement de Geologie,Universite Rennes, Rennes, France

ABSTRACTTextures of Precambrian cherty ironstones are very similar to those

of limestones (Dimroth 1958). Accordingly the methods of limestone petrologycan be applied in the study of ironstones.

Ironstones can be conceived to be composed of textural elements.Textural ironstone types are defined by the kind and proportion of texturalelements present. Textural facies types compose thin (3 feet—lOO feet)stratigraphic units; they are either texturally homogeneous, or are com-posed of several textural rock types that form beds alternating in acharacteristic sequence. It is generally possible to determine the paleo—environment in which the various textural facies types formed, and bycorrelation of stratigraphic sections, to gain a view of the paleogeographyof the depositional basin.

We distinguish the following textural elements in the ironstones:

I — Orthochems

1. Femicrite: A past of fine—grained iron silicate (minnesotaite,stilpnomelane) or of siderite derived from iron silicate and iron carbonate

muds.

2. Matrix chert; Chert deposited as a silicagel matrix.

3. Cement chert: Chert that formed between allochem grains after

deposition.

Matrix chert and cement chert can be distinguished only in somehematite irons tones.

IL — Allochems

1. Pellets: Ellipsoidal particles 0.2 twa. across unsharply bounded,

always embedded in matrix chert. Pellets are interpreted as aggregated

particles.

*Published with the premission of the Deputy Minister, Departmentof Natural REsources, Quebec.

-15-

TEXTURAL FACIES ANALYSIS OF PRECAJfBRIAN CHERTY IRONSTONES*

by

Erich Dimroth, Service de l'Exploration geologiqueMinistere des Richesses Naturelles, Quebec, P.Q.

and

Jean-Jacques Chauvel, Departement de Geologie,Universite Rennes, Rennes, France

A B S T R ACT

Textures of Precambrian cherty ironstones are very similar to thoseof limestones (Dimroth 1958). Accordingly the methods of limestone petrologycan be applied in the study of ironstones.

Ironstones can be conceived to be composed of textural elements.Textural ironstone types are defined by the kind and proportion of texturalelements present. Textural facies types compose thin (3 feet-lOO feet)stratigraphic units; they are either texturally homogeneous, or are composed of several textural rock types that form beds alternating in acharacteristic sequence. It is generally possible to determine the paleoenvi~onment in which the various textural facies types formed, and bycorrelation of stratigraphic sections, to gain a view of the paleogeographyof the depositional basin.

We distinguish the following textural elements in the ironstones:

I Orthochems

1. Femicrite: A past of fine-grained iron silicate (minnesotaite,sti1pnome1ane) or of siderite derived from iron silicate and iron carbonatemuds.

2. ~~trix chert: Chert deposited as a silicagel matrix.

3. Cement chert: Chert that formed between allochem grains afterdeposition.

Matrix chert and cement chert can be distinguished only in somehematite ironstones.

II Allochems

1. Pellets: Ellipsoidal particles 0.2 mm. across unsharply bounded,always embedded in matrix chert. Pellets are interpreted as aggregatedparticles.

*Published with the premission of the Deputy Minister, Departmentof Natural REsources, Quebec.

—16—

2. Entraclasts: Fragments of the unconsolidated sediment thathave been redeposited. Previously (James, 1954) described from intra—formational conglomerates (large intraclasts) and as "granules" (= sand—size intraclasts).

3. Oolites and Pisolites.

4. Shards: Shards are complex textures. Two sub—types predominate(a) welded and extremely deformed oolites and intraclasts (accommodationshards). (b) shards composed mainly of peeled off fragments of oolites andintraclasts (exfoliation shards).

Textural rock types are defined by the combination of textural elementspresent. Main types are:

I. Femicrites: Laminated or ribboned silicate—carbonate ironstones.

II. Matrix chert: Laminated or ribboned cherts, generally with pellets.

III. Cemented intraclastjc or oolitic ironstones.

IV. Intraclastic or oolitic rocks with chert matrix.

V. Intrafemicrjtes: Intraclasts embedded in a femicrite matrix.

Some other types, particularly containing shard textures, are quantitativelyless important.

The paleogeographic application of the method will be demonstrated atthe example of the lower jaspilite member of the Sokoman Ironstone in thewestern half of the Labrador trough between latitudes 54°45'N and 55°15'N.

Close to the western margin of the trough this stratigraphic unit islocally represented by a massive or thickly bedded finely intraclasticand o.olitic chert cemented hematite ironstone (facies type 1). Towardthe east this type grades into interbedded oolitic—intraclastic hematiteironstone with chert cement or matrix, alternating with laminated matrixchert containing hematite (fades type 2). This type is thin to mediumbedded (2—30 cm.) andcharacteristically shows lenticular bedding. It grades

basinwards into a type that contains intercalated beds, 2—100 cm. thick,of laminated femicrite (facies type 3). Farther east the oolitic intra—clastic beds are lacking in type 4 and finally type 5 is composed only oflaminated femicrite with interbeds of laminated femicrite bearing matrixchert. Pacies types 3 and 4 are characterized by siump structures andstructures indicating strong synsedimentary deformation. Toward the centreof the trough types 4 and 3 reappear, and grade into a facies containinginterbeds with very coarse (2 cm.) intraclasts and pisolites embedded inmatrix chert (facies type 6); the intraclasts and pisolites were verysoft at time of deposition. Facies type 1 is tentatively interpreted asa sand bar facies that separates a lagoonal environment (not discussedhere) farther west from the open basin. Facies type 2 has been depositedin the intratidal. and shallow sub—tidal zone in the foreshore of the sandbars. Types 3 and 4 represent a relatively deep subtidal facies and

-16-

2. Intraclasts: Fragments of the unconsolidated sediment thathave been redeposited. Previously (James, 1954) described from intraformational conglomerates (large intraclasts) and as "granules" (= sandsize intraclasts).

3. Oolites and Pisolites.

4. Shards: Shards are complex textures. Two sub-types predominate(a) welded and extremely deformed oolites and intraclasts (accommodationshards). (b) shards composed mainly of peeled off fragments of oolites andintraclasts (exfoliation shards).

Textural rock types are defined by the combination of textural elementspresent. Main types are:

I. Femicrites: Laminated or ribboned silicate-carbonate ironstones.

II. Matrix chert: Lamina~ed or ribboned cherts, generally with pellets.

III. Cemented intraclastic or oolitic ironstones.

IV. Intraclastic or oolitic rocks with chert matrix.

V. Intrafemicrites: Intraclasts embedded in a femicrite matrix.

Some other types, particularly containing shard textures, are quantitativelyless important.

The paleogeographic application of the method will be demonstrated atthe example of the lower jaspilite member of the Sokoman Ironstone in thewestern half of the Labrador trough between latitudes 54°45'N and 55°l5'N.

Close to the western margin of the trough this stratigraphic unit islocally represented by a massive or thickly bedded finely intraclasticand oolitic chert cemented hematite ironstone (facies type 1). Towardthe east this type grades into interbedded oolitic-intraclastic hematiteironstone with chert cement or matrix, alternating with laminated matrixchert containing hematite (facies type 2). This type is thin to mediumbedded (2-30 em.) andcharacteristically shows lenticular bedding. It gradesbasinwards into a type that contains intercalated beds, 2-100 ern. thick,of laminated femicrite (facies type 3). Farther east the oolitic intraclastic beds are lacking in type 4 and finally type 5 is composed only oflaminated femicrite with interbeds of laminated femicrite bearing matrixchert. Facies types 3 and 4 are characterized by slump structures andstructures indicating strong synsedimentary deformation. Toward the centreof the trough types 4 and 3 reappear, and grade into a facies containinginterbeds with very coarse (2 em.) intraclasts and pisolites embedded inmatrix chert (facies type 6); the intraclasts and pisolites were verysoft at time of deposition. Facies type 1 is tentatively interpreted asa sand bar facies that separates a lagoonal environment (not discussedhere) farther west from the open basin. Facies type 2 has been depositedin the intratidal and shallow sub-tidal zone in the foreshore of the sandbars. Types 3 and 4 represent a relatively deep subtidal facies and

—17—

fades zone 5 was deposited in a relatively deep basinal environment(maybe 50—100 m. water depth). Fades type 6 was likely deposited onshallow sub—tidal shoals that extended in the centre of the Labradortrough. The coarse intraclastic and pisolitic beds may representstorm layers.

References:

James, II. L., 1954: Sedimentary fades of iron formation. Econ. Ceol.,v. 49, p. 235—293.

Dirmroth, 13., 1968: Sedimentary textures, diagenesis and sedimentaryenvironment of certain Precambrian Eronstones. N. Jb. Geol.Palaeont., Abh. v. 130, p. 247—274.

-17-

facies zone 5 was deposited in a relatively deep basinal environment(maybe 50-100 m. water depth). Facies type 6 was likely deposited onshallow sub-tidal shoals that extended in the centre of the Labradortrough. The coarse intraclastic and pisolitic beds may representstorm layers.

References:

James, H. L., 1954: Sedimentary facies of iron formation. Econ. Geol.,v. 49, p. 235-293.

Dimroth, E., 1968: Sedimentary textures, diagenesis and sedimentaryenvironment of certain Precambrian Ironstones. N. Jb. Geol.Palaeont., Abh. v. 130, p. 247-274.

—18—

PRECAMBRIAN IRON FORMATION At COPPER

MOUNTA IN, PREMONT COUNTY, WYOMING

William H. Duhling, Jr.Natural Resources Research Institute

University of Wyoming, Laramie

ABSTRACTPrecambrian banded iron formation is exposed along the south-facing

flanks of Copper Mountain, approximately 15 miles north of Shoshoni, innortheastern Frenont County, Wyoming. The Precambrian metamorphic com-plex consists of interlayered quartzofeldspathic gneisses, amphibolites,amphibole schists, peliric rocks, and iron formation into which granite,pegmatite, and mafic dikes and sills have been intruded. The complexhas been metamorphosed to the kyanite-muscovite subfacies of the alman-dine-amphibolite facies. Sedimentary rocks of Paleozoic or Cenozoicage abut the Precambrian complex on all sides.

The mineralogical composition of the iron formation is very simple,consisting of magnetite, quartz, and any combination of blue-green horn-blend, grunerite, and cummingtonite. Magnetite, in all degrees of alter-ation to hematite, occurs in thin laminae and fine clusters with angular,sharply embayed boundaries and as very fine inclusions in quartz andamphibole grains. Banding is developed by apparent increase in grainsize of the magnetite at the expense of quartz and amphibole grains.

The iron formation, interlayered with amphibolite and quartz-micaschist, is exposed for a distance of about 6 miles over an approximatewidth of 1000 feet. The strike of the formation is N 80°E and the dipis 700 to the south.

Liberation and concentration tests indicate the presence of roughly13¾ million tons of concentrates containing 56% iron and 22% silica. Theamount of very fine magnetite in quartz and amphibole grains and theamount of interstitial quartz in the magnetite-rich bands account forthe high silica content of the concentrates.

Rail transportation comes within ten miles of the outcrop area, butthe nearest markets for a blast furnace feed are over 240 miles away.

The distance to market, high silica content, low tonnage, and steepdip combine to indicate that this iron formation has a questionableeconomic potential.

-18-

PRECAMBRIAN IRON FORMATION AT COPPER

MOUNTAIN, FREMONT COUNTY, WYOMING

William H. Duhling, Jr.Natural Resources Research Institute

University of Wyoming, Laramie

A B S T R ACT

Precambrian banded iron formation is exposed along the south-facingflanks of Copper Mountain, approximately 15 miles north of Shoshoni, innortheastern Fremont County, Wyoming. The Precambrian metamorphic complex consists of interlayered quartzofeldspathic gneisses, amphibolites,amphibole schists, pelitic rocks, and iron formation into which granite,pegmatite, and mafic dikes and sills have been intruded. The complexhas been metamorphosed to the kyanite-muscovite subfacies of the almandine-amphibolite facies. Sedimentary rocks of Paleozoic or Cenozoicage abut the Precambrian complex on all sides.

The mineralogical composition of the iron formation is very simple,consisting of magnetite, quartz, and any combination of blue-green hornblend, grunerite, and curnrningtonite. Magnetite, in all degrees of alteration to hematite, occurs in thin laminae and fine clusters with angular,sharply embayed boundaries and as very fine inclusions in quartz andamphibole grains. Banding is developed by apparent increase in grainsize of the magnetite at the expense of quartz and amphibole grains.

The iron formation, interlayered with amphibolite and quartz-micaschist, is exposed for a distance of about 6 miles over an approximatewidth of 1000 feet. The strike of the formation is N 80 0 E and the dipis 70° to the south.

Liberation and concentration tests indicate the presence of roughlyl3~ million tons of concentrates containing 56% iron and 22% silica. Theamount of very fine magnetite in quartz and amphibole grains and theamount of interstitial quartz in the magnetite-rich bands account forthe high silica content of the concentrates.

Rail transportation comes within ten miles of the outcrop area, butthe nearest markets for a blast furnace feed are over 240 miles away.

The distance to market, high silica content, low tonnage, and steepdip combine to indicate that this iron formation has a questionableeconomic potential.

—19—

Selected References

1. Duhling, William H., Jr, 1970, Oxide facies iron formation in theOwl Creek Mountains, northeastern Fremont County, Wyoming: unpub-lished MS thesis, University of Wyoming, 92 p.

2. Gliozzi, James L., 1967, Petrology and structure of the Precambrianrocks of the Copper Mountain district, unpublished PhD dissertation,University of Wyoming, 141 p.

3. Kopp, Richard S., 1964, Reconnaissance geology of metamorphic struc-tures of the Copper Mountain Mining district, Fremont County, Wyoming:Compass, v. 43, p. 6-20.

4. Millgate, M. L., Gliozzi, James L., 1966, Reconnaissance of iron for-mation in the Copper Mountain area, Fremont County, Wyoming: unpub-lished report Geological Survey of Wyoming files.

-19-

Selected References

1. Duh1ing, William H., Jr, 1970, Oxide facies iron formation in theOwl Creek Mountains, northeastern Fremont County, Wyoming: unpublished MS thesis, University of Wyoming, 92 p.

2. G1iozzi, James L., 1967, Petrology and structure of the Precambrianrocks of the Copper Mountain district, unpublished PhD dissertation,University of Wyoming, 141 p.

3. Kopp, Richard S., 1964, Reconnaissance geology of metamorphic structures of the Copper Mountain Mining district, Fremont County, Wyoming:Compass, v. 43, p. 6~20.

4. Mi11gate, M. L., G1iozzi, James L., 1966, Reconnaissance of iron formation in the Copper Mountain area, Fremont County, Wyoming: unpublished report Geological Survey of Wyoming files.

-20-

Stratigraphy of the North Shore Volcanic GroupNortheast of Silver Bay, Minn.

by

John C. GreenUniversity of Minnesota, Duluth


A B S T RAe T

A sequence of flows and flow vPups, totalling about 21,500 feet,makes up the northeast limb of the North Shore Volcanic Group ofKeweenawan (late Precambrian) age between Tofte and Grand Portage.Exposures are generally good to excellent along the Lake Superiorshore or abandoned wave-cut cliffs, and although cut by severallarge intrusive bodies and obscured by glacial cover over wide areas,many of these lithostratigraphic units can be traced inland fromLake Superior for many miles. Table 1 shows the informal volcanicunits at the northeast limb (top of section at Tofte,base at GrandPortage); Table 2 gives the sequence (less well established becauseof faulting and intrusion) of the upper part of the southwest limbfrom Tofte as far as Palisade Head near Silver Bay. The Schroederbasalts at the top of the southwest limb are at least in partequivalent to the Lutsen basalts of the northeast limb.

The basal 5,000 feet, near Grand Portage, Minn., have reversedmagnetic polarity (Lower Keweenawan) whereas all the rest have normalpolarity (Middle Keweenawan). The lowermost 250 feet of lavas onLucille Island, directly overlying the basal Upper PrecambrianPuckwunge Sandstone, are porphyritic melabasalts with abundantolivine and augite phenocrysts, identical to those in the samestratigraphic position at Nopeming, west of Duluth.

—2]—

Table 1

Stratigraphy of Northeast Linb (Tofte to Grand Portage) of

North Shore Volcanic Group (Exclusive of Interf low Sediments)

Approx.Thickness()

Top (near Tofte —

1020

160

310

360

500

600

-- 1020

— 400—900

1300

1800

1000

3500

4000

200

260

4500

Base

21,430

(est.)

Lithostratigraphic unit

Lu t sen)

Lutsen basalts

Terrace Point basalt flow

Good Harbor Bay andesites

Breakwater trachybasalt flow

Grand Marais rhyolite flow

Croftville basalts

Devil Track felsites

Red Cliff basalts

Kimball Creek felsite

Marr Island lavas

Brule River basalts

Brule River rhyolite flow

Hoviand lavas

Red Rock rhyolite flow

Deronda Bay andesite flow

Grand Portage basalts

(at Grand Portage)

Lithic Character

olivine basalts, olivine tholeittes

Thomsonite—bearing ophitic basalt

brown, porphyritic andesite,trachyandesite

brown, columnar, granulartrachybasalt

pink, red, gray porphyritic rhyolite

various fine—grained basalts

aphyric and porphyritic rhyolite flows

amygdaloidal, ophitic olivine basalts

pink to tan, porphyritic felsite

mixed tholeiltic basalt, intermediate,felsic lavas

granular—diabasic amygdaloidal basalts

pink to gray porphyritic rhyolite

mixed porphyritic basalt, trachybasalt,rhyolite

red, porphyritic rhyolite

gray—brown, aphyric andesite

nixed tholeiitic to diabasic basalts

-2J-

Table 1

Stratigraphy of Northeast Limb (Tofte to Grand Portage) of

North Shore Volcanic Group (Exclusive of Interflow Sediments)

Approx.- Thickness.J ft.) Lithostratigraphic unit Lithic eharacter

Top (near Tofte - Lutsen)

1020

160

310

360

500

600

1020

400-900

1300

1800

1000

3500

4000 (est. )

200

260

4500

Base

2l~430

Lutsen basalts




Grand I1arais rhyolite flow

Croftville basalts


Red Cliff basalts


Marr Island lavas

Brule River basalts

Brule River rhyolite flow

Hovland lavas




(at Grand Portage)

olivine basalts, olivine tholeiites

Thomsonite-bearing ophitic basalt


brown, columnar, granulartrachybasalt


various fine-grained basalts

aphyric and porphyritic rhyolite flows



mixed tholeiitic basalt, intermediate)felsic lavas

granular-diabasic amygdaloidal basalts




gray-brown, aphyric andesite

mixed tholeiitic to diabasic basalts

—22—

Table 2

Stratigraphy of Upper Part of Southwest Limb (Tofte

to Palisade Head) of North Shore Volcanic Group

Approx.Thickness (ft.) Lithostratigraphic unit Lithic character

Top (at Tofte)

4000 Schroeder basalts amygdaloidal, ophitic olivinetholeiites

>300 Manitou trachybasalt flow red—brown, granular trachybasaltto basalt

(more of the Schroeder basalts)

>280 Bell Harbor lavas quartz tholeiites, aphyrictrachybasalts

>300 Palisade rhyolite flow gray to pink, porphyritic rhyolite

Few 100's Baptism River lavas mixed lavas, mostly basalts

-22-

Table 2

Stratigraphy of Upper Part of Southwest Limb (Tofte

to Palisade Head) of North Shore Volcanic Group

Approx.Thickness (ft.)

Top (at Tofte)

4000

>300

>280

>300

Few 100's


Schroeder basalts

Manitou trachybasalt flow


Bell Harbor lavas

Palisade rhyolite flow

Baptism River lavas

Lithic character

amygdaloidal, ophitic olivinetholeiites

red-brown, granular trachybasaltto basalt

quartz tholeiites, aphyrictrachybasalts

gray to pink, porphyritic rhyolite

mixed lavas, mostly basalts

-2 3—

SHALLOW STRUCTURE AND STRATIGRAPHY OF THE LAXE SUPERIOR

BASIN FROM SEISMIC REFRACTION MEASUREMENTS

F1.C. Halls and C.?. West

Geophysics Laboratory, Dept. of Physics, University of Toronto.

ABSTRACT

Between 1966 and 1969 thirty—three seismic refraction profiles wereobtained in Lake Superior using a single ship sonobuoy technique (Halls andWest, 1971). The seismic data have led to a number of conclusions concerningthe shallow structure and stratigraphy of the late Precambrian Keweenawn basinthat underlies the lake:

(1) Faults with their north side downthrown at least 1—2 1cm bound the northshores of Isle Royale and Michipicoten Island.

(2) The northern limb of the Keweenawan basin just southeast of Isle Royaleappears to have undergone a deformation that is perhaps related to movementalong the Isle Royale fault.

(3) In addition to the main basin between Isle Royale and the Keweenaw Peninsula,there is a suggestion of a smaller one southwest of the Slate Islands in thenorthern part of the lake.

(4) Late Keweenawan or early Cambrian Bayfield—Jacobsville sandstones appearto underlie most of Lake Superior. Although the site of the lake is governedby the position of the underlying basin, the principal factors determining thesize and shape of the lake depression are the distribution of the Bayfield—Jacobsville sandstones and their susceptibility to erosion compared with olderrocks.

(5) For those seismic profiles that have a certain degree of geologicalcontrol, such as those between Isle Royale and the Keweenaw Peninsula, refractionvelocities generally agree well with those estimated (Halls, 1969) from sasplemeasurements in the laboratory. One notable exception occurs in those profilesthat lie just off the Minnesota shore. Here velocities of 5 km/s are recorded.These values are typical of Freda—Copper Harbor sandstones, but the presenceof these rocks adjacent to the Minnesota coast is discounted on magneticevidence. Instead the velocities of S km/s are assigned to the North Shorevolcanics that crop out along the mainland. The apparently low velocities ofthese rocks compared to those for volcanics between Isle Royale and the KeweenawPeninsula (5.7—6.2 kmfs) can be explained if the North Shore sequence containsa greater proportion of interflow sediment and/or amygdaloidal flow top material.

(6) The seismic penetration of the refraction profiles was generally insufficientto record the Upper Refractor (Berry and West, 1966) with velocity 6.7± km/s.However, one profile northeast of Isle Royale recorded a velocity of 6.5 km/sat a depth of about 6 km. This observation demonstrates the existence of UpperRefractor—type velocities in the marginal parts of the Keweenawan basin.

~23-

SHALLOW STRUCTURE AND STRATIGRAPHY OF THE LAKE SUPERIOR

BASIN FROM SEISMIC REFRACTION MEASUREMENTS

H.C. Halls and G.F. West


ABSTRACT

Between 1966 and 1969 thirty-three seismic refraction profiles wereobtained in Lake Superior using a single ship sonobuoy technique (Halls andWest, 1971). The seismic data have led to a number of conclusions concerningthe shallow structure and stratigraphy of the late Precambrian Keweenawn basinthat underlies the lake:

(1) Faults with their north side downthrown at least 1-2 krn bound the northshores of Isle Royale and Michipicoten Island.

(2) The northern limb of the Keweenawan basin just southeast of Isle Royaleappears to have undergone a deformation that is perhaps related to movementalong the Isle Royale fault.

(3) In addition to the main basin between Isle Royale and the Keweenaw Peninsula,there is a suggestion of a smaller one southwest of the Slate Islands in thenorthern part of the lake.

(4) Late Keweenawan or early Cambrian Bayfield-Jacobsville sandstones appearto underlie most of Lake Superior. Although the site of the lake is governedby the position of the underlying basin, the principal factors determining thesize and shape of the lake depression are the distribution of the BayfieldJacobsville sandstones and their susceptibility to erosion compared with olderrocks.

(5) For those seismic profiles that have a certain degree of geologicalcontrol, such as those between Isle Royale and the Keweenaw Peninsula, refractionvelocities generally agree well with those estimated (Halls, 1969) from samplemeasurements in the laboratory. One notable exception occurs in those profilesthat lie just off the Minnesota shore. Here velocities of 5 km/s are recorded.These values are typical of Freda-Copper Harbor sandstones, but the presenceof these rocks adjacent to the Minnesota coast is discounted on magneticevidence. Instead the velocities of 5 km/s are assigned to the North Shorevolcanics that crop out along the mainland. The apparently low velocities ofthese rocks compared to those for volcanics between Isle Royale and the KeweenawPeninsula (5.7-6.2 km/s) can be explained if the North Shore sequence containsa greater proportion of interflow sediment and/or amygda10idal flow top material.

(6) The seismic penetration of the refraction profiles was generally insufficientto record the Upper Refractor (Berry and West, 1966) with velocity 6.7± km/s.However, one profile northeast of Isle Royale recorded a velocity of 6.5 km/sat a depth of about 6 km. This observation demonstrates the existence of UpperRefractor-type velocities in the marginal parts of the Keweenawan basin.

—24—

References

Berry, M.J. and G.F. West. 1966. An interpretation of the first arrival dataof the Lake Superior experiment by the time—term method, Bull. Seismol.Soc. Amer., 56, 141—171.

Halls, H.C. 1969. Compressional wave velocities of Keweenawan rock specimensfrom the Lake Superior region, Can. Jour. Earth Sci., 6, 555—568.

Halls, H.C. and G.E. West. 1971. A seismic refraction survey in Lake Superior,Can. Jour. Earth Sci.(In press).

-24-

References

Berry, M.J. and G.F. West. 1966. An interpretation of the first arrival dataof the Lake Superior experiment by the time-term method, Bull. Seismol.Soc. Amer., 56, 141-171.

Halls, H.C. 1969. Compressional wave velocities of Keweenawan rock specimensfrom the Lake Superior region, Can. Jour. Earth Sci., ~, 555-568.

Halls, H.C. and G.F. West. 1971. A seismic refraction survey in Lake Superior,Can. Jour. Earth Sci. (In press).

—25—

THE ISLE ROYALE FAULT

B.C. Halls and G.F. West


ABSTRACT

A fault bounding the northwest shore of Isle Royale was originally postu-lated by Irving and Chamberlin (1885) on physiographic grounds. However, withthe exception of recent aeromagnetic studies (Wold and Ostenso, 1966; Hinze etal., 1966) very little extra evidence has been produced as to whether the faiTtreally exists. The aeromagnetic data show a linear anomaly that follows thenorthwest shore of Isle Royale and extends eastward to Superior Shoal. Althoughthe anomaly indicates a contact between Keweenawan volcanics and sediments, itdoes not reveal whether the contact is a stratigraphic or faulted one (Halls,1970). Evidence to date for the so—called Isle Royale fault has therefore beenrather inconclusive. This paper discusses some lines of evidence that supportthe existence of the fault, in the light of new magnetic and seismic data:

(1) Seismic data (Halls and West, 1971) show that the uppermost layer that

underlies the channel north of Isle Royale and also the region further northeasttoward the Slate Islands has a velocity of about 3.7 kin/s and a thickness ofabout 1—2 km. This seismic layer is all but continuous with a similar one ineastern Lake Superior that can be firmly identified as Eayfield—.Jacobsvillesandstones. These rocks are thus thought to underlie the Isle Royale channeland if so, their existence necessitates the inclusion of the Isle Royale faultwith its downthrown side to the north.

(2) Paleomagnetic data (e.g. Books, 1968; Palmer, 1970) show a remarkablyconsistent pattern for Keweenawan extrusive rocks. Whereas the lower parts ofthe volcanic sequence (such as the Osler and South Range lavas) tend to bereversely magnetised, the upper parts such as the Isle Royale and Portage Lakelavas are normal. In the Isle Royale channel a linear magnetic anomaly (C inFig.l), which is attributed to Keweenawan volcanics, is strongly positive withan attendant minimum to the north, signifying that the volcanics are normallymagnetised. Anomaly C continues beyond the channel to both the east and westwhere it broadens considerably to form anomalies A and D (Fig.l). The sharpnessof anomalies A,C and D together with the seismic data indicate that the volcanicsare overlain by Keweenawan sedimentary rocks. Thus north of the postulated IsleRoyale fault there are buried, normally magnetised, Keweenawan volcanics. If

the magnetic Isle Royale 'fault' anomaly were due to a stratigraphic contactbetween volcanics and sediments it would imply that the sedimentary unit wassandwiched between two thick sequences of normally magnet3sed volcanics (i.e.the Isle Royale lavas and those causing anomalies A,C and D). Such a sequenceis of course possible but it is not a known feature of Keweenawan stratigraphy.The only thick sedimentary unit that occurs in the Keweenawan volcanic sequenceis that in Michigan and Wisconsin but it lies between the normal and thereversely magnetised parts of the volcanic sequence (e.g. Meshref and Hinze,1970). Thus in northern Lake Superior a duplication of the normally magnetisedvolcanics through movement along the Isle Royale fault is favoured over astratigraphic sequence of a sedimentary unit between two normal volcanic ones.

(3) The magnetic map of Fig.l shows that the Isle Royale 'fault' anonaly(E)gradually assumes a more southerly trend toward the west. A weakening of theanomaly in this direction is more compatible with the presence of a fault ratherthan a stratigraphic volcanic—sediment contact (Balls, 1970).

-25-

THE ISLE ROYALE FAULT

H.C. Halls and G.F. West


ABSTRACT

A fault bounding the northwest shore of Isle Royale was originally postulated by Irving and Chamberlin (1885) on physiographic grounds. However, withthe exception of recent aeromagnetic studies (Wold and Ostenso, 1966; Hinze etal., 1966) very little extra evidence has been produced as to whether the faurtreally exists. The aeromagnetic data show a linear anomaly that follows thenorthwest shore of Isle Royale and extends eastward to Superior Shoal. Althoughthe anomaly indicates a contact between Keweenawan volcanics and sediments, itdoes not reveal whether the contact is a stratigraphic or faulted one (Halls,1970). Evidence to date for the so-called Isle Royale fault has therefore beenrather inconclusive. This paper discusses some lines of evidence that supportthe existence of the fault, in the light of new magnetic and seismic data:

(1) Seismic data (Halls and West, 1971) show that the uppermost layer thatunderlies the channel north of Isle Royale and also the region further northeasttoward the Slate Islands has a velocity of about 3.7 km/s and a thickness ofabout 1-2 km. This seismic layer is all but continuous witih a similar one ineastern Lake Superior that can be firmly identified as Bayfield-Jacobsvillesandstones. These rocks are thus thought to underlie the Isle Royale channeland if so, their existence necessitates the inclusion of the Isle Royale faultwith its downthrown side to the north.

(2) Paleomagnetic data (e.g. Books, 1968; Palmer, 1970) show a remarkablyconsistent pattern for Keweenawan extrusive rocks. Whereas the lower parts ofthe volcanic sequence (such as the Osler and South Range lavas) tend to bereversely magnetised, the upper parts such as the Isle Royale and Portage Lakelavas are normal. In the Isle Royale channel a linear magnetic anomaly (C inFig.l), which is attributed to Keweenawan volcanics, is strongly positive withan attendant minimum to the north, signifying that the volcanics are normallymagnetised. Anomaly C continues beyond the channel to both the east and westwhere it broadens considerably to form anomalies A and D (Fig.l). The sharpnessof anomalies A,C and D together with the seismic data indicate that the volcanicsare overlain by Keweenawan sedimentary rocks. Thus north of the postulated IsleRoyale fault there are buried, normally magnetised, Keweenawan volcanics. Ifthe magnetic Isle Royale 'fault' anomaly were due to a stratigraphic contactbetween volcanics and sediments it would imply that the sedimentary unit wassandwiched between two thick sequences of normally magne~ised volcanics (i.e.the Isle Royale lavas and those causing anomalies A,C and D). Such a sequenceis of course possible but it is not a known feature of Keweenawan stratigraphy.The only thick sedimentary unit that occurs in the Keweenawan volcanic sequenceis that in Michigan and Wisconsin but it lies between the normal and thereversely magnetised parts of the volcanic sequence (e.g. Meshref and Hinze,1970). Thus in northern Lake Superior a duplication of the normally magnetisedvolcanics through movement along the Isle Royale fault is favoured over astratigraphic sequence of a sedimentary unit between two normal volcanic ones.

(3) The magnetic map of Fig.l shows that the Isle Royale 'fault' anomaly (E)gradually assumes a more southerly trend toward the west. A weakening of theanomaly in this direction is more compatible with the presence of a fault ratherthan a stratigraphic volcanic-sediment contact (Halls, 1970).

—26—

(4) Anomaly.C in Fig.l is essentially continuous with a belt of prominentpositive anomalies that extends along much of the Minnesota shore before turningsouth and terminating in a hook—shaped anomaly over the Eayfield—Peninsula(Wold and Ostenso, 1966). White (1966) concludes that this anomaly is due tovolcanics in the upper part of the Keweenawan extrusive sequence. The volcanicscausing anomaly C should thus be equivalent, at least in part, to the IsleRoyale lavas. Again, a fault would be necessary to explain the apparentduplication of the sequence.

The foregoing observations therefore all tend to suggest that the IsleRoyale fault does exist. A two—dimensional magnetic model interpretation ofthe Isle Royale fault anomaly at its eastern end (Balls, 1970) suggests that thefault dips to the south. The fault is thus of reversad type as its downthrownside is to the north. The displacement along the fault is at least 1—2 km. Thereversed nature of the fault and its increasing southerly trend toward the west(Fig.l) support the idea initially raised by Irving and Chamberlin (1885) thatit is an easterly continuation of the Douglas fault in Wisconsin.

References

Books, K.G. 1968. Magnetisation of the lowermost Keweenawan lava flows in theLake Superior area, USGS Prof. Paper 600—0, 248—254.

Halls, B.C. 1970. Geological interpretation of geophysical data from the LakeSuperior region, Ph.D. Thesis, University of Toronto, 203 pp.

Halls, B.C. and G.E. West. 1971. A seismic refraction survey in Lake Superior,Can. Jour. Earth Sci. (In press).

Hinze, W.J., O'Hara, N.W., Trow, J.W. and Secor G.B. 1966. Aeromagnetic studiesof eastern Lake Superior, AGU Mono., 10, 95—110.

Irving, R.D. and Chamberlin, T.C. 1885. Observations on the junction betweenthe eastern sandstone and the Keweenaw Series on Keweenaw Point, LakeSuperior, Bull.US. Geol. Surv., 23, 385—498.

Meshref, W.M. and Hinze, W.J. 1970. Geologic interpretation of aeromagnetic datain western Upper Peninsula of Michigan, Mich. Geol. Sun., Kept. of lxiv.,12, 25 pp.

Palmer, B.C. 1970. Paleomagnetisni and correlation of some Middle Keweenawanrocks, Lake Superior, Can. Jour. Earth Sd., 7, 1410—1436.

White, W.S. 1966. Tectonics of the Keweenawan basin, western Lake Superior region,USGS Prof. Paper 525—E, 23 pp.

Wold, R.J. and Ostenso, N.A. 1966. Aeromagnetic, gravity and sub—bottomprofiling studies in western Lake Superior, AGU Mono., 10, 66—94.

-26-

(4) Anomaly.C in Fig.l is essentially continuous with a belt of prominentpositive anomalies that extends along much of the Minnesota shore before turningsouth and terminating in a hook-shaped anomaly over the Bayfield-Peninsula(Wold and Ostenso, 1966). White (1966) concludes that this anomaly is due tovolcanics in the upper .part of the Keweenawan extrusive sequence. The volcanicscausing anomaly C should thus be equivalent, at least in part, to the IsleRoyale lavas. Again, a fault would be necessary to explain the apparentduplication of the sequence.

The foregoing observations therefore all tend to suggest that the IsleRoyale fault does exist. A two-dimensional magnetic model interpretation ofthe Isle Royale fault anomaly at its eastern end (Halls, 1970) suggests that thefault dips to the south. The fault is thus of reversed type as its do'~thrown

side is to the north. The displacement along the fault is at least 1-2 km. Thereversed nature of the fault and its increasing southerly trend toward the west(Fig.l) support the idea initially raised by Irving and Chamberlin (1885) thatit is an easterly continuation of the Douglas fault in Wisconsin.

References

Books, K.G. 1968. Magnetisation of the lowermost Keweenawan lava flows in theLake Superior area, USGS Prof. Paper 600-D, 248-254.

Halls, H.C. 1970. Geological interpretation of geophysical data from the LakeSuperior region, Ph.D. Thesis, University of Toronto, 203 pp.

Halls, H.C. and G.F. West. 1971. A seismic refraction survey in Lake Superior,Can. Jour. Earth Sci. (In press).

Hinze, W.J., O'Hara, N.W., Trow, J.W. and Secor G.B. 1966. Aeromagnetic studiesof eastern Lake Superior, AGU Mono., la, 95-110.

Irving, R.D. and Chamberlin, T.C. 1885. Observations on the junction betweenthe eastern sandstone and the Keweenaw Series on Keweenaw Point, LakeSuperior, Bull.US. Geol. Surv., ~, 385-498.

Meshref, W.M. and Hinze, W.J. 1970. Geologic interpretation of aeromagnetic datain western Upper Peninsula of Michigan, Mich. Geol. Surv., Rept. of Inv.,12, 25 pp.

Palmer, H.C. 1970. Paleomagnetism and correlation of some Middle Keweenawanrocks, Lake Superior, Can. Jour. Earth Sci., l? 1410-1436.

White, W.S. 1966. Tectonics of the Keweenawan basin, western Lake Superior region,USGS Prof. Paper 525-E, 23 pp.

Wold, R.J. and Ostenso, N.A. 1966. Aeromagnetic, gravity and sub-bottomprofiling studies in western Lake Superior, AGU Mono., 10, 66-94.

c1t:) ct=' —600 -

E._—a.—-—ç Vy TI V V V

ISLE ROYALE VOLCANICSTI, 1V/sCOPPER HARBOR CLASTICS

X——'-Y— NORTHERLY LIMIT OF KEWEENAWAN VOLCANICS

Total intensity magnetic map o tnt Isle Royale channel. Survey by N. C. ILdIsand C. F. West in 1966, using a shipborne proton precession spacing: About 3 miles.Profile orientation: N 300 W., c1:csjt in extreme northeast where it is more northerly

89° 40'

FIGURE I

48°OO'

89°20' $9°0O' 88°40' 88°20 B8°0O'

— ROVE FORMATION — —

NORTH SHORE VOLCANICS

a-60$47° 50'

89c2O• $9000' 88° 40

SCALE

n — —,

F')'-I

88° 20'

CONTOURS IN GAMMAS 1* tOO)

ES

87°40'

FI GURE

CONTOURS IN GAMMAS (X 100)

4a"OO'

+ D

THUNDER BAY----------

COPPER HARBOR CLASTICS

880 20' 880 00'

88°00'

I 6••• i

MILES

IN-...lI

_.- X-·-··Y- NORTHERLY LIMIT OF KEWEENAWAN VOLCANICS

Total intensjty magnetic map of the Isle Royale channel. Survey by H. C. Hallsand G. F. West in 1966, using a shipborne proton precession spacing: About 3 miles.Profile orientation: N 30 0 W.,except in extreme northeast where it is more northerly

-28-

RELIABILITY OF U-PB AGES OF SPHENE IN NORTHEASTERN MINNESOTAAND NORTHWESTE~~ ONTARIO

G. N. Hanson

Department of Earth and Space SciencesState University of New York

Stony Brook, New York

and

E. J. CatanzaroDepartment of Geology

Southampton CollegeSouthampton, New York

A B S T R ACT

U-Pb ages have been determined on sphenes from early Precambrian granitic rocks in relatively undisturbed areas and ina thermal aureole. The granitic bodies investigated are theIcarus pluton and the Saganaga tonalite in Ontario and the LindenSyenite and Giants Range Granite in Minnesota. A total of sixsphene concentrates were analyzed. One sphene from the LindenSyenite had an unfavorable U/Pb ratio and no age could be calculated. Three sphenes from the Saganaga tonalite, the Icarus pluton,and the Giants Range Granite gave internally concordant U-Pb agesof 2710 ± 30 m.y. 'T'ivo sphenes from the Giants Range Granite andthe Linden Syenite were internally discordant, but gave Pb207/Pb206ages of 2730 and 2740 m.y., respectively. It is suggested that thediscordance may be related to a relatively high uranium content inone sphene and to shearing of the host rock in the other. Sphenefrom near the contact of the Giants Range Granite with the 1100 m.y.Duluth complex gives internally concordant 2700 m.y. ages at 7.4kilometers from the contact. As the contact is approached the spheneages become internally discordant suggesting lead loss at 1100 m.y.At about one-half kilometer from the contact, where biotite andhornblende have lost essentially all of their radiogenic argon,sphene still retains about 60% of its radiogenic lead.

-29-

CONTINENTAL RIFTS

William J. HinzeDept. of GeologyMichigan State Univ.

Donald M. DavidsonDept. of GeologyUniv. of Minnesota,

Duluth

A B S T R ACT

Robert F. RoyDept. of Geology

and GeophysicsUniv. of Minnesota

Comparison of late Precambrian, linear, tectonic featurestransecting the Mid-Continent region of the United States (e.g., theMid-Continent and Mid-Michigan Anomalies) with relatively recentcontinental rift zones, such as the East African Rift System, indicatesmany similarities suggesting a common mode of origin. Hence, amechanism is proposed for the development of linear Precambrian riftsand subsequent overlying sedimentary basins based upon observed geologicaland geophysical characteristics of modern rifts. Dissimilarities instage of development, depth of erosion and geological age are parameterswhich limit the extent of comparison.

Rift formation is initiated by plate splitting and subsequentupwelling of low velocity layer material into the upper mantle alongthe base of the crust. Although inherently denser than adj acent mantlerocks, the low velocity layer material becomes lighter and thereforerises upon partial melting and fractionation. Uplift of the earth'ssurface and igneous activity along pre-existing zones of weakness isassociated with the vertical rise of this material, while lateral movement results in thinning and rupture of the crust producing extensionalrift grabens. Subsequent magmatic activity in the rift zones resultsin local structural downwarping.

Upon cooling, the low velocity layer residuml becomes denseand ultimately sinks into the mantle causing the uplifted rift zone todeflate. The convergent movement of adjacent mantle material intothe void produced by the sinking residuum places the crust under compression, thus accounting for not only compressional features associatedwith rift systems, but the subsequent development of sedimentary basinsover rift zones.

—30—

Keweenawan Geology of the Porcupine Mountains,Western Upper Peninsula, Michigan

Harold A. Hubbard

U. S. Geological SurveyWashington, 0. C. 20242

ABSTRACT

The Porcupine Mountains, Michigan, are underlain by tue uppernorthwarLl-dipping limb of an overturned asymmetric antiline, not adome with quaquaversal dios as usually shown. The anticline is faultednear its axis1 and at the southern margin o the mountains middleKeweenawan volcanic rocks are thrust over steeply diDpi1i to overturnedupper Keweenawan sedimentary rocks. The middle Keweenawan rocks, whichare younger than the Porta3e lake lava Series, can be divided into twosequences. The older sequence, which induces nost of he volcanicrocks in the mountains, co:isists of fine-grained northward-dippingmafic to intermediate lava flows and two interbedded felsic lava flows.The southern felsic flow is nonporphyr itic and constitutes about halfthe outcrrp area. The younger sequence, which is present only in theeasternmost part of the mountains, consists of gently eastward and.northeastward-dipping fine-grained and aphanitic flows overlain byvolcanic conElonerate and sandstone. The younger sequence is eitherin fault contact or unconformable on the older. Both sequences arerepeao1L in part in imbricate fault slices in the southeasteru part ofthe mountains. A tear fault separates these rocks from the upper Keweena-wan rocks to the east.

In the north, the Copper Harbor Conglomerate consists of saidstonecontaining interbedded lavas. It appears to be conformable on middleKeweenawan lavas in the lake of the Clouds valley. To the south o tlemountains, the upper Keweenawan rocks are folded in an asymmetricalsyncline with a steep north limb. The structural relIef bets-een themountains iid the synciline is more than 8000 feet. The size of the Porcu-pine Mountain structure sug3ests that the Keweenawan rocks of westernicichigan were st;unly folded in post-Freda time, and not just broadly

warped.

-30-

Keweenawan Geology of the Porcupine Mountains,Western upper Peninsula, Michigan

Harold A. Hubbard

u. S. Geological SurveyWashington, D. C. 20242

ABSTRACT

The Porcupine Mountains, Michigan, are underlain by the uppernorthward-dipping limb of an overturned asymmetric anticline, not adome with ~ua~uaversal dips as usually shown. The anticline is faultednear its axis, and at the southern margin of the mountains middleKeweenawan volcanic rocks are thrust over steeply dipping to overturnedupper Keweenawan sedimentary rocks. The middle Keweenawan rocks, whichare younger than the Portage Lake Lava Series, can be divided into twose~uences. The older se~uence, which includes most of the volcanicrocks in the mountains, consists of fine-grained northward-dippingmafic to intermediate lava flows and two interbedded felsic lava flows.The southern felsic flow is nonporphyritic and constitutes about halfthe outcrop area. The younger se~uence, which is present only in theeasternmost part of the mountains, consists of gently eastward andnortheastward-dipping fine-grained and aphanitic flows overlain byvolcanic conglomerate and sandstone. The younger se~uence is eitherin fault contact or unconformable on the older. Both se~uences arerepeated in part in imbricate fault slices in the southeastern part ofthe mountains. A tear fault separates these rocks from the upper Keweenawan rocks to the east.

In the north, the Copper Harbor Con81omerate consists of sandstonecontaining interbedded lavas. It appears to be conformable on middleKeweenawan lavas in the Lake of the Clouds valley. To the south of themountains, the upper Keweenawan rocks are folded in an as~nmetrical

syncline with a steep north limb. The structural relief between themountains and the syncline is more than 8000 feet. The size of the Porcupine Mountain structure suggests that the Keweenawan rocks of westernMichigan were strongly folded in post-Freda time, and not just broadlywarped.

—31--

ThE KEWEENAWN GEOLOGY OF ISLE ROYALE, MICHIGAN

N. King HuberU.S. Geological Sury

Menlo Park, California 94025

ABSTRACT

Isle Royale lies on the north limb of the Lake Superior syncline,and the stratigraphic section exposed on the island is correlative withthe much—studied middle Keweenawan Portage Lake Lava Series and upper}ZeHr.awan Copper Harbor Cunglomerate on the Keweenaw Peninsula, on thesouth limb of the syncline. Dips of the strata range from less than100 to over

5Q0; they are generally steeper on the north side of theisland than on the south and average less than 20°.

Exposures on Isle Royale indicate a minimum thickness of 10,000feet for the Portage Lake Lava Series at this locality. The base of tiiei

series is not exposed. As on the peninsula, the series consists largelyof basaltic and andesitic lava flows, with lesser amounts of interfiowsadiotaoy and tuffaceous rooks. No felsie flows are known fromoutcrops, although one has been reported from diamond drilling, about6,200 feet below the top of the lava series. The interflow clastic rocksgenerally do not crop out, and mast of the individual units are known onlyfrom diamond drilling. Within the lava series, which probably containsover a hundred flows in the exposed section, certain stratigraphic unitsrepresenting individual lava flows or groups of flows can be identifiedand traced on the basis of characteristic textures or structures, andrelative stratigraphic position. Twelve such units have been distinguishedwithin the sequence and provide stratigraphic and structural control forgeologic mapping.

On both the Keweenaw Peninsula and Isle RoyaTh, the Copper HarborConglomerate is largely derived from lowermost Keweenawan volcanic sourceterra as, which shed debris into the subsiding Lake Superior basin fromopposite sides; for Isle Royale, this would be from the North ShoreVolcano Group of Gehman (1958) in Minnesota. The depositional en'i:onmentis interpreted as being one of a combination of fluvial and lacustrineconditions resulting in piedmont fanglomerates and playa lake or floodplain deposits.

On Isle Royale, various sedimentary features indicate that thegeneral direction of sediment transport was easterly, with a range fromnortheast to southeast. In thio same direction the Copper HarborConglomerate Jr.i-eases in thickness and in textural and compositionalmaturity——namely from a boulder conglomerate, through a mixture of cobbleand pebble conglomerates and sandstone, to sandstone and inudstone. In adistance of 20 miles this clastic wedge thickens from a minimum of1,500 feet to over 6,000 feet between stratigraphic marker horizons; thetop of the formation is nowhere exposed and the total thickness must beappreciably greater.

-31-

THE KEWEENAWAN GEOLOGY OF ISLE ROYALE, MICHIGfu~

N. King HuberU.S. Geological Survey

Menlo Park, California 94025

A B S T R ACT

Isle Royale lies on the north limb of the Lake Superior syncline,and the stratigraphic section exposed on the island is correlative withthe much-studied middle Keweenawan Portage Lake Lava Series and upperKeweenawan Copper Harbor Conglomerate on the Keweenaw Peninsula, on thesouth limb of the syncline. Dips of the strata range from less than10° to over 50°; they are generally steeper on the north side of theisland than on the south and average less than 20°.

Exposures on Isle Royale indicate a minimum thickness of 10,000feet for the Portage Lake Lava Series at this locality. The base of theseries is not exposed. As on the peninsula, the series consists largelyof basaltic and andesitic lava flows, with lesser amounts of interflowsedimentary and tuffaceous rocks. No felsic flows are known fromoutcrops, although one has been reported from diamond drilling, about6,200 feet below the top of the lava series. The interflow clastic rocksgenerally do not crop out, and most of the individual units are known onlyfrom diamond drilling. Within the lava series, which probably containsover a hundred flows in the exposed section, certain stratigraphic unitsrepresenting individual lava flows or groups of flows can be identifiedand traced on the basis of characteristic textures or structures, andrelative stratigraphic position. Twelve such units have been distinguishedwithin the sequence and provide stratigraphic and structural control forgeologic mapping.

On both the Keweenaw Peninsula and Isle Royale, the Copper HarborConglomerate is largely derived from lowermost Keweenawan volcanic sourceterranes, which shed debris into the subsiding Lake Superior basin fromopposite sides; for Isle Royale, this would be from the North ShoreVolcanic Group of Gehman (1958) in Minnesota. The depositional environmentis interpreted as being one of a combination of fluvial and lacustrineconditions resulting in piedmont fanglomerates and playa lake or floodplain deposits,

On Isle Royale, various sedimentary features indicate that thegeneral direction of sediment transport was easterly, with a range fromnortheast to southeast. In this same direction the Copper HarborConglomerate increases in thickness and in textural and compositionalmaturity--namely from a boulder conglomerate, through a mixture of cobbleand pebble conglomerates and sandstone, to sandstone and mudstone. In adistance of 20 miles this clastic wedge thickens from a minimum of1,500 feet to over 6,000 feet between stratigraphic marker horizons; thetop of the formation is nowhere exposed and the total thickness must beappreciably greater.

32—

SOME PRIMARY SEDIMENTARY STRUCTURES IN THE LOWER CHERTY MEMBER OF THEBIWABIK IRON FORMATION : VIRGINIA HORN AREA

ERIC FRODESEN

Department of Geology and GeophysicsUniversity of Wisconsin, Madison, Wisconsin 53706

A BSTRACT

The writer discovered and collected some unusual sedimentarystructures in the Lower Cherty Member oF the Biwabik Iron Formationat a locality nea;r the Midway addition of Virginia, Minnesota. Polishedsections and x—radiographs reveal overturned and disturbed bedding re-sembling convolute—type bedding, load casts with flame structures, andother structures characteristic of soft sediment deformation. Small—scale cross—bedding and small local thrust faults also are present.Recent experiments on the Formation of contorted structures(McKee andGoldberg, 1969) show that convolute—type bedding and small—scale thrustfaults, similar to those found in the Biwabik, can be formed by loadinga semi—cohesive mud which was deposited on an existing slope of 15—20degrees.

The preservation of the fine alternating dark and light laminae,small—scale cross—bedding, and possible graded bedding, suggest thatthese rocks were deposited below wave base with only periodic currentactivity.

The alternation of the laminae is generally attributed to sea-sonal changes(Hough, 1958), or periodic influxes of iron—rich and iron—impoverished layers due to tectonic activity coupled with isostaticadjustment of a. semi—rigid crust, or a combination oF the two procecess(Cullen, 1963). Cullen's idea that banded iron formations can he re-garded as corresponding to a syn—orogenic "f1ysch' type of depositionmay have applications in explaining some of the structures found in theLower Cherty Member. Similar sedimentary structures are found in thelaminites of documented flysch facies.

The presence ofallochthonous black chert pebbles within the bedsposes a problem as to mode of emplacement. Turbidity currents, subaqueoussliding or gliding down a slope, or ice rafting mechanisms have to be in-voked to explain their presence

If iron formations represent the products of chemical weathering

under warm humid conditions in semi—restricted basins, then ice raftingis not a very likely possibility. If, however, the iron formations were

-32-

SOME PRIMA.RY SEDIMENTARY STRUCTURES IN THE LOWER CHERTY tJfMBER OF THEBI~/ABIK IRON FORMA.TION : VIRGINIA HORN AREA

ERIC FRODESEN

Department of Geology and GeophysicsUniversity of \.Jisconsin, Madison, Wisconsin 53706

A B S T R ACT

The wri ter di scoverecl and collected some unusua I sedimentarystructures in the Lower Cherty t-lember of the Biwabik Iron Formationat a locality near the Midway addition of Virginia, Minnesota. Polishedsections and x-radiographs reveal overturned and disturbed bedding resembling convolute-type bedding, load casts with flame structures, andother structures characteristic of soft sediment deformation. Smallscale cross-bedding and small local thrust faults also are present.Recent experi ments on the formati on of contorted s truc tures (Mcl<ee andGoldberg, 1969) show that convolute-type bedding and sm~ll-scale thrustfaults, simi lar to those found in the Biwabik, can be formed by loadinga semi-cohesive mud which was deposited on an existing slope of 15-20degrees.

The preservation of the fine alternatin~ dark and light laminae,small-scale cross-bedding, and possible graded bedding, suggest thatthese rocks were deposited below wave base with only periodic currentactivity.

The alternation of the laminae is generally attributed to seasonal changes{Hough, 1950), or periodic influxes of iron-rich and ironimpoverished layers due to tectonic activity coupled with isostaticadjustment of a semi-rigid crust, or a cornbina,tion of the two procecess(Cullen, 1963). Cullen's idea that banded iron formations can be regarded as corresponding to a syn-orogenic "flyschll type of depositionmay have applications in explaining some of the structures found in theLower Cherty Member. Similar sedimentary structures are found in thelaminites of documented flysch facies.

The presence of allochthonous black chert pebbles within the bedsposes a problem as to mode of emplacement. Turbidity currents, subaqueoussliding or gliding down a slope, or ice rafting mechanisms have to be invoked to exrlain their presence g

If iron formations represent the proc'ucts of chp.mical weatheringunder warm humid conditions in semi-restricted basins, then ice raftingis not a very likely possibility. If, however, the iron formations were

—33—

deposited in a cool polar climate as suggested by paleonagnetic data(Symons, 1966), then these pebbles may he the product of ice transport.f thesc sedimentary structures are characteristic of the entire for—

mation, and iron Formations in general, then a detailed regional studycould produce some enlightening results.

1'efercnces:

Cul Ten, U.J., 1963, Tectonic implications of banded iron formations:Jour. Sed. ret., v.33, p.327—392.

Gruner, J.'., l96, Mineralogy and geology of the Mesabi Range:Office of the Comisioner of Iron Range Resources andRehabilitation, St. Paul, tlinn. 127 Pp.

Gundersen, J.N., end G.M. Schwartz, 196Z, The geology of the meta-morphosed Siwabik Iron Formation, Eastern Mesabi district,Minnesota: Minn. Geol. Stir. Bull. 43, 137pp.

Hough, J.L., 1958, resh_water environment of deposition of Pre-cambrian iron formations: Jour. Sed. Pet., v.28, p. 41t+_1130.

James, H.L., 1966, Chemistry of the iron—rich sedimentary rocks:'J.S).S. Prof. Paper (-4O41, 6lpp.

Mcee, E.fl., and M. Goldberg, 1969, Experiments on formation of con-torted structures in mud: G.S.A. Bull., v.80, p.23l—24-+.

Symons, P.T.A., 1966, A paleomagnetic study on the Gunflint, Mesabi,and Cuyuna iron ranges in the Lake Superior Region;Econ. Geol., v.G1, p. 1336—1361.

hite, [l.A., 1954, The stratigraphy and structure of the MesabiRange, Minnesota: Minn. Geol. Sur. Bull. 38, 92pp.

-33-

rlepositerl in a cool pot~r climate as suggested by paleomagnetic data(Symons, 1966), then these pebbles may be the product of ice transport.If these sedimentary structures are characteristic of the entire formation, and iron formations in ~eneral, then a detailed regional studycould pro~uce SOMe enlightening results.

refer~nces:

Cullen, D•.J., 1963, Tectonic implications of banded iron formations:Jour. Sed. Pet., v.33, p.387-392.

Gruner, J.lt!., 1946, i~ineralogy and geology of the Mesabi Range:Office of the Commisioner of Iron Range Resources andrehabilitation, St. Paul, f1inn& 127 ppe

Gundersen, J.N., and G.M. Schwartz, 1962, The geology of the metamorphosed Ri\-.abik Iron Formation, Eastern ~lesabi district,Minnesota: Minn. Geol. Sur. Bull. 43, l37pp.

Yough, J.L., 1958, Fresh-water environment of deposition of Precambrian iron formations: Jour. Sed. Pet., v.l8, p. 414-430.

J~mes, H.L., 1966, Chemistry of the iron-rich sedimentary rocks:IJ.S.G.S. Prof. Paper 440-Iy" 61pp.

~1c:\ee, E.0., and M. Goldberg, 1969, Experiments on formation of contorterl structures in mud: G.S.A. Bull., v.BO, p.23l-244.

Symons, ~.T.A., 1966, A peleomagnetic study on the Gunflint, Mesabi,and Cuyuna iron ranges in the Lake Superior Region:Econ. Geol., v.61, p. 1336-1361.

\.'hi te, [l.A., 195 1f, The stratigraphy and structure of the ~1esabi

]{angc, fHnneso ta: Hinn. Ceoi. Sur. Bull. 38, 92pp.

—34—

ZEOLITE AND PREHNITE—PUELL?ITE FAdES IN THE KEWF.ENAWAN BASALTSOF NORTHERN MICHIGAN II: THE ROLE OF VOLATILES

Wayne T. Jolly

University of SaskatchewanSaskatoon. Canada

ABSTRACT

The tholeiitic lava flows of the Keweenaw peninsula, :Iichigan.have underone metamorphism of the zeolite and prehnite—punpelly itefacies. In the latter, rocks along fractures and upper flow contactshave been transformed to monomineralic rocks (metadomains) corposedof either pumpellyite or epidote, depending on stratiranhic positionin the pile. These metadomains are enriched in CaO and A1,O retitiveto their unaltered basalts through albiti.zation of pre—exiting plagio—clase. Bulk compositions of the altered parts of the flows, deducedthrough weighted averages of the rock compositions, are sinilar to re

parental basaits. Thus, little or no material was added from extraneoussources. This metamorphic differentiation occurred as a result ofmigration of both volatile and non—volatile components over shortdistances through a fluid pressure gradient with r'f< total Calcium

alinninum silicates were formed preferentially near rupture zones,where P was at its lowest levels. Theoretical considerations suggestthat mdamorphic differentiation of essentially homogeneous bodies ofrock may occur only when Pf is less tian During the Keweenawanmetamorphic event, the rocks underwent etensve dehydration. As aresult, water content of secondary phases decreases uith depth fromwater—rich chlorites and zeolites (1120=12% or more) to water—acor epidote(HO about 2%). At lowest exposed levels P02 may have reached levelssutficient to subdue formation of pumpellyite in favor of pistaciticepidote. Co2 was very low during the peak of the metamorphic event.

-34-

ZEOLITE AND PREHNITE-PUMPELLYITE FACIES IN THE KEHEENAHAN BASALTSOF NORTHERN HICH:LGAN II: THE ROLE OF VOLATILES

Wayne T. Jolly

University of SaskatchewanSaskatoon, Canada

A B S T RAe T

The tholeiitic lava flows of the Ke\\'eena,\v peninsula, ~lichigan,

have undergone metamorphism of the zeolite and prehnite-pumpellyitefacies. In the latter, rocks along fractures and upper flO\\1 contactshave been transformed to monomineralic rocks (metadomains) composedof either pumpellyite or epidote, depending on stratigranhic positionin the pile. These metadomains are enriched in CaO and /\1')03 relativeto their unaltered basalts through albitization of pre-existlnG plagioclase. Bulk compositions of the altered parts of the flows, deducedthrough ,veighted averages of the rock compositions, are similar to ti1eparental basalts. Thus, little or no material was added from extraneoussources. This metamorphic differentiation occurred as a result ofmigration of both volatile and non-volatile components over shortdistances through a fluid pressure zradient \vi th r f<P t l' Calciumalwninum silicates were formed preferentially near rup~S~e zones,where P was at its lowest levels. Theoretical considerations suggestthat metamorphic differentiation of essentially homogeneous bodie~ ofrock may occur only when Pf is less thim P l' During the Ke\\1eenaHanmetamorphic event, the rocks underwent ext~g~rve dehydration. As aresult, water content of secondary phases decreases with depth fromwater-rich chlorites and zeolites (H

20=12% or more) to '\-later-poor epidote

(H20 about 2%). At lowest exposed levels P02

may have reached levelssufficient to subdue formation of pumpellyite in favor of pistaciticepidote. PCOZ was very 10\\1 during the peak of the metamorphic event,

—35—

AN AEROMAGNETIC SURVEY C1" THE SOUTHE PENINSULA OF MIOsiIGAN

Richard L. Kellogg and William 3. Hinze

Department of GeologyMichigan State UniversityEast Lansing, Michigan

ABSTRACT

Only fragmentary direct information is available on the basementcomplex underlying the Phanerozoic sediments of the Michigan Basinbecause of the limi;ed and poorly distributed basement drill tests. To

supplement this limited information a regional aeromagnetic survey hasbeen conducted of the Southern Peninsula.Approximately 17,000 miles oftotal magnetic intensity data were recorded along north—south flightlines spaced at three mile intervals.

A hasement configuration map prepared from magnetic depth estimatesand basement drill tests confirms that the basement surface under theSouthern Peninsula of Ni iar has the form of an oval depression reach-ing a maximum depth of approximately 15,000 feet below sea level on thewestern shore of Saginaw Bay. A basement high underlies the Howell.Anticilne and a roughly north—south striking regional basement trLgplunges into the basin from the common boundary poin of Indiana, Ohio,and Ilichigan to the vicinity of 42°30'N. The map shows a broad basementplatform striking northwest in the extreme southwest corner of thepeninsula.

Interpretation of the :esidual aeromagnetic map in conjunction withgeologic and other regional geophysical data from the Southern PeiLtsulaand adjacent areas indicates that the basement of the :1ichigan Basin hashad a complex geologic history. Several basement provinces are definedon the basis of magnetic and gravity anomalies, lithologies and isotopeages of samples obtained from basement drill holes, and extrapolationof known Precambrian geology from the margin of the basin. The Penokeanprovince can be traced from northern Nichiga and Wisconsin into thenorthern portion of the Southern Peninsula. In this area the provinceis characterized by east—southeast striking anomalies. Central andsouthwestern Michigan is underlain primarily by felsic rocks correlatingwith the Central Province.

Basement rocks in southeastern Michigan, which strike generallynorth—northeast are interpreted as mafic and felsic gneisses andamphibolites. They are correlated with the Grenville province whichis bounded on the west by a line extending south—southwest from SaginawBay to west of the HowellAnticline and then du.e south to the Michigan—Ohio boundary. A Keweenawan rift zone characterized by mafic intrusives,extrusives and uplifted gneisses transects the Peninsula irom theTraverse Bay area to southeastern Michigai Keweenaean ic.neous activitymay also be reflected in the numerous local magnetic anomalies in -southwest Mi-htgan which occur along norLn'est striking treds whithparallel the regional gravity anomaly pattern.

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AN AEROMAGNETIC SURVEY GF THE SOUTHKciilJ PENINSULA OF MI~dIGAN

Richard L. Kellogg and William J. Hinze

Department of GeologyMichigan State UniversityEast Lansing, Hichigan

A B S T R ACT

Only fragmentary direct information is available on the basementcomplex underlying the Phanerozoic sediments of the Michigan Basinbecause of the limited and poorly distributed basement drill tests. Tosupplement this limited information a regional aeromagnetic survey hasbeen conducted of the Southern Peninsula. Approximately 17,000 miles oftotal magnetic intensity data were recorded along north-south flightlines spaced at three mile intervals.

A basement configuration map prepared from magnetic depth estimatesand basement drill tests confirms that the basement surface under theSouthern Peninsula of Michigan has the form of an oval depression reaching a maximum depth of approximately 15,000 feet below sea level on thewestern shore of Saginaw Bay. A basement high underlies the HowellAnticline and a roughly north-south striking regional basement troughplunges into the basin from the common boundary point of Indiana, Ohio,and iIichigan to the vicinlty of 42°30'N. The map shows a broad basementplatform striking north\-lest in the extreme southwest corner of thepeninsula.

Interpretation of the =~sidual aeromagnetic map in conjunction withseologic and other regional geophysical data from the Southern Peninsulaand adjacent areas indicates that the basement of the Hichigan Basin hashad a complex geologic history. Several basement provinces are definedon the basis of magnetic and gravity anomalies, lithologies and isotopeages of samples obtained from basement drill holes, and extrapolationof known Precambrian geology from the margin of the basin. The Penokeanprovince can be traced from northern Michigan and Wisconsin into thenorthern portion of the Southern Peninsula. In this area the provinceis characterized by east-southeast striking anomalies. Central andsouthwestern Michigan is underlain primarily by felsic rocks correlatingwith the Central Province.

Basement rocks in southeastern ~lichigan, which strike generallynorth-northeast are interpreted as mafic and felsic gneisses andamphiboli tes. They are correlated with the Grenville province \vhichis bounded on the west by a line extending south-southwest from SaginawBay to \vest of the HowellAt,ticline and then du.e south to the MichiganOhio boundary. A Keweenawan rift zone characterized by mafic intrusives~

extrusives and uplifted gneisses transects the Peninsula from theTraverse Bay area to southeastern Nichigan. Ke\veenmVan igneous activitymay also be reflected in the numerous local magnetic anomalies in •soutlHvest Nichigan which occur along northvest striking trE!lllds \vhichparallel the regional gravity anomaly pattern.

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PRECAMBRIAN CLASTIC aarna SEDENTATION

GEORGE deVRLES KLEIN

Dept. of Geology, Univ. of Illinois, Urbana, Illinois, 61801

ABSTRACT

The Precrbriari Lower Fine-grained Quartzite of Scotland andboth the Precaubrian Sterling Quartzite and Precanbrian part of theWood Canyon Formation of eastern California and Nevada arecharacterized by primary sedimentazy features which are indicativeof sediment transport and deposition by tidal currents. Sediment-sty structures which occur in these formations are grouped intoseven associations which am produced by seven phases of tidalsediment transport:

ASSOCIATION 1: Cross-stratification organized into herringbone sets,with bipolar—bimodal orientation; parallel laninae; these featuresindicate tidal current bedload transport with bipolar reversals offlow directions (Reirieck, 1963).

ASSOCIATION 2: Reactivation surfaces: Multimodal frequency distrib-utions of cross-strata set thickness and of dip angles; unirnodalorientation of directional current structures parallel to basintopographic strike; all produced by time—velocity assymetry oftidal current flow (Klein, l970a3.

ASSOCIATION 3: Interference ripples; superposition of current ripplesat 900 and 1800 on crests and slip faces of dunes, sand waves andinternal cross-strata; "B-C" sequences of cross-strata overlain bymicro-cross-laninae; highly-var! ant orientation of current ripples;all produced by late-stage emergence runoff prior to anergence ofan intertidal flat (Klein,1963,1970a,l970b).

ASSOCIATION It: Cross—stratification with flasers and clay drapes;flaser bedding; lenticular bedding; "tidal bedding'; convolutebedding; all produced by alternation of bedload and suspensionsedimentation associated with alternating bedload and slack-watertidal current flow (Reineck and Wunderlich,l968a,1965b; Wunderlich,1970).

ASSOCIATION 5. Washout structures, sane filled with mud—chipconglctnerates; rill marks; flute casts; all produced by tidal scour(Van Straaten,l951j; Reineck, 1967; Klein,1970a).

-36-

PRECAMBRIAN CLASTIC PALEarIDAL SEDTI1ENTATION

GEORGE deVRIES KLEIN

Dept. of Geology, Univ. of Illinois, Urbana, Illinois, 61801

ABSTRACT

The Precanbrian Lm;er Fine-grained Quartzite of Scotland andboth the Precanbrian Sterling Quartzite and Preccmbrian part of theWood Canyon Formation of eastern California and Nevada arecharacterized by primary sedimentary features which are indicC'ltiveof sediment transport and deposition by tidal currents. Sedimentary structures which occur in these fonnations are grouped intoseven associ~tions which are produced by seven phases of tidalsediment transport:

ASSOCIATION 1: Cross-stratification organized into herringbone sets,with bipolar-bimodal orientation; parallel laminae; these featuresindicate tidal current bedload transport with bipolar reversals offlow directions (Reineck, 1963).

ASSOCIATION 2: Reactivation surfaces: Multimodal frequency distributions of cross-strata set thickness and of dip angles; unimodalorientation of directional current structures parallel to basintopographic strike; all produced by t:ime-veloci ty assymetry oftidal current flow (Klein, 1970a).

ASS<X;IATION 3: Interference ripples; superposition of current ripplesat 900 and 1800 on crests and slip faces of dunes, sand waves andinternal cross-strata; "B-C" sequences ClIf cross-strata overlain bymicro-cross-lcminae; highly-variant orientation of current ripples;all produced by late-stage emergence runoff prior to emergence ofan intertidal flat ~Klein,1963,1970a,1970b).

ASSOCIATION 4: Cross-stratification with fla.sers and clay drapes;flaser bedding; lenticular bedding; "tidal bedding"; convolutebedding; all produced by alterna.tion of bedload and suspensionsed:imentation associated with alternating bedload and slack-watertidal current flow (Reineck and Wunderlich,1968a,1968b; Wunderlich,197Q) •

ASSOCIATION 5. \lJashout structures, sane filled with mUd-chipconglanerates; rill marks; flute casts; all produced by tidal scour(Van Straaten,1954; Reineck, 1967; Klein,1970a).

—37—

ASSOCIATION 6: Mudc racks; mt rafo tin ational congicrie rates; birdseyestricture; all, produced by exposure and evaporation (Shinn,1968).

ASSOCIATION 7: Tracks and trails; burrowing structures,"escapeburrows; all produced by burrowing orgaxüaus adapted to a tidalregime (Rhoads,l967; Remneck and others, 1968).

Precanbrian paleotidal ranges can be approximated in theserock units finn analysis of fining-upward sequences similar tothose occurring in prograding tidal copstlines. To date, LatePrecambrian paleotidal ranges from 0.3 to 13.0 meters have beenmeasuitd. This Late Precaubrian paleotidal range variation isless than the known variation measured along Holocene tidalcoasts (variation of tidal ranges fran 0 to 17.5 meters). PerhapsPrecanbripn paleotidal range variation is not greatly difforentfran present-day variation. If further work substantiates sucha limited paleotidal range variation fruit the Precambrian, itposes critical problems for various geophysical pxtblms that havebeen proposed for the origin and age of the earth-noon systan.

REBERENCES CITED

Klein, C.deV,1963, Bay of Plindy intertidal zone sediments: Jour.Sedimentary Petrology, v. 33, p. 8tb—85t

-——,l97Oa, Depositional and dispersal dynanics of intertidal sandbars: Jour. Sdimentary Petrology, v. 140, p. 1095-1127.

--—,1970b, Tidal origin of a Precambrian Quartzite - The LowerPinc—graird Quartzite (Dairadian) of Islay. Scotland: Jour.Sedimentary Petrology, v. 140, p. 973—985.

Reineck, H.E., 1963, Sedimentgefuge un Bereic;h der sudliche Nordsee:Abh. Senckenbergischen Naturfor. Gesells. No. 5O5, p. 1-138.

--—,1967, Layered sediments of tidal flats, beaches and shelf bottoms,p. 191-206: in Lauff, (hR., Editor, 1967, Estuaries: Am. Assoc.Adv. Sci. Pub. No. 83.

Reineck, H.E., and Wunderlich, F, 1968a, Classification and origin offlaser and lenticular bedding: Sedimentology, v. U, p. 99-1014.

---, &, ---, 1968b, Zeitanessugnen an Gezeitenschichten: Natur undMusetun, v. 97, p. 193—197

Reineck, H.E., Dorjes, J, Gadow, 5, and Hertwick, 0,1968, Sadiznentologie,Faunenzoniening and Faziesabfolge vor der Ostkuste der innerenJeutschen Bucht: Senck. Lethaea, v. 149, p. 261-309

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ASSOCIATION 6: MUdcracks; intraformational conglanerates; birdseyestructure; all produced by exposure and evaporation (Shinn,1968).

ASSOCIATION 7: Tracks and trails; burrowing structures, "escape"burrows; all produced by burrowing organisns adapted to a tidalregime (Rhoads,1967; Reineck and others, 1968).

Precanbrian paleotidal ranges can be approximated in theserock units from analysis of fining-upward sequences similar tothose occurring in prograding tidal COAstlines. To date, LatePrecambrian paleotidal ranges from 0.3 to 13.0 meters have beenmeasured. This Late Precanbrian paleotidal range variation isless thRn the known variation measured along Holocene tidalcoasts (variation of tidal ranges fran 0 to 17.5 meters). PerhapsPrecambri~n paleotidal range variation is not greatly differentfran present-day variation. If further work substantiates sucha limited paleotidal range variation fram the Precambrian, itposes critical problems for various geophysical problens that havebeen proposed for the origin and age of the earth-moon systen.

REFERENCES CITED

Klein, G.deV,1963, Bay of Fundy intertidal zone sediments: Jour.Sedimenta~ Petrology, v. 33, p. 844-854

---,1970a, Depositional and dispersal dYnanics of intertidal sandbars: Jour. S~dimentar,y Petrology, v. 40, p. 1095-1127.

---,1970b, Tidal origin of a Precambrian Quartzite - The LowerFine-grained Quartzite (Dalradian) of Isl~; Scotland: Jour.Sedimentary Petrology, v. 40, p. 973-985.

Reineck, H.E., 1963, Sedimentgefuge im Bereich der sudliche Nordsee:Abh. ~enckenbergischen Naturfor. Gesells. No. 505, p. 1-138.

---,1967, Layered sediments of tidal flats, beaches and shelf bottoms,p. 191-206: in Lauff, G.H., Editor, 1967, Estuaries: Am. Assoc.Adv. Sci. Pub. No. 83.

Reineck, H.E., and WUnderlich, F, 1968a, Classification and origin offiaser and lenticular bedding: Sedimentology, v. ll, p. 99-104.

---, &, ---, 1968b, Zeitmessugnen an Gezeitenschichten: Natur undMuseum, v. 97, p. 193-197

Reineck, H.E., Dorjes, J, Garlow, S, and Hertwick, 0,1968, S3dimentologie,Faunenzonierung und Faziesabfolge vor der Ostkuste der innerenDeutschen Bucht: Senck. Lethaea, v. 49, p. 261-309

—L8—

Rhoads, D.C.,1967, Biogenic reworking of intertidal, and subtidasediments in Bamstable Harbor and Buzzards Bay, Massachusetts:Jour. Geo1or, 'r. 75, t. b61—L76.

Shinn, E.A.,],968, Practical significance of birdseye structure incarbonate rocks: Jour. Sedimentary Petrology, v. 38, p. 215—223.

Van Straaten, L.N.J.tJ., 195h, Sedimento1o' of Recent tidal flatdeposits and the Psamnites du C0ndroz (Devonian): Cleol. enMijnb., v. i6, p. 25-217.

Wunderlich, F,1970, Genesis and envirornent of the T1Nellenkopfschen-schichten"(Lower sian, Rheinian Devonian) at locus typicus inecinparison with modem coastal enviroanents of the Cern n Bay:Jour. Sedimentary Petrology, v. 240, p. 102-130.

-':;8-

Rhoads, D.C.,1967, Biogenic reworking of intertidal and subtidalsediments in Barnstable Harbor and Buzzards Bay, Massachusetts:Jour. Geology, v. 75, p. 461-476.

Shinn, E.A.,1968, Practical significance of birdseye structure incarbonate rocks: Jour. Sedimentar,y Petrology, v. 38, p. 215-223.

Van Straaten, L.M.J.U., 1954, Sedimentology of Recent tidal fla.tdeposits and the Psanmites du Condroz (Devonian): Geol. enMijnb., v. 16, p. 25-47.

Wunderlich, F,1970, Genesis and environment of the "Nellenkopfschenschichten Il(Lower EInsian, Rheinian Devonian) at locus typicus incanparison with modern coastal environments of the GeImclTI Bay:Jour. Sedimentar,y Petrology, v. 40, p. 102-130.

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SOME GEOLOGY OF THE 'ARATHON COUNTY VOLCANIC BELT

by

Gene L. LaBergeWisconsin Geological Survey and

Department of GeologyWisconsin State University

Oshkosh, Wisconsin

ABSTRACT

The area east of Wausau in Marathon County consists largely ofvolcanic rocks which range in composition from basaltic to rhyolitic.The basaltic rocks are mainly to the southeast and the rhyolitic rocksto the northwest, suggesting that the top of the sequence is northwest:however, no definite evidence of stratigraphic top was fond. Trachytesare also pr?sent at the northwestrn edge of the napped area. Sedimentaryrocks are virtually absent.

The volcanic rocks have been intruded by v;rious sized masses ofgabbro, diorite, granite, and syenite. There appears to have beenseveral ages of granitic intrusion. Available radiometric ages indicatethat the rocks arc late Middle Precambrian.

Structurally, the area is characterized by at least two directionsof large scale faulting with niajor fault zones trending approximatelyN8O°E to N60°E. The faulting, which also shows up well on the aeronagneticmap covering the area mainly west of Wausau, has resalted in many of thelithologli units being in fault contact. Perhaps the most significantfault zone is the N3OF trend whiàh has produced a zone nearly a milewide in the Eau Claire River valley in which mylonite is a major rocktype. This zone truncates the N8O°E—N60°E trend more prevalent to thewest.

The current napping project, funded by the Wisconsin GeologicalS"rvey, has revealed significantly more volcanic rocks than are shown OLIWeidinan's 1907 map. This has important implications for min al explora-tion programs. A previously unreported gold prospect, on which miningwas attempted about 1920, is probably the most interesting locality froman economic point of view.

Dutton, C. B. and Bradley, R. E. (1970) "Lithologic, Geophysical, and:•lineral Commodity Maps of Precambrian Rocks in Wisconsin"; U.S.G.S.Miscellaneous Geologic Investigations Mao 1—631.

Henderson, J. IL, Tyson, N. S., and Page, 3. R. (1963) "AeromagneticMap of the Wausau Area, Wisconsin"; U. S. G. S. GeophysicalInvestigations Map CP—40l.

Laberge, G. U. and Weis, L. W. (1968) 'A Greenstone Belt in CentralWisconsin?, Guidebook for 32nd Annual Tn—State Geological FieldConference.

-39-

SOME GEOLOGY OF THE MARATHON COUNTY VOLCANIC BELT

by

Gene L. LaBergeWisconsin Geological Survey and

Department of GeologyWisconsin State University

Oshkosh, Wisconsin

ABSTRACT

The area east of Wausau in Marathon County consists largely ofvolcanic rocks which range in composition from basaltic to rhyolitic.The basaltic rocks are mainly to the southeast and the rhyolitic rocksto the northwest, suggesting that the top of the sequence is northwest;however, no definite evidence of stratigraphic top \vas found. Trachytesare a1.. so present at the northwestern edge of the mapped area. Sedimentaryrocks are virtually absent.

The volcanic rocks have been intruded by various sized masses ofgabbro, diorite, granite, and syenite. There appears to have beenseveral ages of granitic intrusion. Available radiometric ages indicatethat the rocks are late Niddle Precambrian.

Structurally, the area is characterized by at least two directionsof large scale faulting with major fault zones trending approximately1180°£ to ~~60°E. The faulting, \vhich also shows up well on the aeromagneticmap covering the area mainly west of Wausau, has resulted in many of thelithologic units being in fault contact. Perhaps the most significantfault zone is the N300E trend which has produced a zone nearly a milewide in the Eau Claire River valley in which mylonite is a major rocktype. This zone truncates the N800E-N600E trend more prevalent to the,,,,est.

The current mappins project, funded by the Wisconsin GeologicalSurvey, has revealed significantly more volcanic rocks than are shown on\veidman's 1907 map. This has important implications for mineral exploration programs. A previously unreported gold prospect, on which miningwas attempted about 1920, is probably the most interesting locality froman economic point of view.

References

Dutton, C. E. and Bradley, R. E. (1970) "Lithologic, Geophysical, and;·lineral Commodity Haps of Precambrian Rocks in Wisconsin"; U.S.G.S.Miscellaneous Geologic Investigations Map 1-631.

Henderson, J. R., Tyson, ;i). S., and Page, J. R. (1963) "AeromagneticMap of the Wausau Area, Wisconsin"; U. S. G. S. GeophysicalInvestigations Map GP-40l.

LaBerge, G. 1. and l,Jeis, L. Iv. (1968) "A Greenstone Belt if CentralWisconsin?", Guidebook for 32nd Annual Tri-State Geological FieldConference.

—40—

Weidman, S. (1907) The Geology of North Central Wisconsin; WisconsinGeological and Natural History Survey Bulletin 16.

Weis, L. W. and LaBerge, C. L. (1969) "Central Wisconsin Volcanic Belt,"Guidebook for 15th Annual Institute on Lake Superior Geology.

-40-

Weidman, S. (1907) The Geology of North Central Wisconsin; WisconsinGeological and Natural History Survey Bulletin 16.

Weis, L. W. and LaBerge, G. 1. (1969) "Central Wisconsin Volcanic Belt,"Guidebook for 15th Annual Institute on Lake Superior Geology.

—41—

HEMATITE PSEUDOMORPI-1IC AFTER BIOCENICPYRITE IN THE NEGAUNEE IRON FORMATION

N. S. Lougheed and J. 3. Nancuso

Bowling Green State University, Bowling Green, Ohio 43403

ABSTRACT

Spherules of hematite (limonite), ranging in diameter from5 microns to 20 microns but generally of similar size in eachparticular population, as well as disseminated octahedral crystalsof pseudomorphic hematite occur in many laminations of the Negauneeiron formation. Mineral associations include chert, magnetite, ironcarbonate, and iron silicates. A common associate is fossil fila—mentous mat algae. A tentative conclusion based on two microprobeanalyses indicates carbon in the filaments. Thermal experimentsresult in considerable removal of the filamentous material suggestingthe presence of both amorphous carbon and graphite.

Framboidal pyrite with diameters ranging from 5 microns to 20microns and pyrite octahedra, occurring in two unconsolidated sedi—inents with contrasting environments, one a Pleistocene fresh waterlake deposit, the other a Recent marine tidal lagoon, unequivocallydemonstrate their biogenic origin.

P-Jzoic analogs of somewhat similar lithology to iron formationhave the following associations: chert, carbonate, biogenic euhedralpyrite, biogenic framboidal pyrite 5 microns to 20 microns in diameter,sparce euhedral magnetite and fossil microfcunz: and/or microflora.They further demonstrate the biogenic origin of pyrite.

A finely laminated chert carbonate specimen from the PennsylvanianDimple formation, Texas, clearly shows the transition of biogenicpyrite to pseudomorphic hematite (limonite) by oxidation.

These various features are illustrated by phbtomicrographs tosupport the hypothesis that the spherules of hematite (limonite)and possibly other hematite in the Negaunee iron formation arepseudomorphic after biogenic pyrite, and together with the presenceof probable algal mat strongly support the bio3enic (in part) genesisof the Negaunee iron formation.

-41-

HE~1ATITE PSEUDOMORPHIC AFTER BIOGENICPYRITE IN THE NEGAUNEE IRON FORMATION

M. S. Lougheed and J. J. Mancuso

Bowling Green State University, Bowling Green, Ohio 43403

A B S T R ACT

Spherules of hematite (limonite), ranging in diameter from5 microns to 20 microns but generally of similar size in eachparticular population, as well as disseminated octahedral crystalsof pseudomorphic hematite occur in many laminations of the Negauneeiron formation. Mineral associations include chert, magnetite, ironcarbonate, and iron silicates. A common associate is fossil filamentous mat algae. A tentative conclusion based on two microprobeanalyses indicates carbon in the filaments. Thermal experimentsresult in considerable removal of the filamentous material suggestingthe presence of both amorphous carbon and graphite.

Framboidal pyrite with diameters ranging from 5 microns to 20nlicrons and pyrite octahedra, occurring in two unconsolidated sediments with contrasting environments, one a Pleistocene fresh waterlake deposit, the other a Recent marine tidal lagoon, unequivocallydemonstrate their biogenic origin.

Paleozoic analogs of somewhat similar lithology to iron formationhave the following associations: chert, carbonate, biogenic euhedralpyrite, biogenic framboidal pyrite 5 microns to 20 microns in diameter,sparce euhedral magnetite and fossil microfauna and/or microflora.They further demonstrate the biogenic origin of pyrite.

A finely laminated chert carbonate specimen from the PennsylvanianDimple formation, Texas, clearly shows the transition of biogenicpyrite to pseudomorphic hematite (limonite) by oxidation.

These various features are illustrated by photomicrographs tosupport the hypothesis that the spherules of hematite (limonite)and possibly other hematite in the Negaunee iron formation arepseudomorphic after biogenic pyrite, and together with the presenceof probable algal mat strongly support the bio~enic (in part) genesisof the Negaunee iron formation.

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DISTRIBUTION OF URANIUM AND THORIUM IN PRECAMBRIANROCKS OF THE WESTERN GREAT LAKE: RE3ION

by

Roger C. Malan and David A Sterling

U.S. Atomic Energy ommission, Grand Junction, Thiorado

AECTRACT

Prospecting during the 1950's resulted in the discovery of severaluranium and thorium prospects in Precambrian rocks in the west-ern Great Lakes region of the United States. No economic depositswere delHeated by the few small exploration efforts that were under-taken. Since then there has been very little exploration for radio-active minerals in that region.

In Wisconsin and in upper Michigan, Lower, Middle, and Upper Pre-cambrian silicic and hyperalkalic plutonic rocks contain anomalousamounts of disseminated radioactive minerals. In upper Michigan,the Middle Precambrian Animikie Series contains uranium veins inslate, monazite placers in conglomerate, and irregular concentra-tions of utanium in iron formation adjacent to slate. These pros-pects do not contain important reserves of uranium or thorium thatare economically mineable at present but some may contain largelong-range, low-grade resources. For example, limited samplingindicates that masses of silicic igneous rocks in northeastern Wis-cousin may contain 50 to 100 parts per million uranium. This isgreater than the uranium content in any of about 250 bulk samplesof igneous rocks that have been analyzed in a recent study of thedistribution of uranium and thorium in Precambrian rocks in thewestern United States. A potentially great low-grade resource ofthorium may exist in the monazite placers in Animikie conglom-erates in the Marquette Range, upper Michigan.

In other areas of the world, stratiform uranium deposits in MiddlePrecambrian coarse clastic sediments are of major importance.The world's greatest known resource of uranium is in basal con-glomerates of the Middle Precambrian Huronian Series in theElliot Lake - Blind River district in southern Ontario. Importantstratiform uranium deposits have not been discovered in theAnimikie Series which is in part correlative with the Huronian;

-42-

DISTRIBUTION OF URANIUM AND THOP.IUM IN PRECAMBRIANROCKS OF THE WESTERN GREAT LAKE: RE GION

by

Roger C. N~alan and David 1_ Sterling

U.S. Atomic Energy :::ommission, Grand Junction,:::olorado

AE<:::;TRACT

Prospecting during the 1950 1 s resulted in the discovery of severaluranium and thorium prospects in Precambrian rocks in the western Great Lakes region of the United States. No economic depositswere delineated by the few small exploration efforts that were undertaken. Since then there has been very little exploration for radioactive minerals in that region.

In Wisconsin and in upper Michigan, Lower, Middle, and Upper Precambrian silicic and hyperalkalic plutonic rocks contain anomalousamounts of disseminated radioactive minerals. In upper Michigan,the Middle Precambrian Animikie Series contains uranium veins inslate, monazite placers in conglomerate, and irregular concentrations of Ul anium in iron formation adjacent to slate. These prospects do not contain important reserves of uranium or thorium thatare economically mineable at present but some may contain largelong-range, low - grade re sources. For example, limited samplingindicates that masses of silicic igneous rocks in northeastern Wisconsin may contain 50 to lOO parts per million uranium. This isgreater than the uranium content in any of about 250 bulk samplesof igneous rocks that have been analyzed in a recent study of thedistribution of uranium and thorium in Precambrian rocks in thewestern United States. A potentially great low-grade resource ofthorium may exist in the monazite placers in Animikie conglomerates in the Marquette Range, upper Michigan.

In other areas of the world, stratiform uranium deposits in MiddlePrecambrian coarse clastic sediments are of major importance.The world's greatest known resource of uranium is in '::asal conglomerates of the Middle Precambrian Huronian Series in theElliot Lake - Blind River district in southern Ontario. Importantstratiform uranium deposits have not been discovered in theAnimikie Series which is in part correlative with the Huronian;

43—

however, limited sampling ia the iviarquette Range, upper Michigan,indicates that anomalous amounts of uranium are present in coarseelastic facies. Also uranium/thorium ratios in the Animikie in theMarquette Range and in the Huronian in the Elliot Lake - Blind Riverarea increase in descending stratigraphic positions. More study ofthe distribution of uranium and thorium in the Animikie is warnnted.

CELECTED REFERENCES

T.llsley, C. T., Bills, C. W. , and Pollock, J.W. , 1958, Some geo-chemical methods of uranium exploration, in Survey of RawMaterials Resources: United Nations, New York, Proc. Secondlnternat. Conf. Peaceful Uses Atomic Energy, 1958, v. 2, p.126-130.

James, H. L., 1958, Stratigraphy of pre-Keweenawan rocks in partsof Northern Michigan: U. E Geol. Survey Prof. Paper 314-C, 42 p.

King, J. W., 1960, Report of examination, Little Wolf Mining & Min-erals, Inc., Anklam Property Big Falls, Waupaca County, Wiscon-sin: U.S. Atomic Energy Comm. open file rept.

Malan, R. C., and Sterling, D. A., 1969, An introduction to the dis-tribution of uranium and thorium in Precambrian rocks includingthe results of preliminary studies in the southwestern UnitedStates: U.S. Atomic Energy Comm. AEC-RD.-9, 54 p., open file.

Roscoe, S. M. , 1969, 1-luronian rocks and uraniferous conglomeratesin the Canadian shield; Geol. Survey Canada Paper 68-40.

Stead, F. W., Davis, F. J., Nelson, R. A., and Reinhardt, P. W.,1950, Airborne radioactivity survey of parts of Marquette, Dick-inson, and Baraga Counties, Michigan: U.S. Geol. Curvey open—file map.

Vickers, R. C., 1956a, Geology and monazite content of te GoodrichQuartzite, Palmer area Marquette County, Michigan: U.S. Geol.Survey Bull. 1030-F.

________

1956b, Airborne and ground reconnaissance of part of the

syenite complex near Wausau, Wisconsin: U.S. Geol. SurveyBull. 1042-B.

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however, limited sampling L.1. the Ivlarquette Range, upper Michigan,indicates that anomalous amounts of uranium are present in coarseclastic facies. Also uranium/thorium ratios in the Animikie in theMarquette Range and in the Huronian in the Elliot Lake - Blind Riverarea increase in descending stratigraphic positions. More study ofthe distribution of uranium and thorium in the Animikie is warranted.

:ELECTED REFERENCES

':llsley, C. T., Bills, C. W., and Pollock, J. W., 1958, Some geochemical methods of uranium exploration, in Survey of RawMaterials Resources: United Nations, New York, Proc. SecondInternat. Con£. Peaceful Uses Atomic Energy, 1958, v. 2, p.126-130.

James, H. L., 1958, Stratigraphy of pre-Keweenawan rocks in partsof Northern Michigan: U. E Geol. Survey Pro£. Paper 314-C, 42 p.

King, J. W., 1960, Report of examination, Little Wolf Mining & Minerals, Inc., Anklam Property Big Falls, Waupaca County, Wisconsin: U. S. Atomic Energy Comm. open file rept.

Malan, R. C., and Sterling, Do A., 1969, An introduction to the distribution of uranium and thorium in Precambrian rocks includingthe results of preliminary studies in the southwestern UnitedStates: U.S. Atomic Energy Comm. AEC-RD-9, 54 p., open file.

Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomeratesin the Canadian shield: Geol. Survey Canada Paper 68-40.

Stead, F. W., Davis, F. J., Nelson, R. A., and Reinhardt, P. W.,1950, Airborne radioactivity survey of parts of Marquette, Dickinson, and Baraga Counties, Michigan: U. S. Geol. :urvey openfile map.

Vickers, R. C., 1956a, Geology and monazite content of t>-e GoodrichQuartzite, Palmer area Marquette County, Michigan: U. S. Geo!.Survey Bull. 1030-F.

1956b, Ai rborne and ground reconnaissance of part of the-----syenite complex near Wausau, Wisconsin: U. S. Geol. SurveyBull. 1042-B.

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—45—

LOWER KEWEENAWAN SEDIMENTS OF THE LAKE SUPERIOR REGION

Allen F. Mattis

Department of GeologyUniversity of Minnesota, Duluth

ABSTRACT

Lower Keweenawan sediments directly underlie the Keweenawan

voi:anic series in the Lake Superior region. Studies of the quartz—rich Puckwunge Formation (Minnesota) and the Bessemer Formation (Michiganand Wisconsin), which include thin section petrography, heavy mineral

analysis, and measurement of paleocurrent indicators, provide new data

on Lower Keweenawan events.

The Puckwunge Formation of Cook County, northeastern Minnesota,is LT.termittently exposed along a 25 mile belt extending westward from

Pigeon Point on Lake Superior. Although the lower contact of this

formation is not exposed, outcrops of the formation thicken from 30feet on Grand Portage Island to over 100 feet in the westernmost

exposures. Fifteen feet of basal conglomerate exposed on Grand PortageIsland contains flat chips and pebbles of argillite and slate, probablyderived from the underlying Rove Formation, and rounded pebbles ofquartzite. Thin section examinutioi indicates total feldspar contentranges from 20 percent in the eastern exposures to only a trace in thewestern outcrops. Unit quartz is the dominant quartz uype, with up to20 percent polycrystalline quartz being preseriI. Zircon, apatite, andepidote are the common nonopaque accessory heavy ininerais, with tourma—line also present in the central portion of the outcrop belt.

At Nopeming, just west of Duluth, 25 feet of quartzite and quartz-and quatnJte—pebble conglomerate are •:->.oosed bere&: }T the basal (?)

Keweenawn flow over a distance of half a mile. In the sand—sizedfractions, unit quartz is LUe dominant quartz tye, with polycrystallinequartz also preseit. Zircon is the principal nonopaque accessory heavymineral, with minor amounts of a;atite and tourmaline present. Gravityand magnetic profiles across the na suggest the presence of a tabular,dike—like mafic body beneath the Puckwunge exposures at Nopeming; agreater degree of recrystallization in the lowermos: sediments may berelated to this probable intrusive.

The Bessemer Formation of :-:ohigai-r and Wisconsin is intermittentlyexposed along a 40 :nile belt extending eastward from >leflen, Wisconsin.The formation is over 150 feet thick where both the upper i-td lowercontacts are visible. A basal conglomerate contains rounded pebbles ofquartz, qu;E2ite, flint, and jasper in a qnartzite matrix. In thesand—si-:d fractions, unit qunrtz is the dominant quartz type, withpolycrystalline quartz also present. The average total feidsrar contentis less than 10 percent. Zircon is the dominant nonopaque nccesscryh!avy hliner&I, with apatite, rutile, and tourmaline also present.

-45-

LOWER KEWEENAWAN SEDHlENTS OF THE LAKE SUPERIOk REGION

Allen F. Mattis

Department of GeologyUniversity of Hinnesota, Duluth

A B S T RAe T

Lower Keweenawan sediments directly underlie the Keweenawanvolcanic series in the Lake Superior region. Studies of the quartz-rich Puckwunge Formation (Minnesota) and the Bessemer Formation (Michiganand Wisconsin), which include thin section petrography, heavy mineralanalysis, and measurement of paleocurrent indicators, provide new dataon Lower Keweenawan events.

The Puckwunge Formation of Cook County, northeastern Minnesota,is intermittently exposed along a 25 mile belt extending westward fromPigeon Point on Lake Superior. Although the lower contact of thisformation is not exposed, outcrops of the formation thicken from 30feet on Grand Portage Island to over 100 feet in the \vesternmostexposures. Fifteen feet-of basal conglomerate exposed on Grand PortageIsland contains flat chips and pebbles of argillite and slate, probablyderived from the underlying Rove Formation, and rounded pebbles ofquartzite. Thin section examination indicates total feldspar contentranges from 20 percent in the eastern exposures to only a trace in thewestern outcrops. Unit quartz is the dominant quartz type, with up to20 percent polycrystalline quartz being present. Zircon, apatite,andepidote are the common nonopaque accessory heavy minerals, with tourmaline also present in the central portion of the outcrop belt.

At Nopeming, just west of Duluth, 25 feet of quartzite and quartz-and quartzite-pebble conglomerate are exposed beneath the basal (?)Keweenawan flow over a distance of half a mile. In the sand-sizedfractions, unit quartz is the dominant quartz type, with polycrystallinequartz also present. Zircon is the principal nonopaque accessory heavymineral, with minor amounts of apatite and tourmaline present. Gravityand magnetic profiles across the area suggest the presence of a tabular,dike-like mafic body beneath the Puckwunge exposures at Nopeming; agreater degree of recrystallization in the lowermost sediments may berelated to this probable intrusive.

The Bessemer Formation of Hichigan and lHsconsin is intermittentlyexposed along a 40 mile belt extending eastward from ~le~len, Wisconsin.The fonnation is over 150 feet thick where both the upper and lowercontacts are visible. A basal conglomerate contains rounded pebbles ofquartz, quartzite, flint, and jasper in a rruartzite matrix. In thesand-sized fractions, unit quartz is the dominant quartz type, withpolycrystalline quartz also present. The average total feldspar contentis less than 10 percent. Zircon is tlte dominant nonopaque accessoryheavy ltlineral, \vith apatite, rutile. and tourmaline also presenL

—46—

Exposures of the lower IKeweenawan sediments along both limbsof the Lake Superior Syncline suggest the deposition of a thin sheetof sediment throughout the region. The cross—bedded, rippie—mariced,well sorted quartz—rich sediment composed of well rounded grainssuggests a shallow water environment. Cross—bedding, ripple mark,and parting lineation measurements in these sedimentary rocks indicatea general southerly direction of sediment transport, with the sedimentsbeing derived from the Pre—Keweenawan rocks to the north. Thus, theseLower Keweenawan sediments were probably deposited during the northwardtransgression of a sea into the region, and were apparently the finalsediment deposited in the Lake Superior region prior to formation ofthe Lake Superior Syncline.

-46-

Exposures of the lower Keweenawan sediments along both limbsof the Lake Superior Sync:ine suggest the deposition of a thin sheetof sediment throughout the region. The cross-bedded, ripple-marked,well sorted quartz-rich sediment composed of Hell rounded grainssuggests a shallow water environment. Cross-bedding, ripple mark,and parting lineation measurements in these sedimentary rocks indicatea general southerly direction of sediment transport, with the sedimentsbeing derived from the Pre-Keweenawan rocks to the north. Thus, theseLower Keweenawan sediments were probably deposited during the northwardtransgression of a sea into the region, and were apparently the finalsediment deposited in the Lake Superior region prior to formation ofthe Lake Superior Syncline.

—47—

EXPLORATION GEOLOGY OF DOUGLAS COUNTY, w::scoNSIN

by

Joseph T. Mcnge1, Jr., Professor and Ronald A. HendricksonDepartment of Geology

Wisconsin State University, Superior, Wisconsin 54880and

Wisconsin Geological and Natural History Survey, Madison 53706

ABSTRACT

The oldest bed rock units in Douglas County are the Keweenawanbasaltic fissure flows of the St. Croix horst south of the lineBrule, raple, Wentworth, South Range, Pattison State Park, Patzau.These flows have a N6OE3SS attitude along tha north margin of thehorst and N5OE15S along the south. They are intruded by "red rock"in l5—47N-13W,K/Ar dated at .95 b.y. and by a gabbro mass in 32—48N—12W.

Basaltic flows occur again south of the line Totagatic River—Ounce Creek where the attitude is N4SE3ON and there are extensiveconglomerate and sandstone interbeds with the lavas.

Native copper and copper suif ides are found in ainygdaloidalhorizons and along fractures wherever the Keweenawan lavas outcrop.Almost all lava outcrop is within the following limits:

(1) A 2 mile wide band a,uth of the line Brule—Patzau

(2) A 2 nile wide band north of the line Winnebaujou—St.Croix River in 43N—14W and 44N—13W

(3) The NW half of 43N—1SW

(4) The south half of 44N—l5W and SW quarter 44N—14W

(5) The NE quarter of 45N—l2W

(6) The SE haf of 43N—1OW, especially along the valley ofDingle Creek in sections 12 and 13 where an extensivesection is exposed

During the last century and a half the properties listed belowhave all been the locus of shafts and/or test pits and drilling forcopper in the lavas.

-47-

EXPLORATION GEOLOGY OF DOUGLAS COUNTY, WISCONSIN

by

Joseph T. Mengel, Jr., Professor and Ronald A. HendricksonDepartment of Geology

Wisconsin State University, Superior, Wisconsin 54880and

Wisconsin Geological and Natural History Survey, Madison 53706

A B S T R ACT

The oldest bed rock units in Douglas County are the Keweenawanbasaltic fissure flows of the St. Croix horst south of the lin~

Brule, Maple, Wentworth, South Range, Pattison State Park, Patzau.These flows have a N60E35S attitude along the north margin of thehorst and N50E15S along the south. They are intruded by "red rock"in 15-47N-13W,K/Ar dated at .95 b.y. and by a gabbro mass in 32-48Nl2W.

Basaltic flows occur again south of the line Totagatic RiverOunce Creek where the attitude is N45E30N and there are extensiveconglomerate and sandstone interbeds with the lavas.

Native copper and copper sulfides are found in amygdaloidalhorizons and along fractures wherever the Keweenawan lavas outcrop.Almost all lava outcrop is within the following limits:

(1) A 2 mile wide band south of the line Brule-Patzau

(2) A 2 mile wide band north of the line Winnebaujou-St.Croix River in 43N-14w and 44N-13W

(3) The NW half of 43N-15W

(4) The south half of 44N-15W and SW quarter 44N-14W

(5) The NE quarter of 45N-12W

(6) The SE half of 43N-lOW. especially along the valley ofDingle Creek in sections 12 and 13 where an extensivesection is exposed

During the last century and a half the properties listed belowhave all been the locus of shafts and/or test pits and drilling forcopper in the lavas.

—48—

Location Property Nature

NE 12—43N—1OW Weyerhauser Ainygdaloid and inassoc. sediments

SW 28—43N—1OW Williams Ashbed amygdaloid

NW 6—43N—13W Superior Copper Mines C.F. Irving Mon 5,

p1. 25

SW 8—43N—13W Copper Mine Dam Ainygdaloid (?)

SE 34—44N—J.4W Nowell (Crotty Brook ArnoL) Anygdaioici (?)

SE 14—44N—13W Aac ía Amygdaloid

NE 31-4 7N—liJ Culligan Amygdaloid

SE 28—47N—14W Bardon Amygdaloid ()

SW 1447N—14W Cc1:ier Creek Amygdaloid

NE 8—47N—13W Fond du Lac Amygdaloid

SE 2—47N—l3W Starkweather Fissure vein 4—6' wideWisconsin NE strike 75 NW dipZdwards

Center ll—47N—13W Amnicon Narrow steeply dippingvein

4—47N-12W Houghton Amygdalcid

NE 8—47N—12W Badger Nnygdaloid (?)

NW 1O—47N—12W Chippewa Fractured Amygdaloids

St 1O—47N—12W Copper King Amygdaloid (?)

NE 23--48N--lOW Cascade Miygdaloid (?)

NE 24-48N—lOW Percival, Jr. Amygdaloid

NE 27—48N—1OW Percival Veinlets in amygdaloid

NW 28—48N—1OW Astor Amygdaloid

SE 29—48N—12W Mrnicon Mtygdaloid (?)

SE 34—48N—13W Catlin Amygdaloid

Location

NE l2-43N-lOW

SW 28-43N-lOW

NW 6-43N-l3W

SW 8-43N-l3W

SE 34-44N-l4W

SE l4-44N-l3W

NE 3l-47N-l4W

SE 28-47N-l4W

SW l4-47N-l4W

NE 8-47N-l3W

SE 2-47N-l3W

-48-

Property

Weyerhauser

Williams

Superior Copper Mines

Copper Mine Dam

Nowell (Crotty Brook Arnold)

Arnold

Culligan

Bardon

Copper Creek

Fond du Lac

StarkweatherWisconsinEdwards

Nature

Amygdaloid and inassoc. sediments

Ashbed amygdaloid

C.F. Irving Mon 5,pI. 25

Amygdaloid (? )

Amygdaloid (? )

Amygdaloi<l

Amygdaloid

Amygdaloid ('l)

Amygdaloid

Amygdaloid

Fissure vein 4-6' wideNE strike 75 NW dip

Center ll-47N-l3W Amnicon

4-47N-l2\.J Houghton

NE 8-47N-l2W Badger

NW lO-47N-l2W Chippewa

SW lO-47N-l2W Copper King

NE 23-48N-lOW Cascade

NE 24-48N-lO\.J Percival, Jr.

NE 27-48N-lOW Percival

NW 28-48N-lOW Astor

SE 29-48N-l2W Amnicon

SE 34-48N-l3W Catlin

Narrow steeply dippingvein

Amygdaloid

Amygdaloid (?)

Fractured Amygdaloids

Amygdaloid (?)

Amygdaloid (?)

Amygdaloid

Veinlets in amygdaloid

Amygdaloid

Amygdaloid (?)

Amygdaloid

—49—

The U. S. Bureau of Mines reported on extensive tests of theWeyerhauser Property in 1947 and the Chippewa Property in 1955.

The Keweenawan Oronto Group occupies the Lake Superior synelinesouth of the line Winneboujou, Solon Springs — St. Croix River. Re-portedly the Copper Harbor Conglomerate, the Nonesuch Shale, and theFreda Sandstone units are all present in this area. Prospecting inthe syncline has centered on attempts to locate a copper—bearing faciesof the Nonesuch Shale, but has been hampered by complete absence ofoutcrop except along the lower portion of the St. Croix River Valley.Conglomerates have been reported from exploration tests in: NWNW 10—44N-llW, SW corner 3l—44N—1OW, W half lO—45N—1OW, SWSW l6—45N—llW,NE corner 27—45N-lOW, NENE 34—45N—llW, SE 10—46N—llW, NESE 25—46N—11W;Shale rrom near the center of 34—47N—llw; and Sandstone from NE 3—45N—lOW, SW 8—46N—lOW, NWSW ll—46N—lOW, W half 15—47N—1OW, SE 19—47N—1OW.Waterwells do not penetrate the estimated 200 foot depth of sandyoverburden in this area.

References

Grant, U. S., 1901, Preliminary report on the copper—bearing rocks ofDouglas County, Wisconsin: Wisc. Geol. and Nat. Hist. Surv. Bull.6, 55 p.

Holliday, it. W., 1955, Investigation of Chippewa copper—nickel prospectnear Rockmont, Douglas County, Wisconsin: U.S. Bur. Mines Rept.mv. 5114, 11 p.

Irving, R. D., 1883, Copper—bearIng rocks of Lake Superior: U. S.Ceol. Survey Mon. 5, 464 p.

pSmith, H. C., 1947, Copper deposits of Douglas County, Wisconsin:

U.S. Bur. Mines Rept. mv. 4088, 7 p.

Sweet, F. T., 1880, Geology of the western Lake Superior district inGeology of Wisconsin 1873—1879: Wisconsin Geol. Survey, v. 4,

p. 305—362.

-49-

The U. S. Bureau of Mines reported on extensive tests of theWeyerhauser Property in 1947 and the Chippewa Property in 1955.

The Keweenawan Oronto Group occupies the Lake Superior synclinesouth of the line Hinneboujou, Solon Springs - St. Croix River. Reportedly the Copper Harbor Conglomerate, the Nonesuch Shale, and theFreda Sandstone units are all present in this area. Prospecting inthe syncline has centered on attempts to locate a copper-bearing faciesof the Nonesuch Shale, but has been hampered by complete absence ofoutcrop except along the lower portion of the St. Croix River Valley.Conglomerates have been reported from exploration tests in: NWNW 1044N-llW, SW corner 3l-44N-lOW, Whalf 10-45N-lOW, SWSW l6-45N-llW,NE corner 27-45N-lOW, NENE 34-45N-llW, SE 10-46N-llW, NESE 25-46N-llW;Shale from.near the center of 34-47N-llW; and Sandstone from NE 3-45NlOW, SW 8-46N-lOW, NWSW ll-46N-lOW, Whalf l5-47N-lOW, SE 19-47N-lOW.Waterwells do not penetrate the estimated 200 foot depth of sandyoverburden in this area.

References

Grant, U. S., 1901, Preliminary report on the copper-bearing rocks ofDouglas County, Wisconsin: Wise. Geol. and Nat. Hist. Surv. Bull.6, 55 p.

Holliday, R. W., 1955, Investigation of Chippewa copper-nickel prospectnear Rockmont, Douglas County, Wisconsin: U.S. Bur. Mines Rept.Inv. 5114, 11 p.

Irving, R. Do, 1883, Copper-bearing rocks of Lake Superior: U. S.Geo1. Survey Mon. 5, 464 p.

,Smith, N. C., 1947, Copper deposits of Douglas County, Wisconsin:

U.S. Bur. Mines Rept. Inv. 4088, 7 p.

Sweet, E. T., 1880, Geology of the western Lake Superior district inGeology of Wisconsin 1873-1879: Wisconsin Geol. Survey, v. 4,p. 305-362.

—50--

REVISED KEWEENAWAN SUBSURFACE STRATIGRAPHYSOUTHEASTERN MINNESOTA

G. B. MoreyMinnesota Geological Survey

Minneapolis, Minnesota

ABSTRACT

The Mid—continent Gravity High is the major tectonic feature of thenorthern mid-continent region. Detailed geophysical surveys over the 600mile—long belt show that the structure consists mainly of a sequence of ba-saltic lava flows which form steep—sided blocks that are an average of about40 miles wide and several miles thick. Clastic rocks occur in flanking ba-sins and in grabens and axial basins on top of the blocks. Because much ofthe structure is covered by Paleozoic rocks, little is known about the rocksaway from their outcrop area around Lake Superior. However, the Paleozoiccover is relatively thin in southeastern Minnesota, and several drill holeshave penetrated considerable thicknesses of Iceweenawan strata. Of particularinterest here are the sedimentary rocks which flank and overlie the St. Croixhorst, an uplifted basalt block in southeastern Minnesota.

Keweenawan sandstone and shale have been known from the subsurface for ahundred years. Because of their red color, they have been grouped together un-der the Red Clastic Series, a "temporary" name proposed by Hall and others in1912. Mooney and others (1970, J. Geophys. Res., v. 75, p. 5056—5086) havesubdivided these rocks into a number of seismic units, and concluded that sev-eral of their subdivisions could be correlated with already named formations.A detailed petrographic study of Ii,000 feet of diamond drill core from a numberof localities has demonstrated the presence of at least three lithologicallydistinct intervals which more—or-less correspond to seismic units. Therefore,it will be recommended (Morey, in prep.) that the term Red Clastic Series beabandoned and replaced by a more suitable nomenclature. Accordingly, threeformations will be recognized: (1) Hinckley Sandstone, a buff to tan rock con-taining 95 percent or more quartz. (2) Fond du Lac Formation, consisting ofintercalated moderate red shale and sandstone containing 6o percent quartz, 30percent orthoclase, microcline and sodic plagioclase, and 10 percent "granitic"rock fragments. (3) An as yet unnamed formation, consisting of dark reddish—brown mudstone and sandstone containing variable amounts of quartz, plagioclaseof intermediate composition, and aphanitic igneous rock fragments. The firsttwo formations are known from surface exposures, however the third formation isconfined entirely to the sub—surface.

A stratigraphic analysis indicates that in the flanking basins, the un-named formation is overlain by the Fond du Lac Formation, which in turn is grada-tionally overlain by the Hinckley Sandstone. On top of the horst, the unnamedformation overlies basaltic rocks and is locally overlain by Hinckley Sandstone;at places, a regolith as much as 100 feet thick separates the two formations.Either the Fond du Lac Formation was never deposited on top of the horst, or wasremoved prior to Hinckley deposition.

Any attempt to explain the geologic evolution of this structure must takeinto account these stratigraphic relationships.

-50-

REVISED KEWEENAWAN SUBSURFACE STRATIGRAPHYSOUTHEASTERN MINNESOTA

G. B. MoreyMinnesota Geological Survey

Minneapolis~ Minnesota

ABSTRACT

The Mid-continent Gravity High is the major tectonic feature of thenorthern mid-continent region. Detailed geophysical surveys over the 600mile-long belt show that the structure consists mainly of a sequence of basaltic lava flows which form steep-sided blocks that are an average of about40 miles wide and several miles thick. Clastic rocks occur in flanking basins and in grabens and axial basins on top of the blocks. Because much ofthe structure is covered by Paleozoic rocks, little is known about the rocksaway from their outcrop area around Lake Superior. However, the Paleozoiccover is relatively thin in southeastern Minnesota, and several drill holeshave penetrated considerable thicknesses of Keweenawan strata. Of particularinterest here are the sedimentary rocks which flank and overlie the St. Croixhorst, an uplifted basalt block in southeastern Minnesota.

Keweenawan sandstone and shale have been known from the subsurface for ahundred years. Because of their red color, they have been grouped together under the Red Clastic Series, a IItemporaryll name proposed by Hall and others in1912. Mooney and others (1970, J. Geophys. Res., v. 75, p. 5056-5086) havesubdivided these rocks into a number of seismic units, and concluded that several of their subdivisions could be correlated with already named formations.A detailed petrographic study of 4,000 feet of diamond drill core from a numberof localities has demonstrated the presence of at least three lithologicallydistinct intervals which more-or~less correspond to seismic units. Therefore,it will be recommended (Morey, in prep.) that the term Red Clastic Series beabandoned and replaced by a more suitable nomenclature. Accordingly, threeformations will be recognized: (1) Hinckley Sandstone, a buff to tan rock containing 95 percent or more quartz. (2) Fond du Lac Formation, consisting ofintercalated moderate red shale and sandstone containing 60 percent quartz, 30percent orthoclase, microcline and sodic plagioclase, and 10 percent "granitic"rock fragments. (3) An as yet unnamed formation, consisting of dark reddishbrown mudstone and sandstone containing variable amounts of quartz, plagioclaseof intermediate composition, and aphanitic igneous rock fragments. The firsttwo formations are known from surface exposures, however the third formation isconfined entirely to the sub-surface.

A stratigraphic analysis indicates that in the flanking basins, the unnamed formation is overlain by the Fond du Lac Formation, which in turn is gradationally overlain by the Hinckley Sandstone. On top of the horst, the unnamedformation overl ies basaltic rocks and is locally overlain by Hinckley Sandstone;at places, a regolith as much as 100 feet thick separates the two formations.Either the Fond du Lac Formation was never deposited on top of the horst, or wasremoved prior to Hinckley deposition.

Any attempt to explain the geologic evolution of this structure must takeinto account these stratigraphic relationships.

—51—

LIMNOGEOLOGICAL STUDIES OF THUNDER BAY,

LAKE SUPERIOR, ONTARIO

J. S. Mothersill

LAICEHEAD UNIVERSITY

ABSTRACT

Limnogeological studies were carried out in Thunder Bay,Lake Superior during the 1970 field season. Ponar grab samplesand Phleger cores were taken at one hundred and forty—sevenstations on a 1.5 mile grid for geochemical and mineralogicalanalyses. In addition three echo sounding traverses werecarried out using an MS 26F metric echo sounder.

Thunder Bay which is partly silled to the south by aneast—northeast trending horst from Victoria Island to SparIsland contains three separate basins. Recent sediments whichcover the bay—floor grade from a thin veneer of sand, less than3 centimeters thick, to a sequence of clay—silts, up to 14meters thick, in the central parts of the basins. Tight fold-ing of the Recent clay—silts in the deepest part of the centralbasin is probably caused by gravity slumping. Geochemicalinvestigations of the bottom—sediments show that there appearsto be a tendency towards negative Eh and low pH values in thedeep—water areas. In the vicinity of the Kaministikwia Deltaanomolous Eh and pH measurements were recorded which wereprobably caused by industrial pollutants entering the bay fromthe Kaministikwia River.

The Recent sediments unconformably overlie Pleistocenevarved sediments of undetermined thickness. The Recent clay—silt section and the Pleistocene varved sect±on each form atypical syndiagenetic sequence with a relatively thin upperoxidized zone (initial stage) and a lower reduced zone (earlyburial stage). The oxidized zone is caused by the dissolvedoxygen in the trapped waters of the upper layers of sedimentand the action of aerobic bacteria. The depletion of oxygenresults in the underlying reducing zone with anaerobicconditions and a sharp increase in pH.

-51-

LIMNOGEOLOGICAL STUDIES OF THUNDER BAY,

LAKE SUPERIOR, ONTARIO

J. S. Mothersil1

LAKEHEAD UNIVERSITY

A B S T R ACT

Limnogeological studies were carried out in Thunder Bay,Lake Superior during the 1970 field season. Ponar grab samplesand Ph1eger cores were taken at one hundred and forty-sevenstations on a 1.5 mile grid for geochemical and mineralogicalanalyses. In addition three echo sounding traverses werecarried out using an MS 26F metric echo sounder.

Thunder Bay which is partly silled to the south by aneast-northeast trending horst from Victoria Island to SparIsland contains three separate basins. Recent sediments whichcover the bay-floor grade from a thin veneer of sand, less than3 centimeters thick, to a sequence of clay-silts, up to 14meters thick, in the central parts of the basins. Tight folding of the Recent clay-silts in the deepest part of the centralbasin is probably caused by gravity slumping. Geochemicalinvestigations of the bottom-sediments show that there appearsto be a tendency towards negative Eh and low pH values in thedeep-water areas. In the vicinity of the Kaministikwia Deltaanomalous Eh and pH measurements were recorded which wereprobably caused by industrial pollutants entering the bay fromthe Kaministikwia River.

The Recent sediments unconformably overlie Pleistocenevarved sediments of undetermined thickness. The Recent claysilt section and the Pleistocene varved sectton each form atypical syndiagenetic sequence with a relatively thin upperoxidized zone (initial stage) and a lower reduced zone (earlyburial stage). The oxidized zone is caused by the dissolvedoxygen in the trapped waters of the upper layers of sedimentand the action of aerobic bacteria. The depletion of oxygenresults in the underlying reducing zone with anaerobicconditions and a sharp increase in pH.

—52—

REFERENCES

Bissell, H. J., 1959, Silica in sediments of the UpperPaleozoic of the Cordilleran area. Spec. Pubi.of Soc. Econ. Paleontologists and Nineralogists,7, pp 150—185.

Dapples, E. C., 1962, Stage of diagenesis in the develop-ment of sandstones. Bull, Ceol. Soc. Am., 73,

pp 913—934.

Emery, K. 0. and Rittenberg S. C., 1952, Early diagenesisof California Basin sediments in relation to originof oil. Bull. Am. Assoc. Petrol. Geologists, 36,

pp 735—806.

Larsen, C. and Chilingar C. V., 1967, Introduction. In

Diagenesis in sediments. Univ. of SouthernCalifornia, Los Angeles, pp 1—17.

Twenhofel, W. H., 1942, The rate of deposition of sediments:a major factor connected with aiteration ofsediments after deposition. Jour. SedimentaryPetrology, 12, pp 99—110.

ZoBell, C. E., 1942, Changes produced by micro-organisms in sediments after deposition. Jour.

Sedimentary Petrology, 12, pp 127—136.

-52-

REFERENCES

Bissell, H. J., 1959, Silica in sediments of the UpperPaleozoic of the Cordilleran area. Spec. Publ.of Soc. Econ. Paleontologists and Mineralogists,7, pp 150-185.

Dapples, E. C., 1962, Stagement of sandstones.pp 913-934.

of diagenesis in the developBull, Geol. Soc. Am., 73,

Emery, K. O. and Rittenberg S. C., 1952, Early diagenesisof California Basin sediments in relation to originof oil. Bull. Am. Assoc. Petrol. Geologists, 36,pp 735-806.

Larsen, G. and Chilingar G. V., 1967, Introduction. InDiagenesis in sediments. Univ. of SouthernCalifornia, Los Angeles, pp 1-17.

Twenhofel, W. H., 1942, The rate of deposition of sediments:a major factor connected with alteration ofsediments after deposition. Jour. SedimentaryPetrology, 12, pp 99-110.

ZoBell, C. E., 1942, Changes produced by microorganisms in sediments after deposition. Jour.Sedimentary Petrology, 12, pp 127-136.

—53—

REINVESTIGATION OF "RED ROCKS" IN THEPIGIY)N POINT AREA, MINNESOTA

M. C. M::drey, Jr., and P. W. Weiblen

Minnesota Geological Surveyand

University of Minnesota, Minneapolis

ABSTRACT

Keweenawan granitic rocks of reddish color as exemplifiedby the "red rocks" of the Pigeon Point, Cook County, area aredivisible into red quartzites, porphyritic red rocks and granularred rocks. Retnapping with this division reduces the problem ofexcess granophyre abundance associated with the Pigeon Point sill.

Reddish quartzites contain abundant recrystallized quartzwith minor iicrstitial sericite—biotite granophyric feldspar—quartz intergrowths. A faint foliation defined by biotite linea—tion and relict bedding can be discerned in quartzite inclusionsfound in intermediate rock of dioritic composition.

Porphyritic red rock displays euhedral feldspar phenocrystsin a granophyric quartz—feldspar groundmass. This rock type cutsthe Rove formation and intrudes the Pigeon Point gabbro. Miaro—litic cavities are found filled with calcite and zeolite minerals.

Granular red rock contains granoblastic quartz and feldsparset in a granophy tic groundinass of quartz and feldspar with minorsericite—biotite and rounded grains or sphene and epidote.

Phase relations and overgrowths on quartz suggest that asedimentary parentage is possible, however intrusive relations onPigeon Point require that the granular red rocks must have been acrystal mush.

The abundance of red rock in the Pigeon Point area in excessof that to be expected by differentiation has been noted since1893, and is part of a reinvestigation of the eastern exposures ofKeweenawan rocks in Minnesota.

-53-

REINVESTIGATION OF "RED ROCKS" IN THEPIGEON POINT AREA, MINNESOTA

M. G. Mudrey, Jr., and P. W. Weiblen

Minnesota Geological Surveyand

University of Minnesota, Minneapolis

ABSTRACT

Keweenawan granitic rocks of reddish color as exemplifiedby the "red rocks" of the Pigeon Point, Cook County, area aredivisible into red quartzites, porphyritic red rocks and granularred rocks. Remapping with this division reduces the problem ofexcess granophyre abundance associated with the Pigeon Point sill.

Reddish quartzites contain abundant recrystallized quartzwith minor interstitial sericite-biotite and granophyric feldsparquartz intergrowths. A faint foliation defined by biotite lineation and relict bedding can be discerned in quartzite inclusionsfound in intermediate rock of dioritic composition.

Porphyritic red rock displays euhedral feldspar phenocrystsin a granophyric quartz-feldspar groundmass. This rock type cutsthe Rove formation and intrudes the Pigeon Point gabbro. Miarolitic cavities are found filled with calcite and zeolite minerals.

Granular red rock contains granoblastic quartz and feldsparset in a granophyric groundmass of quartz and feldspar with minorsericite-biotite and rounded grains or sphene and epidote.

Phase relations and overgrowths on quartz suggest that asedimentary parentage is possible, however intrusive relations onPigeon Point require that the granular red rocks must have been acrystal mush.

The abundance of red rock in the Pigeon Point area in excessof that to be expected by differentiation has been noted since1893, and is part of a reinvestigation of the eastern exposures ofKeweenawan rocks in Minnesota.

—54—

THE SEDIMENTOLOGY AND TECTONIC SIGNIFICANCE OF THEBAYFIELD GROUP, WISCONSIN AND MINNESOTA

Wallace Darwin Myers, II

University of Wisccnsin

ABSTRACT

A study of the mineralogical and physical characteristics of theBayfield Group was made to determine 1) depositional environment,paleogeography, and source terrane during Bayfield time, 2) thenature of the contact of the Bayfield Group with the older OrontoGroup, and 3) the early history of the Douglas fault.

Field study included detailed mapping of Bayfield and OrontoGroup rocks to establish the location of all outcrops, the lithologyand stratigraphy of all formations, the geographic and stratigraphicdistribution of sedimentary structures and the structural relationshipsbetween rock bodies. Analyses of rock specimens in the laboratoryincluded 1) petrographic study of the bulk mineralogy and texturalcharacteristics of sandstone specimens, 2) x—ray diffraction study of theclay mineralogy of shale beds and chemical analyses of the boron con-centration of the clay mineral illite, 3) x—radiography study of theinternal stratification of selected sedimentary structures, and 4)statistical study of directional sedimentary structures.

Petrographic studies of Bayfield Group sandstones indicate thatquartz constitutes approximately 80 percent of framework grains, thatthe feldspar population is dominated by microcline, and that BayfieldGroup sandstones are better sorted and exhibit a higher degree ofrounding than the Freda Sandstone of the Oronto Group.

The high compositional and textural maturity of the Bayfield Groupsuggests that the source area for these sediments was not a simplevolcanic terrane. Petrographic data indicate that the Bayfield andFreda sandstones had a similar source; possibly recycling of the FredaSandstone was an important source of sediments during Bayfield time.

The sedimentary structures of the Bayfield Group are those whichare typically developed in fluvial environments, including trough cross—bedding, current ripple marks, channel sandstones, mudcracks, andassociated structures. The stratification and bed forms of the BayfieldGroup are characteristic of those formed in the upper part of the lower—flow regime (Harms and Fahnestock, 1965). By analogy with modern alluvialchannels the general geologic setting of the Bayfield Group can beinferred to have been an alluvial plain characterized by loz—gradient,meandering, perennial streams. The strong N40°E trend of trough cross—bedding indicates the direction of sedimentary transport was from thesouthwest to the northeast. This inference is supported by the orienta-tion of parting lineation and current ripple marks.

-54-

THE SEDIMENTOLOGY AND TECTONIC SIGNIFICANCE OF THEBAYFIELD GROUP ~ HISCONSIN AND MINNESOTA

Wallace Darwin Myers~ II

University of Wisconsin

ABSTRACT

A study of the mineralogical and physical characteristics of theBayfield Group was made to determine 1) depositional environment,paleogeography, and source terrane during Bayfield time, 2) thenature of the contact of the Bayfield Group with the older OrontoGroup, and 3) the early history of the Douglas fault.

Field study included detailed mapping of Bayfield and OrontoGroup rocks to establish the location of all outcrops, the lithologyand stratigraphy of all formations, the geographic and stratigraphicdistribution of sedimentary structures and the structural relationshipsbetween rock bodies. Analyses of rock specimens in the laboratoryincluded 1) petrographic study of the bulk mineralogy and texturalcharacteristics of sandstone specimens, 2) x-ray diffraction study of theclay mineralogy of shale beds and chemical analyses of the boron concentration of the clay mineral illite, 3) x-radiography study of theinternal stratification of selected sedimentary structures~ and 4)statistical study of directional sedimentary structures.

Petrographic studies of Bayfield Group sandstones indicate thatquartz constitutes approximately 80 percent of framework grains, thatthe feldspar population is dominated by microcline, and that BayfieldGroup sand·stones are better sorted and exhibit a higher degree ofrounding than the Freda Sandstone of the Oronto Group.

The high compositional and textural maturity of the Bayfield Groupsuggests that the source area for these sediments was not a simplevolcanic terrane. Petrographic data indicate that the Bayfield andFreda sandstones had a similar source; possibly recycling of the FredaSandstone was an important source of sediments during Bayfield time.

Tl~ sedimentary structures of the Bayfield Group are those whichare typically developed in fluvial environments~ including trough crossbedding, current ripple marks, channel sandstones, mudcracks, andassociated structures. The stratification and bed forms of the BayfieldGroup are characteristic of those formed in the upper part of the lowerflow regime (Harms and Fahnestock, 1965). By analogy ,.,ith modern alluvialchannels the general geologic setting of the Bayfield Group can beinferred to have been an alluvial plain characterized by 100.,-gradient,meandering, perennial streams. The strong N400E trend of trough crossbedding indicates the direction of sedimentary transport was from thesouthwest to the northeast. This inference is supported by the orientation of parting lineation and current ripple marks.

—55—

The contact of the Bayfield and Oronto Groups is not exposed;petrographic data provide the clearest evidence of the nature of this

boundary. On the basis of the high compositional and textural maturityexhibited by Bayfield Group sandstones, it is suggested that theseelastics represent a new cycle of sedimentation in the synclinal LakeSuperior basin. This interpretation is supported by the contrastingclay mineralogy of the two sequences (illite— and chlorite—rich Orontorocks, but kaolin—rich Bayfield rocks).

There is no compelling evidence that:the Douglas fault was activeduring Bayfield time. Conglomerates are known at nitty two exposures ofthe Douglas fault, and in each case their distribution is restricted tothe zone immediately adjacent to the fault. Furthermore, the conglom-erates do not resemble true tectonic conglomerates either texturally orcompositionally. Additional evidence is provided by the orientationof directional sedimentary structures at the Douglas fault localities.These structures which include parting lineation and cross—bedding, showno apparent relation to the Douglas fault. Thus, there is no geologicevidence of major faulting during Bayfield, or at least Orienta, time.

-55-

The contact of the Bayfield and Oronto Groups is not exposed;petrographic data provide~he clearest evidence of the nature of thisboundary. On the basis of the high compositional and textural maturityexhibited by Bayfield Group sandstones, it is suggested that theseclastics represent a new cycle of sedimentation in the synclinal LakeSuperior basin. This interpretation is supported by the contrastingclay mineralogy of the two sequences (illite- and chlorite-rich Orontorocks, but kaolin-rich Bayfield rocks).

There is no compelling evidence that: the Douglas fault was activeduring Bayfield time. Conglomerates are known at only two exposures ofthe Douglas fault, and in each case their distribution is restricted tothe zone immediately adjacent to the fault. Furthermore, the conglomerates do not resemble true tectonic conglomerates either texturally orcompositionally. Additional evidence is provided by the orientationof directional sedimentary structures at the Douglas fault localities.These structures which include parting lineation and cross-bedding, showno apparent relation to the Douglas fault. Thus, there is no geologicevidence of major faulting during Bayfield, or at least Orienta, time.

— f

LAKE MICHIGAN AEROMAGNETIC SURVEY

byNorbert W. O'Hara William J. Hinze

Great Lakes Research Division Department of GeologyUniversity of Michigan Michigan State UniversityAnn Arbor, Michigan E;t Lansing, Michigan

ABSTRACT

The Precambrian basement complex beneath Lake Michigan, which lieson the western and northern flank of the Paleozoic Michig.E.n Basin, is

only known from a few widely scattered basement drill holes around theperimeter of the Lake and a single drill hole -ithin the Lake on 3eaverIsland. Nevertheless, the basement geology of Lake Michigan is signifi-cant to the Precambrian framework of the Midcontinent and is critical tothe extrapolation of basement structural trends from Lake Superior, orth—em Michigan, and Wisconsin into the Michigan Basin. To fill this gap anaeromagnetic survey consisting of 7,000 miles of total magnetic intensitydata were collected along flight traverses separated by six mile intervals.Flight traverses were flown over northern Lake Michigan in a general north-east direction and over southern Lake Michigan in a northwest direction.

The residual total magnetic intensity map prepared from the collecteddata exhibits three regional magnetic positives. To general the magneticanomalies are related in a direct manner to gravity anomalies on the peri-meter of the Lake. The southernmost postive :rikes northwest from theMichigan shore from +2° to 13° 30'N. This anomaly is associated with asoutheast striking gravity positive and local magnetic positives extend—icLg across southwestern Michigan and northeastern Indiana into Ohio. It

is on strike with the positive gravity and magnetic anomaly in northeasternIndiana which has been found by basement drilling to be underlain by basaltssimilar to Keweenawan flows of the Lake Superior region. The central regionalpositive, which is made up of several individual anomalies, strikes roughly east—west across the Lake between t140 and 450 l5'N. The northern components ofthis anomaly can be traced into Wisconsin and across the southern Peninsulaof Lchigan except where they are transected by the Mid—Michigan rift zone.These trends are believed to be assrciated with the Penokean basement prov-ince. A marked regional magnetic anomaly minimum striking east—west occursto the north of the regional positive. The anomaly is on strike with thefelsic rocks of the Mountain—Amberg area of Wisconsin which is also char-acterized by magnetic minimums. The minimum reappears east of the Mid—Michi—gan rift zone anomaly and strikes east—southeast across to Lake Huron. Thenorthern regional positive magnetic anomaly strikes north—south from TraverseBay to north of Beaver Island where it bifurcates with one limb extendingnorth—northwest through Lake Superior to the Keweenawan baa alts on KeweenawPoint. The other limb continues into the eastern portion of the NorthernPeninsula of Michigan and another segment of this branch connects to theKeweenawan flows on Mamainse Point. In the Traverse Bay area the regionalpositive becomes strongly negative and connects to the south with the mid—Michigan gravity and magnetic anomaly.

LAKE MICHIGAN AEROMAGNETIC SURVEY

Norbert W. O'HaraGreat Lakes Research Division

University of MichiganAnn Arbor, Mi chigan

byWilliam J. Hinze

Department of GeologyMichigan State University

East Lansing, Michigan

A B S T R ACT

The Precambrian basement complex beneath Lake Michigan, which lieson the western and northern flank of the Paleozoic Michigan Basin, isonly known from a few widely scattered basement drill holes around theperimeter of the Lake and a single drill hole within the Lake on BeaverIsland. Nevertheless, the basement geology of Lake Michigan is significant to the Precambrian framework of the Midcontinent and is critical tothe extrapolation of basement structural trends from Lake Superior, Northern Michigan, and Wisconsin into the Michigan Basin. To fill this gap anaeromagnetic survey consisting of 7,000 miles of total magnetic intensitydata were collected along flight traverses separated by six mile intervals.Flight traverses were flown over northern Lake Michigan in a general northeast direction and over southern Lake Michigan in a northwest direction.

The residual total magnetic intensity map prepared from the collecteddata exhibits three regional magnetic positives. In general the magneticanomalies are related in a direct manner to gravity anomalies on the perimeter of the Lake. The southernmost positive strikes northwest from theMichigan shore from 420 to 430 30'N. This anomaly is associated with asoutheast striking gravity positive and local magnetic positives extending across southwestern Michigan and northeastern Indiana into Ohio. Itis on strike with the positive gravity and magnetic anomaly in northeasternIndiana which has been found by basement drilling to be underlain by basaltssimilar to Keweenawan flows of the Lake Superior region. The central regionalpositive, which is made up of several individual anomalies, strikes roughly eastwest across ·the Lake between 440 and 450 15'N. The northern components ofthis anomaly can be traced into Wisconsin and across the southern Peninsulaof Michigan except where they are transected by the Mid-Michigan rift zone.These trends are believed to be associated with the Penokean basement prov-ince. A marked regional magnetic anomaly minimum striking east-west occursto the north of the regional positive. The anomaly is on strike with thefelsic rocks of the Mountain-Amberg area of Wisconsin which is also characterized by magnetic minimums. The minimum reappears east of the Mid-Michi-gan rift zone anomaly and strikes east-southeast across to Lake Huron. Thenorthern regional positive magnetic anomaly strikes north-south from TraverseBay to north of Beaver Island where it bifurcates with one limb extendingnorth-northwest through Lake Superior to the Keweenawan basalts on KeweenawPoint. The other limb continues into the eastern portion of the NorthernPeninsula of Michigan and another segment of this branch connects to theKeweenawan flows on Mamainse Point. In the Traverse Bay area the regionalpositive becomes strongly negative and connects to the south with the midMichigan gravity and magnetic anomaly.

—57—

GEOCHRONOLOGY OF THE GIANTS RANGE GRANITE

L. A. FRECE AND G. N. HANSON

State University of New YorkStony Brook, N. Y. 11790

ABSTRACT

Seven whole rock samples of two—mica, foliated quartzmonzonites from the central part of the Giants Range Granite,north of Hibbing, Minnesota, give a Rb—Sr isochron age of 2670± 65 m.y. (Rb 87 A = 1.39 x i0' yr 1) and a 3r87/Sr86 initialratio of 0.7002 ± 0.0019 at the 95% confidence level. Epidote,plagioclase, potassium feldspar, biotite—chiorite, apatite andmuscovite separates from one of these whole rock samples give mineral—whole rock ages ranging from 2350 m.y. for epidote to 26140 m.y.for muscovite. The lowered mineral—whole rock ages suggest thatat least one event occurred after intrusion of the granite, but thatthe individual mineral phases were not completely homogenized withrespect to their 5r87/5r86 ratios. The 2350 m.y. epidote—rockage probably is a maximum for the time of the last event. Themineral data provide no conclusive evidence for the sequence ofevents after intrus on, but are not inconsistent with a regionallow—grade metamorphism at around 1600 m.y.

This whole rock age is in agreement with U—Pb data on spheneand zircon from the Giants Range Granite which suggest an age of2100 n.y. and also with K—Ar ages for hornblende from the GiantsRange Granite which generally are 2600—2100 m.y. These ageshowever are older than most of the Rb—Sr and K—Ar ages for biotitewhich range from 2260—2630 m.y. The age of the post—kinematicLinden Syenite just to the north also limits the ae of the syn—to late—kinematic Giants Range Granite. ?b207_Pb2u6 data on spheneand a b—Sr mineral—whole rock isochron with an initial ratio of0.1009±O.000b suggest an age for the Linden Syenite of about2100 m.y.

The low Sr87/5r86 initial ratios from both the Linden Syeniteand the Giants Range Granite suggest a source with a low Rb/Srratio, perhaps the mantle, and it is unlikely that appreciablemixing with preexisting continental crust took place.

-57-

GEOCHRONOLOGY OF THE GIANTS RANGE GRANITE

L. A. PRINCE AND G. N. HANSON

State University of New YorkStony Brook, N. Y. 11790

A B S T R ACT

Seven whole rock samples of two-mica, foliated ~uartz

monzonites from the central part of the Giants Range Granite,north of Hibbing, Minnesota, give a Rb-Sr isochron age of 2670± 65 m.y. (Rb 87 AS = 1.39 x 10- 11 yr _1) and a Sr87 /Sr86 initialratio of 0.7002 ± 0.0019 at the 95% confidence level. Epidote,plagioclase, potassium feldspar, biotite-chlorite, apatite andmuscovite separates from one of these whole rock samples give mineral-whole rock ages ranging from 2350 m.y. for epidote to 2640 m.y.for muscovite. The lowered mineral-whole rock ages suggest thatat least one event occurred after intrusion of the granite, but thatthe individual mineral phases were not completely homogenized withrespect to their Sr87 /Sr 86 ratios. The 2350 m.y. epidote-rockage probably is a maximum for the time of the last event. Themineral data provide no conclusive evidence for the se~uence ofevents after intrusion, but are not inconsistent with a regionallow-grade metamorphism at around 1600 m.y.

This whole rock age is in agreement with U-Pb data on spheneand zircon from the Giants Range Granite which suggest an age of2700 m.y. and also with K-Ar ages for hornblende from the GiantsRange Granite which generally are 2600-2700 m.y. These ageshowever are older than most of the Rb-Sr and K-Ar ages for biotitewhich range from 2280-2630 m.y. The age of the post-kinematicLinden Syenite just to the north also limits the age of the syn-to late-kinematic Giants Range Granite. Pb207_Pb206 data on spheneand a Rb-Sr mineral-whole rock isochron with an initial ratio of0.7009£-0.0004 suggest an age for the Linden Syenite of about2700 m.y.

The low Sr87 /Sr 86 initial ratios from both the Linden Syeniteand the Giants Range Granite suggest a source with a low Rb/Srratio, perhaps the mantle, and it is unlikely that appreciablemixing with preexisting continental crust took place.

—58—

THE GREAT LOGAN PALEOMAGNETIC LOOP — THE POLARWANDERING PATH FROM CANADIAN SHIELD ROCKS DURING THE HELIKIAN ERA

by

W. A. Robertson w. F. FahrigGeomagnetic Laboratory Geological Survey of CanadaEnergy,I'Iines & Resources Ottawa, OntarioOttawa, Ontario

ABSTRACTNormally magnetized dykes and reversely magnetized sills of

Neohelikian age near the north west shore of Lake Superior formtwo distinct paleomagnetic groups with mean pole positions of179W, 35N, and 140W, 47N respectively. Thermal and alternatingfield paleomagnetic studies and the study of magnetic propertiesand opaque minerals indicate that directions of magnetization ofthese rocks were acquired at the time of their intrusion. Fieldevidence indicates that the reversely magnetized sills are olderthan the normally magnetized dykes and radiogenic age determinationsindicate intrusion between 1000 and 1100 m.y. ago.

These pole positions, together with those for the Franklinintrusions pole at 167E—08N, assigned age 675 m.y., the Abitibidykes, at 134W, 27N, assigned aged 1150 m.y. and the NacKenzieIgneous events, at 171W, 4N, assigned age 1200 iu.y. are used todefine Logan's Loop, the path that. the pole took in Neohelikiantime relative to the Canadian Shield. Other poles well definedmagnetically, but less well dated, from rocks of this era fitthe curve quite well.

Analysis of available data supports the hypothesis that therelative polar movement that gave rise to Logan's Loop was pre-ceded and followed by polar stability vis a vis North America,whereas polar movement may have been quite rapid during the for-mation of the loop. The depositional environment of Neohelikianrocks of the Canadian Shield should be tested against theirprobable paleolatitude as indicated by the 5 key points on Logan's

Loop.

-58-

THE GREAT LOGAN PALEOHAGNETIC LOOP - THE POLARWANDERING PATH FRaN CANADIAN SHIELD ROCKS DURING THE HELIKIAN ERA

by

W. A. }{obertsonGeomagnetic LaboratoryEnergy,Mines & ResourcesOttawa, Ontario

H. F. FahrigGeological Survey of CanadaOttavla, Ontario

ABSTRACT

Normally magnetized dykes and reversely magnetized sills ofNeohelikian age near the north west shore of take Superior formtwo distinct paleomagnetic groups with mean pole positions of179\\1, 35N, and 140W, 47i'J respectively. Thermal and alternatingfield paleomagnetic studies and the study of magnetic propertiesand opaque minerals indicate that directions of magnetization ofthese rocks were acquired at the time of their intrusion. Fieldevidence indicates that the reversely magnetized sills are olderthan the normally magnetized dykes and radiogenic age determinationsindicate intrusion between 1000 and 1100 m.y. ago.

These pole positions, together with those for the Franklinintrusions pole at l67E-08N, assigned age 675 m.y., the Abitibidykes, at 134W. 27N, assigned aged 1150 m.y. and the MacKenzieIgneous events, at l71W, 4N, assigned age 1200 m.y. are used todefine Logan's Loop, the path that the pole took in Neohelikiantime relative to the Canadian Shield. Other poles well definedmagnetically, but less well dated, from rocks of this era fitthe curve quite well.

Analysis of available data supports the hypothesis that therelative polar movement that gave rise to Logan's Loop was preceded and follmved by polar stability vis a vis North America,whereas polar movement may have been quite rapid during the formation of the loop. The depositional environment of Neohelikianrocks of the Canadian Shield should be tested against theirprobable paleolatitude as indicated by the 5 key points on Logan'sLoop.

—59—

CHARACTERISTICS OF SOME ALTERATION MINERALS,PORTAGE LAKE LAVA SERIES, MICHIGAN

A. P. RUOTSALAMichigan Technological University

ABSTRACT

Many silicate, carbonate, and oxide minerals are associated withcopper mineralization in the Portage Lake Lava Series and associatedconglomerates. A complete listing was published by Butler and Burbank'.Amygdule zoning patterns associated with mineralization were studied byStoiber and Davidson . Chemical and x-ray diffraction studies are beingcarried out at Michigan Technological University on a continuing basis.In the past some of these studies were supported by the Calurnet Divisi -'n,Universal Oil Products Company.

Epidote is characterized by wide ranges in unit cell dimensions. Ingeneral, epidotes from amygdaloids have larger unit cell volumes than thosefrom conglomerates.

Calcites show wide ranges in trace element composition, especiaiyin concentratiois of Fe, Mg, and Mn. There is a positive correlation be-tween Mn content and copper mineralization in the Kearsarge amygaloid, andtherefore is potentially useful as an exploration tool in the district.

Chlorite occurs in many forms and at virtually every stage of thealteration sequence in the district. An interesting occurrence is as anessentially pure clay mineral in the fault gouge in the Allouez Gap Faultand hanging wall slip of the Kingston Mine4. This is one of the few, if notthe only occurrence of pure clay chlorite known, It consists essentially ofthe type Ub polytype5. In much of the hanging wall slip, chlorite is intimatelymixed with hematite. The water absorption properties (plastic and liquidlimits) are greater for chlorite-hematite mixtures than for pure chlorites,suggesting the possibility of some mixed layering of chlorite and hematite.

References Cited

1. Butler, B. S., and W. S. Burbank (1929) The copper deposits ofMichigan. U.S. Geol. Surve., Prof. Paper 144, 238 pp.

2. Stoiber, R. E. and E. S. Davidson (1959) Amygdule mineral zoning inthe Portage Lake Lava Series, Michigan. Econ. Geo. v. 54,p. 1250-1277 and p. 1444-1460,

-59-

CHARACTERISTICS OF SOME ALTERATION MINERALS,FJRTAGE LAKE LAVA SERIES, MICHIGAN

A. p. RUOTSALAMichigan Technological University

ABSTRACT

Many silicate, carbonate, and oxide minerals are associated withcopper mineralization in the Portage Lake Lava Series and associatedconglomerates. A complete listing was published by Butler and Burbank l •Amygdule zoning patterns associated with mineralization were studied byStoiber and Davidson2. Chemical and x-ray diffraction studies are beingcarried out at Michigan Technological University on a continuing basis.In the past some of these studies were supported by the Calumet Divisi:::n,Universal Oil Products Company.

Epidote is characterized by wide ranges in unit cell dimensions. Ingeneral, epidotes from amygdaloids have larger unit cell volumes than thosefrom congloITlerates.

Calcites show wide ranges in trace eleITlent cOlnposition, especiallyin concentration3 of Fe, Mg, and Mn. There is a positive correlation between Mn content and copper ITlineralization in the Kearsarge amygdaloid, andtherefore is potentially useful as an exploration tool in the district. 3

Chlorite occurs in ITlany forms and at virtually every sh.ge of thealteration sequence in the district. An interesting occurrence is as anessentially pure clay ITlineral in the fault gouge in the Allouez Gap Faultand hanging wall slip of the Kingston Mine~. This is one of the few, if notthe only occurrence of pure clay chlorite known. It consists essentially ofthe type lIb polytype5. In ITluch of the hanging wall slip, chlorite is intiITlatelymixed with hematite. The water absorption properties (plastic and liquidliITlits) are; greater for chlorite-heITlatite mixtures than for pure chlorites,suggesting the pas sibility of SOITle ITlixed layering of chlorite and heITlatite.

References Cited

1. Butler, B. S., and W. S. Burbank (1929) The copper deposits ofMichigan. U. S. Geol. Surve., Prof. Paper 144, 238 pp.

2. Stoiber, R. E. and E. S. Davidson (1959) AITlygdule mineral zoning inthe Port age Lake Lava Series, Michigan. Econ. Geo!. v. 54,p. 1250-1277 and p. 1444-1460.

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3. Ruotsala, A. P., S. C. Nordeng, and R. 3. Weege (1968) Traceelements in accessory calcite - a potential exploration tool in theMichigan Copper District. Cob. School of Mines Quart., Jour.,v. 64, p. 451-455. (International Geochemical ExplorationSymposium)

4. Ruotsala, A. P. (1968) Clay alteration associated with mineralizationin the Michigan Copper District. Clays and Clay Mm., v. 16,p. 400-402.

5. Brown, B. E.,, and S. W. Bailey (1962) Chlorite polytypism.Am. Mineral. ,v. 47, p. 819-850.

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3. Ruotsala, A. P., S. C. Nordeng, and R. J. Weege (1968) Traceelements in accessory calcite - a potential exploration tool in theMichigan Copper District. Colo. School of Mines Quart., Jour.,v. 64, p. 451-455. (International Geochemical ExplorationSymposium)

4. Ruotsala, A. P. (1968) Clay alteration associated with mineralizationin the Michigan Copper District. Clays and Clay Min., v. 16,p. 400-402.

5. Brown, B. E., and S. W. Bailey (1962) Chlorite polytypism.Am. Mineral.,v. 47, p. 819-850.

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THE GENERAL STRATIGRAPHY OF THUNDER BAY,

LAKE SUPERIOR

R. J. Shegelski

LAKE-lEAD UNIVERSITY

ABSTRACTPetrographic studies of the sediments taken from Thunder Bay

combined with x-ray diffractometer analysis have indicated distinctlithologic types in the bottom sediments of Thunder Bay. Strati-graphic correlation of thc bottom sediments from Thunder Bay hasbeen developed with the aid of echo sounding traces. These tracesindicate the boundaries, nature and thickness of the various litho-facies.

The sediments in Thunder Bay can be divided into five categories;(1) Varved Clay, (2) Weathered Varved Clay, (3) Intermediate Clay,

(4) Upper Deltaic Sediment, (5) Upper Trough Sediment. The Varved

Clay is of Post Valders glacial origin and is discoi:fom*Lj overlainby the Weathered and Intermediate Clays. The latter are productsof weathering of the older Varved Clay. The Intermediate Clay isconformably overlain by Upper Deltaic and Upper Trough Sediment.The latter units are sediments derived dominantly from the KaministikwiaRiver. Thin iron and manganese beds are formed through diageneticsolution, upward migration, and precipitation at a chemical interfacein the Upper Sediments.

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THE GENERAL STRATIGRAPHY OF THUNDER BAY,

LAKE SUPERIOR

R. J. Shegelski

LAKEHEAD UNIVERSITY

A B S T R ACT

Petrographic studies of the sediments taken from Thunder Baycombined with x-ray diffractometer analysis have indicated distinctlithologic types in the bottom sediments of Thunder Bay. Stratigraphic correlation of the bottom sediments from Thunder Bay hasbeen developed with the aid of echo sounding traces. These tracesindicate the boundaries, nature and thickness of the various lithofacies.

The sediments in Thunder Bay can be divided into five categories;(1) Varved Clay, (2) Weathered Varved Clay, (3) Intermediate Clay,(4) Upper Deltaic Sediment, (5) Upper Trough Sediment. The VarvedClay is of Post Valders glacial origin and is disconformably overlainby the Weathered and Intermediate Clays. The latter are productsof weathering of the older Varved Clay. The Intermediate Clay isconformably overlain by Upper Deltaic and Upper Trough Sediment •.The latter units are sediments derived dominantly from the KaministikwiaRiver. Thin iron and manganese beds are formed through diageneticsolution, upward migration, and precipitation at a chemical interfacein the Upper Sediments.

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CHERT IN SEDIMENTS

by

G. SpencerDuluth, Minnesota

ABSTRACTAmorphous silica is produced when a silica solution becomes super-

saturated and a precipitate forms or when some silicates are dissolvedin acids. In either case the silica is in a colloidal form and remainsso for long periods of time. Eventually crystallization begins, andheating the substance can speed up the process to a great extent. In

natural conditions opal is the first step followed by chalcedony andfinally cristobalite. All of these forms of silica have been termedchert.

Chert has long been regarded as a chemical precipitate. This

view is sufficient to explain hot spring deposits and hydrothermallydeposited chert or opal near the surface. Cooling volcanic watersbecome supersaturated with silica in solution and amorphous is depositedas a sinter or in thin layers.

There are, however, many other environments in which chert is foundsuch as:

a) black slate with chert lenses or bedsb) limestones with chert nodules or joint fillingsc) bedded cherts containing sponge spicules or Radiolariad) chert stringers and crusts in oxidized sulphidese) agate fillings of vesicles in lava flowsf) chert as a matrix for iron oxide granulesg) opal or chert replacing organic matter

The author ascribes chert to the decomposition of clays, detritalsilicates, volcanic ash or sand grains in carbonates. In acid environ-ments alumina and alkalies are removed and a silica gel is left. In

alkaline waters silica is dissolved and is deposited in a more neutralzone or will replace a dissolving particle such as organic matter. The

relative solubility of alumina and alkalies compared to silica is thedetermining factor.

iturata (1946) investigated many silicates which produced gels whentreated with acid. He found that a silica to alumina ratio of 1:1 or2:3 was conmion to gel forming silicates. Other silicates prpduced silicaparticles larger than colloidal size or were not affected by acids. Anysilicate structure from which alumina has been removed is halfway alongto becoming chert.

In carbonate and silicate iron formations mixed with quartz, mineralsbecome unstable when the temperature is raised and CO. is removed from thesediment. The ionization of water increases up to 23°C at whicli pointsilica both in quartz and silicates becomes increasingly soluble. Sandgrains in oolites might dissolve and secondary silicates such as

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CHERT IN SEDIMENTS

by

G. SpencerDuluth, Minnesota

A B S T R ACT

Amorphous silica is produced when a silica solution becomes supersaturated and a precipitate forms or when some silicates are dissolvedin acids. In either case the silica is in a colloidal form and remainsso for long periods of time. Eventually crystallization begins, andheating the substance can speed up the process to a great extent. Innatural conditions opal is the first step followed by chalcedony andfinally cristobalite. All of these forms of silica have been termedchert.

Chert has·long been regarded as a chemical precipitate. Thisview is sufficient to explain hot spring deposits and hydrothermallydeposited chert or opal near the surface. Cooling volcanic watersbecome supersaturated with silica in solution and amorphous is depositedas a sinter or in thin layers.

There are, however, many other environments in which chert is foundsuch as:

a) black slate with chert lenses or bedsb) limestones with chert nodules or joint fillingsc) bedded cherts containing sponge spicules or Radiolariad) chert stringers and crusts in oxidized sulphidese) agate fillings of vesicles in lava flowsf) chert as a matrix for iron oxide granulesg) opal or chert replacing organic matter

The author ascribes chert to the decomposition of clays, detritalsilicates, volcanic ash or sand grains in carbonates. In acid environments alumina and alkalies are removed and a silica gel is left. Inalkaline waters silica is dissolved and is deposited in a more neutralzone or will replace a dissolving particle such as organic matter. Therelative solubility of alumina and alkalies compared to silica is thedetermining factor.

i'lurata (1946) investigated many silicates ,vhich produced gels \<Thentreated with acid. He found that a silica to alumina ratio of 1:1 or2:3 was common to gel forming silicates. Other silicates produced silicaparticles larger than colloidal size or were not affected by acids. Anysilicate structure from which alumina has been removed is halfway alongto becoming chert.

In carbonate and silicate iron formations mixed ,.,rith quartz,become unstable when the temperature is raised and CO, is removedsediment. The ionization of \vater increases up to 230° C at ,..dlichsilica both in quartz and silicates hecomes incren3in~ly soluble.grains in oolites might dissolve and secondary silicates such as

mineralsfrom theDoint

Sand

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minnesotaite and stilpnonielone would appear. Chert in this case isclearly diagenetic and came from a breakdown of original elasticseven if the original texture remained the same.

Precambrian iron formations would appear to be examples of latediagenetic chert although they would also contain early diagenetic silicaas well. Silica and iron oxide content is higher in these rocks becauseof the loss of carbon dioxide, water, and small amounts of alkalies insolution.

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minnesotaite and stilpnomelone would appear. Chert in this case isclearly diagenetic and came from a breakdown of original clasticseven if the original texture remained the same.

Precambrian iron formations would appear to be examples of latediagenetic chert although they would also contain early diagenetic silicaas well. Silica and iron oxide content is higher in these rocks becauseof the loss of carbon dioxide, water, and small amounts of alkalies insolution.

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IMPLICATIONS OF CARBON ISOTOPE RATIO VARIATIONS IN CARBONATESFROM THE BIWABIK IRON FORMATION, MINNESOTA

F. C. Tan and E. C. Perry, Jr.

Minnesota Geological SurveyUniversity of Minnesota

Minneapolis, Minnesota 55455

ABSTRACT

Carbon isotope ratios from carbonates of the Biwabik Iron—formationobtained from core samples of Mesabi Deep Drilling Project (Pfleider andothers, 1968) show the following significant features.

(1) SC13 values of carbonates associated with magnetite from holes5 and 7 show a range of from —7 to —19 per nil (relative to a Cretaceousbelemnite standard calcite PDB) in comparison with1he carbonates fromthe magnetite—free horizons (0 to —7 per mu). SC is strongly correlatedwith magnetite content in hole 7.

(2) The SC13 values of carbonates from magnetite and non—magnetitehorizons of hole 2 with one exception, do not show1ignificant differencein contrast with holes 5 and 7. They exhibit a SC range of from —7 to—19 per mU..

(3) The C13 values do not show any significant stratigraphiccorrelation with tlia percent iron.

(4) There appears to be a correlation between C1-3 values and theindividual members of the Biwabik Iron—formation.

13In explaining the observed correlation between magnetite and the

SC values based on our preliminary results, we proposed (Perry andTan, 1970) that a diagenetic oxidation—reduction reaction producingmagnetite from hematite permitted exchange between organic carbon andcarbonate carbon reservoirs:

bFeO +C r 4FeO +CO (1)2 3 (organic) 3 4 2

C1202 + FeC3O3 C1-3O + FeC2O3 (2)

Our recent detailed studies have strengthen our previous observa-tion on hole 7 but have found anomalously low SC values (—18 per nil)from the non—magnetite horizons of hole 2 (Lower Slaty Unit). Theseanomalous values may be related to metamorphic reactions accompanyingthe intrusion of the Duluth Complex because hole 2 is located nearmetamorphic zone (1) of French (1968).

Our observation that there is a correlation between C13 valuesand the individual members of the Biwabik Iron—formation would suggestthat the SC1-3 variations are depositional or early diagenetic featuresand bear no relationship :o the genesis of niagnetite (model above). The

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IMPLICATIONS OF CARBON ISOTOPE RATIO VARIATIONS IN CARBONATESFROM THE BIWABIK IRON FORMATION, MINNESOTA

F. C. Tan and E. C. Perry, Jr.

Minnesota Geological SurveyUniversity of Minnesota


A B S T R ACT

Carbon isotope ratios from carbonates of the Biwabik Iron-formationobtained from core samples of Mesabi Deep Drilling Project (Pfleider andothers, 1968) show the following significant features.

13(1) SC values of carbonates associated with magnetite from holes5 and 7 show a range of from -7 to -19 per mil (relative to a Cretaceousbelemnite standard calcite PDB) in comparison with

13he carbonates from

the magnetite-free horizons (0 to -7 per mil). SC is strongly correlatedwith magnetite content in hole 7.

13(2) The Gc values of carbonates from magnetite and non-magnetite

horizons of hole 2 with one exception, do not show1~ignificant differencein contrast with holes 5 and 7. They exhibit a SC range of from -7 to-19 per mil.

13(3) The bC values do not show any significant stratigraphic

correlation with the percent iron.

13(4) There appears to be a correlation between ~C values and theindividual members of the Biwabik Iron-formation.

13 In explaining the observed correlation between magnetite and theSC values based on our preliminary results, we proposed (Perry andTan, 1970) that a diagenetic oxidation-reduction reaction producingmagnetite from hematite permitted exchange between organic carbon andcarbonate carbon reservoirs:

6 Fe203 + C(organic)~f----~~- 4 Fe304 + CO2

C12

02 + Fec13

03

< c1302

+ Fec12

03

(1)

(2)

Our recent detailed studies have strengthenI~ our previous observation on hole 7 but have found anomalously low SC values (-18 per mil)from the non-magnetite horizons of hole 2 (Lower Slaty Unit). Theseanomalous values may be related to metamorphic reactions accompanyingthe intrusion of the Duluth Complex because hole 2 is located nearmetamorphic zone (1) of French (1968).

Our observation that there is a correlation between ~el3 valuesand the individual members of the Biwabik Iron-formation would suggestthat the eel3 variations are depositional or early diagenetic featuresand bear no relationship ~o the genesis of magnetite (model above). The

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differences in IC13 values may be explained by the variable contributionsof organic carbon and oceanic bicarbonate to the depositional ordiagenetic environments.

We are currently investigating the SC13 values of co—existinggraphite-tarbonate pairs and oxygen isotope fractionation between co-existing quartz—magnetfte at various stratigraphic levels of holes 2and 7 to see if they have different diagenetic or metamorphic histories.Co—existing quartz—carbonate oxygen isotope fractionation In samplesfrom hole 2 compared to that in holes 5 and 7 suggests that the post—depositional history of these two areas is indeed different.

References

French, B. M. , (1968) Progressive contact metamorphism of the BiwabikIron—formation, Mesabi Range, Ninnesc:a, Minn. Geol. Surv. Bull. 45,Univ. of Minn., Minneapolis.

Perry, E. C. , Jr., and Tan, F. C., Significance of carbon isotopevariations in carbonates from the Biwabik Iron—formation, Minnesota.International symposium on the geology and genesis of Precambrianiron/manganese formation and ore deposits, Kiev, 1970 (In Press).

Pfleider, E. P., Morey, C. B., and Bleifuss, F. L. (1968) Mesabi deepdrilling project, progress report no. 1, Minnesota section, AIME forty—first annual meeting, Univ. of Minn., Minneapolis.

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differences in bCl3 values may be explained by the variable contributionsof organic carbon and oceanic bicarbonate to the depositional ordiagenetic environments.

We are currently investigating theoC13

values of co-existinggraphite-carbonate pairs and oxygen isotope fractionation between coexisting quartz-magnetite at various stratigraphic levels of holes 2and 7 to see if they have different diagenetic or metamorphic histories.Co-existing quartz-carbonate oxygen isotope fractionation in samplesfrom hole 2 compared to that in holes 5 and 7 suggests that the postdepositional history of these two areas is indeed different.

References

rrench, B. M., (1968) Progressive contact metamorphism of the BiwabikIron-formation, Mesabi Range, Minnesota, Minn. Geol. Surv. Bull. 45,Univ. of Minn., Ninneapolis.

Perry, E. C., Jr., and Tan, F. C., Significance of carbon isotopevariations in carbonates from the Biwabik Iron-formation, Minnesota.International symposium on the geology and genesis of Precambrianiron/manganese formation and ore deposits, Kiev, 1970 (In Press).

Pfleider, E. P., Morey, G. B., and Bleifuss, R. L. (1968) Mesabi deepdrilling project, progress repo~t no. 1, Ninnesota section, AIME fortyfirst annual meeting, Dniv. of Ninn., Minneapolis.

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Oxygen isotopic studies of Early Precambrian granitic and

metamorphic rocks from the western part of the

Giants Range batholith, Northeastern Minnesota

S. Viswanathan, E.C. Perry, Jr., and P.K. Sims,Minnesota Geological Survey, University of Minnesota,


ABSTRACT

The paper has two main objectives: (1) to demonstrate that oxygenisotope geochemistry is a valuable tool in elucidating granite petrogenesis,provided it is integrated with detailed field and laboratory studies, and(2) to give values for f(O18/016)qrtz in the Early Precambrian graniticand metamorphic rocks, data for which are rather scarce in the literature.

Geologic mapping of a 500—square mile area in the western part of the2.1 b.y. old Early Precambrian Giants Range batholith of northeasternMinnesota has revealed eleven distinct granitic phases (Sims and others, 1970).Field observations, petrography, petrochemistry, and trace elementgeochemistry suggest that five of the phases are magmatic, four are metasomatic,and two are anatectic.

meIo'8/o'6 ratios of quartz separated from 28 rocks, relative toStandard Mean Ocean Water (SNOW), are presented in the accompanying table.The values are consistent with the postulated genetic grouping for theEarly Precambrian granitic succession. The following conclusions can bedrawn from the data:

(1) Values of 9 to 10 permil are characteristic of relativelyuncontaminated, probable mantle—derived granites and granodiorites. Theseare consistent with the S(O18/ol6quartz values tharhave been reported forplutonic granites, granodiorites, and tonalites, which range from about9 to about 11 permil (Taylor, 1968, p.34).

(2) Values greater than 10 permil require one of several specialexplanations, such as:

(a) syntexis of large amounts of 018—rich country rocksinto a primitive or first—cycle granitic magma

18(b) co—mingling of an 0 —enriched magma and a primitive

or first cycle granitic magma

18(c) anatexis of 0 —rich sediments

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Oxygen isotopic studies of Early Precambrian granitic and

metamorphic rocks from the western part of the

Giants Range batholith, Northeastern Minnesota

s. Viswanathan, E.C. Perry, Jr., and P.K. Sims,Minnesota Geological Survey, University of Minnesota,


A B S T R ACT

The paper has two main objectives: (1) to demonstrate that oxygenisotope geochemistry is a valuable tool in elucidating granite petrogenesis,provided it is integrated with detailed field and laboratory studies, and(2) to give values for~(018/016)guartz in the Early Precambrian graniticand metamorphic rocks, data for wnich are rather scarce in the literature.

Geologic mapping of a 500-square mile area in the western part of the2.7 b.y. old Early Precambrian Giants Range batholith of northeasternMinnesota has revealed eleven distinct granitic phases (Sims and others, 1970).Field observations, petrography, petrochemistry, and trace elementgeochemistry suggest that five of the phases are magmatic, four are metasomatic,_and two are anatectic.

r 18 16The ~O 10 ratios of quartz separated from 28 rocks, relative toStandard Mean Ocean Water (SMOW)Jare presented in the accompanying table.The values are consistent with the postulated genetic grouping for theEarly Precambrian granitic succession. The following conclusions can bedrawn from the data:

(1) Values of 9 to 10 permil are characteristic of relativelyuncontaminated, probable mantle-derived granites and granodic~ites. Theseare consistent with the ~(018/016)quartzvalues that-have been reported forplutonic granites, granodiorites, and tonalites, which range from about9 to about 11 permil (Taylor, 1968, p.34).

(2) Values greater than 10 permil require one of several specialexplanations, such as:

18(a) syntexis of large amounts of 0 -rich country rocksinto a primitive or first-cycle granitic magma

18(b) co-mingling of an 0 -enriched magma and a primitiveor first cycle granitic magma

(c) anatexis of 018_rich sediments

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(d) potash metasomatisin of O'8—rich sediments

(e) simple recrystallization (treptomorphisin) ofrelatively 018—rich elastic sediments such asarkoses and graywackes, which thereby assume agranitic fabric and composition

(f) recrystallization of granites under shearing andcrushing to produce aplogranites, accompanied bylate—tectonic metasomatism in mobile zones

(g) post—consolidation eataclasis, and othermetamorphic episodes which facilitate exchangebetween granitic rocks of magmatic origin and

O'8—rich country rocks through a pore—fluid medium

(h) post—consolidation endoblastesis/autometatOrphisminvolving 018—enriched fluids

(i) processes such as petroblastesis and imbibitionin which the participants are O'8—rich sedimentsand 018—enriched fluids

(j) selective exchange between granitic intrusions andO'8—rich host rocks

(k) tectonic styles of emplacement, whether synkineinatic,post—kinematic or late—kinematic, and

(1) early segregation of O'8—depleted minerals whichresults in O'8—enrichment in late granitic differentiates.

(3) The values observed in granitic rocks of anatectic origin are nearlyidentical to those of their source rocks. They are either low or highdepending on whether such granitic rocks were formed from O'8—poor orOIS_rich sources, respectively. The values will be grossly different only if:(i) the anatectic melt undergoes subsequent differentiation, and (ii) theconsolidated anatectic melt is subjected to later metasomatism (see dataunder "C" and "D (2)", of table).

(4) The main intrusive phases of granites and granodiorites of maginaticorigin have values nearly identical to those of their satellitic phases(sve data under "A" of table). Interestingly, a value of 9.2 permil wasobtained for a leucogranite (satellitic late—magmatic differentiate) thattransgresses an 018—rich metasedimentary host rock having a value of 12.4perinil. This result indicates that there was hardly any oxygen communicationbetween the satellitic late—maginatic differentiate and its o8—richmetasedimentary host rock. This is in sharp contrast to the conclusion ofShieh and Taylor (1969, p.353) who report that: "Samples from tiny intrusivebodies and dikes and from the marginal portions of most of the larger plutonshave unusually high 018/016 ratios relative to "normal" igneous rocks from

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18(d) potash metasomatism of a -rich sediments

(e) simple recrystallization (treptomorphism) ofrelatively 018_r ich clastic sediments such asarkoses and graywackes, which thereby assume agranitic fabric and composition

(f) recrystallization of granites under shearing andcrushing to produce aplogranites, accompanied bylate-tectonic metasomatism in mobile zones

(g) post-consolidation cataclasis, and othermetamorphic episodes which facilitate exchangebetween granitic rocks of magmatic origin and018_r ich country rocks through a pore-fluid medium

(h) post-consolidation endoblastesis/autometamorphisminvolving 018-enriched fluids

(i) processes such as petroblastesis and imbibitionin which the participants are 018_r ich sedimentsand 018-enriched fluids

(j) selective exchange between granitic intrusions and018_r ich host rocks

(k) tectonic styles of emplacement, whether synkinematic,post-kinematic or late-kinematic, and

18(1) early segregation of a -depleted minerals whichresults in 018-enrichment in late granitic differentiates.

(3) The values observed in granitic rocks of anatectic origin are nearlyidentical to those of their source rocks. They are either low or highdepending on whether such granitic rocks were formed from 018_poor or018_r ich sources, respectively. The values will be grossly different only if:(i) the anatectic melt undergoes subsequent differentiation, and (ii) theconsolidated anatectic melt is subjected to later metasomatism (see dataunder "e" and "0 (2)", of table).

(4) The main intrusive phases of granites and granodiorites of magmaticorigin have values nearly identical to those of their satellitic phases(sve data under "A" of table). Interestingly, a value of 9.2 permil wasobtained for a leucogranite (satellitic late-magmatic differentiate) thattransgresses an Ol8_r ich metasedimentary host rock having a value of 12.4permi!. This result indicates that there was hardly any oxygen communicationbetween the satellitic late-magmatic differentiate and its alB-richmetasedimentary host rock. This is in sharp contrast to the conclusion ofShieh and Taylor (1969, p.353) who report that: "Samples from tiny intrusivebodies and dikes and from the marginal portions of most of the larger plutonshave unusually high 018/0 16 ratios relative to "normal" igneous rocks from

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the central portions of plutons. This is interpreted to be the resultof large—scale oxygen isotopic exchange between essentially molten igneousrock and metasedimentary country rock, either through a medium of aqueousfluids or by contamination with xenolithic blocks of country rock."

(5) The sequence: sedimentary parent ——)' partially granitizedsediment —>-granitic rock of metasomatic origin reflects a progressivedecrease in the 018/016 ratio, and encompasses only a narrow range of1 to 2 permil, and

(6) Global comparisons of oxygen isotopic ratios from granitizedsequences should not be attempted, unless: Ci) the 018/016 ratios of theparent rocks of two widely separated granitized sequences are comparable,and (ii) their geological settings are nearly identical.

Problems involving a possible time—dependence of the 018/0 16 ratios inminerals from specific rock—series, representing the entire geological column,are under study by one of us (Per;y).

Ref e—ences cited

Shieh, Y.N. and H.P. Taylor, Jr. (1969): Oxygen and hydrogen isotope studiesof contact metamorphism in the Santa Rosa Range, Nevada and other areas:Contr. Mineral. and Petrol., v.20, p.306—356.

Sims, P.R. and others (1970): Geologic map of Minnesota, Ribbing Sheet(scale 1:250,000): Minn. Geol. Survey, University of Minnesota, Minneapolis.

Taylor, H.P. Jr. (1968): The oxygen isotope geochemistry of igneous rocks:Contr. Mineral. and Petrol., v.19, p.'—71.

Other useful References

Allison, 1.5. (1925): The Giants Range batholith of Minnesota: Jour. Geology,

v.33, p.488—508.

Goldich, S.S. and others (1961): The Precambrian geology and geochronologyof Minnesota; Minn.Geol. Survey Bull.41, University of Minnesota,Minneapolis, p.62—65.

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the central portions of plutons. This is interpreted to be the resultof large-scale oxygen isotopic exchange between essentially molten igneousrock and metasedimentary country rock, either through a medium of aqueousfluids or by contamination with xenolithic blocks of country rock. 1I

(5) The sequence: sedimentary parent -~ partially granitizedsediment --~granitic rock of metasomatic origin reflects a progressivedecrease in the 018/0 16 ratio, and encompasses only a narrow range of1 to 2 permil, and

(6) Global comparisons of oxygen isotopic ratios from granitizedsequences should not be attempted, unless: (i) the 018/016 ratios of theparent rocks of two widely separated granitized sequences are comparable,and (ii) their geological settings are nearly identical.

18 16Problems involving a possible time-dependence of the 0 /0 ratios inminerals from specific rock-series, representing the entire geological column,are under study by one of us (PerJY).

References cited

Shieh, Y.N. and H.P. Taylor, Jr. (1969): Oxygen and hydrogen isotope studiesof contact metamorphism in the Santa Rosa Range, Nevada and other areas:Contr. Mineral. and Petrol., v.20, p.306-356.

Sims, P.K. and others (1970): Geologic map of Minnesota, Hibbing Sheet(scale 1:250,000): Minn. Geol. Survey, University of Minnesota, Minneapolis.

Taylor, H.P. Jr. (1968): The oxygen isotope geochemistry of igneous rocks:Contr. Mineral. and Petrol., v.19, p.1-71.

Other useful References

Allison, I.S. (1925): The Giants Range batholith of Minnesota: Jour. Geology,v.33, p.488-508.

Goldich, S.S. and others (1961): The Precambrian geology and geochronologyof Minnesota; Minn.Geol. Survey Bull.41, University of Minnesota,Minneapolis, p.62-65.

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Table

SNOWratios of quartz in Early Precambrian granitic

and metamorphic rocks from the western part of the

Giants Range batholith of Northeastern Minnesota

Range(perlil) (permil)

A. Rocks of magmatic origin:

(1) Granodiorites (main intrusivephase, two samples) 9.4 — 9.9 9.7

(2) Leucogranodiorite (satelliticphase of (1), one sample) 9.8

(3) Granites (main intrusive phase,two samples) 9.5 — 9.6 9.6

(4) Leucogranites (satellitic phaseof (3), two samples) 9.2 — 9.6 9.4

B. Rocks of metasomatic origin:

(1) Granites (five samples) 10.7 — 11.4 10.9

(2) Aplogranite (one sample) 10.2

(3) Partially granitizedvolcanogenic metasediments(three samples) 10.4 — 12.5 11.1

C. Rocks of anatectic origin:(derivatives of relatively0'8—poor source rocks)

(1) Quartz tonalites (four samples) 9.2 — 10.1 9.6

(2) K—f eldspathized tonalite(one sample) 12.1

D. Parent volcanogenic metasediments of:

(1) Group B (four samples) 11.1 — 12.9 12.2

-69-

Table

.{'* 18 16b(O /0 )SMOW ratios of quartz in Early Precambrian granitic

and metamorphic rocks from the western part of the

Giants Range batholith of Northeastern Minnesota

Range(perilil)

A. Rocks of magmatic origin:

Average(permil)

(1) Granodiorites (main intrusivephase t two samples)

(2) Leucogranodiorite (satelliticphase of (1). one sample)

(3) Granites (main intrusive phase t

two samples)

(4) Leucogranites (satellitic phaseof (3)t two samples)

~~.Rocks of metasomatic origin:

(1) Granites (five samples)

(2) Aplogranite (one sample)

(3) Partially granitizedvolcanogenic metasediments(three samples)

c. Rocks of anatectic origin:(derivatives of relativelyalB_poor source rocks)

(1) Quartz tonalites (four samples)

(2) K-feldspathized tonalite(one sample)

D. Parent volcanogenic metasediments of:

(1) Group B (four samples)

9.4 - 9.9

9.5 - 9.6

9.2 - 9.6

10.7 - 11.4

10.4 - 12.5

9.2 - 10.1

1l.1 - 12.9

9.7

9.8

9.6

9.4

10.9

10.2

11.1

9.6

12.1

12.2

—70--

CHEMICALLY ZONED NATIVE COPPER AND CHALCOCITE FROMWHITE PINE, MIChIGAN

T. A. VogelGeology Department

Michigan State UniversityEast Lansing, Michigan

and

T. J. RohrbacherStaff Geologist

White Pine Copper CompanyWhite Pine, Michigan

ABSTRACT

A microprobe study of the copper ore minerals at White Pine,Michigan, has shown that zoned copper—mineral grains occur in theore—bearing horizons of the Nonesuch Shale. This zoning is developedin both chalcocite and native copper, and the most prominent type ofzoning is iron enrichment towards the edge of the grain. Some

chalcocite grains are highly zoned, containing about four times asmuch iron (at least 10%) on the edge of the grain as in the core.Aluminum, magnesium and silicon show the same type of zoning, hutare developed to a lesser extent.

Many native copper grains are also zoned with respect to iron,with the edge of the grain containing about twice as much iron(about 1%) as the core. All zoned copper minerals show an enrich-ment of iron towards the edge of the grain; however, both within andbetween samples, the amount of zoning and the relative proportionof zoned to unzoned grains is highly variable. This variation doesnot appear to be controlled by the major lithologic variations inthe ore zone, but may be controlled by subtle differences in lithologyor by variations in the local chemical environment at the time ofdeposition.

Any model of 're genesis must take into account the occurrence,distribution and type of zoning in these copper minerals.

-70-

CrIEHICALLY ZONED NATIVE COPPER A<\lD CHALCOCITE FRaNWHITE PINE, NICIIIGAN

T. A. VogelGeology Department

~ichigan State UniversityEast Lansing, Michigan

and

T. J. RohrbacherStaff Geologist

\'~ite Pine Copper Company\.Jhite Pine, tHchigan

A B S T R ACT

A microprobe study of the copper are minerals at ~.Jhite Pine,Hichigan, has shown that zoned copper-mineral grains occur in theore-bearing horizons of the Nonesuch Shale. This zoning is develofledin both chalcocite and native copper, and the most prominent type ofzoning is iron enrichment to\vards the edge of the grain. Somechalcocite grains are highly zoned, containing about four times asmuch iron (at least 10%) on the edge of the grain as in the core.Aluminum, magnesium and silicon shaH the same type of zoninf, hutare developed to a lesser extent.

Many native copper grains are also zoned with respect to iron,wi th the edge of the grain containing about t\<7ice as much iron(about 1%) as the core. All zoned copper minerals show an enrichment of iron towards the edge of the grain; ho\·7ever, both ,·7ithin andbetween samples, the amount of zonin~ and the relative proportionof zoned to unzoned grains is highly varia1)le. This variation c:oesnot appear to be controlled by the major litholof,ic variAtions inthe ore zone, but may be controlled uy subtle differences in lithologyor by variations in the local chemical environment rtt the time ofdeposition.

Any j:lotlel of '1re Genesis must take into account the occurrence,distribution and type of zoning in these copper minerals.

—71—

KEWEENAWAN STRATICRAPHY OF WESTERNMOST MICHIGAN

W. S. Wtite E. R. BrooksU. S. Geological Survey Department of Earth and Physical ScienceAgriculture Research Center California State CollegeBeltsville, Maryland Hayward, California

3. A. Hubbard Robert F. JohnsonU. S. Geological Survey 449 Boynton AvenueWashington, D.C. Berkeley, California

J. T. WilbandUniversity of ToledoUepartment of GeologyToledo, Ohio

ABSTRACTRecent geologic and aeromagnetic surveys provide a skeletal

fraciework for throi:ghgoing stratigraphic correlations of theKeweenawan rocks of westernmost Michigan. The stratigraphy of themiddle and upper Fceweenawan rocks of the Keweenaw Peninsula haslong been known in great detail, thanks to the large amount ofdrill—hole and surface geologic information. The recent work per-

mits certain key horizons to be traced, with moderate certainty,westward into Wisconsin.

Stratigrapitic relationships for the middle and upper Keweenawanrocks are best shown in a longitudinal stratigraphic section drawnalong the general strike of the middle Keweenawac rocks and havingthe base of the Nonesuch Shale as a datum plane. The most strikingfeatures of such a diagram are the following: (1) The predominantlybasaltic Portage Lake Lava Series maintains a thickness of 10,000—12,000 feet from 1-loughton to the Black River and is thinnest nearthe Ontonagon River. It is about 8000 feet thick at the MontrealRiver. (2) An unnamed lenticular unit, 10,000 feet thick at thePresque Isle River and pinching out near the Ontonagon Liver on theeast and the Montreal River on the west, lies above the Portage LakeLava Series and below the predominantly sediientary Copper HarborConglomerate. The unit is mainly composed of thin aphanitic to fine—grained andesitic flows and contains minor rhyolite and intermediaterocks, particularly near the top. It seems to represent accumulationwithin a few miles of a volcanic center south of the PorcupinoMountains. (3) The Copper Harbor Conglomerate is less than 500 feetthick over the thick part of the unnamed unit, and 4000—5000 feetthick near the pinchouts of tile unnamed unit. (4) The combined thick-ness of tue three units suggests a strongly asynnetric basin,deepening from a minimum near the Ontonagon River to more than 25,000feet between the Black and Presque Isle Rivers, and then, in the next15 miles to the west, thinning rapidly to less than 10,000 feet at theMontreal River. These findings reinforce the concept that the middleKeweenawan lavas accumulated in separate tectonic basins rather than

in a single large one.

-71-

KEI.vEENAWAN STRATIGRAPHY OF i-JESTERN}[OST HICHIGAN

W. S. i-n1i teu. S. Geological SurveyAgriculture Research CenterBeltsville, Naryland

H. A. HubbardU. S. Geological SurveyWashington, D.C.

E. R. BrooksDepartment of Earth and Physical ScienceCalifornia State CollegeHayward, California

Robert F. Johnson449 Boynton AvenueBerkeley, California

J. T. iVilbandUniversity of ToledoDepartment of GeologyToledo, Ohio

A B S T R ACT

Recent geologic and aeromagnetic surveys provide a skeletalframework for throughgoing stratigraphic correlations of theKeweenawan rocks of westernmost Michigan. The stratigraphy of themiddle and upper Kel.".eenawan rocks of the Keweenaw Peninsula haslong been known in great detail, thanks to the large amount ofdrill-hole and surface geologic information. The recent work permits certain key horizons to be traced, with moderate certainty,westward into Wisconsin.

Stratigraphic relationships for the middle and upper Keweenawanrocks are best shown in a longitudinal stratigraphic section drawnalong the general strike of the middle Keweenawan rocks and havingthe base of the Nonesuch Shale as a rlA.tum plane. The most strikingfeatures of such a diagram are the following: (1) TIle predominantlybasaltic Portage Lake Lava Series maintains a thickness of 10,00012,000 feet from Houghton to the JHack River and is thinnest nearthe Ontonagon River. It is about 8000 feet thick at the MontrealRiver. (2) An unnamed lenticular unit, 10,000 feet thick at thePresque Isle River and pinchinp, out near the Ontonagon River on theeast and the Montreal River on the west, lies above the Portage LakeLava Series and below the predominantly sedimentaty Copper HarborConglomerate. The unit is mainly composed of thin aphanitic to finegrained andesitic flm,s and contains minor rhyolite and intermediaterocks, particularly near the top. It seems to represent accumulation\Vithin a fe\.". miles of a volcanic center south of the Porcupinef·jountains. (3) The Copper Harbor Conglomerate is less than 500 feetthick over tile thick part of the unnamed unit, and 4000-5000 feetthick near tIle pinchouts of the unnamed unit. (4) The combined thickness of tlle three units sugr,ests a strongly asymmetric basin,deepening from a minimum near the Ontonagon River to more than 25,000feet between the Black and Presque Isle Rivers, and then, in the next15 miles to the \Vest, thinning rapidly to less than 10,000 feet at thehontreal River. These findinf's reinforce the concept that the middleKe\VeenaVlan lavas accumulated in separate tectonic basins rather thanin a single large one.

—72—

The lower Keweenawan lavas, which form the sc—called South TrapRange, lie unconforably beneath the Portage Lake Lava Series, from&hich they differ in lithology, metamorphic grade, and magneticproperties. The lowermost 5000 feet consists predominantly of verythin basalt flows, and the next 4000 feet mainly of aphanitic tofine—grained flows of intermediate composition. The uppermost part

of this sequence is everywhere concealed by the Jacobsville Sandstoneor deep overburden, but is inferred from gravity and magnetic datato consist largely of felsic flows. This lower sequence is lC,000and perhaps 20,000 feet thick, the difference reflecting uncertaintyabout the location of the top.

The lower Keweenawan rocks are more metamornhosed than the middleKeweenawan. Their metamorphic grade appears to increase westward —actinolite is found in Wisconsin but is rare near Ironwood.

Kenneth Books has found that the lower and middle Keweenaw€nrocks also differ from one another in magnetic direction.

In contrast with the belts of middle and upper Keweenawan roes,dips of bedding generally decrease with stratirraphic depth in thelower Keweenawan belt. If this represents southward thickeningrather Lian folding, it suggests that the axis of the lower Keweenawanbasin lay to the south.

-72-

The lower Ke,,,eenawan lavas, !'"hich form the so-called South TrapRange, lie unconfonnably beneath the Portage Lake Lava Series, from("hich they differ in lithology, metamorphic grade, and magneticproperties. The lowennost 5000 feet consists predominantly of verythin basalt flows, and the next 4000 feet mainly of aphanitic tofine-grained flows of intermediate composition. The uppermost partof this sequence is everywhere concealed by the Jacobsville Sandstoneor deep overburden, but is inferred from gravity and magnetic datato consist largely of felsic flows. This lower sequence is 10,000and perhaps 20,000 feet thick, the difference reflecting uncertaintyabout the location of the top.

The lm"er Keweenm"an rocks are more metamorphosed than the middleKeweenmvan. Their metamorphic grade appears to increase westl,vardactinolite is found in Hisconsin but is rare near Ironwood.

Kenneth Hooks has found that the lower and middle Keweenawanrocks also differ from one another in magnetic direction.

In contrast with the belts of middle and upper Ke,,,eenal,van rocks,dips of bedding generally decrease !"'ith stratigraphic depth in thelower Ke\veenawan belt. If this represents soutlnvard thickeningrather than folding, it suggests that the axis of the lower Keweenal,vanbasin lay to the south.

—73—

THE NORTH SHORE VOLCANIC GROUP

May 5 and 8, 1971

Prepared by

John C. GreenUniversity of Minnesota, DuluthMinnesota Geological Survey

-73-

THE NORTH SHORE VOLCANIC GROUP

May 5 and 8, 1971

Prepared by

John C. GreenUniversity of Minnesota, Duluth


—74--

The North Shore Volcanic Group

John C. Green

Introduction

Previous Work and Acknowledgments. Detailed mapping of the Minnesotashore of Lake Superior began with A. S. Sandberg's study (1938) of thesection between Duluth and Two Harbors. Grout and Schwartz (1939) andGehman (1957) studied the intrusions and flows in eastern Lake County;Grogan (1940) mapped the lakeshore between Two Harbors and Split RockRiver; Schwartz (1949) studied the Duluth area; and Grout and others(1959) mapped most of Cook County. Most of the data reported in thisaccount derive from studies by the writer who, starting in 1965, hasmapped the shoreline between Silver Bay and Grand Portage, with con-siderable reconnaissance inland (Green, 1966; 1968a; 1968b; 1970).The report does, however, also lean considerably on Grout etal. (1959)and, for the Duluth—Two Harbors area, on Sandberg (1938). The field

studies have been supported by the Minnesota Geological Survey, andmost of the laboratory studies have been supported by the NationalScience Foundation. Sincere gratitude for this support is extended toboth agencies. The writer's ideas have benefitted from discussionswith many other geologists concerned with Keweenawan rocks, esneciallyincluding Bill Bonnichsen, D. M. Davidson, Jr., H. Hubbard, C. B. Morey,W. C. Phinney, P. W. Weiblen, and W. S. White.

Regional Setting. The name "North Shore Volcanic Group" has been usedby Goldich et al. (1961) for the lavas and interbedded sediments ofLate Precambrian age in northeastern Minnesota. These rocks, as wellas all other Late Precambrian volcanic and sedimentary rocks in theLake Superior district, have traditionally been called"Keweenawad'bygeneral lithic and structural correlation with rocks exposed on theKeweenaw Peninsula of Michigan, but recent radiometric and paleomagneticinvestigations as well as geologic mapping indicate that a more precisestratigraphic framework is needed to adequately describe the complexseries of Late Precambrian events and deposits in this area.

In the northeast corner of Minnesota (Grand Portage area) the lowestUpper Precambrian flows overlie a thin quartzite (Puckwunge) which inturn overlies, apparently disconfornably, the shales and graywackes Ofthe Middle Precambrian Rove Formation; here both sequences strike nearlyeast—west and dip at approximately 10° to the south. At the southwestend of the basin immediately west of Duluth (155 miles away), the lowestUpper Precambrian flows also conformably overlie a thin quartzite (Puck—wunge?) which there overlies the vertically folded slates and metagray—wackes of the Middle Precambrian Thomson Formation, which is correlatedwith the Rove. Here the flows strike north and dip at about 25° to theeast. The angular unconformity here reflects the diastrophism and subse-quent erosion associated with the Penokean orogeny, which evidently didnot affect the northeastern corner of the state. Across the axis of theLake Superior Syncline in northern Wisconsin and Michigan the lowest flows

-74-

The North Shore Volcanic Croup

John C. Green

Introduction

Previous Hork and Acknowledgments. Detailed mapping of the Minnesotashore of Lake Superior began with A. E. Sandberg's study (1938) of thesection between Duluth and Two Harbors. Grout and Schwartz (1939) andGehman (1957) studied the intrusions and flows in eastern Lake County;Grogan (1940) mapped the lakeshore between Two Harbors and Split RockRiver; Schwartz (1949) studied the Duluth area; and Grout and others(1959) mapped most of Cook County. Most of the data reported in thisaccount derive from studies by the writer who, starting in 1965, hasmapped the shoreline between Silver Bay and Grand Portage, with considerable reconnaissance inland (Green, 1966; 1968a; 1968b; 1970).The report does, however, also lean considerably on Grout ~ al. (1959)and, for the Duluth-Two Harbors area, on Sandberg (1938). The fieldstudies have been supported by the Minnesota Geological Survey, andmost of the laboratory studies have been supported by the NationalScience Foundation. Sincere gratitude for this support is extended toboth agencies. The writer's ideas have benefitted from discussionswith many other geologists concerned with Keweenffivan rocks, esoecia11yincluding Bill Bonnichsen, D. M. Davidson, Jr., H. Hubbard, G. B. Horey,W. C. Phinney, ·P. W. Weiblen, and W. S. White.

~egional Setting. The name "North Shore Volcanic Group" has been usedby Goldich et al. (1961) for the lavas and interbedded sediments ofLate Precambrian. age in northeastern Minnesota. These rocks, as wellas all other Late Precambrian volcanic and sedimentary rocks in theLake Superior district, have traditionally been called "Ke~veenffiolan"bygeneral lithic and structural correlation with rocks exposed on theKeweenaw Peninsula of Michigan, but recent radiometric and paleomagneticinvestigations as \-lell as geologic mapping indicate that a more precisestratigraphic framework is needed to adequately describe the complexseries of Late Precambrian events and deposits in this area.

In the northeast corner of }linnesota (Grand Portage area) the lowestUpper Precambrian flows overlie a thin quartzite (Puckwunge) \.;rhich 1.nturn overlies, apparently disconformably, the shales and graywackes 6fthe Middle Precambrian Rove Formation; here both sequences strike nearlyeast-west and dip at approximately 100 to the south. At the southwestend of the basin immediately west of Duluth (155 miles away), the lowestUpper Precambrian flows also conformably overlie a thin quartzite (Puckwunge?) which there overlies the vertically folded slates and metagraywackes of the }tiddle Precambrian Thomson Formation, which is correlatedwith the Rove. Here the flmols strike north and dip at about 25 0 to theeast. The angular unconformity here reflects the diastrophism and subsequent erosion associated with the Penokean orogeny, which evidently didnot affect the northeastern corner of the state. Across the axis of theLake Superior Syncline in northern Wisconsin and Michigan the lowest flows

MINNESOTA

'—-7 -

ONTARIO

rHCVL AND

ITLC MARAIS

LUTSEN

KEWEENAWAN

KEWEENAWAN

FIELD

INTRUSION S

LAVAS

•O HARBORS

SCALE

EARLYPRECAMBRIAN

GRAND M AR A IS

JOFTE

uACONITh

LEGEND

ER BAY

JBEAVER BAY

STOP

LAKE SUPERIOR

U'

i.,'\~r KEWEENAWAN INTRUSIONS

Wf0J KEWEENAWAN LAVAS

"'to FIELD TRIP STOP

LEGEND

I-....J\JlI

403020

Miles

SCALE

MICHIGAN

10°!

SUPERIOR

ONTARIO

LAKE

MARAIS

BAY

WISCONSIN

\-nI J)I \

I

MINNESOTA

EARLYPRECAMBRIAN

MINN.

—76—

conformably overlie a similar quartzite (Bessemer) that in turn overliesMiddle Precambrian shale and graywacke with only minor discordance. Thesequartzites have always been referred to as Lower Keweenawan, but noradiometric age determinations are available and they may be much olderthan the volcanic rocks of the iCeweenaw Peninsula.

The North Shore Volcanic Group is cut by a great variety of intrusiverocks that are also of Late Precambrian age. These range from the greatDuluth Complex, dominated by anorthositic and troctolitic rocks, tosmaller sills, stocks, dLkes, and irregular plutons of diabase, 'crro—abbro, troctolite, syenogabbro, trachybasalt, granocdorite, angrarophyricadanellite. Some of these bodies also cut the older rocksto the north, northwest and west of the main Late Precambrian outcroparea (e.g. the Logan intrusives of Cook County and the Thunder Bay Districtof Ontario).

Paleonagnetism and Age. Recent paleomagnetic studies (Dubois, 1962; Beckand Lindsley, 1969; Books, 1968; Palmer, 1970) have shown that tworeversals of magnetic polarity occur within the Late Precambrian volcanicrocks of the Lake Superior district. The lowest stnta show 'nor'ial"

(north—seeking) polarity similar to orientations in the underlying MiddlePrecambrian rocks, but this group of rocks has not been reconized inMinnesota. Books (1y68) has proposed that the Lover — Middle Keweenawanboundary be redefined at the second macnetic reversal where rocks ofreversed po]arity are succeeded by rocks of' normal polarity. The NorthShore Volcanic Group contains at the base of the section at Grand Portageabout 5000 feet of lavas tnat show reversed polarity, and are thus LowerKeweenawan as magnetically defined. The thick wedge of flows west ofDuluth that underlie the Duluth Complex ut overlie the Puckwunge (?)quartzite have not been adequately tested in the laboratory. hut onregional magnetic maps give a negative magnetic anomaly which impliesreversed polarization. Furthermore they are lithically very similar tothe reversed—polarity lavas of Grand Portage. The remainder of the::orth Shore Volcanic Group has normal magnetic polarity, similar to thebulk of the associated intrusive rocks and to the rocks of the KeweenawPeninsula.

Only limited radiometric age determinations are yet available on rocksof the North Shore Volcanic Group. Goldich !! al. (1961) found a1.1 ± 0.1 b.y. age for associated intrusive rocks of the Duluth Complexby Rb/Sr and K/Ar methods, and Silver and Green (1963) found an isotopicage of 1.125 by U/Pb isotopes in zircons of both lavas and intrusive rocksfrom the Duluth and Mellen, Wisconsin areas. Paure et al. (1969) determinedthe age of the Endion sill, which cuts the flows at Duluth, as 1.092 b.y.,and that of the Duluth Complex at Duluth as 1,115 b.y. by theRb/Srmethod.However, all of these sampled rocks are in areas of normal magnetic polarityso no data are available as to the age of the Lower Keweenawan lavas of, forinstance, the Grand Portage area. Hanson and Malhotry (1970) have recentlyfound a 1.380 b.y. age (K/Ar) of a "Logan Intrusive' in southern Ontario,which may indicate the possible age span of the Lower Keweenawan. Isotopic

U/Pb studies over the range of Upper Precambrian rocks iii the district arecurrently in progress.

-76-

conformably overlie a similar quartzite (Uessemer) that in turn overliesMiddle Precambrian shale and graywacke with only minor discordance. Thesequartzites have always been referred to as Lower Keweenawan, but noradiometric age determinations are available and they may be much olderthan the volcanic rocks of the Keweenaw Peninsula.

The North Shore Volcanic Group is cut by a. great variety of intrusiverocks that are also of Late Prec&~brian age. These range from the greatDuluth Complex, dominated by anorthositic and troctolitic rocks, tosmaller sills, stocks, dikes, and irregular plutons of diabase, ferrogabbro, troctolite, syenogabbro, trachybasalt, granodiorite, and.graoQphyric, adamellite. Some of these bodies also cut the older rocksto the north, northwest and west of the main Late Precambrian outcroparea (~.g. the Logan intrusives of Cook County and the Thunder Bay Districtof Ontario).

Paleomagnetism and Age. Recent paleomagnetic studies (Dubois, 1962; Deckand Lindsley, 1969; Books, 1968; Palmer, 1970) have shmm that tworeversals of magnetic polarity occur 'loTi thin the Late Precambrian volcanicrocks of the Lake Superior district. The 1m-rest strata shoYr "normal"(north-seeking) polarity similar to orientations in the underlyinp; MiddlePrecambrian rocks, but this group of rocks has not been recognized inMinnesota. Books (l9~8) ha.s proposed that the Lower - Middle ~eweenawan

boundary be redefined at the second magnetic reversal where rocks ofreversed polarity are succeeded by rocks of normal polarity. The NorthShore Volcanic Group contains at the ba.se of t~e section at Grand Portageabout 5000 feet of lavas that show reversed polarity, and are thus LowerKeweenawan as magnetically defined. The thick wedge of flows west ofDuluth that underlie the Duluth Complex but overlie the Pucki-runge (?)quartzite have not been adequately tested in the laboratory, but onregional magnetic maps give a negative map;netic anonaly which impliesreversed polarization. Furthermore they are lithically very similar tothe reversed-polarity lavas of Grand Portage. The remainder of theNorth Shore Volcanic Group has normal magnetic polarity, similar to thebulk of the associated intrusive rocks and to the rocks of the KeweenawPeninsula.

Only limited radiometric age determinations are yet available on rocksof the North Shore Volcanic Group. Goldich ~~ ~~. (1961) found a1.1 ± 0.1 b.y. age for associated intrusive rocks of the Duluth Complexby Rb/Sr and K/Ar methods, and Silver and Green (1963) found an isotopicage of 1.125 by U/Pb isotopes in zircons of both lavas and intrusive rocksfrom the Duluth and Mellen, Wisconsin areas. Faure et al. (1969) determinedthe age of the Endion sill, which cuts the flows at Duluth, as 1.092 b.y.,and that of the Duluth Complex .at Duluth as 1.115 b.y. by the Rb/Srmethod.However, all of these sampled rocks are in areas of normal rr..agnetic polarity,so no data are available as to the a~e of the Lower Keweenawan lavas of, forinstance, the Grand Portage area. Hanson and Malhotry (1970) have recentlyfound a 1.380 b.y. age (K/Ar) of a "Logan Intrusive" in southern Ontario,which may indicate the possible age span of the Lower Ke'lveena"ran. IsotopicU/Pb studies over the range of Upper Precambrian rocks in the district arecurrently in progress.

—77—

Structure

The general structure of the North Shore Volcanic Group is that of agreat nest of dishes tilted gently to the southeast into Lake Superior.At the northeast end the strata at the base strike slightly north ofwest and dip about 10—12° south, whereas at the southwest end, 155 milesaway, they strike north and dip about 25° east. In between the strikesc-raually converge along the shore of Lake Superior as higher stratigraphiclevels are reached, until the flows strike parallel to the shore in thevicinity of Schroeder, Torte, and i,utsen in southwestern Cook County.Here the highest stratigraphic units are exposed, and the dip is approxi-mately 12° to the southeast.

The lavas are intruded by a great variety and bulk of intrusive rocks,includim several large diabasic sills at Duluth, the Beaver Bay Complex,the 1-iovland and Reservation River diabase complexes and the Logan intrusions.Where these intrusions are discordsnt and abundant they have deformed thelavas considerably, with local strongly divergent strikes and steep tooverturned dius. Along with the thick glacial cover inland, they havealso made difficult to impossible the long-distance tracing of stratigraphicunits in the lava series. Several major flows or roups of similar flows,however, can be traced inland from the lakeshore for at least 15 to 25 miles.Faulting is common in the flows near the areas of abundant intrusions (suchas from Silver Bay to Little :iarais). These faults appear to be of minordisplacement and are mostly transverse and steeply dipping with no stronglypreferred strike or displacennnt, but a few longer strike—faults have beenfound, one of which probably extends for at least five miles.

The thickness of the lava succession has been measured and estimated bySandberg (1938) and Gror-an (l90) as 23,lB feet between Duluth and SplitPock River (the beginning of the Beaver Bay intrusive complex) by addingthe individual flow thicknesses intersected along the lakeshore. Whether

this conforms to the true thickness of the j.ile at Split Rock Biver is not

known. Northeast of the Beaver Bay Complex about 5000 feet of lavas areestimatcd from recent mapping to form the lakeshore section between 'ilverBay and the uppermost flows at °ofte. Rortheast of Tofte and Lutsen, where

the flows are parallel to the shore, lavas totalling about 16,500 feethave been measured down to the Reservation River diabase near }iovland. Below(northeast of) this is an older section of about 5,000 feet of lavas, fora total on this limb of about 21,500 feet.

Estimateof volcanic thicknesses by constructing cross—section profilesgive between 11,000 and 18,000 feet at Tofte, above the Duluth Complex,deDendng on assumed dips between 12° and 20°. Although the average

dip at Tofte is about 12°, there is very little control on dips near thebase of the section as the few inland outcrops rarely expose flow contacts.Farther northeast at the Cascade River, about 15,000 feet of lavas abovethe Duluth Complex are calculated with an average dip of 12°; anotherthick section, possibly as much as 5,000 feet thick, here lies beneaththe Complex.

-77-

Structure

The general structure of the North Shore Volcanic Group is that of agreat nest ofmshes tilted gently to the southeast into Lake Superior.At the northeast end the strata at the base strike slightly north ofwest and dip about 10-12° south, whereas at the southwest end, 155 milesaway, they strike north and dip about 25° east. In between the strikesgradually converge along the shore of Lake Superior as higher stratigraphiclevels are reached, until the flows strike parallel to the shore in thevicini ty of Schroeder, Tofte, and Lutsen in southvrestern Cook County.Here the highest stratigraphic units are exposed, and the dip is approximately 12° to the southeast.

The lavas are intruded by a great variety and bulk of intrusive rocks,including several large diabasic sills at Duluth, the Beaver Bay Complex,the Hovland and Reservation River diabase complexes and the Logan intrusions.Hhere these intrusions are discordant and abundant they have deformed thelavas considerably, with local strongly divergent strikes and steep tooverturned dips. Along with the thick glacial cover inland, they havealso made difficult to impossible the long-distance tracing of stratigraphicunits in the lava series. Several maj or flows or groups of similar flovrs,however, can be traced inland from the lakeshore for at least 15 to 25 miles.Faulting is COffi.J;lon in the flows near the areas of abundant intrusions (suchas from ~jilver Day to Little :'1arais). These faults appear to be of minordisplacement and are :r.lostly transverse and steeply dipping with no stronglypreferred strike or displacemnnt, but a few lon~er strike-faults have beenfound, one of which probably extends for at least five miles.

The thickness of the lava succession has been measured and estimated by~)andberi!, (19::38) and Grof.\an (19)j·0) as 23,148 feet behreen Duluth and SplitRock River (the beginning of the Beaver nay intrusive complex) by addingthe individual flow thicknesses intersected along the lakeshore. ~TIether

this conforms to the true thickness of the pile at Split Rock River is notlmovn. Hortheast of the Beaver Bay Complex about 5000 feet of lavas areestimated from recent mapping to form the lakeshore section between SilverBay and the uppennost flows at 'T'ofte. IJortheast of Tofte and Lutsen, wherethe flows are parallel to the shore, lavas totalling about 16,500 feethave been measured dovn to the Reservation River diabase near Hovland. Below(northeast of) this is an older section of about 5,000 feet of lavas, fora total on this limb of about 21,500 feet.

Estimate50f volcanic thicknesses by constructing cross-section profilesgive between 11,000 and 18,000 feet at Tofte, above the Duluth Complex,depending on ass~~ed dips between 12° and 20°. Although the averagedip at Tofte is about 12°, there is very little control on dips near thebase of the section as the fe,., inland outcrops rarely expose flow contacts.Farther northeast at the Cascade River, about 15,000 feet of lavas abovethe Duluth Complex are calculated with an average dip of 12°; anotherthick section possibly as much as 5,000 feet thick, here lies beneath, ~ '.

the Complex.

—78--

lDesptiq

The North Shore Volcanic Group bears many resemblances, both physicallyand chemically, to plateau lava sequences of various geologic ages.Similarities to the Tertiary plateau lavas of eastern Iceland areparticularly striking. The lavas are almost entirely subaerial, showinghighly vesicular (now amygdaloidal) upper portions and massive interiors,and. various types of jointing, surface features, and textures dependingon their specific composition. Evidence of submarine extrusion is almostentirely limited to the base of the section both at Grand Portage and atDuluth; at Nopeming, west of Duluth, the lowest flow is piflowed and onGrand Portage Island the lowest flow shows spheroidal forms that couldpossibly be pillows, but excellent, thick—rinded, vesicular pillowsconstitute a flow on the lakeward side of the island a few flows abovethe base of the section. Unequivocal hut less well—formed pillows andpillow—breccia have been seen only rarely higher in the section. These

could have formed in local lakes or stream beds on the lava surface. The

flows are in general tabular, and since some individual flows or flowgroups can be traced along strike for at least 20 miles, the generalimpression is that of a broad, rather flat volcanic terrain. In contrast

to the situation in eastern Iceland, however (Walker, l96), no clearevidence of volcanic centers, representing shield or composite volcanoescontemporaneous with the plateau volcanism, has yet been found. White

(1960) has drawn attention to the remarkable extent of some Keweenawanflows (especially in Michigan) and with ample justification calls themflood basalts.

Interfiow sediments make up a minor part (l—3) of the section. They

are principally red, cross—bedded sandstones, that occur sporadically asbeds a few inches thick between flows, but a few local accumulations ofover 100 feet are found. Conglomerate is rare. Some sand has filtereddown into cavities in the upper parts of flows, and also forms a matrixfor flow—top breccia in others. These sediments appear to have beendeposited by occasional temporary streams winding across the volcanicsurface. There is little evidence of erosion. Pyroclastic deposits areextremely scarce, but welded tuff and mixed sand and shards have beenreported from the Cascade River in Cook County (Johnson and Foster, 1965)and basaltic to andesitic breccia, other than flow top breccia, is presentin a few localities.

With the exception of a high potassium content in some oaf ic and intermediatemembers and the relative abundance of rhyolite, the compositions of the lavasare also very similar to those of plateau lava series in Iceland and else-where. Table 1 shows the general characteristics and abundance of themajor types.

The most abundant general type is olivine basalt of several varieties;one widespread, important, and distinctive variety is mottled (ophitic),and is similar to what has been called olivine tholeiites in other areas.These typically have ropy surfaces and were very fluid. Rough columnarjoints are common. Other olivine basalts are coarser, some with diabasicand some with other characteristic textures. In the Tofte—Lutsen area,high in the section, is a group of olivine basalts with abundant, small(1—3 mm) bytownit&phenocrysts or crystal clots. At the base of the sectionboth t Duluth and on Lucille Island east of Grand Portage are distinctivebasalts that contain abundant phenocrysts, 2—3 mm across, of augite and

-78-

General Description,

The North Shore Volcanic Group bears many resemblances, both physicallyand chemically, to plateau lava sequences of various geolo~ic ages.Similarities to the Tertiary plateau lavas of eastern Iceland areparticularly striking. The lavas are almost entirely subaerial, showinghighly vesicular (nmr amygdaloidal) upper portions and massive interiors,and various types of jointing, surface features, and textures dependingon their specific composition. Evidence of submarine extrusion is almostentirely limited to the base of the section both at Grand Portage and atDuluth; at Nopeming, west of Duluth, the lowest flow is pillowed and onGrand Portage Island the lowest flow shmrs spheroidal forms that couldpossibly be pillows, but excellent, thick-rinded, vesicular pillowsconstitute a flow on the lakeward side of the island a few flmrs abovethe base of the section. Unequivocal but less well-formed pillows andpillow-breccia have been seen only rarely higher in the section. Thesecould have formed in local lakes or stream beds on the lava surface. Theflows are in general tabular, and since some individual flows or flowgroups can be traced along strike for at least 20 miles, the generalimpression is that of a broad, rather flat volcanic terrain. In contrastto the situation in eastern Iceland, however (Walker, 1964), no clearevidence of volcanic centers, representing shield or composite volcanoescontemporaneous with the plateau volcanism, has yet been found. \'mite(1960) has drm-rn attenti on to the remarkable extent of some Keweenm-ranflows (especially in Michigan) and with ample justification calls themflood basalts.

Interflow sediments make up a minor part (1-3%) of the section. They, -

are principally red,cross-beddedsandstones, that occur sporadically asbeds a few inches thick between flows, but a fe"T local accumulations ofover 100 feet are found. Conglomerate is rare. Some sand has filtereddown into cavities in the unper parts of f10vs, and also forms a matrixfor flow-top breccia in others. These sediments a.ppear to have beendeposited by occasional temporary streams winding across the volcanicsurface. There is little evidence of erosion. Pyroclastic deposits areextremely scarce, but welded tuff and mixed sand and shards have beenreported from the Cascade IIiver in Cook County (,Tohnson and Foster, 1965)and basaltic to andesitic breccia, other than flow top breccia, is presentin a few localities.

With the exception of a high potassium content in some mafic and intermediatemembers and the relative abundance of rhyolite, the compositions of the lavasare also very similar to those of plateau lava series in Iceland and elsevhere. Table 1 shows the general characteristics and abundance of themajor types.

The most abundant general type is olivine 'basalt of several varieties;one widespresd, important, and distinctive variety is mottled (ophitic),and is similar to what has been called olivine tholeiites in other areas.These typically have ropy surfaces and were very fluid. Rough col~~nar

joints are common. Other olivine basalts are coarser, some i-rith diabasicand some ,.ith other characteristic textures. In the Tofte-Lutsen area,high in the section, is a group of olivine base.1ts "ri th abundant, small(1-3 nun) bytownite' phenocrysts or crystal clots. At the base of the sectionboth at Duluth and on Lucille Island east of Grand Portage are distinctivebasa.lts that contain abundant phenocrysts, 2-3 mm across, of augite and

TABLE 1

Generalized Characti±ritics of Major Lava Types of North Shore Volcanic Group

Wt % MgO

I Olivine Quartz Andesite—

I

Tholeilte Tholelite Trachyandesite

46-49 50—51 4,,52-57

0.1-0.5 0.6-0.9 1 1.9—2.7

5.9:6.8 1 11:9—4.5

Quartz Latiteithyolite

2.8—5.0 3.9—6.2

aphanitic, aphanitic tomostly pot— felsitic;aphyricphyritic (plag., or porphyriticaugite, oh) (plag., orthoclase,

quartz, mag., pyrox.)occas. spherulitic

—

Thickness, feetRange <1 to >100 30—150 50—240

10—40 80 varIable—Structuresflow tops smooth, ropy scoriaceous rubble scoriaceous,

- rubblyjointing sheeted tops[" small, 1rregiIar small, irregular

columnar centers

vesic1es round or 1rregu1ar stretched

4

Other

______ _____

Characteristics

Characteristic

Wt % Si02

Wt % 1(20

IntermediateQuartz Latite

Textures

62—65 72—75

OphiticI Occasionally

porphyritie(plagioclase)

0.0—0.4

Very fine—grainS, inter—granular. Someflow structure,fine oxidation—banding

Very fine—grained,

commonly pot—phyritic (plag.,augite)

very_fluidpipe amygdulesat basesegregation veins,vesicle cylinders

80—20050—1300

perhaps 3500120 50—500

vesicular vesicular, rolled,wrinkled flow—banded

irejilai tâ platy, sub—horizontalsubhorizontal, big columns in thickplaty flows

stretched or stretched, roundround

few flow pink, red, orcontacts exposed light gray

stretched or -

round

brown—weathering;omewhat variable

more_viscoussome contain

more 1(20,Ksrquartz, agatecommon in cavities

TABLE 1

Generalized Characteristics of Major Lava Types of North Shore Volcanic Group

:ic I 1 Olivine Quartz Andesit~:it"P. I {Intermediate Quartz Latite___~ Characteri~~~i~te Tholeiite Tr~rtz L~~ __ .~yoliteWt % Si02 ' 46-49 1 50-51 52-57 62-65 72-75

~-"----", .. ,. • .~-w•••_ -" ~".~~~•••~.-+,~.-_ ~ -_..~--~·_""""~:~'.....,-'T-'·_.-.~ ..~~~'-~·~~,.. ..,..-".-·.-'''' --..~•...".-.-...."....-.~...,.--~ -"'. ~~-~"~---'-'-'-~---~ _.V·_·__P_>'_ • -._••- •... , _ _~--~

Wt % K20 I 0.1-0.5 I 0.6-0.9 ! 1.9-2.7 2.8-5.0 3,.9-6.2-, --~ ,. --...--.......... ·~"-··""'''''''·~''-'''''''''''-''''-'''~~··i''''~· ~ · ~'~l.·.~ ~ol.AA' ••.!::..~< 1-,,- 1,--.' •• _.,,-, ."~.- ~"'-" ·~·~,.._'S...·_"' t '::- I__ "',.. ,""~•••.,J ••••~ • '- ._- '_<'~~', ,"'_ ••.•• -:...~ ~ < - .-

Wt % MgO !I' 5. 9-6 . 8 I 4. 3-5. 9 I 1. 9-4 . 5 0.9- 1• 9 . 0.0-0.4-..---~.--'.<.~."'-"'.'.. _ ..•,,_'__"~_ _~__w .•~.• ~;~'-<'~~""_.•Y. '"-->-"'"'-'~/"""~-"';""'~1--~-'>""'.w.".>--."'-•.,./A~_~C'~~'_"h,,·_, t-'_.,··",._·.·r;.,,·~ ..,..'.,. ".~., --. --.,,' j'" -" <J..,.,--,~__;· ·",· ,,,n"~"·.·~,.,-.•~,,·,,,. ,'-~.~ ..,< , .•~"'_'_, < •• ,,..•, .•--.- .. --.

1 ' , Il\ I . .

Textures ~ Ophitic 1 Very fine- t Very fine- I aphanitic~ aphanitic toOccasionally l grained, inter- ~ grained, mostly por- felsitic; aphyricporphyritic { granular. Some ! commonly porphyritic (plag.,. or porphyritic(plagioclase) ~ flow structure, ~ phyritic (plag., augite, 01.) (plag., orthoclase,,. ): i, fine oxidation- .1 augite) . quartz, mag., pyrox.

--------.--.--------!..-----~---~-~t~:~~~~-.-~--+----- ~.--_ .._,--.;,::::~:lt::l~~~.~ ... --Thickness, feet J ~ r I' . 50-1300

Range ]- <1 to >100 ' 30-150 1 50-240 80-200~ ~ ~ i perhaps 3500

c'oIiIIDo~ .'.. -- -~." ·I6~46 ·"' -,·--···l 'so - 1".. var:Gibier' 120'" · ·_..5..0=500 ..·· -

···------struc~~:-t:=-:-:py--i~c==~=tc=:=::--i·:::=:--====:-j oi;ti';;- ··t-~~~:i~:}~::T.::-t~~~ii,irreg;;i;r-ts~if;~~~;~g;;iarh~~~~;!!~:~~l,'~~~~~:~:h~:i~~~~~lve·si-~i;;~·_..-·--l--·;(;~d·-~-;· 'i;~~g~i~·~+·;t~et"~h'ed ...._,··_- ..--+;tretch~d· O~.. t"';~~:iche~f 'o"r" ···... i .. ~~~:~·ched·: round

1 ! i round I round ~--- ~-. -"'-'--- -----..- ~--_ ·-··-~·i ---..-.-.-..-..~ -"..~ -1 ,-- ,-- - - ~ .. _·--_····_ ..·..l·· .-,-.----- .- ..-.- ---,..~."i'----. __ -- -.~~- - - ~~"Q -""-._..~• .,~_.-.. -,- - '".~-~--'-

Other ! very fluid 1more viscous ~ brown-weathering I few flow : pink, red, orCharacteristics J, pipe amygdules l some contain tomewhat variableIcontacts exposedt light gray

I at~ ~ more K20, !l segregation veins, i Kspar; ~ Ii vesicle cylinders ~: quartz, agate I Il Icommon in cavities'

~~ j f~ I .

—80—

(serpentinized) olivine: these are particularly unusual in havingferromagnesian instead of plagioclase phenocrysts. Another moderatelyabundant and distinctive rock ty-pe is the "quartz tholeiite which isaphanitic or very fine grained and slightly more siliceous and viscousthan the olivine basalts. The quartz—tholeiites characteristically havea rubbly or brecciated top with the highly vesicular fragments set in amatrix of washed—in red sand or occasionally calcite and zeolites. Theyalso commonly show narrow oxidation bands, 1-3 mm thick, along subhorizontalflowage planes. This quartz—tholeiite grades into more potassium—richvarieties (trachybasalt, trachyandesite) that can be distinguished onlyby chemical analysis and microscopic study; patches of interstitial Kfeldspar are present in these rocks but are invisible in hand specimen.

Intermediate varieties are nearly all porphyritic with plagioclase, augite,magnetite, and in some specimens iron—rich olivine phenocrysts; they havethe compositions of andesites, trachyandesites, and intermediate quartzlatites. Most are aphanitic, but one unusual flow, here called the Manitoutraciybasalt, is exceptionally thick (at least 300 feet) and granular,and can be traced for 5 miles although it originally continued for anunknown distance in both directions. These flows are commonly brown orred and irregularly jointed or with platy,subhorizontal joints.

The felsic lavas are anomalously abundant for a simple differentiationseries from a basaltic parent magma. They are red, pink, or light gray.and have the composition of quartz latites. These flows tend to be muchthicker than the other types: the thickest is 1300 feet, a few miles eastof Grand Marais; the 3500' Brule Flyer rhyolite west of Hovland may be alava dome. Their top surfaces are mostly strongly flow—banded, vesicular,and. contorted, but not brecciated, and their bases are commonly flow—bandedand locally brecciated. Spherulites are occasionally present. Jointingranges from large columns 4 feet across in the thickest flows to sub—horizontal platy joints; small tectonically—produced parallel fracturesets a few mm apart commonly break up the ccooling joint fragments intosmall pieces. Most of the felsites are porphyritic, with quartz andfeldspar phenocrysts (oliogclase—andesine and/or orthoclase) but someare only weakly porphyritic or arhyric. Poikilitic quartz surroundingstout alkali—feldspar laths ("snowflnke texture:) is a common microscopetexture in the thicker flows. Even these siliceous lavas have evidentlyflowed a great distance; one lava or fow group can be traced for atleast 23 miles west from the Devil Track Fiver, Grand Marais (see man,fig. 1).

Alteration

The lavas have been strongly but irregularly affected by secondarysolutions that have deposited low—temmerature minerals in vesicles,fractures, and other cavities, and -altered some of the minerals of thelavas themselves. For instance, no fresh divine has been detected inany of the lavas, although it is common in the intrusive diabases.broad zonation of this alteration is anparent; at Duluth and at GrandPortage (in the lower parts of the lava section) r'uch of the groundmasspyroxene has been converted to actinolite (although many larger auritesare unaffected) and some plagioclase has been saussuritized. Iere alsothe arnygdule minerals are characteristically quartz, prehnite, calcite,

-80-

(serpentinized) olivine: these are particularly unusual in havingferromagnesian instead of plagioclase phenocrysts. Another moderatelyabundant and distinctive rock type is the "quartz tholeiite" which isaphanitic or very fine grained and slightly more siliceous and viscousthan the olivine basalts. The quartz-tholeiites characteristically havea rubbly or brecciated top with the highly vesicular fra~ments set in amatrix of washed-in red sand or occasionally calcite and zeolites. Theyalso commonly show narrow oxidation bands, 1-3 ~ thick, along subhorizontalflowage planes. This quartz-tholeiite grades into more potassium-richvarieties (trachybasalt, trachyandesite) that can be distin~uished onlyby chemical analysis and microscopic study; patches of interstitial Kfeldspar are present in these rocks but are invisible in hand speci~en.

Intermediate varieties are nearly all porphyritic with plagioclase, ausite,magnetite, and in SOTIe specimens iron-rich olivine phenocrysts; they havethe compositions of andesites, trachyandesites, and intermediate quartzlatites. Most are aphanitic, but one unusual flow, here called the Manitoutrac~ybasalt, is exceptionally thick (at least 300 feet) and granular,and can be traced for 5 miles although it originally continued for anunknown distance in both directions. 'I'hese flows are cOTlJ1!1only brown orred and irregularly jointed or with platy,subhorizontal joints.

The felsic lavas are anomalously abundant for a simple differentiationseries from a basaltic parent mae;m.a. They are red, pink, or li~ht gray,and have the composition of quartz latites. 'l'hese flows tend to be muchthicker than the other types: the thickest is 1300 feet, a few ~iles eastof Grand Marais; the 3500' Brule River rhyolite west of Hovland may be alava dome. Their top surfaces are mostly strongly flow-banded, vesicular,and contorted, b~t not brecciated, and their bases are cornmonly flow-bandedand locally brecciated. Spherulites are occasionally present. Jointin~

ranges from large columns 4 feet across in the thickest flows to subhorizontal platy joints; small tectonically-produced parallel fracturesets a few mID apart commonly break up the ccooling joint fra~ments intosmall pieces. Most of the felsites are porphyritic, ,-rith quartz andfeldspar phenocrysts (oliogclase-andesine and/or orthoclase) but someare only weakly porphyritic or aphyric. Poikilitic quartz surroundingstout alkali-feldspar laths ("snowfla1:e texture';) is a common microscopetexture in the thicker flows. Even these siliceous lavas have evidentlyflowed a great distance; one lava or f ..J..mr group can be traced for B.tleast 23 miles \-lest from the Devil Track River, Grand Marais (see Man,fig. 1).

Alteration

The lavas have been stronlJ,ly but irrerularl;v affected b;f secondarysolutions that have denosi ted lml-temner?ture minerals in vesicles>fractures, and other cavities, and altered sot'!.e of the minerals of thelavas themselves. For instance, no fresh olivine has been detected inany of the lavas, although it is co~mon in the intrusive diabases. Abroad zonation of this alteration is anparent; at Duluth and R.t GranelPortage (in the lower parts of the lava section) ~uch of the ~round~ass

pyroxene has been converted to actinolite (althoup::h nan'.' laT£ser aur.j te:=>are unaffected) and SOIne plar:ioclase has been saussuritized. [ere alsothe amygdule p.dnerals are characteristically quartz, prehnite, calcite,

—81—

epidote, and chlorite, the same basic assemblage as is found in thePortage Lake hava Series on the Keveenaw Peninsula (Stoiber and Davidson,1959). In and northeast of Duluth K—feldspar is also occasionally found,arid laumontite becomes abundant. Higher in the section variouS zeolites,along vith caThite, are dosnant except In the quartz tholeiites andsimilar levas where aaate, crystalline quartz and chlorite are common.The most abundant zeolites are laumontite, stilbite, heulandite,thorrsonite and scolecite but analcite, natrolite, mesolite, mordenite,and apotbvllite have also been found. Saponite is common in olivinebasalts. Andradite carnets have been discovered in several localitiesin amydules and veins from a wide range of lava tres (basalts torhyclites) and levels in the sequence, and traces of native copper havebeen found in several localities. Thus the secondary zonation in the:rorth Shore 1oThanic Grout spans both the deeper—level, higher—temperatretyte of the Keweenaw Peninsula and the higher—level, cooler tyte character-istic of the icuer parts of the Tertiary plateau lavas of eastern Icelandas described by Walker (1960). Walker's upper, zeolite—free zone isapparently not represented in innesota. According to his estirsates, thepresently exposed top of the section on the Lake Sunerior shore could havebeen anrrcxirratel'r 5,000 feet below the surface during mineralization.Although (tetal led work has not yet been done, no clear cross—cuttingrelations of' zeolite zones to stratigraph within the lavas have beenrecocnized. hut the evident Upper Precambrian, nostvolcanic unconformitywit' ch probably follows the lorth Shore must have postdated the mineralization,since it does crosscut the zeolite zones.

It should be stressed, however, that none of the flows has been entirelyconverted to secondary minerals. in fact, the plaioclase and augite aretvicall'r unaltered or only locally altered in most mafic and intermediaterocks, although no fresh olivine has been discovered, There has typicallybeen sore oxidation of the opsnue minerals, especially of nagnetite, andpi'eonite is coirnonly oxidized at its borders. In many interrsediate andfelsic lavas. plarioclase phenocrysts have been albitized and/or zeolitized:some of this alteration could be deuteric. Fresh, undevitrified volcanicglass is still present in occasional samples, notably in a basalt from abouttwo miles west of the mouth of the Prule River.

Etnra•ivThe Thias of the ;orth Shore Volcanic Group can be conveniently

divided into several lithostratir'ranhic units of coherent petrographiccharacter nrimarily on the basis of exposures at or near the Lake Superiorshore. qir of these units can he traced for a considerable distanceinland, buU nterruntions and structural corwlicatipns by intrusive bodiesas well -as lacial denosits prevent the reconstruction of' a complete, con-tinuous sequence. o clear trend of connositional change is evident frombase to top; in fact, although the most ferrora-'nesian lavas occur at thebase, the unnermost flows, in the Tofte—Lutsen area, are entirely olivineoasalts. It should be kept in rind also that a major stratigraphic breakmay occur heween the Grand Portage lava section (sho'ing reversed magneticpolarity) and the lhgher strata.

Table 2 lists the informal stratigraphic units nroposed for thenortheast limb (Torte to Grand Portage) forth Shore Volcanic Group, withtheir estimated thicknesses and general lithic characters, and Table 3Ltives sinlar data for the southwest limb. This latter table depends largelyon the work of Sandberrr (1038) and Grogan (iqito), pending restudy.

-81-

epidote, and chlorite, the same basic assemblage as is found in thePorta~e Lake Lav~ Series on the Keweenaw Peninsula (Stoiber and Davidson,1959). In and northeast of Duluth K-feldspar is also occasionally found,and laumontite becomes abundant. Higher in the section various zeolites,along with calcite, are dominant except in the quartz tholeiites andsimilar lavas where agate, crystalline quartz and chlorite are common.The most abundant zeolites are laumontite, stilbite, heulandite,thomsonite and scolecite but analcite, natrolite, mesolite, mordenite,and apophyllite have ~lso been found. Saponite is common in olivinebasalts. Andradite garnets have been discovered in several localitiesin ~!'lY8dules and veins from a wide range of lava types (basalts torhyolites) and levels in the sequence, and traces of native copper havebeen found in several localities. Thus the secondary zonation in theiTorth Shore Volcanic Group spans both the deeper-level, higher-temperaturetype of the lCe,.,eenaw Peninsula and the higher-level, cooler type characteristic of the Imler parts of the Tertiary plateau lavas of eastern Icelandas described by 'ilalker (1960). Halker' s uDper, zeolite-free zone isapparently not represented in Hinnesota. According to his estinates, thepresently exposed top of the section on the Lake Superior shore could havebeen approximately 5,000 feet belm., the surface during mineralization.JUthou~h detailed 1-Tork has not yet been done, no clear cross-cuttingrelations of zeolite zones to stratirr,raphy witp_~~ the lavas have beenreco~nized, but the evident Upper Precambrian, postvolcanic unconformity;·rhi-ch ;;robe.bly follm.,s the ~Torth Shore must have postdated the mineralization,since it does crosscut the ZGolite zones.

It should be stressed, however, that none of the flows has been entirelyconverted to secondary minerals. In fact, the plagioclase and augite aretynic8,11y unaltered or only locally altered in most mafic and intermediaterocks. althour:h no fresh olivine has been discovered. There h~s typicallybeen sor.e oxidation of the opaque minerals, especially of magnetite, andni~eonite is com~only oxidized at its borders. In nany intermediate andfelsic lavas, pladoclase phenocrysts have been albitized and/or zeolitized;some of this alteration could be deuteric. Fresh, undevitrified volcanicr,lass is still present in occasional samples, notably in a basalt from aboutt"m r'liles ,{est of the r10uth of the Brule River.

The lavas of the !.Jorth ;3hore Volcanic Group can be convenientlydivided into several lithostratipTanhic units of coherent petrographiccharacter nrimarily on the basis of exposures at or near the L~ke Superiorshore. Hany of these units ca,n be traced for Do considerable distanceinland, but interruntions and structural complicatiens by intrusive bodiesas \·rell as ;o;lacial denosi ts prevent the reconstruction of a complete, continuous sequenc p • fJo clear trend of compositional chanF':e is evident frombase to top; in fact, althou~h the r'lost ferro~arrnesinn lavas occur at the~ase, the u:!Jnermost flmrs, in the Tofte-Lutsen area, are entirely olivinebasa,lts. It 3hould be kent in r;ind also that a ma,jor stratigraphic break.f:lay occur be t"teen the Grand Portap-e lava section (shO\.,ing reversed magneticpolo.ri ty) and the llig-her strnta.

To.ble 2 lists the informal strntif.rcmhic units proposed for thenortheast liMb (Tofte to Gr8.nd Porta,g;e) ~Jorth ;3hore Volcanic Group, "liththeir estimated thicknesses and ~eneral lithic characters, and Table 3p:ives similar data for the southwest limb. 'I'his latter table depends largelyon the "lork of Sandberr:: (1°38) and Grogan (1940), penclin.Q; restudy.

—82—

Table 2

Stratigraphy of Northeast Limb (Tofte-Grand Portage)

North Shore Volcanic Group

(Exclusive of interflow sediments)

Approx.

ThigkneaL&Q

Top

1020

160

310

360

500

600

1020

400—900

1300

1800

1000

3500

4000 (est.)

200

260

4500


Lutsen basalts




Grand Narais rhyolite flow

Croftville basalts


Red cliff basalts


Marr Island lavas

Brule River basalts

Brule River fhyolite flow

Havland lavas




Lithic character

divine basalts, olivine tholeiites

thomsonite—bearing ophitic basalt


brown, columnar, granular trachybasalt


various fine—grained basalts

aphyric and porphyritic rholite fJows



mixed tholeiitlc basalt, intermediate,felsic lavas

granular—diabasic basalts




gray—brown, aphyric andesite

mixed tholeiltic to diabasic basalts

-82-

Table 2

Stratigraphy of Northeast Limb (Tofte-Grand Portage)


(Exclusive of interflow sediments)

Approx •..IhiCkness(ft. )

'fop

1020

160

310

360

500

600

1020

400-900

1300

1800

1000

3500

4000 (est.)

200

260

4500

Base


Lutsen basalts




Grand Marais rhyolite flow

Croftville basalts


Red cliff basalts


Marr Island lavas

Brule River basalts

Brule River fhyolite flow

Hovland lavas




Lithic character

olivine basalts. olivine tholeiites

thomsonite-bearing ophitic basalt

brown. porphyritic andesite,trachyandesite

brown, columnar, granular trachybasalt


various fine-grained basalts

aphyric and porphyritic rholite flows



mixed tholeiitic basalt, intermediate,felsic lavas

granular-diabasic basalts



red. porphyritic rhyolite

gray-brown, aphyric andesite

mixed tholeiitic to diabasic basalts

—83—

Table 3

Generalized Stratigraphy of Southwest Limb (Tofte—Nopeming)


(exclusive of interflow sediments)

Approx.

TbLekness_'ft) Lithostratigraphic Unit Lithic character

Tp

4000 Schroeder basalts ainygdaloidal ophitic olivine tholeiltes

>300 Manitou trachybasalt flow red—brown granular trachybasalt tobasalt


> 280 Bell Harbor lavas mostly quartz tholeiites, otherbasalts

> 3CC' Palisade rhyolite flow gray to pink, porphyritic rhyolite

few 100's Baptism River lavas mixed lavas, mostly basalts

— — — Beaver Bay intrusive complex —

3200 Gooseberry River basalts mixed basalts, one felsite

— --—. LaFayette Bluff, Silver Creek Cliff intrusions — — —

1025 Two Harbors fine—grained basalts "melaphyres", some quartz tholeiites

1615 Larsmont ophitic basalts amygdaloidal ophitic divine basalts.

— — — Knife River diabase intrusion — — — — —

4930 Sucker River basalts mixed basalts, mostly ophitic

4400 Lakewood basalts mixed basalts, mostly non—ophitic

— — Lester River diabase sill

3600 Lakeside lavas mixed basalts, andesites, felsites

— — — Endion diabase sill — — —

2560 Leif Erickson Park lavas mixed basalts, andesites

— — — Duluth Complex — — —

2300 Nope'iuing basalts porphyritic meLbasalts, diabasicbasalts

Base Puckwunge Sandstone

-83-

Table 3

Generalized Stratigraphy of Southwest Limb (Tofte-Nopeming)


(exclusive of interflow sediments)

Approx.Thickness (it)

4000

>300

>280

>3CO

few 100's

Lithostratigraphic Unit

Schroeder basalts

Manitou trachybasalt flow


Bell Harbor lavas

Palisade rhyolite flow

Baptism River lavas

Lithic character

amygdaloidal ophitic olivine tholeiites

red-brown granular trachybasalt tobasalt

mostly quartz tholeiites, otherbasalts

gray to pink, porphyritic rhyolite

mixed lavas, mostly basalts

3200

Beaver Bay intrusive complex

Gooseberry River basalts mixed basalts, one felsite

LaFayette Bluff, Silver Creek Cliff intrusions

1025

1615

4930

4400

3600

2560

2300

Base

Two Harbors fine-grained basalts

Larsmont ophitic basalts

Knife River diabase intrusion

Sucker River basalts

Lakewood basalts

Lester River diabase sill

Lakeside lavas

Endion diabase sill

Leif Erickson Park lavas

Duluth Complex

Nopeming basalts

Puckwunge Sandstone

"melaphyres", some quartz tholeiites

amygdaloidal ophitic olivine basalts.

mixed basalts, mostly ophitic

mixed basalts, mostly non-ophitic

mixed basalts, andesites, felsites

mixed basalts, andesites

porphyri tic melc.basal ts, diabasicbasalts

—84--

References

Beck, M. E., and Lindsley, N. C., 1969, Paleomagnetism of the Beaver BayComplex, Minnesota: Jour. Geophys. Res.,v.74, p. 2002—2013.

Books, K. G., 1968, Magnetization of the Lowermost Keweenawan lava flows inthe Lake Superior area, in Geological Survey Research 1968: U. S. Geol.Survey Prof. Paper 600—n, p. D248—254.

Dubois, P. H., 1962, Paleomagnetism and correlation of Keweenawan rocks:Geol. Sun. Canada Bull. 71, 75p.

Faure, G., Chaudhuri, S., and Fenton, M. D., 1969, Ages of the Duluth GabbroComplex and of the Endion Sill, Duluth, Minnesota: Jour. Geophys. Res.,v. 74, p. 720—725.

Gehman, H. M., 1957, The Beaver Bay Complex, Lake Co., Minn.: unpub. Ph.D.Thesis, Univ. of iinnesota.

Goldich, S. S., Nier, A. 0., Baadsgaard, Halfdan, Hoffman, J.H., and Krueger,11. tJ., 1961, The Precambrian geology and geochronology of Minnesota:Minn. Geol. Survey Bull. 41, 193 p.

Green, J. C., 1966, New field studies of the Keweenawan lavas of Minnesota(abs.): Program, 12th Ann. Inst. on Lake Superior Geology, Sault Ste.Marie, Mich., p. 9.

_____

1968a, Cheaical and physical characteristics of Late Precambrian lavasof northeastern Minnesota (abs.): Amer. Geophys. Union Trans., v. 49, p.363.

_____

1968b, Types and structures of flows of the North Shore Volcanic Group,Minnesota (Sununary): Program, 14th Ann. Inst. on Lake Superior Geology,p. 52—53.

_____

1970, Geology of North Shore Volcanic Group, in Summary of Fieldwork1970, Sims and Westfall, Ed's., Minn. Geol. Survey Inf. Circular 8, p. 19—20.

Grogan, R. M., 1940, Geology of a part of the Minnesota shore of Lake Superiornortheast of Two Harbors, 1-finn.: unpub. Ph.D. thesis, Univ. of Minn.

Greut, F. F., Sharp, R. P., and Schwartz, G. M., 1959, The geology of CookCounty, Minn.: Minn. Geol. Survey Bull. 39, 163 p.

Hanson, C. N., and Maihotra, R., 1970, K—Ar ages of mafic dikes in northeasternMinnesota (abs.): Program, 16th Ann. Inst. on Lake Superior Geology, ThunderBay, Ontario, p. 19.

Johnson, C. H., and Foster, R. L., 1964, Contaminated Precambrian ash—flow cuff,Cascade River, Minnesota (abs.): Geol. Soc. Amer. Special Paper 82, p. 102.

Palmer, H. C., 1970, Paleomagnetism and correlation of some Middle Keweenawanrocks, Lake Superior: Can. Jour. Earth Sci., v. 7, No. 6, p. 1410—1436.

Sandberg, A. E., 1938, Section across Keweenawan lavas at Duluth, Minnesota:Geol. Soc. Amer. Bull., v. 49, p. 795—830.

Schwartz, G. >i. 1949, The geology of the Duluth metropolitan area: Mfnn.Geol. Survey Bull. 33, 136 p.

-84-

References

Beck, M. E., and Lindsley, N. C., 1969, Paleomagnetism of the Beaver BayComplex, Minnesota: Jour. Geophys. Res.,v.74, p. 2002-2013.

Books, K. G., 1968, Magnetization of the Lowermost Keweenawan lave flows inthe Lake Superior area, in Geological Survey Research 1968: U. S. Geol.Survey Prof. Paper 600-D:-p. D248-254.

Dubois, P. M., 1962, Paleomagnetism and correlation of Keweenawan rocks:Geol. Surv. Canada Bull. 71, 75p.

Faure, G., Chaudhuri, S., and Fenton, M. D., 1969, Ages of the Duluth GabbroComplex and of the Endion Sill, Duluth, Minnesota: Jour. Geophys. Res.,v. 74, p. 720-725.

Gehman, H. M., 1957, The Beaver Bay Complex, Lake Co., Minn.: unpub. Ph.D.Thesis, Univ. of Minnesota.

Goldich, S. S., Nier, A. 0., Baadsgaard, Halfdan, Hoffman, J.H., and Krueger,H. W., 1961, The Precambrian geology and geochronology of Minnesota:Minn. Geol. Survey Bull. 41, 193 p.

Green, J. C., 1966, New field studies of the Keweenawan lavas of ~linnesota

(absJ: Program, 12th Ann. lnst. on Lake Superior Geology, Sault Ste.Marie, Mich., p. 9.

, 1968a, Ch~ical and physical characteristics of Late Precambrian lavas---of northeastern Minnesota (abs.): Amer. Geophys. Union Trans., v. 49, p.363.

, 1968b, Types and structures of flows of the North Shore Volcanic Group,---Minnesota (Summary): Program, 14th Ann. lnst. on Lake Superior Geology,p. 52-53.

, 1970, Geology of North Shore Volcanic Group, in Summary of Fieldwork----1970, Sims and Westfall, Ed's., Minn. Geol. Survey lnf. Circular 8, p. 19-20.

Grogan, R. M., 1940, Geology of a part of the Minnesota shore of Lake Superiornortheast of ~yo Harbors, Minn.: unpub. Ph.D. thesis, Univ. of Minn.

GrEiut, F. F., Sharp, R. P., and Schwartz, G. N., 1959, The geology of CookCounty, Minn.: Minn. Geol. Survey Bull. 39, 163 p.

Hanson, G. N., and Malhotra, R., 1970, K-Ar ages of mafic dikes in northeasternMinnesota (abs.): Program, 16th Ann. lnst. on Lake Superior Geology, ThunderBay, Ontario, p. 19.

Johnson, C. H., and Foster, R. L., 1964, Contaminated Precambrian ash-flow tuff,Cascade River, Minnesota (abs.): Geol. Soc. Amer. Special Paper 82, p. 102.

Palmer, H. C., 1970, Paleomagnetism and correlation of some Middle Keweenawanrocks, Lake Superior: Can. Jour. Earth Sci., v. 7, No.6, p. 1410-1436.

Sandberg, A. E., 1938, Section across Keweenawan lavas at Duluth, Minnesota:Geol. Soc. Amer. Bull., v. 49, p. 795-830.

Schwartz, G. 1'1., 1949, The geology of the Duluth metropolitan area: Hinn.Geol. Survey Bull. 33, 136 p.

—85—

page 2

Silver, L. T., and Green, J. C., 1963, Zircon ages for Middle Keweenawan rocksof the Lake Superior Region (abs.): Amer. Geophys. Union Trans., v. 44,

p. 107.

Stoiber, R. E., and Davidson, E. S., 1959, Amygdule mineral zoning in thePortage Lake lava series, Michigan copper district: Econ. Geol. v. 54,p. 1250—1277.

Walker, G. P. L., 1960, Zeolite zones and dike distribution in relation to thestructure of the basalts of eastern Iceland: Jour. Geology, v. 68, p. 515—528

— , 1964, Geological investigations in eastern Iceland: Bull. Voic., v. 27,p. 351—363.

-85-

page 2

Silver, L. T., and Green, J. C., 1963, Zircon ages for Middle Keweenawan rocksof the Lake Superior Region (abs.): Amer. Geophys. Union Trans •• v. 44,p. 107.

Stoiber, R. E.• and Davidson, E. S., 1959, Amygdule mineral zoning in thePortage Lake lava series, Michigan copper district: Econ. Geol. v. 54,p. 1250-1277.

Walker, G. P. L., 1960, Zeolite zones and dike distribution in relation to thestructure of the basalts of eastern Iceland: Jour. Geology, v. 68, p. 515-528

, 1964, Geological investigations in eastern Iceland: Bull. Vole., v. 27,---p. 351-363.

—86—

Field Trip A


Leader: John C. GreenUniversity of Minnesota, Duluth

The stops described below are intended to give a broad picture ofthe chemical, petrographic, and structural varieties of lavas of the Group,some representative exposures of the minor intrusions that cut the flows,and the general structural characteristics of these Upper Precambrian racks.The area northeast of Silver Bay, where Green has done most of his work, isstressed. The trip excludes the Duluth Complex, aspects of which have beenor are covered elsewhere (GSA 1956 Guidebook, ILSG 1968 Guidebook, and FieldTrip B of this program). Many more stops are listed below than will bepossible to examinein a one—day trip, but they are included for the benefitof those who can visit or revisit the area at a later time. The U.S.G.S7 1/2 minute quadrangle name is given for each stop. The stops which areplanned as a minimal framework for Field Trip A are designated with anasterisk after the number. Mileages are listed for distances betweeneasily identifiable points along US Highway 61 (not cumulative mileage forwhole trip), for travel either southwest (starting at the U.S.A.fCanadaborder — Pigeon River: left—hand column) or northeast at the Midway Road,(St. Louis Co. 13), Nopeming, WESt of Duluth: right—hand column). Mileagesfor side trips off Hwy 61 are in parentheses or not given. All stops areshown on Fig. 1. Descriptions between stops are written for southwestward

travel. The total distance covered is approximately 160 miles each way.

At several stops volcanic structures are well preserved. Visitors areurged to refrain from loosening, removing or otherwise destroying them,

since they constitute irreplaceable evidence for flow direction, etc.,

and are valuable teaching features for local students and future visitors.

GoingGoingSW NE

0.0 7.4 Start of Trip A — Pigeon River, Minnesota—Ontario (Pigeon Point,

Minn. — Mich. quad.). Travel SW along flat post—glacial lake bed,then rise along shoulder of a large NE—trending dike of theIceweenawan Logan Intrusions. Several road cuts in Middle PrecambrianRove Formation shales cut by small branch dikes. Excellent views

from parking rest areas to the east over Wauswaugoning Bay, PigeonPoint (thick complex Keweenawan sill), Susy Islands (Rove Fm. andLower Keweenawan flows), Isle Royale (Keweenawan flo's), Hat Pointwith Mount Josephine (Logan dike).

3.7 3.7 Stop 1. Logan dike and Roc Faauction (Grand Portage quad.) At

the top of the rise is a deep cut trirough the thick Keweenawandiabase dike that forms Mt. Josephine and flat oint. Fartherdown the highway to the southwest are good cuts in the Rove

Formation which is here dominr4lc-J by graywacke.

-86-

Field Trip A


Leader: John C. GreenUniversity of ~finnesota, Duluth

The stops described below are intended to give a broad picture ofthe chemical, petrographic, and structural varieties of lavas of the Group,some representative exposures of the minor intrusions that cut the flows~

and the general structural characteristics of these Upper Precambrian rocks.The area northeast of Silver Bay~ where Green has done most of his work, isstressed. The trip excludes the Duluth Complex, aspects of which have beenor are covered elsewhere (GSA 1956 Guidebook, ILSG 1968 Guidebook, and FieldTrip B of this program). Many more stops are listed below than will bepossible to examine in a one-day trip, but they are included for the benefitof those who can visit or revisit the area at a later time. The U.S.'G.S7 1/2 minute quadrangle name is given for each stop. The stops which areplanned as a minimal framework for Field Trip A are designated with anasterisk after the number. Mileages are listed for distances betweeneasily identifiable points along US Highway 61 (not cumulative mileage forwhole trip)~ for travel either southwest (starting at the u.S.A./Canadaborder - Pigeon River: left-hand column) or northeast at the ~fidway Road,(St. Louis Co. 13), Nopeming, west of Duluth: right-hand column). Mileagesfor side trips off Hwy 61 are in parentheses or not given. All stops areshown on Fig. 1. Descriptions between stops are written for southwestwardtravel. The total distance covered is approximately 160 miles each way.

At several stops volcanic structures are well preserved. Visitors areurged to refrain from loosening~ removing or otherwise destroying them~

since they constitute irreplaceable evidence for flow direction, etc.,and are valuable teaching features for local students and future visitors.

GoingGoingSW NE

-0:0 7.4

3.7 3.7

Start of Trip A - Pigeon River, Minnesota-Ontario (Pigeon Point,Minn. - Mich. quad.). Travel SW along flat post-glacial lake bed,then rise along shoulder of a large NE-trending dike of theKeweenawan Logan Intrusions. Several road cuts in Middle PrecambrianRove Formation shales cut by small branch dikes. Excellent viewsfrom parking rest areas to the east over Wauswaugoning Bay, PigeonPoint (thick complex Keweenawan sill)~ Susy Islands (Rove Fm. andLower Keweenawan flows), Isle Royale (Keweenawan flows), Hat Pointwith Mount Josephine (Logan dike).

Stop 1. Logan dik~ and Rove Formation (Grand Portage quad.) Atthe top of the rise is a deep cut through the thick Keweenawandiabase dike that forms Mt. Josephine and Hat Point. Fartherdown the highway to the southwest are good cuts in the RoveFormation which is here dominated by graywacke.

—87—

Going GoingSW NE

(At the base of the slope an interesting detour can bemade to the SE to Grand Portage Chippewa village and bay and

Grand Portage National Monument. The base of the UpperPrecambrian sequence forms Grand Portage Island. Continue

out the west end of the village to Highway 61).

7.4 0.0 Stop 2.* Basal Lower Keweenawan basalts, Grand Portage (Grand

0.0 14.8 Portage quad.). At the top of the next rise (at the Junctionof Cook Co. 17) are low cuts exposing basalts near or at thebase of the Keweenawan reversed polarity sequence. These basaltshave a somewhat diabasic texture and have been strongly though notcompletely retrograded to prehnite—pumpellyite facies minerals.Amygdules contain weathered agate, prehnite, and epidote.

The basal Upper Precambrian sandstone (Puckwunge) underliesthe gentle slope to the north but is not exposed at the highway.It can be seen by bushwhacking about 1/4 mile to the NNW. It is

a clean—looking,somewhat feldspathic quartz sandstone, in marked con-trast to the red, immature, volcanic interf low sandstones above. Thelowland beyond is underlain by the Rove Fm. , anC all the ridges areheld up by large Logan dikes.

3.0 11.9 Stop 3. Tholeiitic basalt and porphyry dike (Grand Portage quad.).Walk or drive off Highway 61 on an inconspicuous, unmaintained littlebranch on the lake side to an old cabin site at a small cove (1/8mile). On R (SW) is the basal, massive portion of a fine—grainedtholeiitic basalt of the Lower Keweenawan Grand Portage lavas. Onthe L is a thick, compound dike of porphyritic trachybasalt thattrends E into lake. A large swarm of similar dikes is present inthis area and is being studied. They are unusually rich ji Fe and K.

3.7 11.1 Stop 4. Red Rock rhyolite (Mineral Center quad.). At DerondaBay the beach leads out to the SE to the breccia—rubble base ofa thick (over 600 feet) porphyritic rhyolite flow which is theuppermost flow found to show magnetic reversal by Books (1968).More scenic exposure can be seen by climbing up the ridge andwalking out to the point on the open lake. Less scenic but moreaccessible exposure is in a road cut 1/2 mile beyond.

4.6 10.2 Stop 5. Reservation River Diabase and abandoned beach ridges.(Mineral Center quad.) At the top of the next rise is the easternedge of the extensive Reservation River diabase complex: onecharacteristic phase is exposed here that shows faint banding onsonic surfaces. Lcrge gravel pits n the lowland to the northeast,mined out for the new highway, were large abandoned beaches oflate postglacial Lake Superior. Excellent smaller, later ridgescan still be seen on a little track that goes to the lake shorefrom the base of the slope.

GoingSW

-87-

GoingNE

(At the base of the slope an interesting detour can bemade to the SE to Grand Portage Chippewa village and bay andGrand Portage National Monument. The base of the UpperPrecambrian sequence forms Grand Portage Island. Continueout the west end of the village to Highway 61).

7.40.0

0.014.8

Stop 2.* Basal Lower Keweenawan basalts, Grand Portage (GrandPortage quad.). At the top of the next rise (at the Junctionof Cook Co. 17) are low cuts exposing basalts near or at thebase of the Keweenawan reversed polarity sequence. These basaltshave a somewhat diabasic texture and have been strongly though notcompletely retrograded to prehnite-pumpe11yite facies minerals.Amygdu1es contain weathered agate, prehnite, and epidote.

The basal Uppe~ Precambrian sandstone (Puckwunge) underliesthe gentle slope to the north but is not exposed at the highway.It can be seen by bushwhacking about 1/4 mile to the NNW. It isa clean-looking, somewhat feldspathic quartz sandstone, in marked contrast to the red, immature, volcanic interf10w sandstones above. Thelowland beyond is underlain by the Rove Pm., and all the ridges areheld up by large Logan dikes.

3.0

3.7

4.6

11.9 Stop 3. Tholeiitic basalt and porphyry dike (Grand Portage quad.).Walk or drive off Highway 61 on an inconspicuous,unrnaintained littlebranch on the lake side to an old cabin site at a small cove (1/8mile). On R (SW) is the basal, massive portion of a fine-grainedtholeiitic basalt of the Lower Keweenawan Grand Portage lavas. Onthe L is a thick, compound dike of porphyritic trachybasalt thattrends E into lake. A large swarm of similar dikes is present inthis area and is being studied. They are unusually rich in Fe and K.

11.1 Stop 4. Red Rock rhyolite (Mineral Center quad.). At DerondaBay the beach leads out to the SE to the breccia-rubble base ofa thick (over 600 feet) porphyritic rhyolite flow which is theuppermost flow found to show magnetic reversal by Books (196 8).More scenic exposure can be seen by climbing up the ridge andwalking out to the point on the open lake. Less scenic but moreaccessible exposure is in a road cut 1/2 mile beyond.

10.2 Stop 5. Reservation River Diabase and abandoned beach ridges.(Mineral Center quad.) At the top of the next rise is the easternedge of the extensive Reservation River diabase complex: onecharacteristic phase is exposed here that shows faint banding onsome surfaces. Large gravel pits in the lowland to.the northeast,mined out for the new highway, were large abandoned beaches oflate postglacial Lake Superior. Excellent smaller, later ridgescan still be seen on a little track that goes to the lake shorefrom the base of the slope.

—88—

Going GoingSW NE

9.0 5.8 Cross Reservation River and out of Grand Portage IndianReservation. Slope is at contact of Reservation River diabasecomplex and lavas (a rhyolite here).

10.0 4.8 Stop 6.* Hovland porphyry lavas. (Hoviand quad.) Opposite aon the lake side are exposures of a remarkable porphyritic

trachyandesite lava flow with platy plagioclase phenocrysts upto 10 cm across. By following the low scarp to the NE behind ahouse (private property) two flows can be ueen, the top of thelower one being vesicular and showing a slightly uneven crust.These are near the base of the Middle Keweenawan (normal polarity)lava sequence, and are assigned to the Hovland lavas. They arehere cut by a large dike, at least 2 miles long, of browntrachyandesite with small. elinopyroxene phenocrysts, that alsocrops out across the road.

Highway descends to old lake—bed flat; then, past Big Bay,rises onto higher ground of Hovland Diabase complex. Cross Flute

il.8 0 r Reed River, pass through village of Hovland at Chicago Bay, and up

00 onto a large sill—like body of syenogabbro. Many road cuts; someshow good foliation of plagioclases, dipping gently S.

Stop 7. Syenogabbro, basalt pillow—breccia, and rhyolite at BruleRiver (Marr Island quad.). Opposite Naniboujou Lodge, park in lot

0.0 of Judge Hagney State Park. Walk up trail, cross rivet on footbridge,10.6 and follow fisherman's trail and bushwhack up W bank. Take care —

steep and unstable slope in places. At footbridge is medium—grained,foliated syenogabbro of "Hovland diabase complex." This is cut bya later basalt dike, then gives way to a rather coarse—grained basaltlava with chlorite scraps and amygdules. Soon this is overlain bybasaltic—scoriaceous tuff—breccia. After a short gap in exposure,steep bank resumes which is made of basalt pillow—breccia, withaltered volcaniclastic matrix. This is one of the few places whereevidence for underwater extrusion can be seen in the North ShoreVolcanic Group (others are on Grand Portage Island and W of Duluthin the Lower Keweenawan). Farther upstream this can be seen to overliethe altered, flow—banded and swirled top of a very large porphyricicrhyolite flow through which the Bride has cut a deep gorge above.Return by fisherman's trail at top of bank.

After crossing the Brule (Arrowhead) River, the highway risesover and cuts through three hasalts of the Brule River group, thencrosses a porphyritic trachyandesite or intermediate quartz latiteof the Marr Island lavas at Paradise Beach. A few more low cuts ofthis mixed group are passed in next 3 miles.

3.8 5.8 Stop 3•x Porphyritic intermediate cuartz latite p Marr Islandlavas one mile past Cook Co. 14 (Kadunce Creek quad.) Two largeroad cuts on N. side of a thick, intermediate quartz latitelava (62% SiO , 4.1% K 0) with plagioclase, ferroaugite, and rareex—olivine an magnetie phenocrysts. Contacts not exposed.

Going GoingSW NE

9.0 5.8

-88-

Cross Reservation River and out of Grand Portage IndianReservation. Slope is at contact of Reservation River diabasecomplex and lavas (a rhyolite here).

10.0 4.8 Stop 6.* Hovland porphyry lavas. (Hovland quad.) Opposite ahouse on the lake side are exposures of a remarkable porphyritictrachyandesite lava flow with platy plagioclase phenocrysts upto 10 em across. By following the low scarp to the NE behind ahouse (private property) two flows can be ~een, the top of thelower one being vesicular and showing a slightly uneven crust.These are near the base of the Middle Keweenawan (normal polarity)lava sequence, and are assigned to the Hovland lavas. They arehere cut by a large dike, at least 2 miles long, of browntrachyandesite with small.elinopyroxene phenocrysts, that alsocrops out across the road.

14.8---0.0

Highway descends to old lake-bed flat;rises onto higher ground of Hovland DiabaseReed River, pass through village of Hovlandonto a large sill-like body of syenogabbro.show good foliation of plagioclases, dipping

then, past Big Bay,complex. Cross Fluteat Chicago Bay, and upMany road cuts; somegently S.

4.0----".0.0

3.8

0.010.6

Stop 7. Syenogabbro, basalt pillow-breccia, and rhyolite at BruleRiver (Marr Island quad.). Opposite Naniboujou Lodge, park in lotof Judge tmgney State Park. Walk up trail, cross river on footbridge,and follow fisherman's trail and bushwhack up W bank. Take care steep and unstable slope in places. At footbridge is medium-grained,foliated syenogabbro of "Hovland diabase complex." This is cut bya later basalt dike, then gives way to a rather coarse-grained basaltlava with chlorite scraps and mnygdules. Soon this is overlain bybasaltic-scoriaceous tuff-breccia. After a short gap in exposure,steep bank resumes which is made of basalt pillow-breccia, withaltered volcaniclastic matrix. This is one of the few places whereevidence for underwater extrusion can be seen in the North ShoreVolcanic Group (others are on Grand Portage Island and W of Duluthin the Lmyer Keweenawan). Farther upstream this can be seen to overliethe altered, flow-banded and styi rled top of a very large porphyriticrhyolite flow through which the Brule has cut a deep gorge above.Return by fisherman's tr~il at top of bank.

After crossing the Brule (Arrowhead) River, the highway risesover and cuts through three basalts of the Brule River group, thencrosses a porphyritic trachyandesite or intermediate quartz latiteof the }larr Island lavas at Paradise Beach. A few more low cuts ofthis mixed group are passed in next 3 miles.

Stop 8.~ Porphyritic intermediate quartz latite of ~arr Islan~

lavas one mile past Cook Co. 14 (Kadunce Creek quad.) Two largeroad cuts on N. side of a thick, intermediate quartz latitelava (62% Si02 , 4.1% K20) with plagioclase, ferroaugite, and rareex-olivine ana magnetite phenocrysts. Contacts not exposed.

—89--

Going GoingSW NE

1 1/2 miles to the west at Kadunce Creek (Kodonce River)State Park a thick porphyritic felsite is exposed, especiallyin a deep and narrow canyon that begins about 1/8 mile upstreamfrom highway — this is part of the Kimball Creek felsite group,also exposed in Kimball and Cliff creeks farther west. Excellentabandoned wave—cut cliffs of Nipissing stage.

7.3 3.2 Stop 9. Olivine basalts of Red Cliff series (Kadunce Creek quad.).Just past large gravel pit and creek gully, highway rises onto aseries of 5 or 6 ophitic olivine basalt flows totalling between400 and 900 feet in thickness. Amygdules contain saponite,laumontite,calcite, quartz, and agate. Plagioclase phenocrysts have floated totop in some, sunk to bottom in others. 47% Si02 0.5% K20.

Past Durfee Creek (near top of Red Cliff basalts) highway (andNipissing cliff) pass onto Devil Track felsite group, here composedof two thick flows.

9.6 1.0 Stop 10.* Felsite of Devil Track sEries, at promontory of abandonedNipissing wave—cut cliff. N side of highway, 0.85 miles west ofDurfee Creek (Kadunce Creek quad.). Pink, non— or weakly porphyriticrhyolite or quartz latite (72% SiO , 5.5% K20) with slabby jointingand faint flow—banding. Bushwhack?ng along old cliff to E for 1/4mile one eventually passes down into vesicular, locally spheruliticand flow banded top of a porphyritic flow of similar composition.These two flows total about 1020 feet in thickness. Five Mile(Guano) Rock, a mile out in Lake Superior, is made of diabase.

10.6 0.0 Cross Devil Track River. This cuts a deep gorge just upstream

0.0 3.7 in the upper felsite flow. Continue on Hwy 61 into Grand Marais, oralternatively turn off just before rise on small side road (Cook Co.87) toward lake to Croftville settlement and optional Stop 11.

(off Hwy 61)(1.3) (0.45) Stop 11. Spherulitic hasal phase, Grand Marais rhyolite, Croftville

(0.0) (Grand Marais quad.) Drive about 1.1 mile along Nipissing terraceover Croftville basalts (about 0.45 mi. from W end of this road).

Private Property. Ask permission at the house in birches on lake

side, examine outcrops at modem beach—back. Slabby, spherulitic

red rhyolite is exposed here (73.4% SiO , 4.65% K20) that contains

andesine—oligoclase and rare hedenbergie, ex—fayalite and magnetitephenocrysts. Nearby is cross—bedded, calcite—cemented interflow sand.

(0.45) (0.0) Strata have been steeply tilted by diabase intrusions. Continue on

(2.1) Croftville road until it re—joins Hwy 61.

Continue SW to Grand Marais.

(off Hwy 61) Stop 12. Breakwater trachybasalt (Good Harbor Bay quad.). Drive

to Coast Guard Station at E end of Grand Marais harbor. Tombolo

here is made by gravel bar connecting mainland to resistant islandand ledges of a massive, locally columnar—jointed, porphyritictrachybasalt or basalt with small phenocrysts of plagioclase, augite,and rare olivine. It is about 360 feet thick. It is not knownfor certain whether this is a big flow or sill; to the west is hasa sharp, chilled basal contact against felsite but its topcontact is covered. It becomes amydgaloidal and zeolitized near

(off Hwy 61)_~~ (0.45)

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1 1/2 miles to the west at Kadunce Creek (Kodonce River)State Park a thick porphyritic felsite is exposed, especial~y

in a dl~ep and narrow canyon that begins about 1/8 mile upstreamfrom highway - this is part of the Kimball Creek felsite group,also exposed in Kimball and Cliff creeks farther west. Excellentabandoned wave-cut cliffs of Nipissing stage.

Stop 9. Olivine basalts of Red Cliff series (Kadunce Creek quad.).Just past large gravel pit and creek gully, highway rises onto aseries of 5 or 6 ophitic olivine basalt flows totalling between400 and 900 feet in thickness. Amygdules contain saponite,laumontite,calcite, quartz, and agate. Plagioclase phenocrysts have floated totop in some, sunk to bottom in others. 47% Si0

2, 0.5% K

20.

Past Durfee Creek (near top of Red Cliff basalts) highway (andNipissing cliff) pass onto Devil Track felsite group, here composedof two thick flows.

Stop 10.* Felsite of Devil Track series, at promontory of abandonedNipissing wave-cut cliff. N side of highway, 0.85 miles west ofDurfee Creek (Kadunce Creek quad.). Pink, non- or weakly porphyriticrhyolite or quartz latite (72% Si02 , 5.5% K20) with slabby jointingand faint flow-banding. Bushwhacking along old cliff to E for 1/4mile one eventually passes down into vesicular, locally spheruliticand flow banded top of a porphyritic flow of similar composition.These two flows total about 1020 feet in thickness. Five Mile~uano) Rock, a mile out in Lake Superior, is made of diabase.

Cross Devil Track~. This cuts a deep gorge just upstreamin the upper felsite flow. Continue on Hwy 61 into Grand Marais, oralternatively turn off just before rise on small side road (Cook Co.87) toward lake to Croftville settlement and optional Stop 11.

Stop 11. Spherulitic hasal phase, Grand Marais rhyolite, Croftville(Grand Marais quad.) Drive about 1.1 mile along Nipissing terraceover Croftville basalts (about 0.45 mi. from Wend of this road).Private ?roperty. Ask permission at the hous~ in birches on lakeside, exwline outcrops at modern beach-back. Slabby, spheruliticred rhyolite is exposed here (73.4% SiO , 4.65% K20) that containsandesine-oligoclase and rare hedenbergite, ex-fayalite and magnetitephenocrysts. Nearby is cross-bedded, calcite-cemented interflow sand.Strata have been steeply tilted by diabase intrusions. Continue onCroftville road until it re-joins Hwy 61.

Continue SW to Grand Marais.

(off H'YY 61) Stop 12. Breakwater trachybasalt (Good Harbor Bay quad.). Driveto Coast Guard Station at E end of Grand Marais harbor. Tombolohere is made by gravel bar connecting mainland to resistant islandand ledges of a massive, locally columnar-jointed, porphyritictrachybasalt or basalt with small phenocrysts of plagioclase, augite,and rare olivine. It is about 360 feet thick. It is not knownfor certain whether this is a big flow or sill; to the west is hasa sharp, chilled basal contact against felsite but its topcontact is covered. It becomes amydgaloidal and zeolitized near

Going Going —90—

SW NE

its top and is assumed to be a flow. As can be seen from thisvantage point, it forms one of the major strike—ridges of the"Sawtooth Range" to the west (as does the big "thomsonite flow"starting at Good Harbor Bay). The harbor at Grand Marais is

probably eroded from rhyolite.

3.7 0.0 Highway 61 passes corner of harbor, near start of Gunf lint Trail0.0 9.2 (Cook Co. 12).

Then highway rises to W past a good norphyritic rhyolite cutonto higher level (Breakwater trachybasalt cuesta ahead), thenback down across the Breakwater trachybasalt and across Fall River(Rosebush Creek). About 1 mile past this creek, low road cuts startin two thick porphyritic trachyandesite to andesite flows (55% Si02,2.7% K 0) with small plagioclase and clinopyroxene phenocrysts andvesicuar—rubble tops. Big cuesta ahead is held up by Terrace Pointthomsonite—bearing basalt flow. Continue across Cut face (GoodHarbor) Creek.

5.2 3.9 Stop 13.* Thomsonite—bearing basalt, interf low sediments (GoodHarbor Bay quad.). In this large road cut one of the major cliffformers of the "Sawtooth Range" overlies a thick (130') section ofinterf low sediments. The Terrace Point basalt is dominantly amassive, fine—grained, ophitic basalt that characteristically containsthomsonite in amygdules, hut in its lengthy exposure (including inthis cut) several flow units of varying character show complexrelations with the major, massive part of the flow.

The interfiow sediment Is here mainly thin—bedded siltstoneand silty shale, but by walking up the bed of Cutface Creek atthe bottom of this hill one passes outcrops of the basal contactof the sediments resting on the amygdaloidal—scoriaceous top ofan andesite flow and eventually reaches large banks cut into well—bedded sandstone showing abundant ripple marks.

Highway 61 passes low cuts of the complex upper parts of theTerrace Point flow complex, with big cuesta on this flow visibleahead, then at jct. of Cook Co. 7 passes into coarse—grained olivine

9.2 — 0.0 basalt of the Lutsen basalt series. Cross Cascade River in State0.0 16.9 Park. Trail up W side provides access to good river outcrops of

Terrace Point thomsonite basalt, the underlying sandstones, andseveral andesite flows of the Good Harbor Bay series. Drive pastCascade Lodge.

0 5 16 4Stop 14* Cascade olivine basalt of Lutsen basalt series (DeerYard Lake quad.). 0.5 miles W. of Cascade River. Shore outcropsof a thick (100 feet or more), relatively coarse—grained olivinebasalt of a distinctive group in this Cascade—Lutsen area.Segregation cylinders up to 6" or so in diameter and segregationlenses or sills can be seen within this flow.

Highway continues on long straight stretch roughly parallelto strike of Lutsen basalts, topmost series of the North ShoreVolcanic Group. Pass through Lutsen village (big ridges toW held up by Leveaux trachybasalt sill), to Poplar River.

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its top and is assumed to be a flow. As can be seen from thisvantage point, it forms one of the major strike-ridges of the"Sawtooth Range" to the west (as does the big "thomsonite flm.,"sta~ting at Good Harbor Bay). The harbor at Grand Harais isprobably eroded from rhyolite.

Highway 61 passes corner of harbor, near start of Gunflint Trail(Cook Co. 12).

Then highway rises to W past a good porphyritic rhyolite cutonto higher level (Breakwater trachybasa1t cuesta ahead), thenback down across the Breakwater trachybasa1t and across Fall River(Rosebush Creek). About 1 mile past this creek, low road cuts startin two thick porphyritic trachyandesite to andesite flows (55% 5i02 ,2.7% K

20) with small plagioclase and clinopyroxene phenocrysts and

vesicular-rubble tops. Big cuesta ahead is held up by Terrace Pointthomsonite-bearing basalt flow. Continue across Cut face (GoodHarbor) Creek.

Stop 13.* Thomsonite-bearing basalt, interflow sediments (GoodHarbor Bay quad.). In this large road cut one of the major cliffformers of the "Sawtooth Range" overlies a thick (130') section ofinterflow sediments. The Terrace Point basalt is dominantly amassive, fine-grained, ophitic basalt that characteristically containsthomsonite in amygdu1es, but in its lengthy exposure (including inthis cut) several flow units of varying character show complexrelations with the major, massive part of the flow.

The interflow sediment is here mainly thin-bedded siltstoneand silty shale, but by walking up the bed of Cut face Creek atthe bottom of this hill one passes outcrops of the basal contactof the sediments resting on the amygdaloidal-scoriaceous top ofan andesite flow and eventually reaches large banks cut into wellbedded sandstone showing abundant ripple marks.

Highway 61 passes low cuts of the complex upper parts of theTerrace Point flow complex, with big cuesta on this flow visibleahead, then at jct. of Cook Co. 7 passes into coarse-grained olivinebasalt of the Lutsen basalt series. Cross Cascade River in StatePark. Trail up W side provides access to good river outcrops ofTerrace Point thomsonite basalt, the underlying sandstones, andseveral andesite flows of the Good Harbor Bay series. Drive pastCascade Lodge.

Stop 14* Cascade olivine basalt of Lutsen basalt series (DeerYard Lake quad.). 0.5 miles W. of Cascade River. Shore outcropsof a thick (100 feet or more), relatively coarse-grained olivinebasalt of a distinctive group in this Cascade-Lutsen area.Segregation cylinders up to 6" or so in diameter and segregationlenses or sills can be seen within this flow.

Highway continues on long straight stretch roughly parallelto strike of Lutsen basalts, topmost series of the North ShoreVolcanic Group. Pass through Lutsen village (big rid?,es toW held up by Leveaux trachybasa1t sill), to Poplar Rive~,

—91—Going GoingSN NE

9.3 7.1 Stop 15. Poflyritic olivine basalts of Lutsen series (Lutsenquad.), Poplar River. Watch your step - dangerous. Private land.Several flows of a distinctive ophitic olivine basalt characterizedby abundant small (1—3 nun) blocky bytownite phenocrysts are exposedfrcrn the mouth of the Poplar River at Lutsen Resort, up through animpassable canyon to the highway bridge and upstream to left bend(1/10 nile from highway). Take fisherman's trail on W side. This?low type has the most "primitive' composition (lowest K, highest Mg)of the North Shore lavas (1+7% Si02, 0.l2I K20). At L bend in riverit overlies breccia—rubble top of a basaltic andesite flow; red sandhas been washed into all the interstices between the lava blocks,a typical situation.

12.3 4.6 Stop 16. Leveaux — Onion Mtn. trachlbasalt porphyry !411. (Toftequad.). 2.5 ml. SW of Poplar R.. at slight L bend cross RollinsCreek and irnediately turn up U. S. Forest Service gravel road

(No. 336). Continue on it as it contours back to SW, then cuts upinto Onion Mver gap in big ridge held up by a big trachybasaltporphyry sill. This sill forms Leveaux Mtn., the high ridge witha fire tower on it to the SW, and comes out to the lake shore toform the islands at Taccnite Harbor SN of Tofte. Its contacts arenot exposed but it crosscuts the lavas. Park as near as possibleto the S corner of the hill on the NE side of the gap, and bushwKacka short distance N to the steeper, rocky rise. Here the lower partof the sill is exposed — a fine—grained, pigeonite—augite trachy—basalt to trachyandesite. Farther up the Glope a bit abundant large(1 cm) blocky labradorite phenocrysts appeartruptly; they appear tohave floated. This porphyritic phase forms the upper part of thesill throughout its extent.

2.... 0.0 Continue SW past Sawbill Trail (Cook Co. 2) at village of Tofte.0.0 12.1 Town park on lake shore 0.3 mi. SW of Edgewater Motel has good

exposures of thin—bedded ophitic olivine basalts. DO NOT DESTROYFEATI.ffiES. These lie at the top of the section of the orth ShoreVolcanic Group.

1.7 10.1+ Stop 17. Carlton Peak anorthosite in diabase (Tofte quad.) 1.7 mi.SW of Sawbill Trail jct., opposite large Superior National ForestSign, turn up road of Erie Mining Co. to large quarry in side ofCarlton Peak, which is held up by massive anorthosite xenoliths ina gabbroic intrusion. Private Property. The complex rela.tionsbetween the anorthosite and various phases of olivine gabbro are wellexposed.

2.5 9.6 Stop 18. Thin—bedded ophitic olivine tholeiites of Schroederbasalts, Temperance River (Tofte quad.). Park at State Park lots,walk down trail to bridge near river mouth. Note excellent erosionalpotholes. Several thin flows or flow units, with ropy surfaces, pipeanygdules and lensing shape are well exposed here. If more time isavailable, walk up NE side of river above highway to the main gorgewhere the river has cut a very deep and narrow slot with largerpotholes into thicker olivine basalts. Warning people have beenkilled trying to jump across. Large joint columns visible on trailand overlooks, and vesicle cylinders are present in thicker flows.

GoingSW

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Stop 15. Porphyritic olivine basalts of Lutsen series (Lutsenquad.), Poplar River. Hatch your step - dangerous. Private land.Several flows of a distinctive ophitic olivine basalt characterizedby abundant small (1-3 mm) blocky bytownite phenocrysts are exposedfrom the mouth of the Poplar River at Lutsen Resort, up through animpassable canyon to the highway bridge and upstream to left bend(1/10 mile from highway). Take fisherman's trail on W side. Thisflow type has the most Ilprimitive" composition (lowest K, highest Mg)of the North Shore lavas (47% Si02 , 0.12% K20). At L bend in riverit overlies breccia-rubble top of a basaltic andesite flow; red sandhas been washed into all the interstices between the lava blocks,a typical situation.

Stop 16. Leveaux_ - Onion HtIl=- trachybasalt porphyry Eill (Toftequad. ) . 2.5 mi. SH of Poplar R•..at slight L bend cross RollinsCreek and immediately turn up U. S. Forest Service gravel road(No. 336). Continue on it as it contours back to SW, then cuts upinto Onion River gap in big ridge held up by a big trachybasaltporphyry sill. This sill forms Leveaux Mtn., the high ridge witha fire tower on it to the SW, and comes out to the lake shore toform the islands at Taconite Harbor SH .of Tofte. Its contacts arenot exposed but it crosscuts the lavas. Park as near as possibleto the S corner of the hill on the NE side of the gap, and bush~acka short distance n to· the steeper, rocky rise. Here the lower partof the sill is exposed - a fine-grained, pigeonite-augite trachybasalt to trachyandesite. Farther up the slope a bit abundant large(1 cm) blocky labradorite phenocrysts appearroruptly; they appear tohave floated. This porphyritic phase forms the upper part of thesill throughout its extent.

Continue svr past Sawbill Trail (Cook Co. 2) at village of Tofte.Town park on lake shore 0.3 mi. SW of Edgewater Motel has goodexposures of thin-bedded ophitic olivine basalts. DO NOT DESTROYFEATURES. These lie at the top of the section of the North ShoreVolcanic Group.

Stop 17. Carlton Peak ?lloythosite in diabase (Tofte quad.) 1.7 mi.SW of Sawbill Trail jet., opposite large Superior National ForestSign, turn up road of Erie Mining Co. to large quarry in side ofCarlton Peak, which is held up by massive anorthosite xenoliths ina gabbroic intrusion. Private Property. The complex relationsbetween the anorthosite and various phases of olivine gabbro are wellexposed.

Stop 18. Thin-bedded ophitic olivine tholeiites of Schroederbasalts, Temperance River (Tofte quad.). Park at State Park lots,walk down trail to bridge near river mouth. Note excellent erosionalpotholes. Several thin flows or flow units, with ropy surfaces, pipeamygdules and lensing shape are well exposed here. If more time isavailable, walk up NE side of river above highway to the main gorgewhere the river has cut a very deep and narrow slot with largerpotholes into thicker olivine basalts. Warning: people have beenkilled trying to jump across. Large joint colUmns visible on trailand overlooks, and vesicle cylinders are present in thicker flows.

—92—

Going GoingSW NE

3.8 8.3 Drive SW through Schroeder and Taconite Harbor (powerplant and taconite pellet shipping facility of Erie fining Co.)

with occasional low cuts of ariygdaloidal or ophitic olivine tholeiite.

9.14 2.7 Stop 19*. Thin—bedded ophitic olivine tholeiite of Schroeder basaltsat Sugar Loaf Point (Little Marais quad.). Private property. Drive

down side road (just opposite gravel road from uphill) at ConsolidatedPaper Co. pupwood storage and handling facility. Drive down to cove,end ask permission to walk opoint at office. Notice great oldpine boom—logs used to raft pulpwood cross lake. Walk aroundSugarloaf Point clockwise from end of beach. Excellent exarrioles

of thin flow units (6" and up) and thicker flows of ophitic "olivThetholeiites," with ropy surfaces, bent pipe amyvdules, and clasticdikes where sand was washed into open ;oints in the tops or flows.Please do not remove or destroy these structures! On the sides ofthe high knob at the end can be seen vertical tube—like concentrationsof arnydules ("vesicle cylinders") in the massive lower tart of the

topmost flow.

11.7 0.14 Cross Cook/Lake County line.

12.1 0.0 Stop 20. Ophitic olivine tholeiites, Ma! tracybasalt, and

0.0 11.0 strike—fault, Caribou River (Little arais quad.) Park at CaribouFalls State Park 1°t on the side of the highway and walk ut thetrail. (Between the highway and Lake is private property includingfalls over typical ophitic olivine basalts). In the river and aiocgthe trail are a few outcrops of the red volcanic breccia and basaltthat underly the "Manitou trachybasalt," then at a L bend is the NE—most outcrop of the trachybasalt. As the trail anproaches the fallsthe river has cut through a thick section of volcanic breccia, nuta large strike—fault intervenes between this and the typical ophiticolivine thoeiites (flio2=5-.7%, :20=o.3_o.5z) that Torn tt:e cli'over which the river fells. Several flows, 10—30' each, are visiblein the cliff each with a massive lower part and an amygdaloidal andslabbv-jointed top. These are tynical of nearly all the lavas between1:ittle arais and Lutsen, the uppermost sequence of tbe North ChoreVolcanic Group.

2.2 8.8 Drive SW, cross anitou River, '.rhich has cut a deep aorge throu'hdrift and olivine basalts (trail up streaxn in State Land accessibleon SW side via gravel pit road), Cornniercialized gorge downstrean hasfalls, sea arches.

Continue LW almost to I? bend at Little Marais .

14.5 6.5 Stop 21. Manitou trachsasait (Little i'larais quad.). Turn do'.inroad at Ben Fenstad's Resort, Private property; ask permissionfor entry at office. Continue down to lakeshore, bearing left toend of driveway loop at fishhouse. here at the end of the slit isexposed the base of a 1arre flow of granular trachvbasalt, at least300' thick and 5 rLles long, that forns !'uch of t1roreledges Croathis breakwater on, both SW and HE of Menitou FLyer. It containsabout 52% Si02 and 2.3 K20, and has labradorite. augite, andaltered rare olivine phenocrysts and abundant K feldspar andpigeonite in the groundviass. higher in the flow it is coarser—

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Drive SI'! through Schroeder and Taconite Harbor (powerplant and taconite pellet shipping facility of Erie Mining Co.)with occasional low cuts of amygdaloidal or ophitic olivine tholeiite.

Stop 19*. Thin-bedded ophitic olivine tholeiite of Schroeder basaltsat Sugar Loaf Point (Little Marais quad.). Private property. Drivedown side road (just opposite gravel road from uphill) at ConsolidatedPaper Co. pulpwood storage and handling facility. Drive down to cove,and ask permission to walk onpoint at office. Notice great oldpine boom-logs used to raft pulpwood across lake. Halk aroundSugarloaf Point clockwise from end of beach. Excellent examplesof thin flmv units (6" and up) and thicker flo1>ls of ophitic "olivinetholeiites ,It with ropy surfaces, bent pipe amye:dules, and clasticdikes where sand was washed into open joints in the tops of flows.Please do not remove or destroy thes~_Et~.~ures! On the sides ofthe high knob at the end can be seen vertical tube-like concentrationsof amygdules ("vesicle cylinders") in the Massive Imler nart of thetopmost flo",.

Cross Cook/Lake County line.

Stop 20. Qphiti c_ olivine !-holeii:~s, ~If~it0l.! !-S.?-chY.Easalt., andstrike-fault, Caribou River (Little ~larais quad.) Park at CaribouFalls State Park lot on the HH side of the hip-;h,.,ray and "Talk up thetrail. (Between the highway and Lake is private property includingfalls over typical ophitic olivine basalts). In the river and alonr;the trail are a few outcrops of the red volcanic breccia and basaltthat underly the "lvlanitou trachybasalt," then at a L bend is the NEmost outcrop of the trachybasalt. lis the trail ap]Jroaches the fallsthe river has cut through a thick section of volcanic breccia, buta large strike-fault intervenes between this and the typical ophiticolivine tholeiites (Si02==45-47%, K20"'0. 3-0.5%) that form the cli ffover which the river fa.lls. Several flovs, 10-30' each, are visiblein the cliff each with a massive lower part and an amygdaloidal andslabby-jointed top. These are typical of nearly all the lavas betweenLittle Marais and Lutsen, the uppermost sequence of the North ShoreVolcanic Group.

Dri ve 8\1, cross ~ftanitou River, lIhi ch has cut a deep f!;orge throup;hdrift and olivine basalts (trail up stream. in State Land accessibleon S1:T side via gravel pit road). Commercialized gonre dmmstrear'l hasfalls, sea arches.

Continue SH almost to H bend at Little I·,farais R.

4.5 6.5 Stop 21. !5~j.~.sm !-!_a~J1_x.:!?~a1.!-. (Little Marais quad.). 'I'urn downroad at Ben Fenstad's Resort. Private nrqperty; ask permissionfor entry at office. Continue down to lakeshore, bearinp. left toend of drivevay loop at fishhouse. here B.t the end of the slin isexposed the base of n larp,e flow of granular trachybasalt, at least300' thick and 5 miles lonp" that forms much of th~SDreledges fro~

this breaki-mter on, both G\l and tIE of M8ni tou Pi vel". It containsabout 52;~ Si02 and 2.3% K2(), and has labradorite, aup.;i te, andaltered rare olivine phenocrysts and ahundant K feldspar andpiGeonite in the groundmass. hip:,her in the fIm, it is coarser-

—93—

Going GoingSW SE

grained, less potassic and less porphyritic. Its top is notexoosed. It overlies a sequence of interLedded basalts and redvolcanic breccia or conglorierate.

Continue SW past Little 'arais. high ridge inland is held up bya thick intrusive diabase sill containing some anorthosite blocks.

8.0 2.6 Stop 22.* .iartz tholejite—trachybasalt, Kennedy Landing(Finland quad.) Good cuts for about 1/3 mile through severaltypical auartz tholeiites to trachybasalts. They show the char-acteristic fine—grained, anhyric texture, oxidation lamellae, andscoria—rubble tops. They contain interstitial alkali feldsparand about 5l—53 5102 and up to 2% K20. Red sand, locally cross—bedded, fills the interstices between fragments. Large knoboverlooking the bay is made of the thickest of these flows.

9.1 7.8 Stop 23. * Anorthosite in syenoabbro; overturned lavas(Illgen City quad.). After crossing Kennedy Creek come to deepvertical cut in an irregular discordant intrusion (part of BeaverBay complex) of altered syenogabbro that contains a great, massiveblock of rather pure anorthosite. After examining this, walk ordrive SW downhill to lower cut that is composed of a sequence ofseveral basaltic lavas with an interflow sediment bed — all of whichhave been overturned, probably as a result of forceful intrusionof nearby diabases. These basalts show a variety of structures,including some lobes that look like pillows and at the NE end somered scoria—rubble that is characteristic of the top part of thefine—grained tholeiites, basaltic andesites, and trachybasalts.

42 0.0 Continue SW to lilgen City, Lc• with Minc. Rte.ito Ely.0.0 It.9 Just to NE are cuts in quartz tholeiite flows, to SW cuts in

altered rhvolite.

0.45 4.5 Stop 24.* Palisade Head pohyritic rhyolite, Shovel Point(Ingert City quad.3. 0.b5 mile SW of Illgen City jct., or 0.145 mileNE of Baptism River, search for unmafled trail leading to shoreat Shovel Point in Baptism 9. State Park. Here is well exposedthe upper—middle part of the thick (>300'? porphyritic rhyolite(quartz latite) that also forms Palisade Head to the SW and theroad cuts at Tllgen City. Take care on clifftop. View to SWover underlying lavas ("Baptism basalts") and some mafic intrusivebodies, to Palisade Head. Follow trail down dipslope to end and

corner: view TIE toward overlying quartz tholeiite flows. Returnon sa1Ie trail

Drive SW across Baptism River to Palisade head (big hill withradio beacon).

2.1 2.7 Stop 25. Palisade Head r1yaUiq. (Iliren (Tity quad.), Drive upnarrow, winding road (just past Palisade Creek) to top of cliffnext to raOio beacon. careful — joint columns are se-naratingfrom rest of hill. Excellent views to .N, NE, SE, SW. Ridges to

GoingSW

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GoingHE

grained, less potassic and less porph~ritic. Its top is noteXDosed. It overlies a sequence of interbedded basalts and redvolcanic breccia or conglomerate.

Continue S\J past Little ~1arais. High ridge inland is held up bya thick intrusive diabase sill containin~ some anorthosite blocks.

8.0

9.1

2.6

7.8

Stop 22. * QJ.artz tholeii te-trachybasalt, Kennedy Landing(Finland quad.) Good cuts for about 1/3 mile throu~h severaltypical quartz tholeiites to trachybasalts. They show the characteristic fine-grained, aphyric texture, oxidation lamellae, andscoria-rubble tops. They contain interstitial alkali feldsparand about 51-53% Si02 and up to 2% K20. Red sand, locally crossbedded, fills the interstices between fragments. Large knoboverlooking the bay is made of the thickest of these flows.

Stop 23. * An_,?rth,?sitE:. in ~y~nogllR..br~; ove!:turned l~vas

(Illgen City quad.). After crossing Kennedy Creek come to deepvertical cut in an irregular discordant intrusion (part of BeaverDay complex) of altered syenogabbro that contains a great, massiveblock of rather pure anorthosite. After examining this, walk ordrive SW downhill to lower cut that is composed of a sequence ofseveral basaltic lavas with an interflow sediment bed - all of whichhave been overturned, probably as a result of forceful intrusionof nearby diabases. These basalts show a variety of structures,including some lobes that look like pillows and at the NE end somered scoria,-rubble that is characteristic of the top part of thefine-~rained tholeiites, basaltic andesites, and trachybasalts.

11.0_._----~

0.00.0 Continue SW to Ulgen City, ~• •Tith Minn. Rte. -.l to Ely.~ .Just to NE are cuts in quartz tholeiite flows, to SW cuts in

altered rhyolite.

0.45 4.5

2.1

Stop 21~. * ~~~isad~, .!1~_a~ P.s>E1?-~)'Ti tic:: rhyolite" Shovel Point(Illgen City quad.). 0.45 mile SW of Illgen City jet., or 0.45 mileNE of Baptism River, search for unmarked trail leading to shoreat Shovel Point in Baptism R. State Park. Here is well exposedthe upper-middle part of the thick (>30d: porphyritic rhyolite(quartz latite) that also forms Palisade Head to the SW and theroad cuts at III~en City. Take care on clifftop. View to SWover underlying lavas ( "Baptism basalts ") and some mafic intrusivebodies, to Palisade Head. Follow trail down dipslope to end andNE corner: view NE toward overlying quartz tholeiite flows. Returnon same trail

Drive 811 across Baptism !liver to Palisade Head (big hill withradio beacon).

Stop 25. Palisade Head rhvolite (Illgen City quad.). Drive upnarrow, winding road--C,ju;t'" past--Palisade Creek) to top of cliffnext to radio beacon. ,9areful - joint colwnns are separatingfrom rest of hill. ~xcellent views to ~, NE, SE, SW. Ridges to

—94—

GoinI GoingSW NE

N. NE aie held up by intrusions of Beaver Bay complex. Uplakeshore to NE is columnar—jointed Shovel Point, made of samerhyolite flow, and far beyond is smoke from Taconite Harborpower plant and Canton Peak anorthosite knob at Tofte. To Sacross lake are Apostle Islands and Bayfield Peninsula, Wisc.To SW is Reserve Mining Co. taconite plant at Silver Bay and humpytopography of Beaver Bay complex. Palisades made of thick, por—phyritic rhyolite with quartz and feldspar phenocrysts, flow—banded at the base. Return to highway.

1.9 0.0 Drive 2.75 ml. to traffic light at Silver Bay, site of0.0 i1.8 Reserve Mining Co.'s taconite plant where about 100,000 tons of

rock from the eastern Mesabi Range are processed per day.

0.14 l.3 Stop 26. Anorthosite in diabase of Beaver Bay complex, SilverBay (Silver Bay 15' quad.). Stop at road cut directly oppositethe main building of the Reserve Mining Co. 's taconite plant (afterthe 2nd traffic lights). Here several large blocks of anorthositeare included in diabase, a typical assemblage in the Beaver Baycomplex. Large patches of interstitital, poikilitic olivine occurin some parts of the anorthosite. No definite source for theanorthosite is known. Small red veins and dikelets of "granophe"cut the diabase and anorthosite.

Continue SW 2.8 miles across Beaver River to Beaver Bay.

3.2 11.6 Stop 27. Beaver y ferrogabbro and Black - (names afterGebman) of Beaver Bay complex (Silver Bay 15' quad.), Drive orwalk down to shore on gravel road opposite the main restaurant InBeaver Bay. Walk E through woods across little point to shoreledges. These are well—foliated ferrogabbro of the Beaver BayFerrogabbro, one of two similar plugs in this area. On walking NE alongthe shore, the contact with an enclosing ring—dike of Black Bay Gabhrois reached; such rings surround both ferrogabbro plugs. It is ratherinhomogeneous and locally contains very abundant apatite. The ophiticBeaver River Gabbro can be seen farther along the shore below somecabins. Return to highway.

Continue SW through Beaver Bay complex; most outcrop is ofcoarsely mottled, ophitic olivine gabbro ("Beaver River Gabbrot'of Gehman), well seen opposite Kings's Landing Marina 3 miles fromBeaver Bay. After 2.2 more miles road branches to Split RockLighthous,buil on anorthosite in diabase. At Day Hill (parkingand trail), 1.0 mile beyond, is last exposure of Beaver Baycomplex. Re—enter volcanics, continue 5.14 miles to GooseberryFalls State Park.

114.8 0.0 Stop 28. Smooth—surfaced olivine basalt, Gooseberry Falls State Park0.0 l3.i (Split Rock Point quad.). Below highway bridge are good exposures,in. and between falls, of Columnar—Jointed olivine basalt lavas withainygdalojdal tops, and smooth, gently billowing surfaces.

Continue SW through Castle Danger (2.L ml.), across Crow Creek(a diabasic intrusion here returns soon to SW at deeply weatheredLafayette Bluff), through Encampment Forest (privately owned old—growth forest with summer homes) to Silver Cliff (anoth' +1I41-

Goin,:?;SW

4.90.0

0.4

3.2

GoingNE

0.014.8

14.3

11.6

-94-

"[IT, NE B,Te held up by intrusions of Beaver Bay complex. Uplakeshore to NE is columnar-jointed Shovel Point, made of samerhyolite flow, and far beyond is smoke from Taconite Harborpower plant and Carlton Peak anorthosite knob at Tofte. To Sacross lake are Apostle Islands and Bayfield Peninsula, Wise.To SW is Reserve Mining Co. taconite plant at Silver Bay and humpytopography of Beaver Bay complex. Palisades made of thick, porphyritic rhyolite with quartz and feldspar phenocrysts, flowbanded at the base. Return to highway.

Drive 2.15 mi. to traffic light at Silver Bay, site ofReserve Mining Co. 's taconite plant where about 100,000 tons of 'rock from the eastern Mesabi Range are processed per day.

Stop 26. Anorthosite in diabase of Beaver Bay complex, SilverBay (Silver Bay 15' quad.). Stop at road cut directly oppositethe main building of the Reserve Mining Co. 's taconite plant (afterthe' 2nd traffic lights). Here several large blocks of anorthositeare included in diabase, a typical assemblage in the Beaver Baycomplex. Large patches of interstitital, poikilitic olivine occurin some parts of the anorthosite. No definite source for theanorthosite is known. Small red veins and dikelets of "granoph~y'.(e"

cut the diabase and anorthosite.

Continue SW 2.8 miles across Beaver River to Beaver Bay.

St,up 27. Beaver Bay ferrogabbro and Black_ Bay_ gabbro (names afterGehman) of Beaver Bay complex (Silver Bay 15' quad.). Drive orwalk down to shore on gravel road opposite the main restaurant tnBeaver Bay. Walk E through woods across little point to shoreledges. These are well-foliated ferrogabbro of the Beaver BayFerrogabbro, one of two similar plugs in this area. On walking NE alongthe shore, the contact with an enclosing ring-dike of Black Bay Gabbrois reached; such rings surround both ferrogabbro plugs. It is ratherinhomogeneous and locally contains very abundant apatite. The ophiticBeaver River Gabbro can be seen farther along the shore below somecabins. Return to highway.

Continue SW through Beaver Bay complex; most outcrop is ofcoarsely mottled, ophitic olivine gabbro ("Beaver River Gabbro"of Gehman), well seen opposite Kings's Landing Marina 3 miles fromBeaver Bay. After 2.2 more miles road branches to Split RockLighthouse,built on anorthosite in diabase. At Day Hill (parkingand trail), 1.0 mile beyond, is last exposure of Beaver Baycomplex. Re-enter volcanics, continue 5.4 miles to GooseberryFalls State Park.

14.8 0.00.0 13.1

Stop 28. Smooth-surfaced olivine basalt, Gooseberry Falls State Park~Split Rock P6int quad.). Below highway bridge are good exposures,ln, and between falls, of columnar-jointed olivine basalt lavas withamygdaloidal tops, and smooth, gently billowing surfaces.

Continue SW through Castle Danger (2.4 mi.), across Crow Creek(a diabasic intrusion here returns soon to SWat deeply weatheredLafayette Bluff), through Encampment Porest (privately owned oldgrowth forest with summer homes) to Silver Cliff (anothp~ tni~~

Going Going —95—

SW

gabbroic intrusion) to outskirts of Two Harbors.

12.7 0.5 Stop 29. Quartz tholeiite basalt, Two Harbors city park (Two

I:arbors quad.). Just before entering town, turn toward lake at

road to city camp ground and Wa—Ke—Ya Motel. Drive past Burlington

Bay, and up a hill. Park at hilltop picnic area, walk E to shore,

then S. Well exposed contact between amygdaloidal, weathered(ex—laumontite) top of one basalt and massive basal part of ananalyzed fine—grained, aphyric quartz tholeiite (50% 5102, 0.6%K20). Local thin sand lenses near and at contact; traces ofCu have been found. The tholeiite shows occasional small quartz—agate and chlorite amygdules and typical incipient sheeting—fractures with thin bands of oxidation. Upper zone- .of this thick

tholeiite can be examined to S. toward power plant; it becomesrubbly, vesicular, brecciated, with abundant laumontite. Returnto Hwy 61.

13.1 0.0 Continue SW to traffic light in Two Harbors. At west end of town0.0 20.6 take express highway to Duluth. Many cuts of basalts and minor

diabase intrusions. At-about 17.7 miles Moose Mtn. is seen inland,held up by Lester River diabase sill. At 20 miles jct. with oldHwy 61 (now St. Louis Co. 61), and at 20.6 miles Lester River

20.6 0.0 bridge. Just beyond, either continue straight on Hwy 61 (London Rd.)0.0 5.9 direct to downtown 6 miles ahead, or turn uphill on Minn. 23

(60th Ave. E) across ItR tracks to Superior St; go left.

(3.0) (3.0) Stop 30. Tischer Creek felsite and Endion sill, Congdon Park(Duluth quad.). Examine outcrops beneath bridge; then walk uptrail that starts on W side of creek. There are some impassableplaces where you must climb up to a bank—top trail on W side.Lower part of section cuts through orange, foliated felsite withflow structure, quartz veinlets, occasional inclusions, out by twoor three basaltic dikes. Upstream at top of steeper part of gorgethe stream cuts down into red granophyre top of Endion sill. Up-stream the creek cuts gradually down into intermediate, then gabbroicrocks of main part of Endion sill. At Vermilion Road bridge (secondbridge above Superior St.) still in lower—middle part of sill;return via trail or Congdon Parkway on W. side. See Ernst, 1960.

Continue SW on Superior St. over rise held up by Endion Sill.

5.0 0.9 Stop 31. Basaltic—andesitic lavas, interflow sandstone, LeifErickson Park, Duluth (Duluth quad.). At lath Avenue E. turn offSuperior St. toward lake to London Road at Leif Erickson Park.Walk over footbbridge near "viking ship' t shore. Weakly por—phyritic basalt or basaltic andesite ledges behind stage. Rubbly

(weathered?) top to NE is directly overlain by a thick cross—beddedsandstone, strongly epidotized, that is cut by a small dike at NEend of beach. Several more flows exposed along shore to SW.

5.9 0.0 Lake Avenue and Superior Street, downtown Duluth.0.0 11.2

(approx.)

Drive SW on Hwy 61 past Point of Rocks through Duluth Complex11-2 0.0 up Thomson Hill to Nopeming. Turn R at Midway Road (St. Louis o. 13),approx.) drive uphill about 0.9 mi. almost to top of rise.

GoingSW

12.7

13.10.0

20.60.0

(3.0)

GoingNE

0.5

0.020.6

0.05.9

(3.0)

-95-

gabbroic intrusion) to outskirts of Two Harbors.

Stop 29. Quartz tholeiite basalt, Two Harbors city park (TwoHarbors quad.). Just before entering town, turn toward lake atroad to city camp ground and Wa-Ke-Ya Motel. Drive past BurlingtonBay, and up a hill. Park at hilltop picnic area, walk E to shore,then S. Well exposed contact between amygdaloida1, weathered(ex-laumontite) top of one basalt and massive basal part of ananalyzed fine-grained, aphyric quartz tholeiite (50% Si02, 0.6%K20). Local thin sand lenses near and at contact; traces ofCu have been found. The tholeiite shows occasional small quartzagate and chlorite amygdu1es and typical incipient sheetingfractures with thin bands of oxidation. Upper zone' ,of this thicktholeiite can be examined to S. toward power plant; it becomesrubb1y, vesicular, brecciated, with abundant laumontite. Returnto Hwy 61.

Continue SW to traffic light in Two Harbors. At west end of towntake express highway to Duluth. Many cuts of basalts and minordiabase intrusions. At- about 11.7 miles Moose Mtn. is seen inland,held up by Lester River diabase sill. At 20 miles jet. with oldHwy 61 (now St. Louis Co. 61), and at 20.6 miles Lester Riverbridge. Just beyond,either continue straight on Hwy 61 (London Rd.)direct to downtown 6 miles ahead 9 or turn uphill on Minn. 23(60th Ave. E) across RR tracks to Superior St; go left.

Stop 30. Tischer Creek felsite and Endion sill, Congdon Park(Duluth quad.). Examine outcrops beneath bridge; then walk uptrail that starts on W side of creek. There are some impassableplaces where you must climb up to a bank-top trail on W side.Lower part of section cuts through orange, foliated felsite withflow structure, quartz veinlets, occasional inclusions, Clut by twoor three basaltic dikes. Upstream at top of steeper part of gorgethe stream cuts down into red granophyre top of Endion sill. Upstream the creek cuts gradually down into intermediate, then gabbroicrocks of main part of Endion sill. At Vermilion Road bridge (secondbridge above Superior St.) still in lower-middle part of sill;return via trail or Congdon Parkway on W. side. See Ernst, 1960.

Continue SW on Superior St. over rise held up by Endion Sill.

5.0 0.9 Stop 31. Basaltic-andesitic lavas, interflow sandstone, LeifErickson Park, Duluth (Duluth quad.). At 10th Avenue E. turn offSuperior St. toward lake to London Road at Leif Erickson Park.

IWalk over footbbridge near "viking ship" to shore. Weakly por-phyritic basalt or basaltic andesite ledges behind stage. Rubbly(weathered?) top to NE is directly overlain by a thick cross-beddedsandstone, strongly epidotized, that is cut by a small dike at NEend of beach. Several more flows exposed along shore to SW.

5.9 0.0 Lake Avenue anc Superior Street, downtown Duluth.0.0 11.2

~approx.)

1L 2 o. a:approx. )

Drive SW on Hwy 61 past Point of Rocks through Duluth Complexup Thomson Hill to Nopeming. Turn R at Midway Road (St. Douis ~o. 13),drive uphill about 0.9 mi. almost to top of rise.

—96—

Stop 32. Basal (Puckwunge?) sandstone, basal basalts of LowerKeweenawan ('Grandview Golf Course" locality, Esko quad.).Drive or walk E on road toward Cloquet city water supply tank onhill. Pass low outcrops of vertical Thomson slate and graywacke(Middle Precambrian: more are exposed on rise and roadcuts toN.). At slope, go either R or L along base of slope to outcropsof basal Upper Precambrian quartzite and conformably overlyingmafic, porphyritic basalt with small augite and altered olivinephenocrysts identical to rocks on Lucille Island at Grand Portage.Note pillows subtly outlined by small vesicles and color zones.

-96-

Stop 32. Basal (Puckwunge?) sandstone, basal basalts of LowerKe\veena\van ("Grandview Golf Course" locality, Esko quad.).Drive or walk E on road toward Cloquet city water supply tank onhill. Pass low outcrops of vertical Thomson slate and graywacke(Middle Precambrian: more are exposed on rise and roadcuts toN.). At slope, go either R or L along base of slope to outcropsof basal Upper Precambrian quartzite and conformably overlyingmafic, porphyritic basalt with small augite and altered olivinephenocrysts identical to rocks on Lucille Island at Grand Portage.Note pillows subtly outlined by small vesicles and color zones.

—97-.

GUIDE TO THE PRECAMBRIAN ROCKSOF NORTHWESTERN COOK COUNTY

AS EXPOSED ALONG THE GUNFLINT TRAIL

Prepared by

Paul W. WeiblenDepartment of Geology and Geophysics

University of MinnesotaMinneapolis, Minnesota

G. B. MoreyMinnesota Geological SurveyUniversity of Minnesota

St. Paul, Minnesota

'1. G. MudreyMinnesota Geological Survey

University of MinnesotaSt. Paul, Minnesota

-97-

GUIDE TO THE PRECAMBRIAN ROCKSOF NORTHWESTERN COOK COUNTY

AS EXPOSED ALONG THE GUNFLINT TRAIL

Prepared by

Paul \~. WeiblenDepartment of Geology and Geophysics

University of HinnesotaMinneapolis~ Minnesota

G. B. HoreyMinnesota Geological Survey


H. G. MudreyMinnesota Geological Survey


—98—

Geology of Northwestern Cook County, Minnesota

INTRODUCTION

An exceptionally complete Precambrian section is exposed inthe vicinity of the Cunf lint Trail in northwestern Cook County.Rocks of Early Precambrian age, represented by a volcanicsuccession and the Saganaga Granite, are unconformablyoverlain by the Middle Precambrian Animikie Group,coiLsisting ofthe Gunflint and Rove Fotmations. A low anglular unconformityseparates the Middle Precambrian strata from the nverlying PuckwungeFormation of Late Precambrian age. The Logan Intrusive Rocks andthe Duluth Complex, which intrude and truncate the Middle Precambrianstrata, comprise the major part of the Upper Precambrian section.However, possible remnants of the North Shore Volcanic Group occurat the top of Duluth Complex rocks.

The geology of the area was summarized in 1959 by Grout andothers. Because geologic mapping since 1962 has considerably re-vised the geologic history of the area, and because much of thiswork is as yet unpublished, a comprehensive summary is presentedhere. This discussion is meant to provide a framework for thespecific aspects of the geology which the chosen stops illustrate.

Lower Precambrian

Introduction

The Lower Precambrian rocks in Cook County are the easternextension of the Vermilion greenstone belt (Sims and others, 1971,in press). Gruner (1941) has shown that the district contains manymajor longitudinal faults that divide it into long segments or belts,each with distinct characteristics. They cannot be connectedstratigraphically with each other in any simple way. One segmentextends from Gabimichigami Lake across the Gunflint Trail (Fig. 1).It includes a metavolcanic assemblage consisting of metabasalt,metaandesite, agglomerate and tuff, hornblende andesite porphyry,and intercalated metagraywacke and slate. To the north, the SaganagaGranite has intruded and metamorphosed the volcanic succession; itis overlain to the south by younger rocks of Middle and Late Precambrianage.

-98-

Geology of Northwestern Cook County, Minnesota

INTRODUCTION

An exceptionally complete Precambrian section is exposed inthe vicinity of the Gunflint Trail in northwestern Cook County.Rocks of Early Precambrian age represented by a volcanic,succession and the Saganaga Granite, are unconformablyoverlain by the Middle Precambrian Animikie Group, cOllsisting ofthe Gunflint and Rove Fo~ations. A low anglular unconformityseparates the t-1iddle Precambrian strata from the f1verlying PuckwungeFormation of Late Precambrian age. The Logan Intrusive Rocks andthe Duluth Complex, which intrude and truncate the Middle Precambrianstrata, comprise the major part of the Upper Precambrian section.However, possible remnants of the North Shore Volcanic Group occurat the top of Duluth Complex rocks.

The geology of the area was summarized in 1959 by Grout andothers. Because geologic mapping since 1962 has considerably revised the geologic history of the area, and because much of thiswork is as yet unpublished, a comprehensive summary is presentedhere. This discussion is meant to provide a framework for thespecific aspects of the geology which the chosen stops illustrate.

Lower Precambrian

Introduction

The Lower Precambrian rocks in Cook County are the easternextension of the Vermilion greenstone belt (Sims and others, 1971,in press). Gruner (1941) has shown that the district contains manymajor longitudinal faults that divide it into long segments or belts,each with distinct characteristics. They cannot be connectedstratigraphically with each other in any simple way. One segmentextends from Gabimichigami Lake across the Gunflint Trail (Fig. 1).It includes a metavolcanic assemblage consisting of metabasalt,metaandesite, agglomerate and tuff, hornblende andesite porphyry,and intercalated metagraywacke and slate. To the north, the SaganagaGranite has intruded and metamorphosed the volcanic succession; itis overlain to the south by younger rocks of Hiddle and Late Precambrianage.

—99—

The metavolcanic assemblage constitutes a homoclinal sequencewhich dips southward toward what Grtiner inferred to be the axisof a southeastward—plunging synclinorium. Pillow—top directionsindicate that the sequence alsO becomes stratigraphically youngerto the south. However, some pillows top to the north indicatingfolding, but poor exposure precludes detailed evaluation of thestructure.

Previously, an unconformity was thought to separate themafic and more felsic rocks; thus the succession was subdividedinto two formations. The metabasalts were referred to as part ofthe Ely Greenstone whereas the more felsic volcanic and clasticrocks were placed within the Knife Lake Group. Recent mapping(Fiorey and others, 1969) has shown these two lithologies to begradational, and nappin in other parts of the Vernilion district(lorey and others, 1970; Green, 1970; Sims and others, 1968; 1971,in press) has shown that specific rock types are not diagnosticof any particular stratigraphic horizon—i.e., lithologic unitsare not time—stratigraphic units — therefore no formal stratigraphicnomenclature is now used for these rocks.

Descriptive Stratigraphy

Volcanogenic Rocks

Detailed mapping in the Long Island Lake quadrangle, reconnaissancemappin as far west as Va>' Lake in the Cillis Lake quadrangle, andGrunerts work in the area have indicated that the volcanigenic sequenceconsists of approximately 60 percent inetabasalt and associated frag—mental rocks, 30 percent netnandesitic agglomerate, conglomerate, tuff,and flows, and 10 percent intercalated netagraywacice and slate.

IjuLLaaLs.st±1psis!; Over 95 percent of the meta—basalt is extrusive; there are several tabular bodies of metadiabase,too rLall to show at a scale of 1:24,000. Many of the metabasaltsexhibit pillow structure. The pillows are as much as three feet indiameter, but many have been deformed so that they are now two orthree times as long as they are wide. Chilled rinds are well—developedand are as much as one—half inch thick: typically they are lighter incolor than the dark green or dark greenish—gray interiors. Inter—pillowmaterial locally is well developed and co!nists of tuffaceous materialcEtert, or pillow—rind fragments.

The metaasalts shot: intense retrograde alteration; many thinsections are nearly opaque. Recognizable minerals include relictaugite and calcic plagioclaso, and secondary sodic plagioclase,actinolice, chlorite, epu'ote, calcite, quartz, leucoxene, andopaques.

Property of

C. Patrick Ervin

-99-

The metavolcanic assemblage constitutes a homoclinal sequence\"hich dips southward toward what Gruner inferred to be the axisof a southeastward-plunging synclinorium. Pillow-top directionsindicate that the sequence also becomes stratigraphically youngerto the south. However, some pillm"s top to the north indicatingfolding, but poor exposure precludes detailed evaluation of thestructure.

Previously, an unconformity was thought to separate themafic and more felsic rocks; thus the succession was subdividedinto two formations. The metabasalts were referred to as part ofthe Ely Greenstone whereas the more felsic volcanic and clasticrocks \Vere placed within the Knife Lake Group. Recent mapping(i-iorey and others, 19(9) has sho\Vn these t\VO lithologies to begradational, an(~ mapping in other parts of the Vermilion district(~Iorey and others, 1970; Green, 107(); Sims and others, 1968; 1971,in press) has shm.;n that specific rock types are not diagnosticof any particular strati~raphic horizon - i.e., lithologic unitsare not time-stratigraphic units - therefore no formal stratigraphicnomenclature is no\V used for these rocks.


Volcano;;enic Rocks

Detailed mappinR in the Lonfl Island Lake quadrangle, reconnaissancerr.appin,~ as far \"est as Fay Lake in the Gillis Lake quadrangle, andGruner's Hark in the area have indicated that the volcani8enic sequenceconsists of approximately 60 percent metabasalt and associated fragmental rocks, 30 percent metaandesitic agglomerate, conglomerate, tuff,and flows, and 10 percent intercalated metagraywacke and slate.

:i.eta_~~i1.sa-h.~~_?.~(LQ.~sg..0at~_~.t ro_c±,-s_: Over 95 percent of the metabasalt is extrusive; there are several tabular bodies of metadiabase,too small to shmv at a scale of 1:24,000. Nany of the metabasaltsexhibit pillm-J structure. The pillo\Vs are as much as three feet indiameter, but many have been deformed so that they are no\V two orthree times as lonf, as they are \"ide. Chilled rinds are 1vell-developedand are as much I1S one-half inch thick: typically they are lighter incolor than the dar:( green or dark fJreenish-Rray interiors. Inter-pillm"material locally is well developed and con~ists of tuffaceous materialchert, or pillm,,-rind fragments.

The metabnsalts shoH intense retrograde alteration; many thinsections nre nearly opaque. Recognizable minerals include relictaugite anu cnlcic plagioclase, and secondary sodic plagioclase,actinolite, chlorite, epidote, calcite, quartz, leucoxene, andopaques.

Property ojC. Patrick Ervin

Mikel

Rectangle

—100—

The tabular bodies of metadiabase have a relict poikilitictexture — actinolite pseudomorphs after augite — and a mineralogy

much like that of the metabasalts.

Thin beds of texturally and mineralogically immature meta—clastic rocks are locally intercalated with the metabasalt. Several

beds are crudely graded and have a texture suggestive of a pyroclasticorigin. Most layers, however, appear to be epiclastic with pebble—to silt—size clasts of locally derived metadiabase in a finer—grainedmatrix of chert, plagioclase, hornblende, chlorite, and sericite.

Hornblende andesite prophyry and related rocks: There are twotypes of hornblende—biotlte phenocryst bearing rocks. Both are lightgreenish—gray in color. One type lacks internal structure. The groundmass consists of plagioclase, lesser amounts of biotite, opaques andrare quartz and secondary calcite. The other type has similarphenocryst and groundmass mineralogy but is composed of angular torounded, cobble—s,'ze clasts and is inferred to be a volcanic brecciaor agglomerate.

Metagraywacke and slate: In the vicinity of Fay Lake in theGillis Lake quadrangle, the felsic volcanic rocks apparently inter—finger, or are infolded with agglomerate or conglomerate, meta—graywacke, and slate.

Amphibolites: The metamorphic effects of the Saganaga Graniteon the metabasalts becomes apparent only within several hundred feetof the contact. Away from the contact, the metabasalt containsincipient hornblende; nearer the contact, the rock becomes granularand consists of hornblende and calcic plagioclase; at the contact,the rock is strongly schistose. The schistosity parallels thegranite contact, as does a well—developed foliation within the granite.

Saganaga Granite

General Statement: The Saganaga Granite (A. Winchell, 1888)(Fig. 1) is a late—kinematic composite intrusion emplaced around2,700 m.y. ago in older volcanic rocks and the Northern Light Gneiss.Inch— to foot—sized inclusions of both rock types are found in thegranite. The main phase, which comprises 85 percent of the outcroparea, is a medium—graineci "quartz—eye hornblende—bjotjte tonalite.Other phases include: 1) a border phase, found along the southernmargin of the batholith and along the base of a roof pendant ofthe Northern Light Gneiss; 2) a younger fine—grained tonalite whichlacks conspicuous tquartz_eyes?T; 3) a coarse—grained biotite—fluorite—bearing granodiorite; and 4) a quartz—feldspar pegnatite which occursas veinlets as much as three feet wide in the main phase.

-100-

The tabular bodies of metadiabase have a relict poikilitictexture - actinolite pseudomorphs after augite - and a mineralogymuch like that of the metabasalts.

Thin beds of texturally and mineralogically immature metaclastic rocks are locally intercalated with the metabasalt. Severalbeds are crudely graded and have a texture suggestive of a pyroclasticorigin. Most layers, however, appear to be epiclastic with pebble-to silt-size clasts of locally derived metadiabase in a finer-grainedmatrix of chert, plagioclase, hornblende, chlorite, and sericite.

Hornblende andesite prophyry and related rocks: There are twotypes of hornblende-biotite phenocryst bearing rocks. Both are lightgreenish-gray in color. One type lacks internal structure. The groundmass consists of plagioclase, lesser amounts of biotite, opaques andrare quartz and secondary calcite. The other type has similarphenocryst and groundmass mineralogy but is composed of angular torounded, cobble-s~ze clasts and is inferred to be a volcanic brecciaor agglomerate.

Metagraywacke and slate: In the vicinity of Fay Lake in theGillis Lake quadrangle, the felsic volcanic rocks apparently interfinger, or are infolded with agglomerate or conglomerate, metagraywacke, and slate.

Amphibolites: The metamorphic effects of the Saganaga Graniteon the metabasalts becomes apparent only within several hundred feetof the contact. Away from the contact, the metabasalt containsincipient hornblende; nearer the contact, the rock becomes granularand consists of hornblende and calcic plagioclase; at the contact,the rock is strongly schistose. The schistosity parallels thegranite contact, as does a well-developed foliation within the granite.

Saganaga Granite

General Statement: The Saeanaga Granite (A. Hinchell, 1888)(Fig. 1) is a late-kinematic composite intrusion emplaced arouno2,700 m.y. ago in older volcanic rocks and the Northern Light Gneiss.Inch- to foot-sized inclusions of both rock types are found in thegranite. The main phase, which comprises 85 percent of the outcroparea, is a medium-grained "quartz-eye" hornblende-biotite tonalite.Other phases include: 1) a border phase, found aloTIC' the southernmargin of the batholith and along the base of a roof"pendant ofthe Northern Light Gneiss; 2) a younger fine-grained tonali te ,vhichlacks conspicuous "quartz-eyes"; 3) a coarse-grained biotite-fluoritebearing granodiorite; and 4) a quartz-feldspar pegmatite which occursas veinlets as much as three feet wide in the main phase.

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:iain Phase: This phase is a "quartz eye"bearing tonalitecontaining 20 percent quartz, 60 percent oligoclase, 1—6 percentmicrocline, up to 8 percent biotite, 6 percent hornblende, and1—4 percent epidote. Other minerals include clino—pyroxene,muscovite, chlorite, sphene, apatite, zircon, calcite and opaqueoxides. Cataclastic deformation of the tonalite is indicated bystrong undulose extinction and granulation of the quartz and abun-dant mortar and mylonite zones.

Border Phase: The border phase is a quartz bearing, hornblendediorite that is gradational with the "quartz—eye" tonalite. This

phase is well developed along the southern edge of the batholith inthe Long Island Lake quadrangle where it is as much as 1000 feet wide,

and in the Moose Bay area in Ontario where it is found along theunderside of a roof pendant of the Northern Light Gneiss. Foliations

in the border phase and in the country rock are everywhere similar,and it is inferred that the border phase was produced by partialassimilation of country rocks of mafic composition (Halford, 1969).

Younger Tonalite: A tonalite characterized by a lack of"quartz—eyes," a fine—grained equigranular texture, and abundanthornblende and biotite crops out in the Horseshoe Island area ofSaganaga Lake. This tonalite is not sheared which indicates thatit was emplaced after deformation of the main phase.

Internal Structure: Linear structural elements marked by analignment of 'quartz eyes" and/or elongate hornblende grains arewell developed only in the border phase. Lineations in the borderphase plunge gently southeastward. In contrast, lineations in themain phase are random.

Structure

All the minor structural elements in the pre—Saganaga rocks inthe Long Island Lake quadrangle reflect a southeasterly plungingsynclinorium. in addition, a major, northwesterly—trending fault(Lookout Fault, Sims and others, 1969) separates rocks showing nometamorphic effect of the Saganaga Granite frorn.those that do. The

fault is marked by a topographic low developed in a breccia zoneas much as 50 feet wide. The breccia consists of highly shearedmetabasalt in a matrix of massive quartz and calcite. Fracturecleavage is present throughout the area and commonly parallels theLookout Fault. Southeastward plunging lineations formed by bedding/cleavage intersections are locally well developed. These lineationsare consistent in orientation with mineral lineations developed inthe amphibolites and in the granite itself; obviously all of thesestructures are related to the same tectonic event.

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Hain Phase: This phase is a "quartz eye"-bearing tonalitecontaining 20 percent quartz, 60 percent oligoclase, 1-6 percentmicrocline, up to 8 percent biotite, 6 percent hornblende, and1-4 percent epidote. Other minerals include clino-pyroxene,muscovite, chlorite, sphene, apatite, zircon, calcite and opaqueoxides. Cataclastic deformation of the tonalite· is indicated bystrong undulose extinction and granulation of the quartz and abundant mortar and mylonite zones.

Border Phase: The border phase is a quartz bearing, hornblendediorite that is gradational with the "quartz-eye" tonalite. Thisphase is well developed along the southern edge of the batholith inthe Long Island Lake quadrangle where it is as much as 1000 feet wide,and in the Noose Bay area in Ontario ,,,here it is found along theunderside of a roof pendant of the Northern Light Gneiss. Foliationsin the border phase and in the country rock are everYWhere similar,and it is inferred that the border phase was produced by partialassimilation of country rocks of mafic composition (Halford, 1969).

Younger Tonalite: A tonalite characterized by a lack of"quartz-eyes," a fine-grained equigranular texture, and abundanthornblende and biotite crops out in the Horseshoe Island area ofSaganaga Lake. This tonalite is not sheared ,,,nich indicates thatit was emplaced after deformation of the main phase.

Internal Structure: Linear structural elements marked by analignment of "quartz eyes" and/or elongate hornblende grains arewell developed only in the border phase. Lineations in the borderphase plunge gently southeastward. In contrast, lineations in themain phase are random.

Structure

All the minor structural elements in the pre-Saganaga rocks inthe Long Island Lake quadrangle reflect a southeasterly plungingsynclinorium. In addition, a major, northwesterly-trending fault(Lookout Fault, Sims and others, 1969) separates rocks showing nometamorphic effect of the Saganaga Granite from;those that do. Thefault is marked by a topographic 1m" developed in a breccia zoneas much as 50 feet wide. The breccia consists of highly shearedmetabasalt in a matrix of massive quartz and calcite. Fracturecleavage is present throughout the area and commonly parallels theLookout Fault. Southeastward plunging lineations formed by bedding/cleavage intersections are locally well developed. These lineationsare consistent in orientation with mineral lineations developed inthe amphibolites and in the granite itself; obviously all of thesestructures are related to the same tectonic event.

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The Lookout Fault also apparently was related to the finalemplacement of the Saganaga Granite. Previously it was assumed

(Grout, 1936) that the granite was emplaced into its presentposition via a westward rotation of about 700 about a north—southaxis. This implies that the eastern part of the granite and theNorthern Light Gneiss represent roots of the batliolith that were onceas much as 25 miles deep. However, Goldich and others (1968) havesuggested that the granite was originally intruded at a shallow depth—an observation more consistent with the regional Abukurna greenschistfacies metamorphism (Mudrey, 1969) — and brought to its presentrelative position by dominantly vertical movements along severalwesterly—trending border faults. The granite—greenstone contactmapped by Harris (1968) on the north shore of Saganaga Lake wasinterpreted to be the northern border fault, and it is here inferredthat the Lookout Fault is its southern analog.

The block was emplaced during E:ly Precambrian time, in asmuch as detritus now found in the Knife Lake Group of Gruner (1941)was derived from the Saganaga Granite. Although there has been latermovement on the Lookout Fault, it is dominantly a Lower Precambrianstructure.

MIDDLE PRECAMBRIAN

Introduction

The Animikie Groun consisting of the Gunflint and Rove Formationscomprises the entire Middle Precanibrian in this area. These rocksunconformably overlie Lower Precambrian rocks and in turn are intrudedand truncated by gabbroic rocks of Niddle Keweenawan age.

The time of Animilcie deposition has not been completely documented.Hurley and others (1962) have suggested that deposition of the iron—formation occurred around 1,900 ± 200 x io6 years. Although this K—Aage includes an arbitrary and perhaps unnecessary correction of 20percent assumed argon loss, it nevertheless has been widely quotedin the literature. In contrast, Faure and Kovac (1969) obtained aRb—Sr whole—rock isochron age of 1,685 ± 24 x 10 years from theGunflint Iron—formation. They interpret this age to be the time ofdiagenesis, however, it is similar to other ages obtained from rocksmetamorphosed during the Penokean orogeny. Later Misra and Faure(1970) showed that argillite from three Gunflint Iron—formationlocalities have Rb—Sr ages that decrease from '. . .1.7 b.y. at the easternend to 1.2 b.y. near... the western.. . this systematic variation ofapparent ages may be related to metamorphic effects caused by Keweenawandiabase sills. . . . '. Thus there is no unequivical depositional age forthese rocks.

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The Lookout Fault also apparently was related to the finalemplacement of the Saganaga Granite. Previously it was assumed(Grout, 1936) that the granite was emplaced into its presentposition via a west\vard rotation of about 70° about a north-southaxis. This implies that the eastern part of the granite and theNorthern Light Gneiss represent roots of the batholith that were onceas much as 25 miles deep. However, Goldich and others (1968) havesuggested that the granite was originally intruded at a shallow depthan observation more consistent with the regional Abukuma greenschistfacies metamorphism (Mudrey, 1969) - and brought to its presentrelative position by dominantly vertical movements along severalwesterly-trending border faults. The granite-greenstone contactmapped by Harris (1968) on the north shore of Saganaga Lake wasinterpreted to be the northern border fault, and it is here inferredthat the Lookout Fault is its southern analog.

The block was emplaced during E8~ly Precambrian time, in asmuch as detritus now found in the Knife Lake Group of Gruner (19/+1)t.,as derived from the Saganaga Granite. Although there has been latermovement on the Lookout Fault, it is dominantly a Lower Precambrianstructure.

~UDDLE PRECANBRI!u~

Introduction

The Animikie Group consisting of the Gunflint and '\ove Formationscomprises the entire :oliddle Precambrian in this area. These rocksunconformably overlie Lm.,er Precambrian rocks and in turn are intrudedand truncated by gabbroic rocks of i'·liddle KeweenaT,oJan age.

The time of !mimikie deposition has not been completely documented.Hurley and others (1962) have suggested that deposition of the iron-

formation occurred around 1,900 ± 200 x 106 years. Although this K-Aage includes an arbitrary and perhaps unnecessary correction of 20percent assumed argon loss, it nevertheless has been widely quotedin the literature. In contrast, Faure and Kovacg (1969) obtained aRb-Sr tvhole-rock isochron age of 1,685 ± 24 x 10 years from theGunflint Iron-formation. They interpret this age to be the time ofdiagenesis, however, it is similar to other ages obtained from rocksmetamorphosed during the Penokean orogeny. Later Nisra and Faure(1970) showed that argillite from thr2e Gunflint Iron-formationlocalities have Rb-Sr ages that decrease from " ... 1.7 b.y. at the easternend to 1.2 b.y. near ... the \vestern ... this systematic variation ofapparent ages may be related to metamorphic effects caused by Keweenawandiabase sills •••• ". Thus there is no uriequivical depositional age forthese rocks.

—10 3—


Gunflint Iron—formation

The Gunflint Iron—fori:iation crops out in a northeasterly—trending belt that extends from Thunder Bay on Lake Superior toa point in Minnesota 12 miles west of Gunflint Lake where it is

truncated by the Duluth Complex.

In Canada the iron—formation is only slightly, if at aF.,metamorphosed and consists of silica, much of which is chalcedonic,iron oxides, iron carbonates, and greenalite. Consequently, Goodwin(1956) recognized six sedimentary facies which serve to subdividethe iron—formation into four members. In Minnesota the oriina1nature of the iron—formation is -obscured by metamorphism by theDuluth Complex. The carbonates and greenalite are replaced by amphibole,pyroxene, fayalite, and locally by garnet and other silicates. In

addition, many of the small—scale sedimentary textures have beenalmost completely destroyed; however larger structures, and especiallycomplex bedding relationships are still preserved. Therefore thefour—fold nomenclatural scheme — originally outlined in the BiwabikIron—formation (Wolff, 1917) and later extended to the Gunflint Iron—formation (l3roderick, 1920) — was retained by Morey and others (1969)because it emphasizes various bedding aspects. Accordingly, fourmembers are recognized; Lower Cherty, Lower Slaty, Upper Cherty,and Upper Slaty. Although the boundaries of these members do notcoincide with those recognized by Goodwin (1956), the two schemes canbe equated with only slight difficulty.

Lower ChertLMemhe.r: The Lower Chterty Member is thin, rangingin thickness from 15 to 45 feet. Feldspathic quartzite that containsgranite and greenstone cobbles is present locally at Lie base of theformation; these beds are equivalent to Goodwin's basal conglomeratemember. A persistent magnetite—rich, silicate—bearing unit five to15 Feet thick occurs within this member and serves as an excellentmarker—horizon. :c'st commonly it lies directly upon basement rocks,but locaii.v it overlies either the feldspathic qtiartzite or a chert—cemented conglomerate containing fragr.ents of algal structures; it inturn is overlain by a massive, chert—rich, magnetite—poor unit about15 feet thick.

Lower Slaty Member: This member is 80 to 95 feet thick. Thelowermost, nearly magnetite—freo 10 feet is a black, thin—beddedargillite composed dominantly of volcanically derived material. Itis equivalent to the intermediate slate on the 'esahi range and tothe lower tuffaccous shale f,icies in Canada. The beds immediatelyaove the Intermediate slate are massive and clierty and resemblethe upor aert of the lower Chcrty :-;ember. This unit passes abruptlyipuird into cherty s iJ.icate— carjn beds uith sparse inagnetite inter-calated with a few thinly—laminated silicate—rich beds. The remaining50 feet is a thiin—bndciel to laminated rock containing various silicatesand Cro 20 to 35 percent magnetite. A few cherty—silicate beds,definitely subordinate in amount are intercalated in this interval.

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Gunflint Iron-formation

The Gunflint Iron-formation crops out in a northeasterlytrending belt that extends from Thunder Bay on Lake Superior toa point in Minnesota 12 miles west of Gunflint Lake where it istruncated by the Duluth Complex.

In Canada the iron-formation is only slightly, if at al:_,metamorphosed and consists of silica, much of \"hich is chalcedonic,iron oxides, iron carbonates, and greena1ite. Consequently, Goodwin(1956) recognized six sedimentary facies which serve to subdividethe iron-formation into four members. In Hinnesota the originalnature of the iron-formation is obscured by TIetamorphism by theDuluth Complex. The carbonates and greenalite are replaced by amphibole,pyroxene, fayalite, and locally by garnet and other silicates. Inaddition, many of the small-scale sedimentary textures have beenalmost completely destroyed; however larger structures, and especiallycomplex bedding relationships are still preserved. Therefore thefour-fold nomenclatural scheme - originally outlined in the BiwabikIron-formation (Holff, 1917) and later extended to the Gunflint Ironformation (Broderick, 1920) - ,vas retained by 1'1orey and others (1969)because it emphasizes various bedding aspects. Accordingly, fourmembers are recognized; Lower Cherty, Lower Slaty, Upper Cherty,and Upper Slaty. Although the boundaries of these members do notcoincide witll those recognized by Goodwin (1956) the two schemes can,be equated \vith only slight difficulty.

Lo\ver Cherty Hember: The Lo,ver Cherty Nember is thin, rangingin thickness from 15 to 45 feet. Feldspathic quartzite that containsgranite and greenstone cobbles is present locally at the base of theformation; these beds are equivalent to CoodHin' s basal conglomeratemember. A persistent magnetite-rich, silicate-bearing unit five to15 feet thick occurs IVithin this member and serves as an excellentmarl(er-horizon. :Iost commonly it lies directly upon basement rocks,but loc<llly it overlies either the feldspathic quartzite or a chertcemented conglomerate contctininr, frctgments of algal structllres; it inturn is overlain hy a massive, chert-rich, magnetite,-poor unit about15 feet thick. ' \

Lower -,:,?-latY---l~~emher: This member is 80 to 95 feet thick. ThelOHennost, nearly magnetite-free 10 feet is a black, thin-beddedargillite conlposcd dominantly of volcanically derived material. Itis equivalent to the InterPlcdi"tc slate on the ;lesabi range and tothe lOh·er tuffaceous shale fD-cLcs in l,<ln"da. The beds immediately.:1iJove tile IntenneJiate slate are massive and cherty and resemblethe upper part of the LOIver Cherty ;'[e1l1her. This unit passes ahruptlyuTl\!;]rd into cherty si.licate-hci1rini; beds ':lith spilrse magnetite intercalateJ \vi til a fe\\' tilinly-la:ninated silicate-rich heels. The remaining50 feet is i1 thin-beJded to lill\linated rock containing various silicatesand from 2C to 35 percent map;netite. A fe'" cherty-silicate beds,elefillitely subordinate in illllount are intercalated in this interval.

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Upper Cherty Member: There is a complete gradation between

the Lower Slaty and Upper Cherty Members. The Upper Cherty Member,

as presently defined, is approximately 50 feet thick. The lower

part consists of irregularly bedded to lenticular cherty layers

intercalated with thinly—laminated silicate—rich layers that increase

in abundance upward. Thin irregular layers of magnetite are commonin the cherty beds near the bottom of the member, but become less

abundant upward. The top of the member—equivalent to Goodwin'supper algal chert facies—is characterized by several granular

chert beds containing algal structures, conglomerate fragments,and abundant magnetite.

Upper S1ay Member: The Upper Slaty Member is approximately

150 feet thick. Thick lenticular chert beds with disseminated

magnetite characterize the lower few tens of feet, but most of themember consists of a thin—bedded to laminated quartz—silicate rockinterbedded with thinly laminated layers of graphitic argillite,and one to two inch thick beds of relatively pure chert. Theupper 10 feet is nearly magnetite free and consists of limestoneand chert interbedded with argillite.

Rove Formation

The Rove Formation gradationally overlies the Cunflint Iron—formation and is intruded by Logan Intrusive Rocks and truncatedby the Duluth Complex. A detailed description of the formationis presented by Morey (1969). In the Long Island Lake, GunflintLake, Southlake, and Hungary Jack Lake quadrangles, the formationis characterized by intercalated black to grayish black, locallycarbonaceous argillite, argillaceous siltstone and fine—grainedgraywacke. In general the silt and sandstone beds become coarse—grained, thicker, and more abundant upward in the formation. tn

addition, there are several lenses and irregular bodies of lime-stone, dolomite, and chert and a number of calcite—dolomite con—cretions of various shapes and sizes, similar to those describedin the formation by Tanton (1931) and Moarhouse (1963) scatterednear the base of the formation.

Morey (1969) has suggested that deposition started in a deepbasin in which fine—grained sediment accumulated under reducingconditions. The siltstone and sandstone beds contain many primarysedimentary structures indicative of turbidite deposition. Thereis also evidence of an increase infrequency of this type of depositionupward in the section. The graywackes contain abundant frameworkgrains of quartz, feldspar and Igranitici rock fragments indicativeof a granitic source area. Sedimentary structures including cross—bedding and various kinds of sole marks indicate that sedimenttransport dominantly was from north to south. These observationsand the graywacke mineralogy led Morey (1969) to conclude that thesource area was the Lower Precambrian terrane now exposed north of theoutcrdp area of the Rove Formation.

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Upper Cherty Member: There is a complete gradation betweenthe Lower Slaty and Upper Cherty Members. The Upper Cherty Member,as presently defined, is approximately 50 feet thick. The 10loJerpart consists of irregularly bedded to lenticular cherty layersintercalated with thinly-laminated silicate-rich layers that increasein abundance upward. Thin irregular layers of magnetite are commonin the cherty beds near the bottom of the member, but become lessabundant upward. The top of the member-equivalent to Goodwin'supper algal chert facies-is characterized by several granularchert beds containing algal structures, conglomerate fragments,and abundant magnetite.

Upper Slaty Member: The Upper Slaty Member is approximately150 feet thick. Thick lenticular chert beds with disseminatedmagnetite characterize the lower few tens of feet, but most of themember consists of a thin-bedded to laminated quartz-silicate rockinterbedded with thinly laminated layers of graphitir:: argillite,and one to two inch thick beds of relatively pure chert. Theupper 10 feet is nearly magnetite free and consists of limestoneand chert interbedded with argillite.

Rove Formation

The Rove Formation gradationally overlies the Gunflint Ironformation and is intruded by Logan Intrusive Rocks and truncatedby the Duluth Complex. A detailed description of the formationis presented by Morey (1969). In the Long Island Lake, GunflintLake, Southlake, and Hungary Jack Lake quadrangles, the formationis characterized by intercalated black to grayish black, locallycarbonaceous argillite, argillaceous siltstone and fine-grainedgray\oJacke. In general the silt and sandstone beds become coarsegrained, thicker, and more abundant upward in the formation. Inaddition, there are several lenses and irregular bodies of limestone, dolomite, and chert and a number of calcite-dolomite concretions of various shapes and sizes, similar to those describedin the formation by Tanton (1931) and Moorhouse (1963) scatterednear the base of the formation.

Horey (1969) has suggested that deposition started in a deepbasin in which fine-grained sediment accumulated under reducingconditions. The siltstone and sandstone beds contain many primarysedimentary structures indicative of turbidite deposition. Thereis also evidence of an increase infrequency of this type of depositionupward in the section. The graY\oJackes contain abundant frameworkgrains of quartz, feldspar and "granitic" rock fragments indicativeof a granitic source area. Sedimentary structures including crossbedding and various kinds of sole marks indicate that sedimenttransport dominantly \oJas from north to south. These observationsand the gray\oJacke mineralogy led Horey (1969) to conclude that thesource area was the Lower Precambrian terrane now exposed north of theoutcrop area of the Rove Formation.

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Structure

The Animikie strata form a homocline that dips 1O—l5° SEexcept where intrusive bodies and secondary structures have dis-torted or disturbed the beds. For example, complex foldingcommonly occurs in narrow zones adjacent to many of the LoganIntrusive Rocks and is most likely related to the forceful em-placement of the sills into restricted space. There is a fairlyregular increase in dip to as much as 600 near the Duluth Complex.However, this structure is most likely pre—Duluth Complex in age.Lack of marker beds in the Animikie strata inakestecognition offaults difficult; however, a number of northwesterly— and northerly—trending faults have been mapped.

Most faults have displacements of less than 50 feet; however,the Lookout fault has a displacement in the iron—formation of atleast 200 feet. All faults so far recognized displace LoganIntrusive rocks, but none extend into the Duluth Complex; thusmovement apparently occurred in Middle Keweenawan time prior toemplacement of the Duluth Complex. The fault pattern is similarto that in Canada (Moorhouse, 1960; Goodwin, 1960) near what canbe inferred to be the northern hinge of the Lake Superior syncline.This structure must have formed at least in part prior to emp1acenen.tof the Duluth Complex.

Movement on the Lookout fault during Keweenawan time duplicatesthe iron—formation west of the Gunflint Trail, and a northerly—trending segment of the fault separates the iron—Formation into twostructurally distinct terranes. East of the Gunflint Trail, theAnimikie rocks dip gently southward; accordingly the outcrop areaof the iron—formation is relatively wide and most of the apparentirregularities in the map pattern result from the super—positionof a rugged topography on gently—dipping strata. The south sideof the Lookout fault is upthrown, and the Anirnikie strata on thisside dip steeply and thus form a narrow outcrop belt.

A number of smaller structures also can be recognized. Justeast of the Gunflint Trail the iron—formation and the sills arefolded into a southeasterly plunging anticline with a steeplydipping north limb and a gently dipping south limb. This structurewhich is obviously post—Logan in age has an anticlinal axis whichprojects into the trace of the Lookout Fault.

Metamorphism

In the Animikie rocks there is no textural or atineralogicevidence for a pervassive regional metamorphism beyond minorrecrystallization of the clay—size detritral. fraction. The obviousmetamorphic effects in the Animikie strata are associated with theemplacement of the Duluth Complex and Logan Intrusive Rocks of MiddleKeweenawan age.

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Structure

The Animikie strata form a homocline that dips 10°-15° SEexcept where intrusive bodies and secondary structures have distorted or disturbed the beds. For example, complex foldingcommonly occurs in narrow zones adjacent to many of the LoganIntrusive Rocks and is most likely related to the forceful emplacement of the sills into restricted space. There is a fairlyregular increase in dip to as much as 60° near the Duluth Complex.However, this structure is most likely pre-Duluth Complex in age.Lack of marker beds in the Animikie strata makestEcognition offaults difficult; however, a number of northwesterly- and northerlytrending faults have been mapped.

Most faults have displacements of less than 50 feet; however,the Lookout fault has a displacement in the iron-formation of atleast 200 feet. All faults so far recognized displace LoganIntrusive rocks, but none extend into the Duluth Complex; thusmovement apparently occurred in Middle Keweenawan time prior toemplacement of the Duluth Complex. The fault pattern is similarto that in Canada (Moorhouse, 1960; Goodwin, 1960) near ,.,hat canbe inferred to be the northern hinge of the Lake Superior syncline.TIlis structure must have formed at least in part prior to emplacementof the Duluth Complex.

Hovement on the Lookout fault during Keweenawan time duplicatesthe iron-formation west of the Gunflint Trail, and a northerlytrending segment of the fault separates the iron-formation into twostructurally distinct terranes. East of the Gunflint Trail, theAnimikie rocks dip gently southward; accordingly the outcrop areaof the iron-formation is relatively wide and most of the apparentirregularities in the map pattern result from the super-positionof a rugged topography on gently-dipping strata. The south sideof the Lookout fault is upthrown, and the Animikie strata on thisside dip steeply and thus form a narrow outcrop belt.

A number of smaller structures also can be recognized. Justeast of the Gunflint Trail the iron-formation and the sills arefolded into a southeasterly plunging anticline with a steeplydipping north limb and a gently dipping south limb. This structurewhich is obviously post-Logan in age has an anticlinal axis whichprojects into the trace of the Lookout Fault.

i'letamorphism

In the Animikie rocks there is no textural or mineralogicevidence for a pervassive regional metamorphism beyond minorrecrystallization of the clay-size detritral frac'don. The obviousmetamorphic effects in the Animikie strata are associated with theemplacement of the Duluth Complex and Logan Intrusive Rocks of MiddleKe,,,eenawan age.

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The contact aureole of the Duluth Complex has not been studiedin detail in the Gunflint Trail Area. Preliminary work in theGunflint Iron—formation shows there are a number of metamorphicfacies similar to those described from the Biwabik Iron—formationby French (1968) and Bonnichsen (1969). Unmetamorphosed iron—formation like that described by Goodwin (1956) is found in Minnesotaonly in a small area between Gunf lint and North Lakes. There theiron—formation consists of chert, iron carbonates, greenalite, abundantamounts of finely disseminated hematite, and traces of magnetite.However, west of Gunf lint Lake, the iron—formation has been metamorphosedand three metamorphic zones have been distinguished by changes inmineralogy along tI strike of the formation toward the Duluth Complex.

Zone 1, or slightly metamorphosed iron—formation, occurs in thearea immediately west of Gunflint Lake. It consists of quartz, ironcarbonates, greenalite, minnesotaite, and stilpnomelane. Finely—divided hematite occurs in the east end of the zone, hut disappearsmid—way in it. Disseminated and interlocking grains of magnetite areabundant, especially as rims around granules. This part of the iron—formation is much like that described by French (1968) in 'unmetaniorphosedBiwabik Iron—formation.

Zone 2, or moderately metamorphosed iron—formation, is about 1.2 mileswide and extends to within 0.3 miles of the Duluth Complex. Grunerite—cummingtonite, hornblende and actinolite, as well as quartz and magnetitecharacterize this zone. As in zone 1, much of the magnetite is between0.002 and 0.02 mm in diameter; a size—range similar to that observed invarious other Lake Superior iron—formations. Small—scale pre—metamorphicsedimentary structures such as granules and oolites are partly destroyed,but larger—scale primary structures and bedding features are littleaffected.

Zone 3, or highly metamorpl-iosed iron—formation, occurs adjacentto the Duluth Complex and is characterized by a wholly metamorphicfabric. The rock is composed chiefly of quartz, magnetite, iron—richpyroxenes, and fayalite. Very commonly, euliedral or subliedral grainsof magnetite are poikilitically enclosed 'Tithin large silicate grains;they are of essentially the same size as in the lower grade rocks.However a significant part of the ragnetite is extensively recrystallized,and grains as much as a millimeter in diameter are concentrated alongbedding planes. Actinolite is common in magnetite—rich layers, andboth prograde and retrograde cummingtonite :Ls±un(lantly oresent. In

general this zone is very similar to that described in the J)unka Piverarea on the I'lesabi range by Bonnichsen (1)69).

Preliminary work on the contact aureole o the Duluth Complex inthe Rove Formation indicates a complex mixture of rock tyPes suggestingpartial melting, and mineral and textural variations due to originalinhoinogenities and degree of netainorphilari. Tue metamorphosed rocks arecommonly layered and have a granoblastic texture. Individual layerscontain; 1) cordierite and Iyperstliene with minor hiotite and ilmenire,2) hypcrsthene, plagioclase, hiotite, and ilmanite, 3) augite,plagioclase ± minor olivine, biotite, and ilmentte, or 4) hvperstliene,

plagioclase, K—feldspar, and hiotite. Calcareous beds near the contacthave a skarn—minera.logy consisting of wollastonite, diopside, tremolite,and grossularite garnet. Away from the contact and from piece to eLace

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The contact aureole of the Duluth Complex has not been studiedin detail in the Gunflint Trail Area. Preliminary work in theGunflint Iron-formation shows there are a number of metamorphicfacies similar to those described from the Bh\Tabik Iron-formationby French (1968) and Bonnichsen (1969). Unmetamorphosed iron-formation like that described by Good\vin (1956) is found in Hinnesotaonly in a small area between Gunflint and North Lakes. There theiron-formation consists of chert, iron carbonates, greenalite, abundantamounts of finely disseminated hematite, and traces of magnetite.However, west of Gunflint Lake, the iron-formation has been metamorphosedand three metamorphic zones have been distinguished hy changes inmineralogy along tie strike of the formation tm'Jard the Duluth Complex.

Zone 1, or slightly metamorphosed iron-formation, occurs in thearea immediately west of Gunflint Lake. It consists of quartz, ironcarbonates, greenalite, minnesotaite, and stilpnomelane. Finely-divided hematite occurs in the east end of the ZOne, but disappearsmid-way in it. Disseminated and interlocking grains of magnetite areabundant, especially as rims around granules. This part of the i ronformation is much like that described by French (1968) in "unmetamorphosed"Biwabik Iron-formation.

Zone 2, or moderately metamorphosed iron-formation, is about 1.2 mileswide and extends to \vithin 0.3 miles of the Duluth Complex. Gruneritecummingtonite, hornblende and actinolite, as well as quartz and map,netitecharacterize this zone. As in zone 1, much of the magnetite is hetHeen0.002 and 0.02 mm in diameter; a size-range similar to that observed invarious other Lake Superior iron-formations. S:'1all-scale pre-metAmorphicsedimentary structures such as granules and oolites are partly destroyed,but larger-scale primary structures and beddinr, features are littleaffected.

Zone 3, or highly metan,orphosed iron-formation, occurs adjacentto the Duluth Complex and is characterized by a ",holly metamorphicfabric. The rock is composed chiefly of quartz, magnetite, iron-richpyroxenes, and fayalite. Very commonly, eultedral or suhl1edral (.~rains

of magnetite are poikilitically Ptlclose.d \orithin large silicate (~rains;

they are of essentially the Si1Iile size .-:IS in the lOHer grade rocks.Hmvever a significant part of tile magnetite is extensively recrystallized,and grains as much <18 a millimeter in diiwleter arc concentrated alongbedding planes. Actinoli te is cor:lJl1on in rnagm~tite-rich layers, andboth pro~rade and retro0rrtde cummin;>:tonitc is -:hundnntly nresent. Ingeneral this zone is very similar to thClt de~;cribed in the Dunka!~oiver

area on the :olesabi ranre by nonnicbsen (l,)6~l).

Preliminary l\Tork on the contnc t aureole 0 f the lluluth Complex inthe Rove Formation indicates a complex mixture of rock tyoes sUfgcstinRpartial meltint, and mineral and textural variations due to originalinhornogenities and degree of liletamorr1dsr,l. The met"mop)!losed rocKs :HC

commonly layered ane! have il granohlastic texture. Individual layerscontain; 1) corcJierite and hynersthene '\Tith loinor biotite and ilmenite,2) hypersthene, plagioclase, hiotite, and ilmenite, J) aur-ite,plagioclase ± minor olivine, biotite, "at! ilmeni.t0., or 1+) hypersthe.ne,plagioclase, K-feldsrar, and biotite. Calcareous GeJs ncar the contacthave a skarn-mineralogy consisting of lvollastonitc, diopsi<le, tremolite,and 8rossularite p:arn0.t. AI<7ay frol'l tile contflct - nnd fron plnee to Dlace

—10 7—

this distance varies from several tens to several hundreds of feet —most peletic rocks are rich in biotite, and cordierite may or maynot be present.

The Logan Intrusive Rocks also have metamorphosed the Gunf lintand Rove Formations. The width of the metamorphic aureoles are relatedto sill thicknesses and range from less than one foot to more than 30feet. In the Gunflint Iron—formation it is difficult to recognizeunique metamorphic assemblages adjacent to sills. In zone 2, mineralassemblages characteristic of zone 3 occurs in aureoles around the sills.Similarly In zone 1, ininnesotaite, which is characteristic of zone 2metamorphism is found next to the sills.

In the Rove Formation thick sills have assemblages that can beassigned to the pyroxene—hornfels facies whereas thinner sills haveassemblages characterisitc of the hornblende—hornfels facies. Locallyandalusite (Morey, 1969) and chloritoid (Grant, 1971) have been identifiedin certain beds.

UPPER PRECAMBRIAN

Introduction

In northeastern Minnesota two units are recognized in the UpperPrecambrian: the Lower Keweenawan represented by the PuckwungeFormation and the Middle Keweenawan which consists of the Logan IntrusiveRocks, the North Shore Volcanic Group (Green, this volume), and theDuluth Complex.

Puckwunge Formation

The Puckwunge Formation consists of conglomerate and sandstonewhich unconformably overlie the Rove Formation and underlie the NorthShore Volcanic Group. The type locality was described by N. N. Winchell(1897) on the Stump River in Sec. 25, T. 64 N., R. 3 B. where about 18feet of section is exposed. Exposures of similar lithology are frnind inthe Grand Portage area (Grout and others, 1959).

If these isolated exposures are equivalent to the Sibley Seriesin Canada (Tanton, 1931) they represent a period of sedimentation around1376 * 36 m.y. ago (Franklin and Kustra, 1970) followed by possibleuplift and erosion prior to the onset of volcanic activity in MiddleKeweenawan time.

Logan Intrusive Rocks

The name Logan sills was applied to tabular, diabase intrusiverocks in the Rove Formation (Lawson,. 1893, p. 48). Similar intrusiverocks in Animikie and Lower Keweenawan formations are currentlyreferred to as Logan Intrusive Rocks. As now defined, they includesills and dikes of diabasic gahhro which range in measured age from1300 n.y. (hanson and [alhotra, 1971) to 963 m.y. (Franklin, citedLn Wanless and others, 1970, p. 57). Thus the Logan Intrusive Rocks,

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this distance varies from several tens to several hundreds of feetmost peletic rocks are rich in biotite, and cordierite mayor maynot be present.

The Logan Intrusive Rocks also have metamorphosed the Gunflintand Rove Fonnations. The width of the metamorphic aureoles are relatedto sill thicknesses and range from less than one foot to more than 30feet. In the Gunflint Iron-formation it is difficult to recognizeunique metamorphic assemblages adjacent to sills. In zone 2, mineralassemblages characteristic of zone 3 occurs in aureoles around the sills.Similarly in zone 1, minnesotaite, which is characteristic of zone 2metamorphism is found next to the sills.

In the Rove Fonnation thick sills have assemblages that can beassigned to the pyroxene-hornfels facies whereas thinner sills haveassemblages characterisitc of the hornblende-hornfels facies. Locallyandalusite (Horey, 1969) and chloritoid (Grant, 1971) have been identifiedin certain beds.

UPPER PRECAMBRIAN

Introduction

In northeastern Minnesota two units are recognized in the UpperPrecambrian: the Lower Keweenawan represented by the PuckwungeFormation and the Middle Keweenawan which consists of the Logan IntrusiveRocks, the North Shore Volcanic Group (Green, this volume), and theDuluth Complex.

Puckwunge Fonnation

The Puckwunge Formation consists of conglomerate and sandstonewhich unconfonnably overlie the Rove Fonnation and underlie the NorthShore Volcanic Group. The type locality was described by N. H. Winchell(1897) on the Stump River in Sec. 25, T. 64 N., R. 3 E. where about 18feet of section is exposed. Exposures of similar lithology arefuund inthe Grand Portage area (Grout and others, 1959).

If these isolated exposures are equivalent to the Sibley Seriesin Canada (Tanton, 1931) they represent a period of sedimentation around1376 ± 36 m.y. ago (Franklin and Kustra, 1970) followed by possibleuplift and erosion prior to the onset of volcanic activity in MiddleKeweenawan time.

Logan Intrusive Rocks

The name Logan sills was applied to tabular, diabase intrusiverocks in the Rove Formation (Lm"rson, 1893, p. 48). Similar intrusiverocks in Animikie and Lm.,rer Keweenawan formations are currentlyreferred to as Logan Intrusive Rocks. As now defined, they includesills and dikes of diabasic gabbro which range in measured age from~300 m.y. (Hanson and Ha1hotra, 1971) to 963 m.y. (Franklin, citedIn Hanless and others, 1970, p. 57). Thus the Logan Intrusive Rocks,

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units of the Duluth Complex, and the North Shore Volcanic Group mayhave overlapping time—stratigraphic relationships.

In Cook County the predominant exposed volume of the Logan rocks

is sill—like, and the present sawtooth topography results from the

differential erosion of the inclined sills and Rove Formation. The

sills range in thickness from a few feet to over a thousand feet and

thick sills can be mapped along strike for several miles. In three

dimensions they form a boxwork pattern, their emplacement having beennntrolled by bedding, joint and possibly fault plane connections betweensills but in the Grand Portage in northeastern Cook County area thereare major northwest—trending dikes comparable in size and extent to the

sills (Grout and Schwartz, 1933, P. 36—59).

Early detailed studies of the petrogenesis of the Logan IntrusiveRocks were primarily restricted to the sill on Pigeon Point (Bayley,1893; Daly, 1917; Grout and Schwartz, 1933). Reinvestigation of these

rocks is currently in progress and the following generalized descrip-tion of textures, and mineralogy is based on the unpublished work ofJ. A. Grant, E. Mathez, G. B. Morey, N. Mudrey and P. Weiblen.

Texture: Rock types include aphyric chilled margins, fine— tomedium—grained diabase with ophitic clinopyroxene enclosing plagioclase,porphyritic diabase with plagioclase phenocrysts, plagioclase cumulates,and granophyre. Chilled margins form sharp contacts with country rocks.Diabase grades from fine to medium toward the center of sills. Clino—

pyroxene remains ophitic even though the grain size changes and enclosedplagioclase crystals rarely exceed 5 mm in length. Plagioclase pheno-

crysts however are as much as 10 cms long in thick sills. Diabasicrocks with plagioclase phenocrysts grade into accumulations of essentiallycoarse—grained plagioclase. Such accumulations are found in the upperparts of some large sills and their origin has been attributed to thefloating of plagioclase (Grout and Schwartz, 1933, p. 50).

M1neraly: Minor olivine is found in the lower part of somesills. Plagioclase both as phenocrysts and as small grains enclosed inpyroxene is generally highly seriticized. Clinopyroxene is altered toamphibole within single oikocrysts. The remnant pyroxene varies incomposition from pigeonite through augite and has a mottled birefringencewhich resembles that of the complex lunar pyroxenes. lirnenite and minormagnetite appear at distinct horizons in some sills. The ilmenite iscommonly skeletal. Minor interstitial quartz is characteristic ofmuch of the diabase. Biotite is ubiquitous in the diahase and texturallyappears to be of both igneous and metamorphic origin. Granophyricintergrowths are common in the upper part of the diabase where theyimpart a pink mottling which can be recognized In hand specimen. Separateoccurrences of granophyre in mappable units are rare, except for thePigeon Point sill (Mudrey and Weiblen, 1971).

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units of the Duluth Complex, and the North Shore Volcanic Group mayhave overlapping time-stratigraphic relationships.

In Cook County the predominant exposed volume of the Logan rocksis sill-like, and the present smvtooth topography results from thedifferential erosion of the inclined sills and Rove Formation. Thesills range in thickness from a fe\v feet to over a thousand feet andthick sills can be mapped along strike for several miles. In threedimensions they form a boXtVork pattern, their emplacement having been~ntro11ed by bedding, joint and possibly fault plane connections betweensills but in the Grand Portage in northeastern Cook County area thereare major northwest-trending dikes comparable in size and extent to thesills (Grout and Schwartz, 1933, p. 36-59).

Early detailed studies of the petrogenesis of the Logan IntrusiveRocks were primarily restricted to the sill on Pigeon Point (Bayley,1893; Daly, 1917; Grout and Sch\vartz, 1933). Reinvestigation of theserocks is currently in progress and the follmving generalized description of textures, and mineralogy is based OIl the unpublished work ofJ. A. Grant, E. Mathez, G. B. Morey, N. Mudrey and P. Weiblen •

.Texture: Rock types include aphyric chilled margins, fine- tomedium-grained diabase with ophitic clinopyroxene enclosing plagioclase,porphyritic diabase with plagioclase phenocrysts, plagioclase cumulates,and granophyre. Chilled margins form sharp contacts with country rocks.Diabase grades from fine to medium toward the center of sills. Clinopyroxene remains ophitic even though the grain size changes and enclosedplagioclase crystals rarely exceed 5 rom in length. Plagioclase phenocrysts however are as much as 10 ems long in thick sills. Diabasicrocks with plagioclase phenocrysts grade into accumulations of essentiallycoarse-grained plagioclase. Such accumulations are found in the upperparts of some large sills and their origin has been attributed to thefloating of plagioclase (Grout and Schwartz, 1933, p. 50).

Mineralogy: Minor olivine is found in the lower part of somesills. Plagioclase both as phenocrysts and as small grains enclosed inpyroxene is generally highly seriticized. Clinopyroxene is altered toamphibole within single oikocrysts. The remnant pyroxene varies incomposition from pigeonite through augite and has a mottled birefringencewhich resembles that of the complex lunar pyroxenes. Ilmenite and minormagnetite appear at distinct horizons in some sills. The ilmenite iscommonly skeletal. Minor interstitial quartz is characteristic ofmuch of the diabase. Biotite is ubiquitous in the diabase and texturallyappears to be of both igneous and metamorphic origin. Granophyricintergrowths are common in the upper part of the diabase \vhere theyimpart a pink mottling which can be recognized in hand specimen. Separateo~currences of granophyre in mappable units are rare, except for theP~geon Point sill (Hudrey and Weib1en, 1971).

—10 9—

Structure: In plan view the Duluth Complex truncates the variousLogan sills along strike at a low angle (Fig. 3). Where exposed, thebase of the Duluth Complex dips gently southward, whereas both theLogan sills and country rocks may dip as much as 60° to the south.Drilling in the vicinity of the contact indicates that the base of theComplex steepens to as much as 60° and levels off to about 30° 1 km.down—dip (Johnson, 1970, p. 82). This structural configuration izuch like that described by Mancuso and Dolence (1970) in the East?Iesabi district. They suggested that the emplacement of the DuluthComplex in that area was in part controlled by a pre—Complex structure.

>inera1ization: :1inor hydrothermal mineralization is found withinthe Logan Intrusive Rocks. The mineralization is probably similar tothe Thunder Bay silver deposits, but no commercially important occurrenceshave been found in Minnesota. On Susie Island where a shaft was sunkalong a fracture zone filled with calcite—barite and lesser amounts ofquartz, bornite, chalcocite, chalcopyrite, pyrite, covellite andmalacite, ore was recovered which contained 6.22 percent copper andtrace amounts of silver (Grout and Schwartz, 1933, p. 64). At LoonLake, S. Blankenburg has investigated a prospect in a quartz—calcitevein that contains arsenopyrite with minor cobalt (Johnson, 1968).

Duluth Complex

The Duluth Complex consists of a variety of anorthositic, troctolitic,granodioritic, and granophyric rocks which crop out in an arcuate beltfrom Duluth to east of the Gunflint Trail (Fig. 1). A brief review ofthe early literature and recent work up to 1969 is given by Phinney(1969). The results of subsequent mapping and study of the northeasternlimb of the Complex are presented by Nathan (1969), Morey and others(1969), Johnson a970), and Davidson (1970).

Nathan (1969) mapped a layered series of sheet—like intrusions acrossthe Gunflint, South Lake and Hungry Jack Lake quadrangles (Fig. 2). To

the west, in the Long Island quadrangle, Nathan's layered series istruncated by the Tuscarora Intrusion and other associated rocks.

Layered Series of Nathan

The layered series extends from the east edge of the Hungry JackLake quadrangle across the South Lake quadrangle and into the GunflintLake quadrangle where it is truncated by the Tuscarora Intrusion. Thistruncation is marked by an irregular northwest trending scarp (Fig. 3).For the most part, the series consists of a sequence of conformablesheets having a regional dip of 15—25° to the south. The sheets thickento the west and are locally interrrupted by minor crosscutting stock—and dike—like bodies. On the east side of the Hungry Jack Lakequadrangle a northwest trending fault offsets the series with an unknownamount of displacement, but as much as 140 feet of vertical displacementof the northeast side is inferred (Fig. 3).

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Structure: In plan view the Duluth Complex truncates the variousLogan sills along strike at a low angle (Fig. 3). Where exposed, thebase of the Duluth Complex dips gently southward, whereas both theLogan sills and country rocks may dip as much as 60° to the south.Drilling in the vicinity of the contact indicates that the base of theComplex steepens to as much as 60° and levels off to about 30° 1 km.down-dip (Johnson, 1970, p. 82). This structural configuration ismuch like that described by Hancuso and Dolence (1970) in the EastMesabi district. They suggested that the emplacement of the DuluthComplex in that area was in part controlled by a pre-Complex structure.

Mineralization: Hinor hydrothermal mineralization is found withinthe Logan Intrusive Rocks. The mineralization is probably similar tothe Thunder Bay silver deposits, but nO commercially important occurrenceshave been found in Minnesota. On Susie Island where a shaft was sunkalong a fracture zone filled with calcite-barite and lesser amounts ofquartz, bornite, chalcocite, chalcopyrite, pyrite, covellite andmalacite, ore was recovered which contained 6.22 percent copper andtrace amounts of silver (Grout and Schwartz, 1933, p. 64). At LoonLake, R. Blankenburg has investigated a prospect in a quartz-calcitevein that contains arsenopyrite with minor cobalt (Johnson, 1968).

Duluth Complex

The Duluth Complex consists of a variety of anorthositic, troctolitic,granodioritic, and granophyric rocks which crop out in an arcuate beltfrom Duluth to east of the Gunflint Trail (Fig. 1). A brief review ofthe early literature and recent work up to 1969 is given by Phinney(1969). The results of subsequent mapping and study of the northeasternlimb of the Complex are presented by Nathan (1969), Horey and others(1969), Johnson Q970), and Davidson (1970).

Nathan (1969) mapped a layered series of sheet-like intrusions acrossthe Gunflint, South Lake and Hungry Jack Lake quadrangles (Fig. 2). Tothe west, in the Long Island quadrangle, Nathan's layered series istruncated by the Tuscarora Intrusion and other associated rocks.

Layered Series of Nathan

The layered series extends from the east edge of the Hungry JackLake quadrangle across the South Lake quadrangle and into the GunflintLake quadrangle \vhere it is truncated by the Tuscarora Intrusion. Thistruncation is marked by an irregular northwest trending scarp (Fig. 3).For the most part, the series consists of a sequence of conformablesheets having a regional dip of 15-25° to the south. The sheets thickento the west and are locally interrrupted by minor crosscutting stockand dike-like bodies. On the east side of the Hungry Jack Lakequadrangle a nortlnvest trending fault offsets the series with an unknownamount of displacement, but as much as 140 feet of vertical displacementof the northeast side is inferred (Fig. 3).

—110—

The series consists of troctolitic, gabbroic, and associatedfelsic rocks. Several of the major units represent uniqud occur-rences in the Duluth Complex of oxide—rich gabbros and two—pyroxenegabbros. For the most part, fine—grained rocks are not chilledmargins but occur principally as separate intrusions or inclusionsof mappable size. Planar orientation of minerals is common,indicating flow or crystal setting. Differentiation resulting fromthese processes can be demonstrated within some units, however thelayered series does not form a regular stratigraphic sequence.Intrusive relationships for 27 different units were establishedusing cross—cutting structs, fine—grained margins, inclusionsand thermal effects, the latter being principally a developmentof dark clouded plagioclase near intrusive contacts, Nathan (1969,

p. 99). On the basis of field relationships, mineralogy, and composi-tion, Nathan concluded the 27 units could be combined into eightcogenetic groups.

Detailed rock descriptions and interpretations are given byNathan (1969, p. 38—185) and are summarized here using his nomen-clature. The descriptive rock names have several textural prefixesthat characterize the primary mineral assemblages. A size class-ification for these rocks, based on visually—estimated mean graindiameters, is: >10 mm, very coarse—grained; 4—10 mm, coarse—grained;1—4 mm, medium—grained; 1/2—1 mm, fine—grained; <1/2 mm, very fine—grained. Four basic fabrics are recognized. A 'granular' rock has

only equidimensional minerals. If elongate grains are present andare randomly oriented, the rock is "decussate," if the grains define

a plane, the rock is "foliated," and if the grains are aligned, the

rock is "lineated." All rocks are named by the characterizingprimary mineral assemblage, in order of increasing abundance; theprimary mineral assemblage refers to early crystallizing phases in

contrast to late interstitial phases. Modes are indicated by sub—

scripts. If a significant part of the primary assemblage wastransported in the magma the rock is referred to as an allocrystallate,

a cumulate being a special case in which gravity settling has occurred.

Rocks formed by crystallization in place are called autocrystallates.

Subscripts are used to indicate modal composition. Thus a descriptive

rock name for a troctolite formed by gravity settling could he a fine—

grained foliated olivine20—plagioclase70 cumulate with minor inter—

stitital augite5and oxides5.

Group 1 The oldest unit (da) in the layered series now appearsin the upper part of the section as dilated septa as

da much as 200 feet thick. It is a fine—grained foliated

olivine - —plagioclase63 cumulate with minor pigeonite9

and aug?e which are also cumulus in the middle of

the unit. his unit grades into a 1,000 foot—thick

sheet of medium—grained, foliated augite15—pigeonite26

db plagioclase59curnulate (dh). A very fine—grained

dc ollvine1 —augite27—plagioclase61 rock (dc) occurs as

masses o9 various shapes and sizes near the base of

the layered series. Nathan speculated that it mightrepresent a chilled margin of the Group 1 intrusive

rocks.

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The series consists of troctolitic, gabbroic, and associatedfelsic rocks. Several of the major units represent unique occurrences in the Duluth Complex of oxide-rich gabbros and two-pyroxenegabbros. For the most part, fine-grained rocks are not chilledmargins but occur principally as separate intrusions or inclusionsof mappable size. Planar orientation of minerals is cornman,indicating flow or crystal setting. Differentiation resulting fromthese processes can be demonstrated within some units, however thelayered series does not form a regular stratigraphic sequence.Intrusive relationships for 27 different units were establishedusing cross-cutting structures, fine-grained margins, inclusionsand thermal effects, the latter being principally a developmentof dark clouded plagioclase near intrusive contacts, Nathan (1969,p. 99). On the basis of field relationships, mineralogy, and composition, Nathan concluded the 27 units could be combined into eightcogenetic groups.

Detailed rock descriptions and interpretations are given byNathan (1969, p. 38-185) and are summarized here using his nomenclature. The descriptive rock names have several textural prefixesthat characterize the primary mineral assemblages. A size classification for these rocks, based on visually-estimated mean graindiameters, is: >10 mm, very coarse-grained; 4-10 rnrn, coarse-grained;1-4 mm, medium-grained; 1/2-1 mm, fine-grained; <1/2 mm, very finegrained. Four basic fabrics are recognized. A "granular" rock hasonly equidimensiona1 minerals. If elongate grains are present andare randomly oriented, the rock is "decussate," if the grains definea plane, the rock is "foliated," and if the grains are aligned, therock is l'lineated. II All rocks are named by the characterizingprimary mineral assemblage, in order of increasing abundance; theprimary mineral assemblage refers to early crystallizing phases incontrast to late interstitial phases. Modes are indicated by subscripts. If a significant part of the primary assemblage wastransported in the magma the rock is referred to as an allocrystallate,a cumulate being a special case in which gravity settling has occurred.Rocks formed by crystallization in place are called autocrystallates.Subscripts are used to indicate modal composition. Thus a descriptiverock name for a troctolite formed by gravity settling could he a finegrained foliated 01ivinezo-plagioclase70 cumulate \-lith minor interstitital augitesand oxides S•

Group 1

da

dbde

The oldest unit (da) in the layered series now appearsin the upper part of the section as dilated septa asmuch as 200 feet thick. It is 8 fine-grained foliated01ivine~s-plagioclase63cumulate with minor pigeonite9and aug1te which are also cumulus in the middle ofthe unit. ~his unit grades into a 1,000 foot-thicksheet of medium-grained, foliated augiteIS-pigeonite26piagioclase

S9cumulate (dh). A very fine-grained

olivine o-augite27-plagioclaseGl rock (de) occurs asmasses ~f various shapes and Slzes near the base ofthe layered series. Nathan speculated that it mightrepresent a chilled margin of the Group 1 intrusiverocks.

—111—

Groip2 This group comprises the major oxide—rich part of thedg layered series. The main unit (dg) is a coarse—grained

foliated ilmenite—titanomagnetite —augite —olivine

plagioclase6

cumulate. Within unit e followingsequence of5crystallization appears: (1) ilmenite—

olivine—plagioclase; (2) ilmenite—titanomagnetite—olivine—augite—plagioclase; and finally (3) apatite—pigeonite—titanoinagnetite—ilmenite—olivine—augite—plagioclase.

dd Very fine—grained foliated olIvine—oxide—augite—plagioclasede (dd) and fine—grained granulo—decussate augite — plagioclase

(de) rocks occur in unit dg as inclusions as much as 400feet across. A number of mappable units are gradational withunit dg: (1) at the base of the complex, a fine— to coarse—grained decussate augite—olivine—plagioclase autocrystallate

df (df) grades into rocks having a texture and mineralogy similarto that in unit dg,and may be the base of the cumulates ofunit dg. Unit df shows sulfide mineralization similar tounit ttf of the Tuscarora Intrusion (Johnson, 1970, p. 68).(2) A fine—grained, olivine2Q—p1agioclase6—o1ivine90cumulate with miner augite —oxide4—pigeonie —apatite1occurs as a thin sheet within unit dg. (3) * fine—grained,decussate, oxide2 —augite28—plagioclase

5autocrystallate

containing minor livine2 and apatite2 tntrudes unit dg anddi is presumed to be a late differentiate (di). (4) Unit dg

grades upward into 100—200 foot—thick discontinuous sheetsof coarse—grained, foliated pigeonite4—titanomagnetite4—augite6—plagioclase83 cumulatescontaining minor potassium

dj feldspar2 and quartz1 (dj). A fine—grained granulardk plagioclase, quartz, orthoclase rock (dk) occurs as dikes

cutting other Group 2 rocks.

Groyp3 Unit din, the upper part of the layered se•:ies, is a

2,000 foot—thick sheet of coarse—grained decussatedin pigeonite11—augite24--plagioclase59 autocrystallates

and cumu1aes containing minor oxides2, quartz2and potassium feldspar1. This unit is thought tohave been emplaced along foliation planes and thus tohave dilated the earlier units. Variations in grainsize and modal mineralogy suggest th.t this unit maybe a multiple intrusion. A related intrusion may be

dl unit dl, a inedluin—grained decussate pigeonite17—augite20—plagioclase50 rock containing minor oxide,quartz, and potassium feldspar, which occurs as asmall stock about 1—1/2 miles across in the centralpart of the Hungry Jack quadrangle. Another relatedintrusive unit is a fine—grainS granular oxide19—

dn plagioclase41—augite4 rock (dxi) which occurs as asheet 6 feet thick anI 4 miles long in the northernpart of the Souti Lake quadrangle. Felsic dikes which

do intrure unit dcL were given a separate designation (do)and may represent a late—stage differentiate of unit djor partially fused country rock (Nathan, p. 115).

Group 2dg

ddde

df

di

djdk

dm

c..ll

dn

do

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This group comprises the major oxide-rich part of thelayered series. The main unit (dg) is a coarse-grainedfoliated ilmenite-titanOmagnetitell-augitel~-olivineI4

plagioclaseS6

cumulate. Within tff~s unit f e followrngsequence of crystallization appears: (1) ilmeniteolivine-plagioclase; (Z) ilmenite-titanomagnetite-olivineaugite-plagioclase; and finally (3) apatite-pigeonitetitanomagnetite-ilmenite-olivine-augite-plagioclase.Very fine-grained foliated olivine-oxide-augite-plagioclase(dd) and fine-grained granulo-decussate augite - plagioclase(de) rocks occur in unit dg as inclusions as much as 400feet across. A number of mappable units are gradational withunit dg: (1) at the base of the complex, a fine- to coarsegrained decussate augite-olivine-plagioclase autocrystallate(df) grades into rocks having a texture and mineralogy similarto that in unit dg,and may be the base of the cumulates ofunit dg. Unit df shows sulfide mineralization similar tounit ttf of the Tuscarora Intrusion (Johnson, 1970, p. 68).(Z) A fine-grained, olivinezo-plagioclase66-olivine20cumulate with minor augite5-oxide4-pigeon~te4-apatiEel

occurs as a thin sheet ,vitfiin unit dg. (3) A fine-graine4,decussate, oxideZ3-augite2S-plagioclase45 autocrystallatecontaining minor olivineZ and apatitez intrudes unit dg andis presumed to be a late differentiate (di). (4) Unit dggrades upward into 100-ZOO foot-thick discontinuous sheetsof coarse-grained, foliated pigeonite4-titanomagnetite4augite

6-p1agioclase

S3cumulates containing minor potass~um

feldspar2

and quartzl

(dj). A fine-grained granularplagioclase, quartz, orthoclase rock (dk) occurs as dikescutting other Group 2 rocks.

Unit dm, the upper part of the layered se~ies, is a2,000 foot-thick sheet of coarse-grained decussatepigeonite13-augite24-~lagi~clase59autocrystallatesand cumulates conta~n~ng nanor oX~desZ' quartz

2and potassium feldsparl

. This unit is thought tohave been emplaced along foliation planes and thus tohave dilated the earlier units. Variations in grainsize and modal mineralogy suggest that this unit maybe a multiple intrusion. A related intrusion may beunit dl, a medium-grained decussate pigeonite

17

augite20-plagioclase60 rock containing minor oxide,quartz, and potassium feldspar, ''''hich occurs as asmall stock about 1-1/2 miles across in the centralpart of the Hungry Jack quadrangle. Another relatedintrusive unit is a fine-grained granular oxide

19

plagio~lase4l-a~gite40 rocl~ (dn) 1vhich occurs as asheet 6 feet tlack and 4 mlles long in the northernpart of the South Lake quadranp;le. Felsic dikes whichintrude unit dm \vere given a separate designation (do)and may represent a late-stage differentiate of unit djor partially fused country rock (Nathan, p. 115).

—112—

Group 4 The next intrusive unit (dp) occurs as a concordantdp sheet as much as 1,200 feet thick between units dg and

db. It is a fine—grained foliated augite32—plagioclase58cumulate with olivine3—hypersthene.7 and trace amounts ofoxides. Plagioclase and augite occur in a granularfabric, hyperstheneas oikocrysts, and olivine as pheno—crysts. The absence of late—stage interstitial materialsuggests the unit formed by flow or crystal settlingwith exchange between the magma and the inner cumulusmelt. Near the top of unit dp, a 100—foot—thick sheetof medium—grained foliated olivine17—plagioclase83

dq cumulate (dq) is gradational with unit dp.

Group 5 In the southwestern part of the Gunf lint Lake quadrang!lea heterogeneous assortment of pyroxene—plagioclase andolivine—plagioclase rocks possibly related to unit ttaof the Tuscarora Intrusion truncate the layered series.A typical example is a medium—grained decussate tironals5—

dr ollvine5—augite 5—plagioclase75 autocrystallate (dr).Near the base o the layered series a medium—grainedfoliated augite1 —plagioclase77 rock with minor oxide3—

ds pigeonite and oivine (ds) is intruded as a sheet300 feet hick between units df and dg. Unit ds ishighly altered with montmorillonite after plagioclase,and amphibole and chlorite after pyroxene. Locallychalcopyrite and bornite occur within interstitial,altered pyroxene.

Group 6 A number of oxide—rich stock— and dike—like bodies3/4 miles across occur near the base of the Complexin the South Lake quadrangle. Nathan recognized fourvarieties: (1) medium—grained granular olivine16—oxide73 rocks with a minor amount of plagioclase and

dt augite (dt). The ilmenite andtitanomagnetite occuras primary phases; the latter generally has exsolvedcoarse ilmenite lamellae with intra—titanomagnetitegranules which have in turn exsolved to a fine reticulateintergrowth of magnetite and hercynite.(')On Little IronLake in the South Lake quadrangle a mediunv-grained

du granular plagioclase 1—oxide24—olivine (du) rockhaving minor hyperstLne and augite fogs a 1/2 milelong composite sheet within unit dt.(3)A coarse--grainedgranular plagioclase8—oxide23_pjgeonjt2, —augiterock containing minor olivine occurs as inall dicordant masses in the South Lake quadrangle. (4) Twooccurrences of a coarse—grained decussate oxide —

dw augite31_p1agioc autocrystallate (dw) wer0mappedwithin unit dg. This unit exhibits an amphibolealteration similar to that in unit ds, but is con-sidered to be part of group 5 rocks because of itslarge oxide content.

Group 4dp

dq

Group 5

dr

ds

Group 6

dt

du

dw

-112-

The next intrusive unit (dp) occurs as a concordantsheet as much as 1,200 feet thick bet'}7een units dg anddb. It is a fine-grained foliated augite32-plagioclaseS8cumulate with olivine -hypersthene"? and trace amounts ofoxides. Plagioclase ~nd augite occur in a granularfabric, hypersthene as oikocrysts, and olivine as phenocrysts. The absence of late-stage interstitial materialsuggests the unit formed by flow or crystal settlingwith exchange between the magma and the inner cumulusmelt. Near the top of unit dp, a 100-foot-thick sheetof medium-grained foliated olivine17-plagioclase83cumulate (dq) is gradational with unit dp.

In the southwestern part of the Gunflint Lake quadrangiLea heterogeneous assortment of pyroxene-plagioclase andolivine-plagioclase rocks possibly related to unit ttaof the Tuscarora Intrusion truncate the layered series.A typical example is a medium-grained decussate tironals

S

olivines-augitelS-plagioclase75 autocrystallate (dr).Near the base of the layered series a medium-grainedfoliated augite17-plagioclase77 rock with minor oxide

3

pigeonite? and olivine (ds) is intruded as a sheet300 feet chick between units df and dg. Unit ds ishighly altered with montmorillonite after plagioclase,and amphibole and chlorite after pyroxene. Locallychalcopyrite and bornite occur within interstitial,altered pyroxene.

A number of oxide-rich stock- and dike-like bodies3/4 miles across occur near the base of the Complexin the South Lake quadrangle. Nathan recognized fourvarieties: (1) medium-grained granular olivine

l6

oxide 73 rocks with a minor amount of plagioclase andaugite (dt). The ilmenite anddtanomagnetite occuras primary phases; the latter generally has exsolvedcoarse ilmenite lamellae with intra-titanomagnetitegranules which have in turn exsolved to a fine reticulateintergrowth of magnetite and hercynite.C)On Little IronLake in the South Lake quadrangle a medium-grainedgra~ular,plagioclase1-oxide

24-olivine

49(du) rock

hav1ng m1nor hypersttene and augite forms a 1/2 milelong composite sheet within unit dt.G)A coarse-grainedgranular plagioclase -oxide -pigeonite -augiterock t '· i 8 I' ,23 24 44con a1n1ng m nor 0 1Vlne occurs as small dis-cordant masses in the South Lake quadrangle. UI) Twooccurrences of a coarse-grained decussate oxide _

't l' 1 10a~g1,e3l-~ ag10c aseS5 autocrystallate (dw) were mappedwlth1n unlt dg. This unit exhibits an amphibolealteration similar to that in unit ds but is considered to be part of group 5 rocks b;cause of itslarge oxide content.

—113—

Groupj Several fine—grained decussate rocks occur asdiscontinuous thin sheets along the base of the com-

plex and as small stocks and dikes higher in thesection. They may represent either fused fractionsof country rock or contaminated melts, but all arethought to be intrusive. They consist of: (1) fine—

grained decussate oxide7—augite2plagioclase66dx rock having minor olivine2—quartz2 and potassium1

feldspar (dx); (2) fine—grained decussate oxideç

dy hypersthone —augite31—plagioclase55 autocrystallate(dy); and (?Y fine—grained augite3—oxide9 decussate

dz quartz10—orthoclase15—hornblende29—plagioclase34 rock(dz).

Group 8 The youngest intrusive unit within the layered seriesoccurs as dikes and stocks as much as 1—1/2 miles

across in the Hungary Jack Lake quadrangle. This unit,

a inedium-grained granular quartz18—alkali feldspar77daa rock with minor augite and oxide2 (daa), truncates

unit din. Across an inerval over a mile wide at the

east end of unit din, there is a progressive increase inthe amount of late stage interstitial material that hasa composition similar to unit daa. This unit (dmaa)

dmaa might be a late stage differentiate of din. Nathan foundhowever that unit dx has been intruded and altered by daa.Therefore, it is presumed that din was cold when unit daawas intruded and that the gradational zone representsmelt from daa that was introduced into unit dm.

Tuscarora Intrusion and Associated Rocks

In the Long Island quadrangle (Fig. 2) a sequence of rock typescommon to other parts of the Duluth Complex appear in the followingsuccession away from the base: (1) a fine—grained poikilitic augitegabbro (tp), (2) a fine—grained granoblastic gabbro (hornfels) (th),(3) a fine— to medium—grained troctolite (ttf—ttm), (4) interlayeredtroctolite and poikilitic gabhro (tta), (5) anorthositic gabbro (ag),(6) ferrograndiorite (tg), (7) granophyre (gr), and (7) metamorphosedflows (:mv). Although the outcrop pattern suggests a simple differentiatedlayered sequence, the stratigraphic position of units tp, ag, tg and grhave not been unequivocally established. However, units ttf, ttn, andtta are clearly parts of a separate troctolite intrusion that truncatespart of Nathan's layered series (Fig. 3).

Units tL,ad tta: The main unit of the Tuscarora Intrusionis a niedium—grained troctolite (ttrn, Fig. 2), consisting of 65—70percent cumulus plagioclase (An 56 and 10—15 percent cumulusdivine (Fo0). Relative amouns 09 poikilitic augite and iron—titaniumoxides varies locally. Orthopyroxene mantles olivine and occurs insitaplectic intergrowth with plagioclase. biotite is associated withthe iron—titanium oxides. Planar orientation of plagioclase and modal—mineral layering are locally well—developed and mutually concordant.

dx

dy

dz

Group 8

daa

dmaa

-113-

Several fine-grained decussate rocks occur asdiscontinuous thin sheets along the base of the complex and as small stocks and dikes higher in thesection. They may represent either fused fractionsof country rock or contaminated melts, but all arethought to be intrusive. They consist of: (1) finegrained decussate oxide7-augite 2-plagioclase66rock having minor olivine -quarrz 2 and potassluffi1feldspar (dx); (2) fine-g~ained decussate oxide hypersthene -augite -plagioclaseSS autocrystal1ate(dy); and (~t fine-g1~ined augite

3-oxideg decussate

quartzlO-orthoclaselS-hornblende29-p1agioclase34 rock(dz) .

The youngest intrusive unit within the layered seriesoccurs as dikes and stocks as much as 1-1/2 milesacross in the Hungary Jack Lake quadrangle. This unit,a medium-grained granular quartzls-alkali feldspar nrock ,vitl, minor augite

1and oxide

2(daa) , truncates

unit dm. Across an interval over a mile wide at theeast end of unit dm, there is a prog,ressive increase inthe amount of late stage interstitial material that hasa composition similar to unit daa. This unit (dmaa)might be a late stage differentiate of dm. Nathan foundhm-lever that unit dx has been intruded and altered by daa.Therefore, it is presumed that dm \Vas cold ,vhen unit daawas intruded and that the gradational zone representsmelt from daa that was introduced into unit dm.

Tuscarora Intrusion and Associated Rocks

In the Long Island quadrangle (Fig. 2) a sequence of rock typescommon to other parts of the Duluth Complex appear in the followingsuccession away from the base: (1) a fine-grained poikilitic augitegabbro (tp), (2) a fine-grained granoblastic gabbro (hornfels) (th),(3) a fine- to medium-grained troctolite (ttf-ttm), (4) interlayeredtroctolite and poikilitic gabbro (tta), (S) anorthositic gabbro (ag),(6) ferrograndiorite (tg), (7) granophyre (gr), and (7) metamorphosedflows (kmv). Although the outcrop pattern suggests a simple differentiatedlayered sequence, the stratigraphic position of units tp, ag, tg and grhave not been unequivocally established. Hm.;rever, units ttf, ttm, andtta are clearly parts of a separate troctolite intrusion that truncatespart of Nathan's layered series (Fig. 3).

ynits ttm, ttf, and tta: The main unit of the Tuscarora Intrusionis a medium-grained troctolite (ttm, Fig. 2), consisting of 6S-70percent cumulus plagioclase (An S-60)' and 10-lS percent cumulusol~vine (F~50). Relative amoun~s of poikilitic augite and iron-titaniumoXldes varles locally. Orthopyroxene mantles olivine and occurs insimplectic intersrowth with plagioclase. Biotite is associated withtile iron-titanium oxides. Planar orientation of plagioclase and modalmineral layering are locally well-developed and mutually concordant.

-114—

The troctolite grades into an upper unit which consists ofinterlayered poikilitic augite gabbro and troctolite (Fig. 3).The poikilitic augite consists of about 70 percent plagioclase

15—20 percent augite, 5—10 percentilmenite and ismedium— to coarse—grained with well developed augite orthocrystsas much as 1—1/2" across. The troctolite within the layeredinterval is similar to that in unit ttm. Contacts between layersare generally sharp and in gener& conformable with layering inthe troctolite. Interlayering occurs on a scale of several inchesto several feet, and is undulatory with wave lengths of ten to twentyfeet and amplitudes of two to three feet, but the gross structureis nearly flat—lying.

Unit ttp; A belt of fine—grained rocks occur beneath unitttf. It consists of fine— to medium—grained augite troctolitewith 60—70 percent cumulus plagioclase (An50), 5—10 percent cumulusolivine (Fo35), 15—20 percent poikilitic augite, .5—10 percentiron—titanium oxides, and minor orthopyroxene—plagioclase simplecite.As yet the upper contact of tins unit has not been observed and itis not clear from outcrop data if it is a separate intrusion or thebasal unit of the overlying rocks. Johnson (1970, p. 76) concludedfrom drill core data that it is a separate intrusion.

Unit th: Several areas of fine—grained, granoblastic gabbroconsisting of 50 to 60 percent short tabular plagioclase, 30 to40 percent rounded augite, minor subhedral iron—titanium oxides,olivine, and blades of biotite are exposed on topographic highs.These rocks may be a remnant capping over the troctolite; howeverthere is no noticeable chilling of the troctolite next to thehornfels and they more likely represent large inclusions.

Unit ap; A distinct break in plagioclase content separatesunits tta and ag. Unit ag — an anorthositic gabbro — contains 75 toS5 percent plagioclase An5560 ( contrast to the 50 to 70 percentof unit tta). Plagioclase is the only cumulus phase, and inter-stitial minerals, occurring in various proportions are augite,olivine, and iron oxides in a poikilitic texture. Orthopyroxeneoccurs in siciplectic intergrowth with late—stage plagioclase.Biotite is associated with the iron—titanium oxides. Planarorientation of plagioclase is developed locally. At the top ofunit ag, quartz and potassium feldspar occur interstitially.

Tue flat—lying outcrop pattern of the ferrogranodiorite (tg)and granophyre (gr) is concordant with the structure within thetroctolite (ttm) and the anorthositic galibro could be just anotherpart of the layered intrusion. however some field evidence contra-dicts this interpretation. llornfels inclusions and sulfide mineral-ization are found in unit tta at several localities along the contactwith unit ag. Thus the troctolite may be intrusive into theanorthositic gabbro as in the Gabbro Lake quadrangle (Green andothers, 1966). Phinney (1969) has extended the older anorthositicgabbro from the Gabbro Lake quadrangle eastward and this extensionprojects toward unit ag. Thus unit ag may be a thin wedge of anareally extensive older anorthositic gabbro. In this case, unitsfg and gr could be either related to the troctolite and intrudedinto the ancirthositic gabbro or a differentiated part of theanorthositic gabbro.

-114-

The troctolite grades into an upper unit which consists ofinterlayered poikilitic augite gabbro and troctolite (Fig. 3).The poikilitic augite consists of about 70 percent plagioclase(AnSO- 60 )' 15-20 percent augite, 5-10 percent ilmenite and ismed1um- to coarse-grained with well developed augite orthocrystsas much as 1-1/2 11 across. The troctolite \vithin the layeredinterval is similar to that in unit ttm. Contacts between layersare generally sharp and in general conformable with layering inthe troctolite. Interlayering occurs on a scale of several inchesto several feet, and is undulatory with wave lengths of ten to twentyfeet and amplitudes of two to three feet, but the gross structureis nearly flat-lying.

Unit ttp: A belt of fine-grained rocks occur beneath unitttf. It consists of fine- to medium-grained augite troctolitewith 60-70 percent cumulus plagioclase (An

50), 5-10 percent cumulus

~livin~ (F~35)' ~5-20 perce~t poikilitic augite, 5-10 percent .1ron-t1tan1um oX1des, and mlnor orthopyroxene-plagioclase simplect1te.As yet the upper contact of this unit has not been observed and itis not clear from outcrop data if it is a separate intrusion or thebasal unit of the overlying rocks. Johnson (1970, p. 76) concludedfrom drill core data that it is a separate intrusion.

Unit th: Several areas of fine-grained, granoblastic gabbroconsisting of 50 to 60 percent short tabular plagioclase, 30 to40 percent rounded augite, minor subhedral iron-titanium oxides,olivine, and blades of biotite are exposed on topographic highs.These rocks may be a remnant capping over the troctolite; hOlveverthere is no noticeable chilling of the troctolite next to thehornfels and they more likely represent large inclusions.

Unit ag: A distinct break in plagioclase content separatesunits tta and ago Unit ag - an anorthositic gabbro - contains 75 to85 percent plagioclase An

55-

60(in contrast to the 50 to 70 percent

of unit tta). Plagioclase is the only cumulus phase, and interstitial minerals, occurring in various proportions are augite,olivine, and iron oxides in a poikilitic texture. Orthopyroxeneoccurs in sil:lplec tic intergrm-Jth W'i th late-stage plagioclase.Biotite is associated \'lith the iron-titanium oxides. Planarorientation of plagioclase is developed locally. At the top ofunit ag, quartz and potassium feldspar occur interstitially.

The flat-lying outcrop pattern of the ferrogranodiorite (tg)and granophyre (gr) is concordant with the structure \vithin thetroctolite (ttm) and the anorthositic gabbro could be just anotherpart of the layered intrusion. IImvever some field evidence contradicts this interpretation. Hornfels inclusions and sulfide mineralization are found in unit tta at several localities along the contactwith unit ago Thus the troctolite may be intrusive into theanorthositic gabbro as in the Gabbro Lake quadrangle (Green andothers, 1966). Phinney (1969) has extended the older anorthositicgabbro from the Gabbro Lake quadrangle eastward and this extensionprojects toward unit ago TIluS unit ag may be a thin wedge of anareally extensive older anorthositic gabbro. In this case, unitsfg and gr could be either related to the troctolite and intrudedinto the anorthositic gabbro or a differentiated part of theanorthositic gabbro.

—115—

Unit f: This unit is restricted to a topographically higharea i the southwest corner of the Long Island Lake quadrangle(Fig. 3). It is a medium—grained ferrogranodiorite which contains50 to 60 percent cumulus plagioclase, 10 to 15 percent amphibole,minor clinopyroxene, and varying amounts of quartz, potassiumfeldspar, and magnetite. Contacts with the underlying anorthositicgabbro and overlying granophyre are gradational over tens of feet.The former could represent replacement by intrusive ferrograno—diorite or differentiation within the anorthositic gabbro. Furtherstudy is needed to clarify the stratigraphic relationships.

Unit gr: The ferrogranodiorite grades into and is cut byfine— to medium—grained granophyre. It consists of quartz,plagioclase, potassium feldspar and magnetite. The texture rangesfrom granophyric to granitoid.

Unit kmv: The granophyre intrudes black fine—grained meta—volcanic rocks whch are interpreted to be remnants of MiddleKeweenawan flows. The groundmass is highly altered. Acicularblades of ilmenite are common and plagioclase phenocrysts areclouded similar to intruded rocks it'. the layered series.

Mineralization: Two types of mineralization are found inthe Duluth Complex in this area: (1) low grade copper—nickelconcentrations are associated with the basal rocks of theTuscarora Intrusion and several of the layered series intrusions(Johnson, 1970), (2) Ilmenite— and titanomagnetite—rich rocksoccr in several units of Nathan's layered series.

With regard to the mineralization, the unpublished Ph.D.thesis of Johnson (1970) warrants special mention. The thesissummarizes the results of an exploration program conducted bythe Cleveland—cliffs Iron Company and the Amerada—Hess CorporationCompany from 1966 to 1969. This program assessed the economicpotential of the base of the Duluth Complex in a 38 km corridoralong the Gunflint Trail adjacent to the Boundary Waters CanoeArea. The drilling program (10 holes) provides an unique opportunityto assess the effects of drilling on the area in comparison with otheractivities such as logging and recreation. More importantly the re-lease of geophysical, drill core, and assay data, along with Johnson'sstudy represents a major contribution by a mining company to theconcern for the environment of the area. The correlation of geo-physical and drill core data, discussed below, makes possible amore accurate evaluation of mineral resources in adjacent areasand in other areas of similar geology using less expensive anddisruptive preliminary investigations.

Coç—Nicke1_Iinera1izatjon: Discontinuous areas of gossanand visible sulfide mineralization within unit ttf have been mappedacross the Long Island quadrangle. Similar isolated exposures havebeen found in unit tta at the contact with anorthositic gabbro. Thesulfide assemblage, consisting of cI:alcopyrjte,pyrrhoi, and minorpentlandjte occurs interstitially to plagioclase and olivine. Becauseof the smaller grain size of the troctolite a distinct interstitialtexture, like that found in the sulfide mineralization in the iCawishiwiarea in the Gabbro Lake quadrangle, is not apparent in hand specimen.

-115-

Unit fg: This unit is restricted to a topographically higharea in the southwest corner of the Long Island Lake quadrangle(Fig. 3). It is a medium-grained ferrogranodiorite which contains50 to 60 percent cumulus plagioclase, 10 to 15 percent amphibole,minor clinopyroxene, and varying amounts of quartz, potassiumfeldspar, and magnetite. Contacts with the underlying anorthositicgabbro and overlying granophyre are gradational over tens of feet.The former could represent replacement by intrusive ferrogranodiorite or differentiation within the anorthositic gabbro. Furtherstudy is needed to clarify the stratigraphic relationships.

Unit gr: The ferrogranodiorite grades into and is cut byfine- to medium-grained granophyre. It consists of quartz,plagioclase, potassium feldspar and magnetite. The texture rangesfrom granophyric to granitoid.

Unit kmv: The granophyre intrudes black fine-grained metavolcanic rocks whch are interpreted to be remnants of MiddleKeweenawan flows. The groundmass is highly altered. Acicularblades of ilmenite are common and plagioclase phenocrysts areclouded similar to intruded rocks in the layered series.

Mineralization: Two types of mineralization are found inthe Duluth Complex in this area: (1) 1m., grade copper-nickelconcentrations are associated with the basal rocks of theTuscarora Intrusion and several of the layered series intrusions(Johnson, 1970), (2) Ilmenite- and titanomagnetite-rich rocksoccur in several units of Nathan's layered series.

\-lith regard to the mineralization, the unpublished Ph.D.thesis of Johnson (1970) warrants special mention. The thesissummarizes the results of an exploration program conducted bythe Cleveland-Cliffs Iron Company and the Amerada-Hess CorporationCompany from 1966 to 1969. This program ass~ssed the economicpotential of the base of the Duluth Complex in a 38 km corridoralong the Gunflint Trail adjacent to the Boundary Waters CanoeArea. The drilling program (10 holes) provides an unique opportunityto assess the effects of drilling on the area in comparison with otheractivities such as logging and recreation. More importantly the release of geophysical, drill core, and assay data, along with Johnson'sstudy represents a major contribution by a mining company to theconcern for the environment of the area. The correlation of geophysical and drill core data, discussed below, makes possible amore accurate evaluation of mineral resources in adjacent areasand in other areas of similar geology using less expensive anddisruptive preliminary investigations.

~0J?E.£!:.-Nickel ~'lineralization: Discontinuous areas of gossanand visible sulfide min~r-;li~-ation ,.,ithin unit ttf have been mappedacross the Long Island quadrangle. Similar isolated exposures havebeen.found in unit tta at the contact with anorthositic gabbro. Thesulflde assemblage, consisting of chalcopyrite pyrrhotite and minorpentlandite occurs interstitially to plagiocla~e and olivine. Becauseof the smaller grain size of the troctolite a distinct interstitialtexture, like that found in the sulfide mineralization in the Kawishiwiarea in tQe Gabbro Lake quadrangle, is not apparent in hand specimen.

—116—

Drilling across the quadrangle (Johnson, 1969) has indicated atabular, possibly continuous volume of low grade ore (0.3% combinedcopper—nickel) about 50 feet thick in the unit ttf. A thinner 10—20

foot—thick zone, 50—100 feet above the lower mineralized zone, hasa higher combined copper—nickel content that approaches one percent(Johnson, p. 84). The mineralization can be correlated3with adetectable resistivity anomaly on the order of 700 x 10 ohmcentimeters

in the troctolite (ttf, ttm).

Titanium Mineralization: The primary titanium oxide phasesare i].menite solid solution (Fe20 —MgTiO —FeTiO3) and titanomagnetite(Fe304—Fe TiO ). Subsolidus exsoLtion as resulted in the complexintergrosAhs âescribed above. Johnson (1970, p. 87) estimatesthat Ti recovered from ilmenite In unit ttf in the Tuscarora Intrusioncould add $1.50 per ton to sulfide ore from this unit.

The largest titanium concentrations however are in units dt and duat Little Iron Lake in the Gunflint Lake quadrangle and other isolatedexposures in the South Lake quadrangle. Unfortunately, most of theseoccurrences at the surface do not exceed about 35 feet in maximumdimension.

A large low—grade titanium resource also is contained withinunit dg (Fig. 3). Oxide—rich layers as much as 5 feet thick arecommon, although individual layers seem too thin and discontinuousto be mined separately. Unit dg should he considered in its entiretyfor commercial evaluation with the potential of developing a verylarge tonnage of low—grade ore. The unit is very heterogeneous andfield exposures are scarce and discontinuous, so only widespreadsystematic drilling will reveal which parts have the greatest promise.

-116-

Drilling across the quadrangle (Johnson, 1969) has indicated atabular, possibly continuous volume of low grade ore (0.3% combinedcopper-nickel) about 50 feet thick in the unit ttf. A thinner 10-20foot-thick zone, 50-100 feet above the lower mineralized zone, hasa higher combined copper-nickel content that approaches one percent(Johnson, p. 84). The mineralization can be correlated

3with a

detectable resistivity anomaly on the order of 700 x 10 ohmcentimetersin the troctolite (ttf, ttm).

Titanium Mineralization: The primary titan1um oxide phasesare ilmenite solid solution (Fe

203-MgTi03-FeTi0

3) and titanomagnetite

(Fe304-Fe2Ti04). Subsolidus exsolution fias resulted in the complexintergrowEhs described above. Johnson (1970, p. 87) estimatesthat Ti recovered from ilmenite in unit ttf in the Tuscarora Intrusioncould add $1.50 per ton to sulfide ore from this unit.

The largest titanium concentrationsat Little Iron Lake in the Gunflint Lakeexposures in the South Lake quadrangle.occurrences at the surface do not exceeddimension.

however are in units dt and duquadrangle and other isolatedUnfortunately, most of theseabout 35 feet in maximum

A large low-grade titanium resource also is contained withinunit dg (Fig. 3). Oxide-rich layers as much as 5 feet thick arecommon, although individual layers seem too thin and discontinuousto be mined separately. Unit dg should be considered in its entiretyfor commercial evaluation with the potential of developing a verylarge tonnage of low-grade ore. The unit is very heterogeneous andfield exposures are scarce and discontinuous, so only widespreadsystematic drilling will reveal which parts have the greatest promise.

—117—

References Cited

Bayley, W. S., 1893, The eruptive and sedimentary rocks on PigeonPoint, Minnesota, and their contact phenomena: U. S. Geol.Survey Bull. 109, 121 p.

Bonnichsen, Bill, 1969, Metamorphic pyroxenes and amphiboles inthe Biwabik Iron—formation, Dunka River area, Minnesota:Mineralog. Soc. America, Spec. Paper 2, p. 217—239.

Broderick, T. M., 1920, Economic geology and stratigraphy ofthe Gunflint iron district, Minnesota: Econ. Geol., v. 15,

p. 422—452.

Daly, R. A., 1917, The geology of Pigeon Point, Minnesota: Amer.

Jour. Science, ser. 4, v. 43, p. 423—448.

Davidson, D. M., Jr., 1970a, Geologic map of Kawishiwi Lakequadrangle, Lake and Cook Counties, Minnesota (with discussion):Minn. Geol. Survey Misc. Map 7.

_____

l970b, Geologic map of Perent Lake quadrangle, Lake County,Minnesota (with discussion): Minn. Geol. Survey Misc. Map 8.

Faure, G. and J. Kovach, 1969, The age of the Gunflint Iron Formationof the Animikie Series in Ontario, Canada: Geol. Soc. AmericaBull., v. 80, P. 1725—1736.

Franklin, J. N., and C. R. Kustra, 1970, Proterozoic rocks in theThunder Bay area: Field trip guide for the 16th Ann. MeetingInst. on Lake Superior Geology, p. 49—68.

French, B. N., 1968, Progressive contact metamorphism of the BiwabikIron—formation, Mesabi Range, Minnesota: Minn. Geol. SurveyBull. 45, 103 p.

Goodwin, A. N., 1956, Fades relations in the Gunflint Iron Formation:Econ. Geology, v. 51, p. 565—595.

_____,

1960, Gunflint Iron formation of the Whitefish Lake area: Ont.Dept. Mines, v.

Grant, J. A., 1971, Geology of the northern part of Gunfl.nt Lakequadrangle: in Summ. of Fieldwork, 1970: P. K. Sims andJ. Westfall eds.: Minn. Geol. Survey Inf. Circ. 8, p. 20.

Green, J. C., 1970, Lower Precambrian rocks of the Gabbro Lakequadrangle, northeastern Minnesota: Minn. Geol. SurveySpecial Pub. 13, 96 p.

Green, J. C., U. L. Phinney, and P. U. Weiblen, 1966, Geologic map ofCabbro Lake quadrangle, Lake County: Minn. Geol. Survey Misc.'tap 2.

Grout, F. F., 1933, Contact metamorphism of the slates of Minnesotaby granite and by gabbro magmas: Geol. Soc. America Bull.,v. 44, p. 989—1040.

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References Cited

Bayley, W. S., 1893, The eruptive and sedimentary rocks on PigeonPoint, Minnesota, and their contact phenomena: U. S. Geol.Survey Bull. 109, 121 p.

Bonnichsen, Bill, 1969, Metamorphic pyroxenes and amphiboles inthe Biwabik Iron-formation, Dunka River area, Minnesota:Mineralog. Soc. America, Spec. Paper 2, p. 217-239.

Broderick, T. M., 1920, Economic geology and stratigraphy ofthe Gunflint iron district, Minnesota: Econ. Geol., v. 15,p. 422-452.

Daly, R. A., 1917, The geology of Pigeon Point, Minnesota: Amer.Jour. Science, sere 4, V. 43, p. 423-448.

Davidson, D. M., Jr., 1970a, Geologic map of Kawishiwi Lakequadrangle, Lake and Cook Counties, Minnesota (with discussion):Hinn. Geol. Survey Nisc. Nap 7.

____, 1970b, Geologic map of Perent Lake quadrangle, Lake County,Minnesota (with discussion): Minn. Geol. Survey Misc. Map 8.

Faure, G. and J. Kovach, 1969, The age of the Gunflint Iron Formationof the Animikie Series in Ontario, Canada: Geo!. Soc. AmericaBull., v. 80, p. 1725-1736.

Franklin, J. M., and C. R. Kustra, 1970, Proterozoic rocks in theThunder Bay area: Field trip guide for the 16th Ann. MeetingInst. on Lake Superior Geology, p. 49-68.

French, B. H., 1968, Progressive contact metamorphism of the BiwabikIron-formation, Hesabi Range, Minnesota: Minn. Geol. SurveyBull. 45, 103 p.

Goodwin, A. 1'1.,1956, Facies relations in the Gunflint Iron Formation:Econ. Geology, v. 51, p. 565-595.

, 1960, Gunflint Iron formation of the Whitefish Lake area: Onto---Dept. Hines, v.

Grant, J. A., 1971, Geology of the northern part of Gunfl:;.nt Lakequadrangle~: in Sunun. of Fieldwork, 1970: P. K. Sims andJ. ~~estfall eds.: fHnn. Geo!. Survey Inf. Circ. 8, p. 20.

Green, J. C., 1970, Lower Precambrian rocks of the Gabbro Lakequadrangle, northeastern Minnesota: Hinn. Geo!. SurveySpecial Pub. 13, 96 p.

Green, J. C., W. L. Phinney, and P. W. Weiblen, 1966, Geologic map ofGabbro Lake quadrangle, Lake County: Minn. Geo1. Survey Misc.:'lap 2.

Grout, F. F., 1933, Contact metamorphism of the slates of Ninnesotaby granite and by gabbro magmas: Geol. Soc. America Bull.,V. 44, p. 989-1040.

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_____

1936, Structural features of the Saganaga Granite of :1thnesotOntario: 16th Internat. Geol. Cong., Rept. 1, p. 255—270.

Grout, F. F., R. P. Sharp, and G. H. Schwartz, 1959, The geology ofCook County, Minnesota: Minn. Geol. Survey Bull. 39, 163 p.

Grout, F. F., and C. M. Schwartz, 1933, The geology of the Roveformation and associated intrusives in northeastern Minnesota:Minn. Geol. Survey Bull. 24, 103 p.

Gruner, J. 14., 1941, Structural geology of the Knife Lake area of

northeastern Minnesota: Geol. Soc. America Bull., v. 52,

p. 1577—1642.

Halford, C. R., 1969, Petrography and structure of the SaganagaGranite, Saganaga—Northern Light Lakes Area, Minnesota—Ontario:unpub. M. S. Thesis, State Univ. of N. Y. at Stony Brook, 7lp.

Hanson, G. N., and R. Malhotra, 1971, K—Ar ages of mafic dikes andevidence for low—grade metamorphism in northeastern Y.innesota:Geol. Soc. America Bull., in press.

Harris, F. R., 1968, Geology af the Saganagons Lake area, Districtof Thunder Bay: Ont. Dept. Mines Rept. 66, 30 p.

Hurley, P. M., H. W. Fairbairn, W. H. Pinson, and J. flower, 1962,Unmetamorphosed minerals in the Gunflint formation used totest the age of the Animikie: Jour. Geol., v. 70, p. 489—492.

Johnson, R. G., 1968, Copper—Nickel mineralization in the basal Duluth- Gabbro complex, northeastern Minnesota: A case study: unpub. >1. S.

Thesis, Univ. of Iowa, 91 p.

_____

1970, Economic geology of a portion of the basal Duluth Complex,northeastern Minnesota: unpub. Ph.D. Thesis, Univ. of Iowa, 136 p.

Lawson, A. C., 1893, The laccolithic sills of the northwest coast ofLake Superior: Minn. Geol. Nat. lUst. Survey Bull. 8, p. 25—48.

Mancuso, J. D., and J. D. Dolence, 1970, Structure of the Duluth gabbro compicin the Babbitt area, Minnesota (abst.): in 16th Ann. Inst. LakeSuperior Geol., p. 27.

Misra, A., and G. Faure, 1970, Restudy of the age of the Gunflint Forma-tion of Ontario, Canada (absr.): in Geol. Soc. America abstractswith programs for 1970, north—central section, v. 2, p. 398.

Hoorhouse, W.W. , 1960, Gunflint Iron Range in the vicinity of Port Arthur,Ontario: Ont. Dept. Mines, v. , pt. 7, p. 1—40.

_____

1963, Concretions from the Animikie of the Port Arthur region,Ontario: Proc. Geol. Assoc. Canada, v. 15, p. 43—59.

Morey, C. B., 1969, The geology of the Middle Precambrian Rove FormationIn northeastern Minnesota: Minn. Ceol. Survey Special Pub. 7, 62 p.

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, 1936, Structural features of the Saganaga Granite of Hinnesoto.---Ontario: 16th Internat. Geo1. Cong., Rept. 1, p. 255-270.

Grout, F. F., R. P. Sharp, and G. M. Schwartz, 1959, The geology ofCook County, Minnesota: Minn. Geol. Survey Bull. 39, 163 p.

Grout, F. F., and G. M. Schwartz, 1933, The geology of the Rovefonnation and associated intrusives in northeastern Hinnesota:Minn. Geo1. Survey Bull. 24, 103 p.

Gruner, J. W., 1941, Structural geology of the Knife Lake area ofnortheastern Minnesota: Geol. Soc. America Bull., v. 52,p. 1577-1642.

Halford, C. R., 1969, Petrography and structure of the SaganagaGranite, Saganaga-Northern Light Lakes Area, Minnesota-Ontario:unpub. ~1. S. Thesis, State Univ. of N. Y. at Stony Brook, 71p.

Hanson, G. N., and R. Malhotra, 1971, K-Ar ages of mafic dikes andevidence for low-grade metamorphism in northeastern Hinnesota:Geo1. Soc. America Bull., in press.

Harris, F. R., 1968, Geology af the Saganagons Lake area, Districtof Thunder Bay: Onto Dept. Mines Rept. 66, 30 p.

Hurley, P. M., H. W. Fairbairn, H. H. Pinson, and J. Hower, 1962,Unmetamorphosed minerals in the Gunflint fonnation used totest the age of the Animikie: Jour. Geol., v. 70, p. 489-492.

Johnson, R. G., 1968, Copper-Nickel mineralization in the basal DuluthGabbro complex, northeastern Minnesota: A case study: unpub. M. S.Thesis, Dniv. of Iowa, 91 p.

_____, 1970, Economic geology of a portion of the basal Duluth Complex,northeastern Minnesota: unpub. Ph.D. Thesis, Dniv. of Iowa, 136 p.

Lawson, A. C., 1893, The 1accolithic sills of the north\-rest coast ofLake Superior: Ninn. Geol. Nat. Hist. Survey Bull. 8, p. 25-48.

Mancuso, J. D., and J. D. Dolence, 1970, Structure of the Duluth gabbro complEin the Babbitt area, Minnesota (abst.): in 16th Ann. Inst. LakeSuperior Geo1., p. 27.

Misra, A., and G. Faure, 1970, Restudy of the age of the Gunflint Fonnation of Ontario, Canada (abst.): in Geol. Soc. America abstractswith programs for 1970, north-central section, v. 2, p. 398.

Moorhouse, W.W., 1960, Gunflint Iron Range in the vicinity of Port Arthur,Ontario: Ont. Dept. Hines, v. ,pt. 7, p. 1-40.

____, 1963, Concretions from the Animikie of the Port Arthur region,Ontario: Proc. Geol. Assoc. Canada, v. 15, p. 43-59.

Morey, G. B., 1969, The geology of the Middle Precambrian Rove Fonnationin northeastern l'linnesota: Ninn. Geol. Survey Special Pub. 7, 62 p.

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Morey, G. B., J. C. Green, R. W. Ojáangas, and P. K. Sims, 1970,Stratigraphy of the Lower Precambrian rocks in the Vermiliondistrict, northeastern Minnesota: Minn. Geol. Survey Rept.mv. 14, 33 p.

Morey, C. B., P. W. Weiblen, J. J. Papike, and U. H. Anderson, 1969,Geologic map of Long Island Lake quadrangle, Cook County,Minnesota: Minn. Geol. Survey open file map.

Mudrey, 21. C., 1969, Petrology of the Northern Light Gneiss, NorthernLight Lake, Thunder Bay district, Ontario, Canada: unpub. M.S.

Thesis, North. Ill. Univ., 66 p.

Mudrey, >1. G. , and P. N. Weiblen, 1971, Reinvestigation of "red rocks"in the Pigeon Point area, Minnesota (abst.): in 17th Ann.Inst. on Lake Superior Geol.

Nathan, Ii. 0., 1969, The geology of a portion of the Duluth Complex,Cook County, unpub. Ph.D. Thesis, Univ. of Ninn., 198 p.

Phinney, W. C., 1969a, The Duluth Complex in the Gabbro Lakequadrangle, Minnesota: Minn. Geol. Survey Rept. trw. 9, 20 p.

Phinney, N. C., 1969h, Geology of Central part of Duluth Complex;in Summary of Fieldwork 1969: P. K. Sims and I. Westfall, eds.:Minn. Geol. Survey Inf. Circ. 7, 2

Sims, P. K., G. B. Morey, R. w. Ojakangas, and N. L. Griffin, 1968,Preliminary geologic map of the Vermilion district and adjacentareas, northern ttinneso ta: lirin. Geol. Survey Misc. Map N—S.

Sins, P. K., C. B. Morey, and J. C. Green, 1969, The potential fornew mineral discoveries in Minnesota: 30th Ann. Mining Symposium,Univ. of Uinn., p. 75—87.

Tanton, T. L., 1931, Fort William and Port Arthur, and Thunder Cape map—areas, Thunder Bay district, Ontario: Geol. Survey Canada Mem.167, 222 p.

Wanless, IL K., IL U. Stevens, G. R. Lachance, and R. N. Dalablo, 1970,z\ge determinations and geological studies K—Ar istopic ages,Report 9: Geol. Surv. Canada Paper 69—2A, 78 p.

Winchell, A. , 1888, Report of a geological survey in Minnesota duringthe season of 1887: tlinn. Geol. Nat. lust. Survey, 16th Ann.Rept., p. 336—337.

1897, Some new features in the geology of northeastern Minnesota:Amer. Geologist, v. 20, p. 41—51.

Wolff, J. F., 1971, Recent geological developments on the Mesabi ironrange: Trans. Amer. Inst. Mm. Engs., v. 56, p. 142—169.

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Morey, G. B., J. C. Green, R. W. Ojakangas, and P. K. Sims, 1970,Stratigraphy of the Lower Precambrian rocks in the Vermiliondistrict, northeastern Minnesota: Minn. Geol. Survey Rept.Inv. 14, 33 p.

Piorey, G. B., P. \v. Weiblen, J. J. Papike, and D. H. Anderson, 1969,Geologic map of Long Island Lake quadrangle, Cook County,Minnesota: Minn. Geo1. Survey open file map.

Hudrey, H. G., 1969, Petrology of the Northern Light Gneiss, NorthernLight Lake, Thunder Bay district, Ontario, Canada: unpub. M.S.Thesis, North. Ill. Dniv., 66 p.

r1udrey, H. G., and P. hi. lveiblen, 1971, Reinvestigation of "red rocks"in the Pigeon Point area, Minnesota (abst.): in 17th Ann.lnst. on Lake Superior Geol.

Nathan, H. D., 1969, The geology of a portion of the Duluth Complex,Cook County, unpub. Ph.D. Thesis, Dniv. of Minn., 198 p.

Phinney, W. C., 1969a, The Duluth Complex in the Gabbro Lakequadrangle, 11innesota: Minn. Geol. Survey Rept. lnv. 9, 20 p.

Phinney, W. C., 1969b, Geology of Central part of Duluth Complex;in Summary of Fieldwork 1969: P. K. Sims and I. Westfall, eds.:t-~inn. Geol. Survey Inf. Circ. 7, p. 18.

Sims, P. K., G.B. Morey, R. l.v. Ojakangas, and W. L. Griffin, 1968,Preliminary geologic map of the Vermilion district and adjacentareas, northern Ninnesota: ;1inn. Geo1. Survey Misc. Hap H-5.

Sims, P. 1(., G. B. Morey, and J. C. Green, 1969, The potential forne~v mineral discoveries in Ninnesota: 30th Ann. Hining Symposium,Dniv. of ~1inn., p. 75-87.

Tanton, T. L., 1931, Fort William and Port Arthur, and Thunder Cape mapareas, Thunder Bay district, Ontario: CeoI. Survey Canada Mem.1G7, 222 p.

hlanless, R. K., R. D. Stevens, G. R. Lachance, and R. N. Da1abio, 1970,Age detenninations and geological studies K-Ar is6topic ages,Report 9: Geol. Surv. Canada Paper 69-2A, 78 p.

Winchell, A., 18$8, Report of a geological survey in ~linnesota duringthe season of 1887: Minn. Geol. Nat. Hist. Survey, 16th Ann.Rept., p. 336-337.

_____, 1897, Some· new features in the geology of northeastern Minnesota:Amer. Geologist, v. 20, p. 41-51.

\-101ff, J. F., 1971, Recent geological developments on the Mesabi ironrange: Trans. Amer. Inst. Min. Engs., V. 56, p. 142-169.

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FIELD TRIP GUIDE TO THE PRECAMBRIAN ROCKS, i'u.._COOK COUNTY ALONG THE GUNFLINT TRAIL

Stop 1 End of Trail Campground - Main Phase of Saganaga Granite

The main phase of the Saganaga Granite at this stop is amedium—grained hornblende—"quartz—eyet' tonalite having quartz2o—plagioclase (An27)60_lO—hornblende6—microljne3_5 and accessorymuscovite, biotite, chlorite, epidote, sphene, apatite, allaniteand magnetite. The apparent lineation of the "quartz—eyes" is25_300 ENE. This structure is parodied in the hornblende—biotiteinclusions. These inch--sized inclusions are probably related tothe greenstones to the south of Seagull Lake.

The bay leading north to Saganaga Lake was considered byGrout (1936) to be a shatter zone. It is here interpreted as afault in the granite. At this stop, a minor secondary east—northeast trending foliation is marked by shears and epidoteveinlets, and may be related to faulting.

Stop 2 Gunf lint Trail near the Campground — Lamprophyre dike in SaganagaGranite

The lamprophyre dike at this stop is 50 feet wide, and can betraced to the north shore of Saganaga Lake where it is found to cutthe northern boundary fault (Harris, 1968, p. 21). This observationsets a lower age limit for faulting and uplift to the west for theSaganaga Granite. Goldich and others (1961, p. 52) date the biotite(KA—70B)fromasmall island to the north at 1.75 h.y.

Sundeen (1936) reviewed the petrography of the larnprophyredikes in the Saganaga Lake area and found biotite, hornblende,and pyroxene as the mafic phenocrysts in a groundmass of eitherplagioclase or "orthoclase" This dike contains plagioc1ase5—pyroxenel5_20—pyroxenei5_20—biotite513 and hornblende5. Accessoryminerals include quartz, apatite, magnetite, chlorite, carbonate,sphene, pyrite, zircon, serpentine, talc and perovskite.

Stop 3a Saganaga Granite — Border Phase

The strongly foliated hornblende diorite exposed at this stopis typical of the border phase of the Saganaga Granite. The foliationis defined by a layering characterized by various proportions of darkand light minerals; it is nearly vertical and strikes N70°W. Elongate

hornblende needles within the foliation plane define an elongateilneation that plunges gently to the east.

A transition from the border to the main phase involving anincrease in quartz with the development of 'quartz—eye' structure

— and a decrease in hornblende can be seen in a number of outcropson either side of the Gunf lint Trail to the north of this stop.

Stop 1

Stop 2

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FIELD TRIP GUIDE TO THE PRECAMBRIAN ROCKS, ~UL' •• _.. _

COOK COUNTY ALONG THE GUNFLINT TRAIL

End of Trail Campground - Main Phase of Saganaga Granite

The main phase of the Saganaga Granite at this stop is amedium-grained hornblende-"quartz-eye" tonalite having quartZ20plagioclase (An27)60-70-hornblende6-microline3_S and accessorymuscovite, biotite, chlorite, epidote, sphene, apatite, allaniteand magnetite. The apparent lineation of the "quartz-eyes" is25-30° ENE. This structure is parodied in the hornblende-biotiteinclusions. These inch-sized inclusions are probably related tothe greenstones to the south of Seagull Lake.

The bay leading north to Saganaga Lake Has considered byGrout (1936) to be a shatter zone. It is here interpreted as afault in the granite. At this stop, a minor secondary eastnortheast trending foliation is marked by shears and epidoteveinlets, and may be related to faulting.

Gunflint Trail near the Campground - Lamprophyre dike in SaganagaGranite

The larnprophyre dike at this stop is SO feet tv-ide, and can betraced to the north shore of Saganaga Lake where it is found to cutthe northern boundary fault (Harris, 1968, p. 21). This observationsets a 10lv-er age limit for faulting and uplift to the Hest for theSaganaga Granite. Goldich and others (1961, p. S2) date the biotite(KA-70B)from a small island to the north at 1. 7S b.y.

Sundeen (1936) reviewed the petrography of the lamprophyredikes in the Saganaga Lake are.a and found biotite, hornblende,and pyroxene as the mafic phenocrysts in a groundmass of eitherplagioclase or "orthoclase. 11 This dike contains plagioclasesopyroxenelS-20-pyroxeneI5_20-biotiteS_IO and hornblendes. Accessoryminerals include quartz, apatite, magnetite, chlorite, carbonate,sphene, pyrite, zircon, serpentine, talc and perovskite.

Stop 3a Saganaga Granite - Border Phase

The strongly foliated hornblende diorite exposed at this stopis typical of the border phase of the Saganaga Granite. The foliationis defined by a layering characterized by various proportions of darkand light minerals; it is nearly vertical and strikes N70oW. Elongatehornblende needles within the foliation plane define an elongatelineation that plunges gently to the east.

A transition from the border to the main phase involving anincrease in quartz - tv-ith the development of "quartz-eye" structure- and a decrease in hornblende can be seen in a number of outcropson either side of the Gunflint Trail to the north of this stoP.

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Stop 3b Ietabasalt and associated rocks

In the general vicinity of this stop, vaguely pillowed meta—basalt and thin—bedded to laminated pyroclastic material typicalof the nafic part of the volcanic succession are exposed. Layeringis nearly vertical and strikes in a northwesterly direction. Fracture

cleavage also is near vertical and strikes in a northeasterly directionparallel to the trace of the Lookout fault.

The mafic rocks are cut by conformable layers of fine—grainedgraphic feldspar granite. Locally a thin layer of iron—formationcomposed of magnetite and chert unconformably overlies the olderrocks.

Iron—rich strata typical of the lower part of the CunflintIron—formation are exposed on the steep north—facing slope immediatelyto the south of these exposures.

Stopj Along Magnetic Rock Trail — metamorphosed Gunf lint Iron—formation

of Zone 2

Thin—bedded, fine—grained, chert—axnphibole—inagnetitebearingstrata assigned to the upper part of the Lower Slaty member areexposed along Magnetic Rock Trail at this locality. These exposures

are near the transition between moderately and strongly metamorphosediron—formation; small, poorly developed pyroxene porphyroblastscan be seen,pecially in the more massive beds at the top of themember.

Along the power line trail to the south, the iron—formation islocally deformed with beds dipping northward at 15°.

Approximately 75 feet farther to the south a northwesterly—trending, medium—grained diabase sill cuts slaty iron—formatin.Approximately 200 feet to the south the slaty beds again dip tothe south and are interlayered with coarse—grained, magnetite—richcherty beds. On the same knob, algal chert—bearing beds characteristicof the Upper Cherty Member also are exposed.

Stop 5 Along the Kekabeic Trail — Metamorphosed Gunflint Iron—formation atZone 3.

The Kekabeic Trail more or less parallels the base of the GunflintIron—formation and the north—facing slope immediately south of the trailcontains exposures of the Lower Slaty Member. The iron—formation every-

where in this area has been extensively metamorphosed and now consistsof various assemblages of quartz—cummingtonite—grunerite—fayalite—magnetite and quartz—cummingtonite—grunerite—pyroxene—magnetite.

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Stop 3b Netabasalt and associated rocks

In the general vicinity of this stop, vaguely pillowed metabasalt and thin-bedded to laminated pyroclastic material typicalof the mafic part of the volcanic succession are exposed. Layeringis nearly vertical and strikes in a northwesterly direction. Fracturecleavage also is near vertical and strikes in a northeasterly directionparallel to the trace of the Lookout fault.

The mafic rocks are cut by conformable layers of fine-grainedgraphic feldspar granite. Locally a thin layer of iron-formationcomposed of magnetite and chert unconformably overlies the olderrocks.

Iron-rich strata typical of the lower part of the GunflintIron-formation are exposed on the steep north-facing slope immediatelyto the south of these exposures.

Stop 4

Stop 5

Along Magnetic Rock Trail - metamorphosed Gunflint Iron-formationof Zone 2

Thin-bedded, fine-grained, chert-amphibole-magnetite-bearingstrata assigned to the upper part of the Lower Slaty member areexposed along Magnetic Rock Trail at this locality. These exposuresare near the transition between moderately and strongly metamorphosediron-formation; small, poorly developed pyroxene porphyroblastscan be seen,especially in the more massive beds at the top of themember.

Along the power line trail to the south, the iron-formation islocally deformed with beds dipping northward at 15°.

Approximately 75 feet farther to the south a northwesterlytrending, medium-grained diabase sill cuts slaty iron-formatin.Approximately 200 feet to the south the slaty beds again dip tothe south and are interlayered with coarse-grained, magnetite-richcherty beds. On the same knob, algal chert-bearing beds characteristicof the Upper Cherty Member also are exposed.

Along the Kekabeic Trail - Metamorphosed Gunflint Iron-formation atZone 3.

The Kekabeic Trail more or less parallels the base of the GunflintIron-fornlation and the north-facing slope immediately south of the trailcontains exposures of the Lower Slaty Hember. The iron-formation everywhere in this area has been extensively metamorphosed and now consistsof various assemblages of quartz-cun~ingtonite-grunerite-fayalite

magnetite and quartz-cummingtonite-grunerite-pyroxene-magnetite.

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Test pits can be seen along the Trail. IIost are in the lowe.:

magnetite—rich part of the Lower Cherty Member. Various sulfides,especially pyrrhotite, also are associated with the magneti:e.

Stop 6 Along the Paulson Mine railcut — Basal Contact of the DuluthComplex

The Paulson Mine railcut exposes the base of the Duluth Complexfrom the Kekabeic Trail to the Tuscarora Lodge road, a distance ofabout 1—1/4 miles. Contacts between beds of the Upper Cherty Memhe:of the Gui-if lint Iron—formation and fine—grained poikilitic augitetroctolite, unit tp of the Duluth Complex, are exposed at the westend of the railcut. Also at the west end truncation of a thin sillof the Logan Intrusive Rocks can he seen. About half—way along therailcut, argillite and graywacke of the Rove Formation are in contactwith the base of the Duluth Complex. The contact aureole here isnarrow with no visible recrystallization of Rove Formation rocksexcept within a few feet of unit tp. This is inferred to be areflection of the thickness of unit tp 100—1,000 feet. The dipof the Gunf lint and Rove Formation varies from 15—60° to the southalong this part of the contact.

Stop 7a Scenic overlook on the Gunf lint Trail above Gunf lint Lake — Copper—nickel mineralization at the base of the Tuscarora Intrusion

The base of unit ttf of the Tuscarora Intrusion is exposed onthe northeast side of the Gunf lint Trail at the overlook. The fine—to mediuin—grairied troctolite shows no regular increase in grain size

away from a contact with unit tp and the upper part of a Logan sill.Visible chalcopyrite, pyrrhotite, pentlandite occur interstitialto plagioclase and olivine in the troctolite. This is a typicalexample of the copper—nickel mineralization of unit ttf, which wasfound by Cleveland—Cliffs Iron Company (in five holes along the baseof the Complex) to occur constantly in a 150 feet thick interval nearthe base of unit ttf. The combined nickel—copper content is about0.3 percent in this interval. A slightly richer zone 10 to 20 feetthick was intercepted about 50—150 feet above the lower zone.

Stop7b 2000 feet northeast of Stop 7a — Logan Intrusive Rocks andthe Rove Formation

Thin bedded argillite is exposed on the north face of a ridgecapped by diabase. The argillite is recrystallized to a biotite—bearing hornfels over an interval of a few inches at the contact.The diabase is typical of fine— to medium—grained diabase in thinsills and in the lower parts of thick sills.

Stop 6

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Test pits can be seen along the Trail. Host are iI:. the 10HE;::magnetite-rich part of the Lower Cherty Member. Various sulfides,especially pyrrhotite, also are associated with the magnetL:e.

Along the Paulson Mine railcut - Basal Contact of dIe DuluthComplex.

The Paulson Mine railcut exposes the base of the Duluth Complexfrom the Kekabeic Trail to the Tuscarora Lodge road, a distance ofabout 1-1/4 miles. Contacts bet\veen beds of the Upper Cherty Hembe:.:of the Gunflint Iron-formation and fine-grained poikilitic augitetroctolite, unit tp of the Duluth Complex, are exposed at the westend of the railcut. Also at the west end truncation of a thin sillof the Logan Intrusive Rocks can be seen. About half-l'1ay along therailcut, argillite and graywacke of the Rove Formation are in contact,-lith the base of the Duluth Complex. The contact aureole here isnarrow with no visible recrystallization of Rove Formation rocksexcept within a few feet of unit tp. This is inferred to be areflection of the thickness of unit tp 100-1,000 feet. The dipof the Gunflint and Rove Formation varies from 15-60° to the southalong this part of the contact.

Stop 7a Scenic overlook on the Gunflint Trail above Gunflint Lake - Coppernickel mineralization at the base of the Tuscarora Intrusion

The base of unit ttf of the Tuscarora Intrusion is exposed onthe northeast side of the Gunflint Trail at the overlook. The fineto medium-grained troctolite shows no regular increase in grain sizeaway from a contact with unit tp and the upper part of a Logan sill.Visible chalcopyrite, pyrrhotite, pentlandite occur interstitialto plagioclase and olivine in the troctolite. This is a typicalexample of the copper-nickel mineralization of unit ttf, which wasfound by Cleveland-Cliffs Iron Company (in five holes along the baseof the Complex) to occur constantly in a 150 feet thick interval nearthe base of unit ttf. The combined nickel-copper content is about0.3 percent in this interval. A slightly richer zone 10 to 20 feetthick was intercepted about 50-150 feet above the lower zone.

Stop 7b 2000 feet northeast of Stop 7a - Logan Intrusive Rocks andthe Rove Formation

Thin bedded argillite is exposed on the north face of a ridgecapped by diabase. The argillite is recrystallized to a biotitebearing hornfels over an interval of a few inches at the contact.The diabase is typical of fine- to medium-grained diabase in thinsills and in the lower parts of thick sills.

—123—

Stop 8 Northwest arm of Poplar Lake on the Gunflint Trail

At this stop typical exposures and intrusive relationships offour of Nathads units will be examined. On the north side of theunit df, a fine—grained decussate augite—olivine—plagioclase rock(troctolite) similar to unit ttf of the Tuscarora Intrusion intrudesunit dc, a very fine—grained granular olivine—augite—plagioclaserock (gabbro) which may represent a chilled margin of the oldestunit of the layered series (da). About 50 feet north of the Trailat the east end of the northwest arm of Popular Lake, a small massof unit dt, a medium—grained granular olivine oxide rock, occurswithin unit dc. A small isolated exposure of unit ds with uncertaincontact relationships occurs between units dc and df about 500 feetnorthwest of Popular Lake and 400 feet north of the Gunflint Trail.South of the Gunflint Trail there are exposures of the large oxide—rich sheet unit dg (coarse—grained, foliated oxide, augite, olivine,plagioclase rock).

Sto2j Cunflint Trail south of Bear Club Lake — Late granitic rocksof the Duluth Complex

Unit daa, a medium—grained granular quartz—feldspar rock, isexposed as a dike in unit dm on the north side of the Gunflint Trail.The main mass of daa occurs south of th Trail as a stock 1—1/2 milesacross. Unit dm, a cumulate sheet of pigeonite—augite—plagioclaserock has visible interstitial quartz and alkali feldspar in anaureole as much as 1 mile wide around the stock of daa. This inter-stitial material is interpreted as replacement associated with theintrusion of unit daa.

Stop 8

Stop 9

-123-

Northwest arm of Poplar Lake on the Gunflint Trail

At this stop typical exposures and intrusive relationships offour of Nathans units will be examined. On the north side of theunit df, a fine-grained decussate augite-olivine-plagioclase rock(troctolite) similar to unit ttf of the Tuscarora Intrusion intrudesunit dc, a very fine-grained granular olivine-augite-plagioclaserock (gabbro) which may represent a chilled margin of the oldestunit of the layered series (da). About 50 feet north of the Trailat the east end of the north~vest arm of Popular Lake, a small massof unit dt, a medium-grained granular olivine oxide rock, occurswithin unit dc. A small isolated exposure of unit ds with uncertaincontact relationships occurs between units dc and df about 500 feetnorthwest of Popular Lake and 400 feet north of the Gunflint Trail.South of the Gunflint Trail there are exposures of the large oxiderich sheet unit dg (coarse-grained, foliated oxide, augite, olivine,plagioclase rock).

Gunflint Trail south of Bear Club Lake - Late granitic rocksof the Duluth Complex

Unit daa, a medium-grained granular quartz-feldspar rock, isexposed as a dike in unit dm on the north side of the Gunflint Trail.The main mass of daa occurs south of the Trail as a stock 1-1/2 milesacross. Unit dm, a cumulate sheet of pigeonite-augite-plagioclaserock has visible interstitial quartz and alkali feldspar in anaureole as much as 1 mile wide around the stock of daa. This interstitial material is interpreted as replacement associated with theintrusion of unit daa.

This page intentionally left blank

Mikel

Typewritten Text

7 ,N /

Ettrus,ve rocks rongrnqin Composition from diem.bosolr to quartz totile and

tocot rt.yoliter4cg lite and Groywocke

Rove Fm. in Cook County

Iond Vrginio and I horneon Fm.

.. in St. Louis County

Figure 1 1ap of northeastern Minnesota showing the geologicsettinE of Cook County and the location of Figure 2.

ADA

St LOUIS

E P AN AT ONK

Kg XX

so'

Duluth Compte.

+ +

ciMci ic nt, usive rocks

rIrirt Shore

U,-c Cto

4?

S/aCto

E

C

L

C

nEC4?

a-

0 10 20 3Orniles

S C

Lrlron —Fottitoijon

ncluces C u,f tilt in Cookand B'wo bitt in SI. Louis

A L if4?C

2

CountyCounty

l'iitiu dod

I/-'NV'1I

c:,,=!.0E

"'"'".t

[+++ ++JMafic intrusive rocks

of North Shore

Marais

[02

Ix x x Ix )( x x

Duluth Complell

1/ ? I"-: I /Extrusive rocks ranging

in composition from olilJinebasalt to Quartz lotite and

local rhyolite

Undiv ided

Iron - Formationincludes Gunflint in Cook Countycnd Biwabik in St. Louis County

o~

Argillite cnd Groywackeincludes Rove Fm. in Cook Countyand Virginia and Thomson Fms.

in St. Louis County

EXPLANATION

'":.;;;'E'c<X

a.:::>~

""

~~{0"".c:<I)",

..r="c-"~~Zo

>

A

~{

.!:!'C

:'2::;;

DA~ N./I.

I 'bJ I

+)" . O~SlIver '"~ '\.Bo.y '9'"~V

c

seA L E

o 1O 20 30 milesI ! I I

y.,eVO

1

xLAKEx X

Ely.

crINDEX MAP

"-"';'''L,

'1''''-, _/ u <.~~,;t?: t-'-"~)(. x"

ST. LOUISI ' x. >. )(

Figure 1 >lap of northeastern Hinnesota showing the geologicsetting of Cook County and the location of Figure 2.

Figure 2 Generalized geologic map of part of northwestern Cook County. The Gunflit-it

Iron—formation is stippled. Only the major units of the Layered Series ofNathan in the Duluth Complex are shown. The area designated df also includesdc,de,dk,ds,du,dy,dx, and dz; similarly ds includes dc,de,dg,dk,dt,du,dv,dw;dg includes dc,dd,dh,di,du,dv,dt, and dz; dp includes dg,dj,and dq; db includesda,dd, and dm; and dm includes da,dd,de,dn,do, and dx. (Nathan, 1969).

I I I I I I I

i-a

0

4800730—H—

SAGANAGAGRANITE

0

ROVE CMATION ANLOGAN INTRUSIVE ROCKS

L —

BLOCK DIAGRAM A BLOCK DIAGRAM

WLUTH COMPLEXLAYERED SERIES OF NATH\

Field hip stop

BWC.A boundary%o3T•3O'

V

II-'N0'I

dm

-~

Gunflint Trail

dm + do

ds

CANADA

df

db

dp

dq

+DULUTH COMPLEX

LAYERED SERIES OF NATHANdm + da

ROVE FORMATION ANOLOGAN INTRUSIVE ROCKS

dgL---------

+

BLOCK DIAGRAM B

I

~)

~ISAGANAGA

GRANITE

tta

DULUTH COMPLEX

TUSCARORA INTRuSION

BLOCK DIAGRAM A

'~f\ ag

~f9+

Quadrangles Studied

t>~~

~ .'" ~~-,'"

.~'*'tl) ~'(§q, ~<b '"c§'t:: ;§ ",0", "-~~G.:J s-v " " ","''I

0/'

5 mi.=

432

seA L E

o

Contact

Fault

•Field trip stop

BWCA boundary

Figure 2 Generalized geologic map of part of northwestern Cook County. The GunflintIron-formation is stippled. Only the major units of the Layered Series ofNathan in the Duluth Complex are shown. The area designated df also includesdc,de,dk,ds,du,dy,dx, and dz; similarly ds includes dc,de,dg,dk,dt,du,dv,dw;dg includes dc,dd,dh,di,du,dv,dt, and dz; dp includes dg,dj,and dq; db includesda,dd, and dm; and dm includes da,6j,de,dn,do, and dx. (Nathan, 1969).

I I I I I I I I I I I I

Figure 3 Block diagrams showing the irif erred geologic relation-ships of rock units in the Long Island Lake and GunflintLake quadrangles (location of diagrams shown in Figure 2).

1700

I Soc

300 FfET

SAGAN.GA &EANtTS\

MET *VOLC ANICS

LGI.JNFLINT lION FM.

M El A V Ot C A N I C S

FL I N1,\.106 A N

.IOVE f N.

dy

in

—1900

Ii iJ0 lm,l

a

dm C190°

1100

db

1300 FEET

It-'tv---JI

1500

1700

1900

BL_L-----------.... 1 300 f EE1

1500

1700

-1900

A

og

.)l ~1 300 FEET

\ \'I.-~-"'-----\ \ \ md /\\ -------\ 9 r \, - -, I

,fg 'I' I, I\ ~

f M.

lmileo

Figure 3 Block diagrams showing the inferred geologic relationships of rock units in the Long Island Lake and GunflintLake quadrangles (location of diagrams shown in Figure 2).

—12 8—

Mesabi Range Magnetite Taconite

May 8, 1971

by

R. W. NarsdenUniversity of Minnesota, Duluth

Duluth, Ninnesota

-128-

Hesabi Range Nagnetite Taconite

May 8, 1971

by

R. W. HarsdenUniversity of Hinnesota, Duluth

Duluth, Hinnesota

—129—

MESABI RANGE FIELD TRIP

INTRODUCTION

The Mesabi Range field trip starts at the Auburn Mine in Virginia,Minnesota and proceeds to the Erie Mine near Aurora, the Reserve Mineat Babbitt and ends at the Dunka Pit at the east end of the Mesabi Rangenear Birch Lake. The trip is designed to show the metamorphic changesin the Biwabik iron formation caused by the intrusion of the DuluthComplex. The trip plan is shown on the index map.

The esabi Range trip is made possible by the cooperation of theMinnesota Ore Operations, United States Steel Corporation, the ErieMining Company and the Reserve Mining Company. Maps and other infor-

mation were furnished by the mining companies. Leaders for each part ofthe trip are shown in the trip log.

Stop 1. AUBURN MINELeader: Wayne L. Plummer

The accompanying map, section and description of the

stratigraphic sequence gives the geologic setting of theAuburn Mine. The unit numbers shown on the stratigraphicsequence are painted on the rock to aid recognition of thehorizons. The upper part of the Pokegama quartzite and theLower Cherty, Lower Slaty and 3*5 feet of the Upper Chertymembers of the Biwabik formation are exposed. Oxidizedand partly leached Biwabik formation is exposed on the westpit wall and the Upper Slaty member and leached Virginiaformations are exposed in the slump structure at the northend of the pit.

-129-

MESABI RANGE FIELD TRIP

INTRODUCTION

The Mesabi Range field trip starts at the Auburn Mine in Virginia,Minnesota and proceeds to the Erie Mine near Aurora, the Reserve Mineat Babbitt and ends at the Dunka Pit at the east end of the Mesabi Rangenear Birch Lake. The trip is designed to show the metamorphic changesin the Biwabik iron formation caused by the intrusion of the DuluthComplex. The trip plan is shown on the index map.

The Mesabi Range trip is made possible by the cooperation of theMinnesota Ore Operations, United States Steel Corporation, the ErieMining Company and the Reserve Mining Company. Maps and other information were furnished by the mining companies. Leaders for each part ofthe trip are shown in the trip log.

Stop 1. AUBURN MINELeader: Wayne L. Plummer

The accompanying map, section and description of thestratigraphic sequence gives the geologic setting of theAuburn Mine. The unit numbers shown on the stratigraphicsequence are painted on the rock to aid recognition of thehorizons. The upper part of the Pokegama quartzite and theLower Cherty, Lower Slaty and 145 feet of the Upper Chertymembers of the Biwabik formation are exposed. Oxidizedand partly leached Biwabik formation is exposed on the westpit wall and the Upper Slaty member and leached Virginiaformations are exposed in the slump structure at the northend of the pit.

.Figure 1 INDEX MAPOF THE

E

I-.

0

MESABI DISTRICT , MINNESOTA

I I

C'

S44

<4RESERVE

-1CQ(\ 0÷

#9

1-Os

If-'woI

,Figure 1 INDEX MAP

OF THE

MESABI DISTRICT, MINNESOTA

IRON FORMATIONS

( BLACK AREAS ARE DIRECT—SHIPPING ORE BODIES )

GRANITEU)I-s

Figure 2 MESABI RANGE

•---ex---

Figure 2 MESABI RANGEIRON FORMATIONS

If-'Wf-'I

( BLACK AREAS ARE DIRECT-SHIPPING ORE BODIES)

—132—

AUBURN MINE

The Auburn Nine is one of a group of open pits located near thecity of Virginia, Minnesota. It was originally developed as an undergroundmine by the Minnesota Iron Company during the period from 1894—1902 and pro-duced 2,143,000 tons of ore prior to closing in 1902 when ownership passedto the Oliver Iron Mining Company, a subsidiary of United States Steel 'Cor-poration. Reopened as an open pit by Oliver in 1951, the mine produced anadditional 11,219,000 tons of ore by the end of 1969 when the mine hecirneinactive because remaining ore is under the approach traccs to the Viridrtiacrushing and rescreening plant located just west of the nine. Most of theopen pit ore was loaded by electric shovels into side dump cars and hauled tothe plant by electric locomotives, but in the last few years, ore from thelower benches was loaded into trucks, hauled to a stockpile beside the trackin the upper part of the pit and reloaded into railroad cars. The AuburnNine and other former Oliver Iron Mining Company mines on the Mesabi Rance arenow operated by U. S. Steel Corporation, Minnesota Ore Operations.

The rocks exposed in the mine starting at the bottom are thePokegama Quartzite and the lower three members of the Biwabik iron formation:Lower Cherty, Lower Slaty and part of the Upper Cherty. These dip from5 degrees to 20 degrees to the northwest as the formatcy lies on the northside of the gently southwestward plunging Eveleth anticline and the centerfold known as the Virginia Horn.

The Auburn ore body was formed in the Biwabik formation by removalof silica from iron bearing rock by leaching ground waters, leaving lesssoluble iron Oxides. The ore body follows down the dip of the formation forabout 3000 feet. Beginning as a Slr::. ii fissure scarcely 50 feet

wide at the southwestern part of the mine, the ore body gradually widenstoward the northwest into a larger trough with a maximum width of about500 feet. In the vertical walled fissure at the southwestern end of the mine,the ore extends from slightly above the Ouartzite (here reduced to a witesand) through 115 feet of the Lower Cherty memher, whereas near the west endof the mine, the ore extends to a depth of about 260 feet from the surfaceto the Lower Cherty-Lower Slaty contact. Due to the leaching ofsilica, ore zones are commonly slumped into structures resembling synclinalfolds. Where slumping occurs adjacent to taconite walls, sltnp faults mayoccur.

Glacial deposits consisting of reddish—gray—brown till containingnumerous boulders of granite and greenstone covered the entire area to adepth of 10 to 35 feet.

An earth slide in the northwestern bank of the mine several yearsago was stabilized with a rock fill which now covers much of previouslyexposed formation in this area.

-132-

AUBURN MINE

The Auburn Mine is one of a group of open pits located near thecity of Virginia, Minnesota. It was originally developed as an undergroundmine by the Minnesota Iron Company durin?, the period from 1894-1902 and produced 2,143,000 tons of ore prior to closing in 1902 when ownership passedto the Oliver Iron Nining Company, a subsidiary of United States Steel 'Corporation. Reopened as an open pit by Oliver in 1951, the mine produced anadditional 11,219,000 tons of ore by the end of 1969 when the mine becameinactive because the remaining ore is under the approach tracks to the Virr,iniacrushing and rescreening plant located just west of the mine. Most of theopen pit ore was loaded by electric shovels into side dump cars and hauled tothe plant by electric locomotives, but in the last few years, ore from thelower benches was loaded into trucks, hauled to a stockpile beside the trackin the upper part of the pit and reloaded into railroad cars. The AuburnMine and other former Oliver Iron }Iining Company mines on the Mesabi Ran?,e arenow operated by U. S. Steel Corporation, Minnesota Ore Operations.

The rocks exposed in the mine startin~ at the bottom arc thePokegama Quartzite and the lower three members of the Biwabik iron formation:Lower Cherty, Lower Slaty and part of the Upper Cherty. These dip from5 degrees to 20 degrees to the northwest as the formation lies on the northside of the gently southwestward plunging Eveleth anticline and the centerfold known as the Virginia Horn.

The Auburn ore body was formed in the Biwabik formation by removalof silica from iron bearing rock by leaching ground waters, leaving lesssoluble iron oxides. The ore body follows drnvn the dip of the formation forabout 3000 feet. Beginning as a small fissure scarcely 50 feetwide at the southwestern part of the mine, the ore body gradually widenstoward the northwest into a larger trough with a maximum width of about500 feet. In the vertical walled fissure at the southwestern end of the mine,the ore extends from slightly above the Quartzite (here reduced to a whitesand) through 115 feet of the Lower Cherty member, whereas near the west endof the mine, the ore extends to a depth of about 260 feet from the surfaceto the Lower Cherty-Lower Slaty contact. Due to the leaching ofsilica, ore zones are commonly slumped into structures resembling synclinalfolds. Where slumping occurs adjacent to taconite walls, slump faults mayoccur.

Glacial deposits consisting of reddish-fray-brown till containinffnumerous boulders of granite and greenstone covered the entire area to adepth of 10 to 35 feet.

An earth slide in the northwestern bank of the mine several yearsago was stabilized with a rock fill which now covers much of previouslyexposcd formation in this area.

—13 3—

Figure 3 GEOLOGTC MAP OF AUBURN M:rE AND VICINITY

IIIII

--l--

I ------

III

I

I "\/Jf'~O~~Oo@ CRUSHERV SCREENING

-L__ PLANT\ ---\ RIqQ:

\ .::>((

\

\

\---\

-133-

III

VIRGIJ1IIAII

-j-----t----

! i I

~II

_--I 1-

ROUCHLEAUMINE

Figure 3 GEOLOGIC MAP OF AUBURN MJr~E AND VTCINITY

-134—

Table 1

STRATIGRAPRIC SEQUENCE IN THE BIWABflC ThON FORMATION

AUBURN ICIE

ThicknessUPPER CHERTY MEMBER in feet1

16.2 Jaspery, conglorieratic and algal chert (a and $ submember x) 10 (eat.)

Covered interval 10 (eat.)

15. Nodular hematitic chert beds interbedded with laminatedhematite-eilicate-magnetite beds 48 +1'

14. Laminated heniatite-ailicate-magnetite beds with subordinatejaspery chert beds and lenses 31

13. Jaspery, conglcaneratic chert beds interbedded with sub-ordinate laminated heinatite-ailicate-magnetite beds 28

12. liberty taconite with thin irregular magnetite beds, mqgfl*tite mottles and disseminated 'retite 16

143

LUiQER $LATY MEMBER3

U. Laminated silicate magnetite taconite with subordinatesilicate chert lenses 101

10. Laminated non-magnetic silicate taconite, fissile in part.6' of fissile "intermediate slate" at bottom (a and Ssubmember Q) 37

138

LO CHERTY ER9. liberty taconite with irregular rnngnetite beds. Upper 10'

has dark-colored silicate rich beds instead of magnetitebeds, mak4 ng base of lower slaty somewhat indefinite 37

8. Mottled silicate-magnetite chert with chert "pebbles" andabundant coarse granules. U

7. liberty taconite with thick (i"±) magnetite beds and mottles 84

6. Mottled cherty taconite with thin, very irregular magnetitebeds. 14

5. Thick jaspery chert beds interbedded with varying propor-tions of thin, regular laminated inngnetite-hematite-. ilicate-carbonate beds • 66

-134-

Table 1

G'mATIGRAPHIC SEQUENCE IN THE BIWABIK IRON FORMATION

AUBURN KmE

48 +f

'l'h1cknessUPPER CHERTY MEMBER in teetl

16.2 Jaspery, conglomeratic and algal chert (G and S submember I) 10 (est.)

Covered interval 10 (est.)

15. Nodular hematitic chert. beds interbedded with laminatedhematite-silicate-magnetite beds

14. Laminated hematite-uilicate-magnetite beds with subordinateJaspery chert beds and lenses 31

13. Jaspery, conglomeratic chert beds interbedded with sub-ordinate laminated hematite-silicate-magnetite beds 28

12. Cherty taconite with thin irregular magnetite beds, magne-tite mottles and disseminated mBgnetite ....,;;1;;,;;6__--=~

11. Laminated silicate magnetite taconite with subordinatesilicate chert lenses 101

138

10. Laminated non-magnetic silicate taconite, fissile in part.6' of fissile "intermediate slate" at bottan (G and S6ubmember Q) .....3;.;7_--=~

LC1WER CHERTY MEMBER

9. Cherty taconite with irregular magnetite beds. Upper 10'has dark-colored silicate rich beds instead of magnetitebeds, making base of lower slaty somewhat indefinite 37

8. Mottled silicate-magnetite chert with chert "pebbles" andabundant coarse granules. 11

7. Cherty taconite with thick (1 1ft) magnetite beds and mottles 84

6. Mottled cherty taconite with thin, very irregular magnetitebeds. 14

5." Thick jaspery chert beds interbedded with varying proportions of thin, regular lBDlinated magnetite-hemat1te-s1l1cate-carbonate beds. 66

—135—

Thickne on

Lover Cherty Member (ConYd) in feet1

Fflyft, henicstitic chert beJa with subordinate laminatedzonos. Seine elastic sand grains near bottcn. Muchcarbonate. 8

3. Jaspery, conc'lomeratic and algal ehert with subordinatelaminated zones. Sand graino common. 4

2. l'lassive chioritic (or hematitic) sandstone 8

1. Jaspery, conglomeratic and algal chert 4

236

Total, thickness exposed

P0iGAl4A QUARTZ ITE Base not exposed

* * * * * *

1. Units 15 and 16 measured on bank between truck road, and railroad near entranceto pit. Units 1 - 5 measured on SW bank, at end, of pit. Remainder measured.above railroad.

2. Unit nwubers correspond to numbers painted on the walls of the Atthun Mine andare not Intended to be a new stratigraphic system.

3. The lower slaty-upper cherty contact is not well—marked, and disagreement existsas to its position.

* * * * * *

The Auburn ore body is of the fissure or trough type and its location andorientation appear to be controlled by a fracture set striking about N 400 W, nearlyat right angles to the strike of the iron formation. At the SE end. of the mine thetrough is only 50' wide and the ore extended upward from the quartzite through aboutU5' of the Lower Cherty member. At the northwest end the trough is 500 feet wideand ore occurs for about 260 feet frcmt the Lover Slaty-Lcwer Cherty contact to theoutcrop. Exceflent examples of ore slump structures can be seen in the ends of thepit, especially at the northwest end..

-135-

Ldwer Cherty Member (Cont'd)

4.. 'rhi c k hem~~ti tic chert beis witb sulJordinate laminatedzones. Some clastic sand grains near bottan. Muchcarbonate.

3. Jaspery, conglomeratic and algal chert with subordinatelnminnted zonea. Sand graino common.

z. l-lilsslve chlorit1c (or hematitic) sandstone

1. Juapcry, conglomeratic and algal chert

Thlckneooin feetl

a

4:

8

4:236

Total thickness exposed 5I7

Base not exposed

* * * * * *1. Units 15 and 16 measured on bank between truck roed and railroad near entrance

to pit. Units 1 - 5 measured on S\rl bank, at BE end of pit. Remainder measuredabove railroad.

2. Unit numbers correspond to numbers painted on the walls of the Auburn Mine andare not intended to be a new stratigraphic system.

3. The lower slaty-upper cherty contact 1s not well-marked and disagreement existsas to its position.

* * * * * *

The Auburn ore body is of the fissure or trough type and its location andorientation appear to be controlled by a fracture set striking about N 400 W, nearlyat right angles to the strike of the iron formation. At the SE end. of the mine thetrough is only 50' wide and the ore extended upward from the quartzite through about115' of the Lower Cherty member. At the northwest end the trough 1s 500 feet wideand ore occurs for about Z60 feet fran the Lower Slaty-Lower Cherty contact to theoutcrop. Excellent examples of ore slump structures can be seen in the eDds ot thepit, especially at the northwest end.

-136—

ERIE MINE — The stops in the Erie Mine are sLown on the accompanyingmap. The Erie geologists designate units in the Biwabik formationas follows:

A—F Upper Slaty member — .av. 110'

G—O Upper Cherty member - av. 185'

P—Q Lower Slaty member — av. 105'

R—W Lower Cherty member — av. 125'

Leader: Forrest W. Boyce

Stop 2. Erie — Pit 1 West

This stop is in the upper part of the Lower Cherty member(units are designated T and S layers by Erie). The Biwabikformation is composed of tine Stilpnomelane, I!innesotaite, magnetiteand cherty quartz and contains bands, mottles and blotches of' pinkto yellow carbonate.

Stop 3. Erie Pit 2 West

This stop is in middle to upper part of the Upper Chertymember and the lower part of the Upper Slaty member (F) with thealgal layer (I) well exposed. (Units in this pit are designatedF, (Upper Slaty), G, H, I, J and K by Erie). A diabase sill about

3 feet in thickness occurs near the bottom of the exposed ironformation in the K layer. Some green mottles of cuminingtoniteoccur in the I layer and carbonate mottles in the K layer.

Stop . Erie Pit 3

This stop is in the western part of Pit 3 in T, S. and Blayers in the middle part of the Lower Cherty member. Therock is a cherty, silicate taconite with layers of cummingtonite.

Stop 5. Erie Pit 3

This stop is in the eastern part of pit 3 in the T, C and F

layers of the Lower Cherty member in about the same stratigraphiczone as stop 4. Much of the chert in the Biwabik formation hasgone to form actinolite and cumniingtonite. The rock is termeda magnetite—sflicate taconite. Locally pyrrhotite occurs in theiron formation.

-136-

ERIE MINE - The stops in the Erie Mine are shown on the accompanyingmap. The Erie geologists designate units in the Biwabik formationas follows:

A-F Upper Slaty member - .av. 110'G-O Upper Cherty member - avo 185'P-Q Lower Slaty member - avo 105'R-W Lower Cherty member - avo 125'

Leader: Forrest W. Bovee

Stop 2. Erie - Pit 1 West

This stop is in the upper part of the Lower Cherty member(units are designated T and S layers by Erie). The Biwabikformation is composed of fine Stilpnomelane, Minnesotaite, magnetiteand cherty quartz and contains bands, mottles and blotches of pinkto yellow carbonate.

Stop 3. Erie Pit 2 West

This stop is in middle to upper part of the Upper Chertymember and the lower part of the Upper Slaty member (F) with thealgal layer (I) well exposed. (Units in this pit are designatedF, (Upper Slaty), G, H, I, J and K by Erie). A diabase sill about3 feet in thickness occurs near the bottom of the exposed ironformation in the K layer. Some green mottles of cummingtoniteoccur in the J layer and carbonate mottles in the K layer.

Stop 4. Erie Pit 3

This stop is in the western part of Pit 3 in T, S. and Rlayers in the middle part of the Lower Cherty member. Therock is a cherty, silicate taconite with layers of cummingtonite.

Stop 5. Erie Pit 3

This stop is in the eastern part of pit 3 in the T, S and Rlayers of the Lower Cherty member in about the same stratigraphiczone as stop 4. Much of the chert in the Biwabik formation hasgone to form actinolite and cummingtonite. The rock is termeda magnetite-silicate taconite. Locally pyrrhotite occurs in theiron formation.

LEGEND

o COAR CRUSHER

® FINE CRUSHER

® CONCENTRATOR

® PELLET PLANT

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LEGEND

CD COARSE CRUSHER

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@ CONCENTRATOR

@ PELLET PLANT

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® STOCKPILE

CD GENERAL SHOPS

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—138—

RESERVE MINING COMPANY

The Reserve Miniqg Company mine is situated in a zone with aconsiderable range in mineralogy and texture shown by the Biwabikformation. The Reserve geologists designate units of the Biwabikformation as follows:

A—G Upper Slaty member — 100130 av.—12011—0 Upper Cherty member — 120—160' av.—140P—Q Lower Slaty member — 75—120' av.'90H—V Lower Cherty member — 30'—50' av.—30

Stop 6. Reserve Mine (Peter Mitchell Mine)Leader: James i1. Emanuelson

This stop is in the western part of the mine, northwest ofCrusher No. 2 in the middle part of the Upper Cherty member in theJ, K and L zones. The rock is a magnetite — silicate taconite withabundant cuznnhingtonite.

Stop 7. Reserve Mine

This stop is in same general stratigranhic zone as stop 6. withlayers from F to 0 exposed. The rock is a magnetite—quartz—silicatetaconite with hedenbergite, ferro—hypersthene, and garnet. There arelocal areas of pegmatite.

Stop 8. Dunka Pit

The Dunka pit of the Erie ['lining Company is situated near theeastern end of the Nesabi Range where the Biwabik formation isintruded by the Duluth Complex. This stop will, show the upperpart of the Upper Cherty member, the lower part of the Upper Slatymember, and gabbro of the Duluth Complex.

The taconite is composed of quartz, magnetite, hedenbergite,fayalite with andradite garnet and locally some hessingerite

The gabbro in this area contains suiphides, chalcopyrite andpyrrhotite with pentlandite.

The iron formation and gabbro will be observed in the pit andin outcrops north of the pit.

-133-

RESERVE MINING COMPANYThe Reserve Minil1g Company mine is situated in a zone ~vith a

considera~le range in mineralogy and texture shown by the Biwabikformation. The Reserve geologists designate units of the Biwabikformation as follows:

A-G Upper Slaty member - 100'-130' av.-120H-O Upper Cherty member - 120-160' av.-140P-Q Lower Slaty member - 75-120' av.-90R-V Lower Cherty member - 30'-50' av.-30

Stop 6. Reserve Mine (Peter HitcheE Hine)Leader: James W. Emanuelson

This s top is in the ~ves tern part of the mine, northwest ofCrusher No.2 in the middle part of the Upper Cherty member in theJ, K and L zones. The rock is a magnetite - silicate taconite withabundant cummingtonite.

Stop 7. Reserve Mine

This stop is in same general stratigraphic zone as stop 6. Hithlayers from F to 0 exposed. The rock is a magnetite-quartz-silicatetaconite with hedenbergite, ferro-hypersthene, and garnet. There arelocal areas of pegmatite.

Stop 8. Dunka Pit

The Dunka pit of the Erie Mining Company is situated near theeastern end of the Hesabi Range where the Bhvabik formation isintruded by the Duluth Complex. This stop Hill show the upperpart of the Upper Cherty member, the lower part of the Upper Slatymember, and gabbro of the Duluth Complex.

The taconite is composed of quartz, magnetite, hedenbergite,fayalite with andradite garnet and locally some hessingerite

The gabbro in this area contains sulphides, chalcopyrite andpyrrhotite with pentlandite.

The iron formation and gabbro will be observed in the pit andin outcrops north of the pit.

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—141—

Geology of the Vermilion Metavolcanic—Metasedimentary Belt, Northeastern Minnesota

May 8, 1971

Prepared by

R. U. Ojakangas, University of iinnesota, Duluth, andNinnesota Geological Survey

P. K. Sims, Minnesota Geological Survey, Minneapolis,Minnesota

C. B. Morey, Minnesota Geological SurveyMinneapolis, Minnesota

J. C. Green, University of Minnesota, Duluth, andMinnesota Geological Survey

-141-

Geology of the Vermilion MetavolcanicMetasedimentary Belt, Northeastern Minnesota

May 8, 1971

Prepared by

R. \v. Ojakangas, University of Ninnesota, Duluth, andNinnesota Geological Survey

P. K. Sims, Minnesota Geological Survey, Minneapolis,Minnesota

G. B. Norey, Hinnesota Geological SurveyMinneapolis, Minnesota

J. C. Green, University of Minnesota, Duluth, andMinnesota Geological Survey

—14 2—

Guide to the Geology of the Vermilion Metavolcanic—Metasedimentary Belt; Northeastern Minnesota

INTRODUCTION

The Vermilion district is a belt of metavolcanic—metasedimentaryrocks more than 100 miles long and as much as 20 miles wide. It isbordered on the north and south by younger granitic batholiths (Fig. 1)of Algoman (Kenoran) age. The region is typical of Lower Precambrian(>2,500 m.y.) greenstone—metasediment--granite complexes of the Superiorprovince.

The metavolcanic—metasedimentary sequence constitutes a complexvolcanic pile, characterized by interfingering of lithologies and localrepetitions of volcanism. Most of the metasediinents are composed ofvolcanic detritus, and probably include volcaniclastic and epiclasticrocks. Numerous coeval and younger igneous rocks occur locally withinthe sequence.

The stratified sequence was metamorphosed and deformed before andduring emplacement of the bordering granitic rocks of the Giants Rangebatholith and the Vermilion batholith. Pervasive greensehist faciesassemblages were developed except adjacent to the intrusive bodies wheremetamorphism attained aniphibolite grade. Deformation consisted of twomajor foldings and of later faulting on a major scale.

ST RATIGRAPHY

The metavolcanic and metasedimentary rocks are assigned to fiveformations (Morey and others, 1970). The oldest formation, the ElyCreenstone — composed mainly of mafic metavolcanic rocks — is overlainstratigraphically in the west by the Lake Vermilion Formation andlocally, the Soudan Iron—formation, and in the central part by theKnife Lake Group (Figs. 2 & 3). Both the Lake Vermilion Formation andthe Knife Lake Group are composed mainly of intermediate—felsic pyro—clastic deposits and volcanogenic sandstones. The Newton Lake Formation,a younger mafic to intermediate—felsic metavolcanic unit, overlies theKnife Lake Group in the central part of the district and interfingers withit to the east (Fig. 3). A generalized and idealized pre—deformationalstratigraphic sequence for the western part of the district is shown infigure 4.

Ely Greenstone

The Ely Greenstone, as redefined (?Iorey and others, 1970), is an

elongate body of dominantly mafic metavolcanic rocks, on the average2—4 miles wide, that extends from the vicinity of Tower eastward toMoose Lake, a distance of about 40 miles (Figs. 2 & 3). Pillowed ormassive metabasaltic lavas and metadiabase dominate the formation.Pillowed lavas of andesitic comnosition, interinediate—felsic pyroclastic

-142-

Guide to the Geology of the Vermilion MetavolcanicMetasedimentary Belt; Northeastern Minnesota

INTRODUCTION

The Vermilion district is a belt of metavolcanic-metasedimentaryrocks more than 100 miles long and as much as 20 miles wide. It isbordered on the north and south by younger granitic batholiths (Fig. 1)of Algoman (Kenoran) age. The region is typical of Lower Precambrian(>2,500 m.y.) greenstone-metasediment-granite complexes of the Superiorprovince.

The metavolcanic-metasedimentary sequence constitutes a complexvolcanic pile, characterized by interfingering of lithologies and localrepetitions of volcanism. Most of the metasediments are composed ofvolcanic detritus, and probably include volcaniclastic and epiclasticrocks. Numerous coeval and younger igneous rocks occur locally withinthe sequence.

The stratified sequence was metamorphosed and deformed before andduring emplacement of the bordering granitic rocks of the Giants Rangebatholith and the Vermilion batholith. Pervasive greenschist faciesassemblages were developed except adjacent to the intrusive bodies wheremetamorphism attained amphibolite grade. Deformation consisted of twomajor foldings and of later faulting on a major scale.

STRATIGRAPHY

The metavolcanic and metasedimentary rocks are assigned to fiveformations (Morey and others, 1970). The oldest formation, the ElyGreenstone - composed mainly of mafic metavolcanic rocks - is overlainstratigraphically in the west by the Lake Vermilion Formation andlocally, the Soudan Iron-formation, and in the central part by theKnife Lake Group (Figs. 2 & 3). Both the Lake Vermilion Formation andthe Knife Lake Group are composed mainly of intermediate-felsic pyroclastic deposits and volcanogenic sandstones. The Newton Lake Formation,a younger mafic to intermediate-felsic metavolcanic unit, overlies theKnife Lake Group in the central part of the district and interfingers withit to the east (Fig. 3). A generalized and idealized pre-deformationalstratigraphic sequence for the western part of the district is shown infigure 4.

Ely Greenstone

The Ely Greenstone, as redefined (Horey and others, 1970), is anelongate body of dominantly mafic metavolcanic rocks, on the average2-4 miles wide, that extends from the vicinity of Tower eastward toMoose Lake, a distance of about 40 miles (Figs. 2 & 3). Pillowed ormassive metabasaltic lavas and metadiabase dominate the formation.Pillowed lavas of andesitic composition, intermediate-felsic pyroclastic

—14 3—

deposits, maf ic—intermediate epiclastic deposits, chert and bandediron—formation, and siliceous carbonaceous tuff (?) comprise theremainder. Andesite and dacite porphyry are conmion hypabyssalintrusive rocks. The green color of most of the rocks is due toabundant secondary chlorite and green amphibole.

The ruaxinul exposed thickness of the Ely is estimated at about15,000 feet; the tops of separate flows are consistently to the north,as indicated by pillows.

Soudan Iron—formation

The Soudan Iron—formation, the thickest and most continuous bandediron—formation in the sequence, extends from Tower and Soudan eastwardfor a distance of about 16 miles (Fig. 2).

In the Tower—Soudan area it is overlain directly by intermediate—felsic volcaniclastic rocks of the Lake Vermilion Formation, whereas eastof Armstrong Lake (Fig. 2) it is overlain directly by at least 7,000 feetof mafic—interinediate metavolcanic rocks with lenses of banded iron—formation, which are assigned to the Ely Greenstone. Thus, the Soudan,which represents a time—stratigraphic unit, is a useful indicator of theessential contemporaneity of intermediate—felsic volcanism in the west(Lake Vermilion Formation) and mafic volcanism in the east (Ely Greenstone).

The Soudan Iron—formation, as redefined (Morey and others, 1910),consists of several types of ferruginous cherts that are interbeddedwith fine—grained carbonaceous and sericitic tuffs (?) and local meta—basalt; all are intruded by metadiabase and dacitic porphyries. The

thickness of the formation has not been determined accurately becauseof complex folding, but probably is less than 1,000 feet. It should benoted that the iron—formation in the Ely trough (Reid, 1956), which hasyielded large quantities of high—grade hematite iron ore, probably isnot equivalent to the Soudan Iron—formation. The high—grade hematiteores that were mined in the Soudan Iron—formation at Soudan (Klinger,1956) and in the banded iron—formation at Ely (Machamer, 1968) areconsidered to have been formed by hydrothermal processes (Gruner, 1926).

Lake Vermilion Formation

In the extreme western part of the district (Fig. 2), the LakeVermilion Formation overlies either the Ely Greenstone or the SoudanIron—formation. Until recently these strata were assigned to theKnife Lake Group. They were reassigned (Morey and others, 1970) tothe Lake Vermilion Formation because they are not demonstrably con-tinuous with strata exposed in the type area of the Knife Lake, in theeastern part of the district.

The Lake Vermilion is a heterogeneous formation which has beendivided (Morey and others, 1910) into four informal members — a feldspathicquartzite member, a metagraywacke—slate member, a volcaniclastic member,and a mixed metagraywacke—felsic conglomerate member.

-143-

deposits, mafic-intermediate epiclastic deposits, chert and bandediron-formation, and siliceous carbonaceous tuff (?) comprise theremainder. Andesite and dacite porphyry are common hypabyssalintrusive rocks. The green color of most of the rocks is due toabundant secondary chlorite and green amphibole.

The maximum exposed thickness of the Ely is estimated at about15,000 feet; the tops of separate flows are consistently to the north,as indicated by pillows.

Soudan Iron-formation

The Soudan Iron-formation, the thickest and most continuous bandediron-formation in the sequence, extends from Tower and Soudan eastwardfor a distance of about 16 miles (Fig. 2).

In the Tower-Soudan area it is ov~rlain directly by intermediatefelsic volcaniclastic rocks of the Lake Vermilion Formation, whereas eastof Armstrong Lake (Fig. 2) it is overlain directly by at least 7,000 feetof mafic-intermediate metavolcanic rocks with lenses of banded ironformation, which are assigned to the Ely Greenstone. Thus, the Soudan,which represents a time-stratigraphic unit, is a useful indicator of theessential contemporaneity of intermediate-felsic volcanism in the west(Lake Vermilion Formation) and mafic volcanism in the east (Ely Greenstone).

The Soudan Iron-formation, as redefined (Morey and others, 1970),consists of several types of ferruginous cherts that are interbeddedwith fine-grained carbonaceous and sericitic tuffs (7) and local metabasalt; all are intruded by metadiabase and dacitic porphyries. Thethickness of the formation has not been determined accurately becauseof complex folding, but probably is less than 1,000 feet. It should benoted that the iron-formation in the Ely trough (Reid, 1956), which hasyielded large quantities of high-grade hematite iron ore, probably isnot equivalent to the Soudan Iron-formation. The high-grade hematiteores that were mined in the Soudan Iron-formation at Soudan (Klinger,1956) and in the banded iron-formation at Ely (Machamer, 1968) areconsidered to have been formed by hydrothermal processes (Gruner, 1926).

Lake Vermilion FO~lation

In the extreme ~~estern part of the district ~Fig. 2), the LakeVermilion Formation overlies either the Ely Greenstone or the SoudanIron-formation. Until recently these strata were assigned to theKnife Lake Group. They were reassigned (Morey and others, 1970) tothe Lake Vermilion Formation because they are not demonstrably continuous with strata exposed in the type area of the Knife Lake, in theeastern part of the district.

The Lake Vermilion is a heterogeneous formation which has beendivided (Morey and others, 1970) into four informal members - a fe1dspathicquartzite member, a metagraywacke-slate member, a volcaniclastic member,and a mixed metagra~vacke-felsicconglomerate member.

—144—

The feldspathic quartzite member, composed dominantly ofvolcanogenic minerals and rock fragments of dacitic composition, -ts

in contact with the Ely Grenstone at the fold nose southwest of Tower(Fig. 2), and is older than the metagraywacke—slate member, as indicatedby graded beds. The metagraywacke—slate member, which Is areally themost extensive member, directly overlies the Ely Greenstone locally, ason the south limb of the fold at Tower, but for the most 'part Is in contactwith the older quartzite. The graywacke generally are well bedded andcommonly are graded; like the feldspathic quartzite, they consist mainlyof volcanogenic debris. A chioritic fades occurs on the shores of theeastern part of Lake Vermilion, but a biotitic fades, containing scatteredamphibole, is dominant elsewhere. The member contains several interbeddedlenses of metabasalt that are sufficiently large to be shown on figure 2.

The volcaniclastic member is of interest because it was interp:etedpreviously to be mainly a conglomerate (Clements, 1903) related to theLaurentian orogeny. Instead, it is dominantly tuff and agglomerate ofdacitic composition, with lesser dacitic lavas, banded iron—formations,and euxenic black slates. Locally, dacite porphyry intrudes the variousrock types. The agglomerates, which are interbedded with tuffs, consistof sub—rounded felsite to felsite porphyry cobbles and boulders in afine—grained matrix of similar composition. txotic rock fragments, mainlyiron—formation and greenstone, constitute only one or two percent of therock.

The mixed metagraywacke—felsic conglomerate member, which occupiesan area of about 30 square miles on the south limb of the fold south ofTower, interfingers with and is stratigraphically overláin by the meta—graywacke—slate member (Fig. 2). It consists of a maximum of about10,000 feet of felsic to mafic volcaniclästic rocks, felsite flows, severaltypes of conglomerates and agglomerates, and metagraywacke (Griffin, 1969;Griffin and Morey, 1969).

The thickness of the Lake Vermilion Formation and its constituentmembers is poorly known because of complex folding and faulting andrather poor exposures. In the Tower quadrangle, the quartzite memberis estimated to be 1,500—2,000 feet thick and the volcaniclastic memberto be a maximum of about 4,000—5,000 feet thick. The metagraywacke—slate

member is at least 3,000 feet thick and is probably much thicker.

Knife Lake Group

Rocks of the Knife Lake Group directly overlie the Ely Greenstonefrom the vicinity of Ely, where the Knife Lake terminates against a fault,eastward to Moose Lake (Fig. 3). The Knife Lake Group, as redefined(Morey and others, 1970), consists dominantly of graywacke, slate, andphyllite but includes substantial amounts of pyroclastic rocks, lava

flows, and conglomerates. Gruner (1941, p. 1624) estimated that the

Knife Lake near the eastern end of the district is about 15,000 feet

thick, but this figure may be conservative.

-144-

The feldspathic quartzite member, composed dominantly ofvolcanogenic minerals and rock fragments of dacitic composition, isin contact with the Ely Greenstone at the fold nose southwest of Tower(Fig. 2), and is older than the metagraywacke-slate member, as indicatedby graded beds. The metagraywacke-slate member, which is area11y themost extensive member, directly overlies the Ely Greenstone locally, ason the south limb of the fold at Tower, but for the most ,part is in contactwith the older quartzite. The graywack~generally are well bedded andcommonly are graded; like the feldspathic quartzite, they consist mainlyof volcanogenic debris. A chloritic facies occurs on the shores of theeastern part of Lake Vermilion, but a biotitic facies, containing scatteredamphibole, is dominant elsewhere. The member contains several interbeddedlenses of metabasalt that are sufficiently large to be sho\vu on figure 2.

The volcaniclastic member is of interest because it was inter~~eted

previously to be mainly a conglomerate (Clements, 1903) related to theLaurentian orogeny. Instead, it is dominantly tuff and agglomerate ofdacitic composition, with lesser dacitic lavas, banded iron-formations sand euxenic black slates. Locally, dacite porphyry intrudes the variousrock types. The agglomerates, which are interbedded with tuffs, consistof sub-rounded felsite to felsite porphyry cobbles and boulders in afine-grained matrix of similar composition. Exotic rock fragments, mainlyiron-formation and greenstone, constitute only one or two percent of therock.

The mixed metagraywacke-felsic conglomerate member, which occupiesan area of about 30 square miles on the south limb of the fold south ofTower, interfingers with and is stratigraphically overlain by the metagraywacke-slate member (Fig. 2). It consists of a maximum of about10,000 feet of felsic to mafic volcaniclastic rocks, felsite flows, severaltypes of cong1.omerates and agglomerates, and metagraywacke (Griffin, 1969;Griffin and Morey, 1969).

The thickness of the Lake Vermilion Formation and its constituentmembers is poorly known because of complex folding and faulting andrather poor exposures. In the Tower quadrangle, the quartzite memberis estimated to be 1,500-2 t OOO feet thick and the volcaniclastic memberto be a maximum of about 4,000-5,000 feet thick. The metagraywacke-slatemember is at least 3,000 feet thick and is probably much thicker.

Knife Lake Group

Rocks of the Knife Lake Group directly overlie the Ely Greenstonefrom the vicinity of Ely, where the Knife Lake terminates against a fault,eastward to Moose Lake (Fig. 3). The Knife Lake Group, as redefined(Morey and others, 1970), consists dominantly of gra~vacke, slate, andphyllite but includes substantial amounts of pyroclastic rocks, lavaflows, and conglomerates. Gruner (1941, p. 1624) estimated that theKnife Lake near the eastern end of the district is about 15,000 feetthick, but this figure may be conservative.

—145—

Newton Lake Formation

The Newton Lake Formation was mapped earlier (Clements, 1903)as Ely Greenstone, but has been renamed (Morey and others, 1970; Green,l970)because it is stratigraphically younger than the Knife Lake Group(Fig. 3). The formation is truncated on the north by the Vermilionfault and along strike to the northeast by granitic rocks of theVermilion batholjth. At its western extremity, near :olf Lake, theformation is truncated by a fault (Fig. 2).

The western part of the Newton Lake formation is composed princi-pally of mafic volcanics and the eastern part of intermediate—felsicvolcanic members, which interfinger in the vicinity of Newton Lake.The mafic volcanic member consists dominantly of metabasalt and meta—andesite lavas, some of which are pillowed, and fine—to—coarse—grainedmetadiabase and tuff or tuff—breccia. Several small bodies of serpen—tinized peridotite are associated spatially with the metabasalt andmetadiabase. Small lenses of siliceous marble and banded iron—formationoccur locally in the formation. The felsic member, east of Newton Lake,is composed of felsic—intermediate volcanics, dominantly tuff—brecciadeposits and lesser flows. At places, metabasalt is interbedded withthe dominantly felsic volcanics.

Intrusive Rocks

Five distinct episodes of intrusive activity are recognized in theregion. In order of inferred age, from oldest to youngest, these are(1) synvolcanic bodies, including hypabyssal porphyries, which have awide range of composition, metadiabase and metagabbro, and serpentinizedperidotite, (2) lamprophyres and related hornblende—bearing rocks, (3)

plutonic rocks of the Giants Range and Vermilion batholiths, which aresyntectonic, (4) altered diorite—gabbro which forms large dikes that arepost—tectonic, and (5) basalt, which forms small, discontinuous, scattereddikes. In addition, the Saganaga Granite of Winchell (1888) at the easternend of the district (Fig. 1) is approximately equivalent in age to therocks of the two batholiths and intrudes the older rnetavolcanics (Grout,1929; Hanson and Goldich, 1970).

The plutonic rocks of the Vermilion and Giants Range batholiths pro-foundly affected the volcanic—sedimentary sequence. Granitic rocks of thecomposite Giants Range batholith, on the south, irregularly intrude thesequence or are in fault contact with it, and have cut out an unknown amountof section at the base of the Ely Greenstone. Where the granite is not infault contact with the lower—grade volcanic—sedimentary rocks, it has normalintrusive relationships to the older strata, with the development ofamphibolite—facies assemblages adjacent to the contact. The Vermilionbatholith, on the north side of the district, transects the upper strati—graphic part of the supracrustal sequence. This leucocratic biotite graniteincludes wide zones of abundant inclusions of biotite schist and amphibolite

STRUCTURE

The metavolcanic and metasedimentary rocks dominantly constitute ahomoclinal, northward—younging sequence in the central part of the district,

-145-


The Newton Lake Formation was mapped earlier (Clements, 1903)as Ely Greenstone, but has been renamed (Morey and others, 1970; Green,1970)because it is stratigraphically younger than the Knife Lake Group(Fig. 3). The formation is truncated on the north by the Vermilionfault and along strike to the northeast by granitic rocks of theVermilion batholith. At its western extremitYt near Wolf Lake, theformation is truncated by a fault (Fig. 2).

The western part of the Newton Lake formation is composed principally of mafic volcanics and the eastern part of intermediate-felsicvolcanic members t which interfinger in the vicinity of Newton Lake.The mafic volcanic member consists dominantly of metabasalt and metaandesite lavas t some of which are pillowed, and fine-to-coarse-grainedmetadiabase and tuff or tuff-breccia. Several small bodies of serpentinized peridotite are associated spatially with the metabasalt andmetadiabase. Small lenses of siliceous marble and banded iron-formationoccur locally in the formation. The felsic member, east of Newton Lake tis composed of felsic-intermediate volcanics, dominantly tuff-brecciadeposits and lesser flows. At places, metabasalt is interbedded withthe dominantly felsic volcanics.

Intrusive Rocks

Five distinct episodes of intrusive activity are recognized in theregion. In order of inferred age, from oldest to youngest, these are(1) synvolcanic bodies, including hypabyssal porphyries, which have awide range of composition, metadiabase and metagabbro, and serpentinizedperidotite t (2) lamprophyres and related hornblende-bearing rocks, (3)plutonic rocks of the Giants Range and Vermilion batholiths, which aresyntectonic, (4) altered diorite-gabbro which forms large dikes that arepost-tectonic, and (5) basalt, which forms small, discontinuous, scattereddikes. In addition, the Saganaga Granite of Winchell (1888) at the easternend of the district (Fig. 1) is approximately equivalent in age to therocks of the two batholiths and intrudes the older metavolcanics (Grout,1929; Hanson and Goldich, 1970).

The plutonic rocks of the Vermilion and Giants Range batholiths profoundly affected the volcanic-sedimentary sequence. Granitic rocks of thecomposite Giants Range batholith, on the south, irregularly intrude thesequence or are in fault contact with it, and have cut out an unknown amountof section at the base of the Ely Greenstone. ~~ere the granite is not infault contact with the lower-grade volcanic-sedimentary rocks, it has normalintrusive relationships to the older strata, with the development ofamphibolite-facies assemblages adjacent to the contact. The Vermilionbatholith, on the north side of the district, transects the upper stratigraphic part of the supracrustal sequence. This leucocratic biotite graniteincludes wide zones of abundant inclusions of biotite schist and amphibolite

STRUCTURE

The metavolcanic and metasedimentary rocks dominantly constitute ahomoclinal, northward-younging sequence in the central part of the district,

—146--

whereas they are both complexly folded and faulted in the western andeastern parts. Deformation was not pervasive, and primary structuresremain in most of the rocks. Graded bedding and other primary featuresremain in the graywacke—slate successions, and pillow structu:es andvariolites are remarkably well preserved in the mafic metavolcanic rocks.At places, however, a penetrative deformation, mainly shearing, hasobliterated the bedding.

In the western part of the area the rocks are complexly folded asa result of two distinct episodes of deformation. The younger folds anda pervasive accompanying cleavage largely obscure the older folds, al-though the older folds were important in determining the distributionof the rocks. Detailed studies in the Tower quadrangle and adjacent areas(Hooper and Ojakangas, 1971) indicate that the metasedimentary strata,and to a lesser degree the metavolcanic rocks, first were folded on west-northwest—trending axes. These (F1) folds were tight to isoclinal, hadsteep axial planes, and probably had gentle or moderate plunges. Majorfold axes, as determined by consistently facing or opposing tops of beds,were spaced from 700 to 1,500 feet apart. The younger (F2) folds, whichcomprise most of the mappable ones, are strongly asymmetric and havesteep axial planes that trend eastward. In most of the area the (F2) foldsare dominantly Z—folds, and the northwest—trending limbs are two or moretimes longer than the southwest—trending limbs; plunges are generallysteep. The intersection of a pervasive, mild, axial plane cleavage withbedding is parallel to F2 fold axes. In biotite— and higher—grade rocks,new minerals are aligned parallel to the cleavage—bedding intersection.In the Tower area, several nearly vertical structures — faults, joints,and kink bands of a third deformation displace the cleavage of the F2deformation.

High—angle faults of two trends, longitudinal and transverse, breakthe metavolcanic—metasedimentary sequence into a number of blocks orsegments and separate it in part from the marginal batholithic rocks.The Vermilion fault (Sims and others, 1968), a longitudinal fault withan inferred length of 300 miles (Sims, 1970) generally separates theVermilion batholith and associated amphibolite facies schists from lower—grade rocks of the district. The amount and directionof the horizontalcomponent of movement is not known, but possibly is several miles. Thevertical displacement is inferred to be on the order of a mile, to bringhigher—temperature--facies rocks on the north against lower—temperature—facies rocks in the district. Other longitudinal faults, some of whichappear to be strands from the Vermilion fault, slice the northern partof the district into separate segments. The transverse faults trendnortheastward or north—northeastward and have dominantly left lateraldisplacements; the principal faults of this Set have measureabledisplacements of about 3—4 miles (Griffin and Morey, 1969).

The major faults of the area are expressed commonly as narrow,linear topographic depressions. Where exposed, they are seen toconsist either of wide zones of crushed and altered rock or of intenselysilicified and altered rocks.

-146-

whereas they are both complexly folded and faulted in the western andeastern parts. Deformation was not pervasive, and primary structuresremain in most of the rocks. Graded bedding and other primary featuresremain in the gray\vacke-slate successions, and pillow structu::es andvariolites are remarkably well preserved in the mafic metavolcanic rocks.At places, hmvever, a penetrative deformation, mainly shearing, hasobliterated the bedding.

In the western part of the area the rocks are complexly folded asa result of two distinct episodes of deformation. The younger folds anda pervasive accompanying cleavage largely obscure the older folds, although the older folds were important in determining the distributionof the rocks. Detailed studies in the Tower quadrangle and adjacent areas(Hooper and Ojakangas, 1971) indicate that the metasedimentary strata,and to a lesser degree the metavolcanic rocks, first ,,,ere folded on 'iVestnorthwest-trending axes. These (fo

l) folds tvere tight to isoclinal, had

steep axial planes, and probably had gentle or moderate plunges. ~lajor

fold axes, as determined by consistently facing or opposing tops of beds,were spaced from 700 to 1,500 feet apart. The younger (F

2) folds, which

comprise most of the mappable ones, are strongly asymmetrlc and havesteep axial planes that trend eastvlard. In most of the area the (F2) foldsare dominantly Z-folds, and the northwest-trending limbs are two or moretimes longer than the southwest-trending limbs; plunges are generallysteep. The intersection of a pervasive, mild, axial plane cleavage withbedding is parallel to fo

2fold axes. In biotite- and higher-grade rocks,

new minerals are aligned parallel to the cleavage-bedding intersection.In the Tower area, several nearly vertical structures - faults, joints,and kink bands of a third deformation displace the cleavage of the F

2deformation.

High-angle faults of two trends, longitudinal and transverse, breakthe metavolcanic-metasedimentary sequence into a number of blocks orsegments and separate it in part from the marginal batholithic rocks.The Vermilion fault (Sims and others, 1968), a longitudinal fault withan inferred length of 300 miles (Sims, 1970) generally separates theVermilion batholith and associated amphibolite facies schists from lm"ergrade rocks of the district. The amount and directioncl the horizontalcomponent of movement is not known, but possibly is several miles. Thevertical displacement is inferred to be on the order of a mile, to bringhigher-temperature-facies rocks on the north against lower-temperaturefacies rocks in the district. Other longitudinal faults, some of ,,,hi-chappear to be strands from the Vermilion fault, slice the northern partof the district into separate segments. The transverse faults trendnortheastward or north-northeastward and have dominantly left lateraldisplacements; the principal faults of this set have measureabledisplacements of about 3-4 miles (Griffin and ~1orey, 19(9).

The major faults of the area are expressed commonly as narrow,linear topographic depressions. I,'here exposed, they are seen toconsist either of wide zones of crushed and altered rock or of intenselysilicified and altered rocks.

—14 7—

Selected References

Clements, J. U., 1903, The Vermilion iron—bearing district of Minnesota:U. S. Geol. Survey Mon. 45, 463 p.

Coldich, S. S., Nier, A. 0., Baadsgaard, Halfden, Hoffman, J. H., andKrue;er, H. U. , 1961, The P:ecambrian geology and geochronology ofilinnesota: [inn. Geol. Survey Bull. 41, 193 p.

Green, J. C., 1970, Lower Precambrian rocks of the Cabbro Lake quad-rangle, northeastern Minnesota: Minn. Geol. Survey, Spec. Pub.ser., SP—10, 96 p.

Griffin, U. L., 1969, Embarrass quadrangle, St. Louis County, Minnesota:Minn. Ceol. Su—vey Misc. Map Sec., MaD M—6.

Griffin, U. L., and ?torey, C. 8., 1969, The geology of the Isaac Lakequadrangle, St. Louis County, Minnesota: Minn. Geol. Survey,

Special Pub. Ser., SP—B, 57 p.

Grout, F. F., 1929, The Saganaga granite of Minnesota—Ontario: Jour.Geology, v. 37, p. 562—591.

Gruner, .3. U., 1926, The Soudan Formation and a new suggestion as to theoriign of the Vermilion iron ores: Econ. Geol., v. 21, p. 629—644.

Cruner, .3. W., 1941, Structural geolony of the Knife Lake area of north-eastern Minnesota: Geol. Soc. America Bull., v. 52, p. 1577—1642.

Hanson, G. N. and Goldich, S. S., 1970, Early Precambrian geology ofthe Saganaga—r4orthern Light Lakes area, Minnesota—Ontario: Inst.Lake Sup. Geology, Proc. 16th Ann. Mtg., Thunder Bay, Ontario,

p. 18.

Hanson, C. N. and >Lalhocra, R., 1971, K—Ar ages of mafic dikes andevidence for low—grade regional metamorphism in northeastern:linriesota: Ceol. Soc. America Bull. (in press, March issue).

[looper, Peter and Ojakangas, R. U., Multinle deformation in the Vermiliondistrict, Minnesota: Can. Jour. Earth Sd. (in press, April issue).

Klinger, F. L., 1956, Geology of the Soudan mine and vicinity; Guide bookSeries, Precambrian ef northeastern Minnesota: Geol. Soc. America,Minneapolis, Minnesota Meeting, p. 120—134.

iachmner, J. F. , 1968, Geology and origin of the iron ore deposits of theZenith mine, Vermilion district, Minnesota: Minn. Geol. Survey Spec.Pub., SP—2, 56 p.

Ojakangas, 11. U., Sims, P. K., & Hooper, Peter, 1971, Geology of theTower Quadrangle: Minn. Geol. Survey (In preparation).

Reid, I. L., 1956, Ceolo1y of the Ely Trough: Guidebook Series, Precambrianof northeastern Minnesota: Geol. Soc. America, Minneapolis meeting,i,. 135—148.

-147-

Selected References

C1e~ents, J. ri., 1903, The Vermilion iron-bearing district of Minnesota:U. S. Geol. Survey Hon. 45, 463 p.

Goldich, S. S., Nier, A. 0., Baadsgaard, Ha1fden, Hoffman, J. H., andKrueger, H. IV., 1961, The P::ecambrian geology and geochronology ofdinnesota: Hinn. Geol. Survey Bull. 41, 193 p.

Green, J. C., 1970, Lower Precambrian rocks of the Gabbro Lake quadrang1e, northeastern }1innesota: :--1inn. Geol. Survey, Spec. Pub.ser., SP-10, 96 p.

Griffin, ,~. L., 1969, Embarrass quadrangle, St. Louis County, Minnesota:~Hnn. Geol. Su-vey Misc. Hap Ser., Map 1'1-6.

Griffin, IV. L., and !'forey, G. B., 1969, The geology of the Isaac Lakequadrangle, St. Louis County, Minnesota: Hinn. Geol. Survey,Special Pub. Ser., SP-8, 57 p.

Grout, F. F., 1929, The Saganaga granite of Minnesota-Ontario: Jour.Geology, v. 37, p. 562-591.

Gruner, J. W., 1926, The Soudan Formation and a new suggestion as to theoriign of the Vermilion iron ores: Econ. Geol., v. 21, p. 629-644.

Gruner, J. W., 1941, Structural geology of the Knife Lake area of northeastern Hinnesota: Geol. Soc. America Bull., v. 52, p. 1577-1642.'

Hanson, G. N. and Go1dich, S. S., 1970, Early Precambrian geology ofthe Saganaga-Northern Light Lakes area, Minnesota-Ontario: Inst.Lake Sup. Geology, Proc. 16th Ann. Mtg., Thunder Bay, Ontario,p. 18.

Hanson, G. N. and Nalhotra, R., 1971, K-Ar ages of mafic dikes andevidence for lOVT-grade regional metamoJ.Tphism in northeasternHinnesota: Geoi. Soc. America Bull. (in press, March issue).

Hooper, Peter and Ojakangas, R. W., Multiple deformation in the Vermiliondistrict, Hinnesota: Can. Jour. Earth Sci. (in press, April issue).

Klinger, F. L., 1956, Geology of the Soudan mine andSeries, Precambrian of northeastern Minnesota:Ninneapolis, Minnesota Heeting, p. 120-134.

vicinity; Guide bookGeol. Soc. America,

Machamer, J. F., 1968, Geology and origin of the iron are deposits of theZenith mine, Vermilion district, Ninnesota: Minn. Geol. Survey Spec.Pub., SP-2, 56 p.

Ojakangas, R. W., Sims, P. K., & Hooper, Peter, 1971, Geology of theTm.,rer Quadrangle: !'linn. Geol. Survey (In preparation).

l:Zeid, 1. L., 19 S6, Geo1of-y of the Ely Trough: Guidebook Series, Precambrianof northeastern Minnesota: Geol. Soc. America, Hinneapolis meeting,TJ. 135-148.

—14 8—

Sims, P. IC., 1970, Geologic map of Minnesota: Minn. Geol. SurveyMisc. Map Set., Map M—l4.

Sims, P. K., Morey, C. B., Ojakangas, R. W., and Griffin, N. L., 1968,Preliminary geologic map of the Vermilion district and adjacent areas,northern Minnesota: Minn. Geol. Survey Misc. Map Ser., Map M—5.

Sims, P. K., Morey, G. B., Ojakangas, R. N., and Viswanathan, S., 1971,(in press), Geologic Map of Minnesota, Hibbing Sheet: Minn. Geol.Survey.

Winchell, H. V., 1888, Report of observations made during the summerof 1887 (northern Minnesota); Minn. Geol. Survey Ann. Rept. v. 16,p. 395—478, map.

-148-

Sims, P. K., 1970, Geologic map of Minnesota: Minn. Geol. SurveyMisc. Map Ser., Map M-l4.

Sims, P. K., Morey, G. B., Ojakangas, R. W., and Griffin, W. L., 1968,Preliminary geologic map of the Vermilion district and adjacent areas,northern Minnesota: Minn. Geo1. Survey Misc. Map Ser., Map M-5.

Sims, P. K., Morey, G. B., Ojakangas, R. W., and Viswanathan, S., 1971,(in press), Geologic Map of Minnesota, Hibbing Sheet: Minn. Geol.Survey.

Winchell, H. V., 1888, Report of observations made during the summerof 1887 (northern Minnesota): Minn. Geol. Survey Ann. Rept. v. 16,p. 395-478, map.

F L A NJ ATIONccC, C

C,V N I

D °Duluth Complex

Vrqinio and Rove Formations-o

Bv,::uk and Gunflint Iron — fornialionsC

1:Gions Range Granile Vermilion GronLte &

=— , /

S Saganaqa Granite

undivided epiclastic, volconiclasticsedimentary rocks, extrusive mafic

igneous rocks, and teSoudan Iron — tormotion (sit)

mainly greensch;st focies

0 '0 20 3OMiies

SCALE

EXPLANATION

Virg1nia and Rove Formations

Duluth Complex

Biwabik and Gunflinl Iron - formations

If-'~

'0Ico

o

J5Eou"-'

0:::

'I ']r:''",; ;->~,"::~;::::'7·{~::/,---

Vermilion Granile

l::~~:1

~+++++J

Giants Range Granite

c c {o 0

~ ~ 3:"-' -'=' 0

~ E ~:::) 0 ill

U 3:"-' ill

0:::'"

~fa.

CO ::Jo 0

~ ~ <..?-o-'='-0 E ill

"'~'lQ... C«

coo

-'='Eou~

Q...

Q;3:o

---.J

t,/~'-J

Sagona go Granite

~Sif

undivided epiclastic, volcaniclasticsedlmenlary rocks, extrusive mafic

igneous rocks, and lheSoudan Iron - formation (sit),

mainly greenschist facies

o jO 20 30 Miles................ IIoowiiIl I

5 CAL E

Figure 1. Generalized geologic map of the Vermilion district, Minnesota (Morey & cthers, 1970)

0

I iI I I i I I i

II

I

II-'lJloI

oco<:r

90°45' -«)

p1a.te

92°00'00"

cr ",".",o,"v'" (:)~

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,'i' <0(:) 'vC::;

Location Map ,.'" ~<>

~qj~ ~r:'JQ.,

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Area. of

<§> "," o,<> ,,<I;> o,'>~ ~C?it'

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CANADA

92°00'

92°00'

L- ...,

L ,.. <

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I- <V A 7 r-

7 1\ L

°Cook.

+ ++ + +

+ + + ++ + + +

+ + + + ++ + + + +

+ + + + T

+ + + ++

90°45'

-\0 92°45'oco

""

Index to quadrangles stud ied

Figure 1. Generalized geologic map of the Vermilion district, Minnesota (Morey & (thers, 1970)

C )C P L A N A T 10 N

North of Vernii/ioii foijli

G,onis Range Granite Ver ni lion Gronile

Bionic Schisi

Newton Lake Farmolionundivided

vii —7-———-—vmv Amphibolile

YQ vVm Knit Lake Group

L oice Verm,tion For molio n

vgs, meiagroywocke membervg. quorrziie membervgv, mired metogroywacke - felsic

conglomerate membervvtn, vaiconicMsic membervif bonded iron - formation

5 cmv. dominantly melobasoli andmeladiab a so

Soudon Iron - format Ondominantly jaspiiite osper and cherl

-- cit

Ely Greenstoneeg, dominantly menobosolticand meiaandesitic p i ilowed flowsand 'nlern,ediate composition

pyrociostic depOsits

9it, banded Iron - formation

Corroci, dotled wiere nte- 'Cd

Vault • oared where inferred

2 4 6 8 loMites

S CAL E

EXPLANATION

North at Vermilioll foull

I' ... '1.....

Glonts Range Grande

Biollle 5chiSl

Newton Lake Formal ionundivIded

Vif.::::.. vmv

vgv ~v.gs.vq .':'.':'." :"> . vvm

Amphibolite

Knife Lake Group(undivided)

Lake Vermilion Formationc.2.D

E3~

Q..

vgs,v q,

vg v,

vvm,vlf >

vmv,

metagraywacke member

quartzite member

mi xed metagraywacke - felsicconglomerate membervolcaniclastic memberbanded iron - formationdominantly metabasalt and

metod ia b a S8

II-'V1I-'I

Soudan Iron -formationdomlnontly jospilite, jasper, and chert

Ely Greenstone

e9, domlnontly metabasalticand metaandesitic pillowed flowsand Intermediate composi lIon

pyroc lost\c deposits

elf, bonded Iron -formation

Contact, dotted where inferred

Faull, dotted where inferred

o'"'

2 4 6 8

5 CAL E

Figure 2. Geology of the western part of Vermilion district, Minnesota (Norey & others, 1970).

II

p j II I

H '7 W92°3

ID

H IS W

z

ID

I-

ton

..4 ,v_t— c a

C -. Jwtfl okes

log1e' /'V'< c-..Lake

I-LIN:

(H

ri7' ,/6 ft

H t 7 W92*30

a is w 8W92° '5

n law

0

0-J

0000-J

91045R ~ 7"'V\T

92'30'

II-'V1t-:1

Zo(j)

b

z

Z(II(j)

b

9j '45'R~-4VV -0

oZooII"'"(j)

f-<

92'00'B. '3"W

R ~5""N

92' 15'

... : : : :::::::::::::::::::: ::::::: ~ .......................

R 16VV

· .· .

· .· .· .

· . . . . . . . . . . . . . . . . . . . . . .· . . . . . . . . . . . . . . . . . . . .. .. . ....................................................... ......... ~-~-=· .

z

Zo(j)

b

Z(II

(j)

f-<

ZI'l(j)

f-<

92'30'-g R~7W'

ooZ"'"II

(j)

f-<

Figure 2. Geology of the western part of Vermilion district, Ninnesota (Norey

I

& others,

I

1970).

I

S.—.

F I I I

E Sc L AN AT S ONC1

LLL Lii

hi Ui

SC £ S. £

-',-''-'-',-'"

EXPLANATION

Ou'''''' Cornplt.

Sno"~OA'" Lo_" StOel "e,,"""O~ G'Qn,'eund,·•• ded ,,,,I"don o~~'O'ed ~(~,~,~

ro. .... ,on La •• formot,on

nm" molK: ,Olcon,e mtmatl

nh, Itl$,(· ,nl.rmed·"·" .....x:on..:mem!>e< oom'l'Io<"ly fe ~.c:-"",ermed>o'.

pyroelau,e ,oc.~ ond I.~~.' lIo ..~(nf,) of ~,m,lc, eompc~",on

~.,~~...tl

Kn,lt La~e G,o"p

'm~. dom,nonlly mtlO\l,ay ..o~'. 0"<:1 $101.,'m~. ~m'l'Ianuy m.,'otO~O,1

Ow. dom.non", .okon.elo~"t 'oe. 01 Itl~,~

'0 ,nr.rmtd,ol. tompo~,"OI'l

Ely Gr.el'l~lol'le

.\!. dJm,nonl" ..,.lotlO$Olh(Ofl<l ""eloonde~,''': p,llo".d

and .... 'e'm.d'O'. (O""O'O$,I,onOy'OCIO$I..: d.PO~,I$

If-JlJlWI

o • 2 ;} 4..._.__-----......t:~=~

seA L E

_~M,l~

4 - - ,e; / —- — —— —- - -

-

t\— c N

— .--.--- / / -:-

Figure 3. Geology of the central part of the Vermilion district, Minnesota (Morey & others, 1970).

I — I I I

'F --

- !)y/

//

/

7 /

I I

Figure 3. Geology of the central part of the Vermilion district , M'l.nnesota (Morey & others , 1970).

Figure 4.

Basalt—andesite

GENERALIZED AND IDEALIZED PRE—DEFORMATIONAL VOLCANIC PILE,WESTERN VERMILION DISTRICT

(Not to scale. Numbers indicate approximate stratigraphic positionsof field trip stops.)

—155—

w E

IGraywacke—slate I 1 Dacite

[77: j Feldspathic quartzite . Dacite porphyry

I

0 00 .,j Dacite agglomerate Iron—formation

-155-

W E

"C? - ./ '- /=:::> ~

0 z::::;. 13......

/

"c=r ' L:)

,;' "12

. . /. \.. •0 0 ,;'

\.

I'6 / \. / \ ,;'

"- / ....... / \. I' "- ..... 3 \.

"- / ./ "- "- ,;'\ \. ,;' " /

,;' ......I " " ./

,;' \ / I'

" "- " ... / / 2/ "- '- ," / .... / / \./ I ,- ....

"

0Graywacke-slate - . Dacite tuff

0 0. . .. Fe1dspathic quartzite .... .,.. .,.. Daci te porphyry. . ..

0 Dacite agglomerate • Iron-formationo 0

Q Basalt-andesite,;' "

Figure 4.

GENERALIZED AND IDEALIZED PRE-DEFOID1ATIONAL VOLCANIC PILE~

WESTERN VERHILION DISTRICT

(Not to scale. Numbers indicate approximate stratigraphic positionsof field trip stops.)

—156—

VERMILION DISTRICT FIELD TRIP

INTRODUCTION

This field trip is designed to be a one—day trip, starting near Elyand ending a few miles west of Tower. Representative outcrops of mostof the major rock types in the Vermilion district are included. Most

stops are in the Lake Vermilion Formation, the Ely Greenstone, and theSoudan Iron—formation. None are in the Knife Lake Group because goodexposures are not easily accessible; however, the Knife Lake rocks aresimilar to those in the Lake Vermilion Formation.

Stop 1 Any of several roadcuts 2—10 miles South of Ely on Hwy. 1.

Giants Range bathalith

Most abundant facies in this area ('Farm Lake Fades' of Green, 1970) ismedium—grained, porphyritic hornblende—biotite quartz—poor adainellite withpink K—spar phenocrysts. Hornblende and K—spar are commonly aligned in flowstructure. Monzonitic, granodioritic, and dioritic phases also occur; allhave been cut by shear zones locally.

Stop 2 South of Ely on Hwy 1. Outcrops under powerline on E side of Hwy 1, 1.4

mi. S of junction with Hwy 169.

Ely Greens tone

Dacitic to andesitic, pillowed lavas characteristic of a zone within the

Ely Greenstone that trends E—W south of Ely. Pillow shapes show tops face

north, as in most of the formation. These volcanics are cut by aplitic

dikes related to the Giants Range batholith; the hidden contact lies inthe slope to the southwest, and granite outcrop can be seen near the baseof the slope.

StoRj Roadcuts on Hwy 169, at curve about 0.25 ml. 14. of Ely.

Ely Greenstone

Pillowed metabasalt that is typical of the màflc lavas of the Ely Creenstone

is exposed in road cut on north side of highway. The pillow structures have

smoothly rounded tops, nearly flat bases, and are somewhat drawn out in

vertical dimension. The chilled rinds are a fraction of an inch thick. The

pillow structures strike approximately N.20° U., dip 800 SE , and facesoutheastward. The long dimension of the pillows is subparailel to theintersection of cleavage and bedding and plunge-s steeply northeastward.

-156-

VERMILION DISTRICT FIELD TRIP

INTRODUCTION

This field trip is designed to be a one-day trip, starting near Elyand ending a few miles west of Tower. Representative outcrops of mostof the major rock types in the Vermilion district are included. Moststops are in the Lake Vermilion Formation, the Ely Greenstone, and theSoudan Iron-formation. None are in the Knife Lake Group because goodexposures are not easily accessible; however, the Knife Lake rocks aresimilar to those in the Lake Vermilion Forma.tion.

Stop 1 Any of several roadcuts 2-10 miles South of Ely on Hwy. 1.

Giants Range batholith

Most abundant facies in this area C'Farm Lake Facies" of Green, 1970) ismedium-grained, porphyritic hornblende-biotite quartz-poor adamellite withpink K-spar phenocrysts. Hornblende and K-spar are con~only aligned in flowstructure. Monzonitic, granodioritic, and dioritic phases also occur; allhave been cut by shear zones locally.

Stop 2 South of Ely on Hwy 1. Outcrops under pmverline on E side of Hwy l, 1. 4mi. S of junction with Hwy 169.

Ely Greenstone

Dacitic to andesitic, pillowed lavas characteristic of a zone within theEly Greenstone that trends E-W south of Ely. Pillow shapes show tops facenorth, as in most of the formation. These volcanics are cut by apliticdikes related to the Giants Range batholith; the hidden contact lies inthe slope to the southwest, and granite outcrop can be seen near the baseof the slope.

Stop 3 Roadcuts on Hwy 169, at curve about 0.25 mi. W. of Ely.

Ely Greenstone

Pillmved metabasalt that is typical of the mafic lavas of the Ely Greenstoneis exposed in road cut on north side of higlmay. The pillow structures havesmoothly rounded tops, nearly flat bases, and are somewhat drawn out invertical dimension. The chilled rinds are a fraction of an inch thick. Thepillmv structures strike approximately N. 20° Eo, dip 80° SE., and facesoutheastward. The long dimension of the pillows is subparallel to thein~ersection of cleavage and bedding and plunges steeply northeastward.

—15 7—

The exposures are on the northwest limb of a tight syncline, the axisof which passes through the Ely trough. The Ely trough contains aniron—formation that was largely altered to hematite, and which was asubstantial source of direct—shipping hematite ore.

On the south side of highway, fine—to—medium—grained metadlabase isexposed in road cut and on hill to south. The metadiabase intrudes andcrosscuts the pillowed metabasalt. A contact can be seen in the southern partof the crest of the hill.

Stop 4. About 2.5 ml. W of Stop 3 on Hwy 169, turn N (right) on road to BurntsideLodge (Co. 88). Continue on road past bridge over Burntside River andpast junction with Van Vac road (on left). About 1.4 miles past Van Vacroad, turn left (on curve) on private road. Stop in about 0.2 milesnorth on private road, at curve to right.


Serpentinized metaperidotite and associated gabbroic rocks are exposedfrom base of hill northward to crest.

Serpentinized peridotite is exposed at base of hill. It is about 150feet thick, is nearly black, and contains some poikilitic augite.Iniinediately north of serpentinized periodite and apparently gradationalinto it is a coarse—grained hypersthene (?) gabbro, which grades in turninto a gabbro (higher on hill). The gabbro appears to have some crudecompositional layering and is in part diabasic. On crest of hill, partof the gabbro (or diorite) contains coarse, radiating pyroxene crystals,as much as an inch long. There is some interstitital quartz and feldspargranophyric material in this phase of the rock. Pyrite is widely scat-tered through the gabbroic rocks.

The ultramafic—mafic body at this stop is near the southwestern end of anintrusive sheet that is about 3.5 miles long and 1,000 feet or more thick,and which underlies little Long Lake. The sheet appears to be a differ-entiated body, from peridotite at the base to gabbro at the top. Northof the eastern end of Little Long Lake, a small body of serpentinizedperidotite, within metadiabase, is exposed in a roadcut along the EchoTrail. The top of the body is truncated by the Vermilion fault.

Stop 5 Roadcut about 13.0 miles W of Ely and about 2.5 ml. W of Eagle's NestLake road (Co. 408) on Hwy 169. Other outcrops of similar rocks are alsopresent along the highway.

Soudan Iron—formation and cross—cutting dacite.

Roadcut in Soudan Iron—formation. A quartz—feldspar (dacite)porphyry dike and Ely Creenstone are exposed at east edge of outcrop.The iron—formation is composed of interlayered red and white chert andopaque iron oxide layers; it is deformed into drag folds that plunge500_600 N.E. Beyond a covered interval of 0.1 miles to the east, the

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The exposures are on the northwest limb of a tight syncline, the axisof which passes through the Ely trough. The Ely trough contains aniron-formation that was largely altered to hematite, and which was asubstantial source of direct-shipping hematite ore.

On the south side of highway, fine-to-medium-grained metadiabase isexposed in road cut and on hill to south. The metadiabase intrudes andcrosscuts the pillowed metabasalt. A contact can be seen in the southern partof tile crest of the hill.

Stop 4. About 2.5 mi. \oJ of Stop 3 on Hwy 169, turn N (right) on road to BurntsideLodge (Co. 88). Continue on road past bridge over Burntside River andpast junction with Van Vac road (on left). About 1.4 miles past Van Vacroad, turn left (on curve) on private road. Stop in about 0.2 milesnorth on private road, at curve to right.


Serpentinized metaperidotite and associated gabbroic rocks are exposedfrom base of hill northward to crest.

Serpentinized peridotite is exposed at base of hill. It is about 150feet thick, is nearly black, and contains some poikilitic augite.Immediately north of serpentinized periodite and apparently gradationalinto it is a coarse-grained hypersthene (?) gabbro, which grades in turninto a gabbro (higher on hill). The gabbro appears to have some crudecompositional layering and is in part diabasic. On crest of hill, partof the gabbro (or diorite) contains coarse, radiating pyroxene crystals,as much as an inch long. There is some interstitital quartz and feldspargranophyric material in this phase of the rock. Pyrite is widely scattered through the gabbroic rocks.

The ultramafic-mafic body at this stop is near the southwestern end of anintrusive sheet that is about 3.5 miles long and 1,000 feet or more thick,and which underlies little Long Lake. The sheet appears to be a differentiated body, from peridotite at the base to gabbro at the top. Northof the eastern end of Little Long Lake, a small body of serpentinizedperidotite, within metadiabase, is exposed in a roadcut along the EchoTrail. The top of the body is truncated by the Vermilion fault.

Stop 5 Roadcut about 13.0 miles W of Ely and about 2.5 mi. W of Eagle's NestLake road (Co. 408) on H\vy 169. Other outcrops of similar rocks are alsopresent along the highway.

Soudan Iron-formation and cross-cutting dacite.

Roadcut in Soudan Iron-formation. A quartz-feldspar (dacite)porphyry dike and Ely Greenstone are exposed at east edge of outcrop.The iron-formation is composed of interlayered red and white chert andopaque iron oxide layers; it is deformed into drag folds that plunge50 0 -60 0 N.E. Beyond a covered interval of 0.1 miles to the east,the

iron—formation consists of thin to thick, black, red and white chertbeds interlayered with black argillaceous beds that contain abundantveins, stringers, and beds of euhedral pyrite. The beds trend aboutN.85° and W. and dip 80°N.

This dacite is representative of the felsic volcanic rockswhich apparently provided the detritus for most of the sedimentaryand tuffaceous rocks of the district.

Stop 6 Outcrop just W of Stuntz Bay road at crest of Soudan Hill, at the N, edgeof village of Soudan, 1,000 ft E. of Soudan mine. Conserve this outcrop.

Soudan Iron—formation

This is a much—visited classic exposure of folded Soudan Iron—formationcomprised of alternating beds of hematite and jasper. The nearby Soudanmine was opened in 1884, and operated continuously until 1962. when it wasdeeded to the state by U. S. Steel for the development of Tower—Soudan StatePark. It was the first iron ore mine in Minnesota; 15.5 million tons ofhigh grade ore (63—66% Fe) were shipped.

Most of the small folds are the result of the second deformation inthe area, and these (F2) folds plunge to the east at steep angles. However,

evidence of an earlier set of folds is provided by structures such asthese in the sketches below.

F1 AND F2 INTERFERENCE PATTERNS

F1 fold axis

F2 fold axis

F1 FOLD MODIFIED BY F2

DEFORMATION

ApproximateN

1H

One Foot

Approximate Scale

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iron-formation consists of thin to thick, black, red and white chertbeds interlayered with black argillaceous beds that contain abundantveins, stringers, and beds of euhedral pyrite. The beds trend aboutN.85° and W. and dip 80 o N.

This dacite is representative of the felsic volcanic rockswhich apparently provided the detritus for most of the sedimentaryand tuffaceous rocks of the district.

Stop 6 Outcrop just W of Stuntz Bay road at crest of Soudan Hill, at the N. edgeof village of Soudan, 1,000 ft E. of Soudan mine. Conserve this outcrop.

Soudan Iron-formation

This is a much-visited classic exposure of folded Soudan Iron-formationcomprised of alternating beds of hematite and jasper. The nearby Soudanmine was opened in 1884, and operated continuously until 1962, when it wasdeeded to the state by U. S. Steel for the development of Tower-Soudan StatePark. It was the first iron ore mine in Minnesota; 15.5 million tons ofhigh grade ore (63-66% Fe) were shipped.

Host of the small folds are the result of the second deformation inthe area, and these (F

2) folds plunge to the east at steep angles. However,

evidence of an earlier set of folds is provided by structures such asthese in the sketches below.

Fl fold axis/"

F2

fold axis

F1

FOLD MODIFIED BY F2

DEFORMATION

Fl

AND F2

INTERFERENCE PATTERNS

ApproximateN

One Foot

Approximate Scale

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Stqpl Several outcrops on peninsula in Lake Vermilion, east of McKinleyBay, I and W of development road.

Lake Vermilion Formation, Volcaniclastic Member

White dacitic tuff, white dacitic agglomerate, black carbon—iferous (?) slate and minor chloritic graywacke, all of the volcani—clastic member, are interbedded in this area. The westernmost exposuresof the Soudan Iron—formation also occur here. Good exposures of theagglomerate are best reached by boat; therefore, we shall only see somelarge glacial erratics of this rock type which are virtually in place.

The dacitic tuff is very difficult (arid commonly impossible) todistinguish from dacite flows. Study of thin sections is usuallynecessary to resolve the question. It is some thmfort to know thatClements (1903) and numerous other workers had similar difficulties.The tuff is composed of three main components — dacitic volcanic rockfragments, plagioclase, and volcanic quartz. Recrystallization causesthe volcanic rock fragments to appear as a fine—grained quartz—plagioclase matrix.

stop 8 Large roadcuts 0.3 mi. W of Tower on Hwy 169,

Lake Vermilion Fornation, Felds athic Quartzite Member

This is a limonite—stained exposure of the conglomeratic faciesof the feldspathic quartzite member. Bedding and clasts are bestobserved on the glaciated surface at the western end of the southroadcut. Pyrite and pyrrhotite are the major sulfides present, andappear to have replaced slaty fragments .in the conglomerate. Mostclasts are volcanic rocks, probably mostly dacitic. The matrix issimilar to the feldspathic quartzite of Stop 10. Note the strongdevelopment of F2 lineation which plunges easterly at about 600.

5t229 Small outcrop S of Hwy 169, in and across ditch, 1.1 mi. W of Tower.(optional)

Lake Vermilion Formation, FeljIiicQuartzite Member

Lapilli tuff (?) in the basal part of the feldspathic quartzitenewiber. This is the only exposure of this rock type in the immediatearea; it appears to be transitional in texture between the dacitictuffs and dacitic agglomerates.

Stop 8

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Several outcrops on peninsula in Lake Vermilion, east of McKinleyBay, E and W of development road.

Lake Vermilion Formation, Volcaniclastic Member

Hhite dacitic tuff, white dacitic agglomerate, black carboniferous (?) slate and minor chloritic graywacke, all of the volcaniclastic member, are interbedded in this area. The westernmost exposuresof the Soudan Iron-formation also occur here. Good exposures of theagglomerate are best reached by boat; therefore, we shall only see somelarge glacial erratics of this rock type which are virtually in place.

The dacitic tuff is very difficult (and commonly impossible) todistinguish from dacite flows. Study of thin sections is usuallynecessary to resolve the question. It is some comfort to know thatClements (1903) and numerous other workers had similar difficulties.The tuff is composed of three main components - dacitic volcanic rockfragments, plagioclase, and volcanic quartz. Recrystallization causesthe volcanic rock fragments to appear as a fine-grained quartzplagioclase matrix.

Large roadcuts 0.3 mi. W of Tower on Hwy 169.

Lake Vermilion Formation, feldspathic Quartzite Member

This is a limonite-stained exposure of the conglomeratic faciesof the feldspathic quartzite member. Bedding and clasts are bestobserved on the glaciated surface at the western end of the southroadcut. Pyrite and pyrrhotite are the major sulfides present, andappear to have replaced slaty fragments ,in the conglomerate. Mostclasts are volcanic rocks, probably mostly dacitic. The matrix issimilar to the feldspathic quartzite of Stop 10. Note the strongdevelopment of F2 lineation ~vhich plunges easterly at about 60°.

~t.-<?.p_J_ Small outcrop S of thvy 169, in and across ditch, 1.1 mi. W of Tower.(optional)

Lake Vermilion formation, feldspathic ~uartzite Member

Lapilli tuff (?) in the basal part of the feldspathic quartzitemelilber. This is the only exposure of this rock type in the immediatearea; it appears to be transitional in texture between the dacitictuffs and dacitic ap,glomerates.

Note:

Stop 10

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South of the highway in the woods on West Two Rivers is tte foldedwestern end of the Ely Greenstone. The fold nose points Wbut thefold plunges E at about 55°. This is an antiformal mass cored byolder, mafic volcanics whose fold axis plunges toward the older rocksthe wrong way. Some small folds at Stop 11 exhibit the same structure.

Roadcuts N & S of Hwy 169, 1.85 mi W of Tower.

Lake Vermilion Formation, Feldspathic Quartzite Member

Feldspathic quartzite is an unfortunate choice of field terms,as the rock is largely composed of plagioclase and volcanic rockfragments with only minor large quartz grains (see below). Faintbedding and lamination are visible on some parts of the outcrop, anda sericitic phyllite band occurs on the north cut. At the east end ofthe south cut, felsic volcanic fragments up to an inch in diameter arevisible. The 20 ft.-thick dike of diabasic gabbro at the tvest endof the outcrop is the youngest rock in the area. It has a minimumK-Ar age of 1570 m.y., and similar dikes a few miles away have agesof 1520 and 1685 m.y. (Hanson & Malhotra, 1971). The dike exhibi~s

excellent chilled contacts and some inclusions of the quartzite.

The feldspathic quartzites contain 20-30% plagioclase; 15-30%felsic volcanic rock fragments; 30% fine recrystallized quartz andplagioclase which probably represents, in large part, recrystallizedvolcanic rich fragments, plagioclase and quartz; 5-10% micaceous matrix;and 5-10% quartz, including some quartz which is definitely of volcanicorigin.

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Stp_11 Long roadcut S of Hwy 169, 2.5 rid. W of Tower. Conserve these folds.

Lake Vermilion_Formation jjrwacke—Slate Member

Strongly folded biotitic metagraywacke and slate. (The compositionof this rock is described at Stop 12, where the rocks are evenly beddedand relatively undeformed.) The complex folding results from two deforma-tions, as sketched below and as described in the text accompanying thisfield trip log. Some geologists have speculated that the folding is softsediment deformation, rather than tectonic. However, these stnicturesare unique in the area, this exposure is located near a major anticlinalaxis, and well—preserved sedimentary structures nearby are not chaotic.Note the excellent grading.

Approximate

ApproximateScale

One Foot

5Q0

axial plunge

F1 fold axis

F2 Cleavage

OVERTURNEJ) F2 FOLD F1 FOLD ANT) F2 CLEAVAGE

P2 cleavage

/-7

1fold axis

F FOLD 1ODIFIED BY F "EYE STRUCTURE" DUE TO EROSIONDkFORMATION.

2OF F FOLDS WHICH WERE DEFOR1€1)BY F FOLDS.

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_~t...2.E..-.l1. Long roadcut S of H\vy 169, Z. 5 mi. W of Tm.]er. Conserve these folds.

Lake Vermilion Formation, Metagraywacke-Slate Member

Strongly folded biotitic metagraywacke and slate. (The compositionof this rock is described at Stop lZ, where the rocks are evenly beddedand relatively undeformed.) The complex folding results from two deformations, as sketched below and as described in the text accompanying thisfield trip log. Some geologists have speculated that the folding is softsediment deformation, rather than tectonic. However, these structuresare unique in the area, this exposure is located near a major anticlinalaxis, and well-preserved sedimentary structures nearby are not chaotic.Note the excellent grading.

ApproximateN

·····eOV ERTU R.:\IED F2 FOLD

50°axial plunge

iOne Foot

ApproximateScale

Fl

fold axis

FZ Cleavage

F1 FOLD A..~D FZ CLEAVAGE

F1

fOLD MODIFIED HY FZDEFORMATION.

cleavage

fold axis

---~•o

F2

cleavage

t1EYE STRUCTURE" DUE TO EROSIONOF FI FOLDS I-.THICH I-.TERE DEFORMEDBY FZ FOLDS.

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;Lposure NJ of bridge across Pike River an Co. 77, 0.55 mi N of'I •. .r:S'Y 169. (Junction of Co. 77 and Hwy 169 is 2.4 miles W of Tower).

-. C ... conserve this.;:

Lake Vermilion Formation, Metagraywacke—s]ate Member

This is an exposure of biotitjc metagraywacke and slate, havingexcellent grading. Two—thirds of the 200 graywacke beds on thisexposure are graded and nine percent of the 100 siltstone beds aregraded. Beds here are thin, but graywacke beds are as much as 12 feetthick.

North—trending kink bands were formed by the latest (F ) deformation.Small scale faults are common. A NNE—trending fault with aout 1000 feetof left—lateral displacement forms the S side of the river channel atthis locality, and extends for several miles to the north and south.

The biotitic graywackes contain l0—20 plagioclase; 15—25% felsicvolcanic rock fragments; 30—50% fine recrystallized quartz and plagio—clase which probably represents, in large part, recrystallized volcanicrock fragments, plagioclase, and quartz; 2—5% quartz, and lO—15biotitic matrix.

Stop 13 Exposures about 10 mi N of Hwy 169 on Co. 77, at Gruben's Resort on(Optional) Arrowhead Point. Best exposures are just E of wooden bridge to Isle

of Pines.

Lake Vermilion Formation, Metagrywacke—Slate Member

Exposures of chloritic graywackes and slates. Both east and westof this locality, pillowed greenstones are interbedded with the graywacke—slate. This locality is near the axis of the Arrowhead Point fold. This

is an ENE—trending (F2) anticline, the axis of which plunges steeply tothe west although the beds at the fold nose are younger to the east.

The chloritic graywackes contain 40—50% plagioclase (albite—oligoclase),20—25% felsic volcanic rock fragments, 10—20% chioritic matrix, 2—6X quartz,some of which is definitely of volcanic origin.

Stop 14 Roadcuts between lanes of Hwy 169 on "Confusion Hill" (the Continental(Optional) Divide) about 3 mi N of Virginia and about 23 miles W of Tower.


This exposure illustrates the complexity of the south edge of the

Giants Range batholith. A gray granite gneiss contaLning ailiphiholiteinclusions is cut by diorite, and minor pink granites cut the above rocks.

At the top of the exposure between the ttro lanes of the hichway areremnants of Lower Precanl,rian thinly bedded sediments (tuffaceous?), maficlapilli tuffs (fl, and massive amphibolites.

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Stop 12 Exposure NW of bridge across Pike River on Co. 77, 0.55 mi N ofHwy 169. (Junction of Co. 77 and Hwy 169 is 2.4 miles W of Tower).Please conserve this outcrop.

Lake Vermilion Formation, Metagraywacke-Slate Member

This is an exposure of biotitic metagraywacke and slate havingexcellent grading. Two-thirds of the 200 graywacke beds on ~his exposure are graded and nine percent of the 100 siltstone beds aregraded. Beds here are thin, but graywacke beds are as much as 12 feetthick.

North-trending kink bands were formed by the latest (F ) deformation.Small scale faults are common. A NNE-trending fault with aiout 1000 feetof left-lateral displacement forms the S side of the river channel atthis locality, and extends for several miles to the north and south.

The biotitic graywackes contain 10-20% plagioclase; 15-25% felsicvolcanic rock fragments; 30-50% fine recrystaliized quartz and plagioclase which probably represents, in large part, recrystallized volcanicrock fragments, plagioclase, and quartz; 2-5% quartz, and 10-15%biotitic matrix.

Stop 13 Exposures about 10 mi N of Hwy 169 on Co. 77, at Gruben's Resort on(Optional) Arrowhead Point. Best exposures are just E of wooden bridge to Isle

of Pines.

Lake Vermilion Formation, Hetagr~acke-Slate i'lember

Exposures of chloritic graywackes and slates. Both east and westof this locality, pillowed greenstones are interbedded with the graywackeslate. This locality is near the axis of the Arrowhead Point fold. Thisis an ENE-trending (F 2) anticline, the axis of ,,,hich plunges steeply tothe west although the beds at the fold nose are younger to the east.

The chloritic gra~vackes contain 40-50% plagioclase (albite-oligoclase),20-25% felsic volcanic rock fragments, 10-20% chloritic matrix, 2-6% quartz,some of which is definitely of volcanic origin.

Stop 14 Roadcuts bet\veen lanes of Hwy 169 on "Confusion Hill" (the Continental(Optional) Divide) about 3 mi N of Virginia and about 23 miles W of Tower.


This exposure illustrates the complexity of the south edge of theGiants Range batholith. A gray granite gneiss containinh amphiboliteinclusions is cut by diorite, and minor pink granites cut the above rocks.

At the top of the exposure het\\'een the tHO lanes of the higlmay areremnants of LOvIer PrecanlHian thinly bedded sediments (tuffaceous?), maficlapilli tuffs (?y, and massive amphibolites.

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