7.

ORGANIZING COMMITTEE

38TH ANNUAL MEETING

INSTITUTE ON LAKE SUPERIOR GEOLOGY

General Chairman: Albert B. Dickas, University of Wisconsin-SuperiorProgram Chairman and Proceedings Editor: Bruce A. Brown,

Wisconsin Geological and Natural History Survey

Volume 38 consists ofPart 1: Program and AbstractsPart 2: Field Trip Guidebook

Published and distributed byInstitute on Lake Superior Geology

M.G. Mudrey, Jr., Secretary-Treasurerdo Wisconsin Geological and Natural History Survey

3817 Mineral Point RoadMadison, Wisconsin 53705-5100

ISSN 1042-9964

General Chairman: Albert B. Dickas, University of Wisconsin-Superior Program Chairman and Proceedings Editor: Brace A. Brown,

Wisconsin Geological and Natural History Survey

Volume 38 consists of Part 1: Program and Abstracts Part 2: Field Trip Guidebook

Published and distributed by Institute on Lake Superior Geology

M.G. Mudrey, Jr., Secretary-Treasurer c/o Wisconsin Geological and Natural History Survey

38 17 Mineral Point Road Madison, Wisconsin 53705-5 100

ISSN 1042-9964

INSTITUTE ON•LAK E•SUPERIORGEOLOGY

PROCEEDINGS

38m ANNUAL MEETING

MAY 6-9,1992

HURLEY, WIscoNsJl4

ORGANIZED BY

ALBERT B. DIcicAs,UNIvERsn'Y O WISCONSIN-SUPERIOR

BRUCE A. BROWN,WISCONSIN GEOLOGICAL

ANI) NATURAL HISTORY SURVEY

VOLUME 38 MAY 1992

PART 2FIELD TRIP GUIDEBOOK

INSTITUTE ON

S U P E R I O R

PROCEEDINGS

CONTENTS

Trip 1: Archean and Early Proterozoic Geology of the Gogebic District,Northern Michigan and WisconsinLeadersGene L. LaBerge, University of Wisconsin—OshkoshRichard W. Ojakangas, University of Minnesota—DuluthKathy J. Licht, St. Norbert College

Trip 2: Evolution of the Keweenawan Sedimentary Sequence 41

LeadersAlbert B. Dickas, University of Wisconsin—SuperiorM.G. Mudrey, Jr., Wisconsin Geological and Natural History Survey

Trip 3: Geology of Keweenawan Supergroup Rocks near the Porcupine Mountains,Ontonagon and Gogebic Counties, Michigan 75

LeadersWilliam F. Cannon, Suzanne W. Nicholson, Cheryl A. Hedgman,Laurel G. Woodruff, and Klaus J. Schulz,U.S. Geological Survey, Reston, Virginia

Trip 4: Geology of the Great Lakes Tectonic Zone in the Marquette Area,Michigan—A Late Archean Paleosuture 103

LeadersP.K. Sims and Z.E. Peterman, U.S. Geological Survey, Denver, Colorado

CONTENTS

Trip 1: Archean and Early Proterozoic Geology of the Gogebic District, Northern Michigan and Wisconsin Leaders Gene L. LaBerge, University of Wisconsin-Oshkosh Richard W. Ojakangas, University of Minnesota-Duluth Kathy J. Licht, St. Norbert College

Trip 2: Evolution of the Keweenawan Sedimentary Sequence Leaders Albert B . Dickas, University of Wisconsin-Superior M.G. Mudrcy, Jr., Wisconsin Geological and Natural History Survey

Trip 3: Geology of Keweenawan Supergroup Rocks near the Porcupine Mountains, Ontonagon and Gogebic Counties, Michigan Leaders William F. Cannon, Suzanne W. Nicholson, Cheryl A. Hedgman, Laurel G. Woodruff, and Klaus J. Schulz, U.S. Geological Survey, Reston, Virginia

Trip 4: Geology of the Great Lakes Tectonic Zone in the Marquette Area, Michigan-A Late Archean Paleosuture Leaders P.K. Sims and Z.E. Peterrnan, U.S. Geological Survey, Denver, Colorado

ARC HEAN AND

EARLY PROTEROZOICGEOLOGY OF THE

GOGEBIC DISTRICT,NORTHERN MICHIGAN

AND WISCONSIN

Gene L. LaBergeUniversity of Wisconsin—Oshkosh

Richard W. OjakangasUniversity of Minnesota—Duluth

WITH A CONTRIBUTION FROM

Kathy J. LichtSt. Norbert College

Gene L. LaBerge University of Wisconsin-Oshkosh

Richard W. Ojakangas University of Minnesota-Duluth

WITH A CONTRIBUTION FROM

Kathy J. Licht St. Norbert College

2

This field trip is the outgrowth of numerous student fieldtrips led by the authors over the past quarter century. Most ofthe stop localities have been suggested by geologists who haveworked in the district. However, some localities are the resultof our own studies in the area. One of us (LaBerge) spentseveral weeks mapping in the area in July, 1991, supported by theU.S. Geological Survey, and Ojakangas worked in this regionintermittently with U.S. Geological support from 1978 until 1990.

This field trip is the outgrowth of numerous student field trips led by the authors over the past quarter century. Most of the stop localities have been suggested by geologists who have worked in the district. However, some localities are the result of our own studies in the area. One of us (LaBerge) spent several weeks mapping in the area in July, 1991, supported by the U.S. Geological Survey, and Ojakangas worked in this region intermittently with U.S. Geological support from 1978 until 1990.

EARLY PROTEROZOIC GEOLOGY OF THE GOGEBIC DISTRICTNORTHERN MICHIGAN AND WISCONSIN

INTRODUCTION

The Gogebic range extends from Lake Gogebic, Michigan,westward approximately 128 km to near Lake Namekagon, Wisconsin(Figure 1). The major structure in the area is a north-facingmonocline that exposes Archean rocks to the south and Middle andLate Proterozoic (Keweenawan Supergroup) rocks to the north.

EXPLANATION

MIDDLE PROTEROZOIC

volcanics andsediments

ARCH EAN

EARLY PROTEROZOIC

I I Graywacke—slate

Iron—Formation

1-4- 4!-4- I Granitoids

). vt.IVolcanics

Generalized map of the Gogebic range, showing the EarlyProterozoic rocks sandwiched between Archean rocks onthe south and Middle Proterozoic Keweenawan Supergrouprocks on the north. (Adapted from Morey and others,1982)

/7LAKE SUPERIOR

—(C LAX

WAKEFIELD ( \000EBIC

HURLE>

I. • t KiARENI3CO

, —- I +

LI. EN -

. 4-s + 4- + 4- -f + 4 Ci

4- 4- 4- -4-

0 5 10 20

KM

vJ-4-

-- -

cl SUE E I

Figure 1.

3

EARLY PROTEROZOIC GEOLOGY OF THE GOGEBIC DISTRICT NORTHERN MICHIGAN AND WISCONSIN

The Gogebic range extends from Lake Gogebic! Michiganl westward approximately 128 km to near Lake Namekagonl Wisconsin (Figure 1). The major structure in the area is a north-facing monocline that exposes Archean rocks to the south and Middle and Late Proterozoic -(~eweenawan Supergroup) rocks to the north.

MIDDLE PROTEROZOIC

- Volcanics and sediments

EXPLANATION

EARLY PROTEROZOIC

Iron-Formation

my Volcan ics

Figure 1. Generalized map of the Gogebic rangel showing the Early Proterozoic rocks sandwiched between Archean rocks on the south and Middle Proterozoic Keweenawan Supergroup rocks on the north. (Adapted from Morey and others, 1982)

3

4

Early Proterozoic rocks lie discordantly between these majorsequences. As a result of the steep northerly dip of themonocline (a consequence of Keweenawan tilting), the geologicalmap of the district is basically a stratigraphic cross-section.

The general geology of the Gogebic range has been knownsince the pioneering work of Irving and Van Hise (1892), Van Hiseand Leith (1911), Hotchkiss (1919), and Aldrich (1929). Althoughsubsequent studies have done much to "refine" the geologicalpicture - - and much still remains to be done - - most of the earlyinterpretations have withstood the test of time. The broad-scalegeology of the eastern Gogebic district is presented on the IronRiver 1° x 2° Quadrangle (Cannon, 1986).

STRATIGRAPHY

The Gogebic range contains excellent exposures of rocks thatrange in age from Archean (about 2,700 Ma) and Early Proterozoic(2,300 - 1,900 Ma), to Middle and Late ProterozoictKeweenawantI(1,200 - -600 Ma). Stratigraphic and structural relationships ofseveral of the rock sequences can be observed in outcrops.Thus,it is possible to demonstrate the relationship of the rocksequences to each other. The general sequence of rock units inthe Gogebic district is shown in Figure 2, a diagrammaticlongitudinal section along the Gogebic range.

Figure 2. Idealized sketch showing generalized stratigraphicsection on the eastern Gogebic range.

w

tr ofl

KeweenawanE

---+

-

Prit+g * -

Puritan -

Early Proterozoic rocks lie discordantly between these major sequences. As a result of the steep northerly dip of the monocline (a consequence of Keweenawan tiltingl1 the geological map of the district is basically a stratigraphic cross-section.

The general geology of the Gogebic range has been known since the pioneering work of Irving and Van Hise (1892)1 Van Hise and Leith (1911)1 Hotchkiss (1919), and Aldrich (1929). Although subsequent studies have done much to I1refineIt the geological picture - - and much still remains to be done - - most of the early interpretations have withstood the test of time. The broad-scale geology of the eastern Gogebic district is presented on the Iron River lo x 2' Quadrangle (Cannonl 1986) .

The Gogebic range contains excellent exposures of rocks that range in age from Archean (about 21700 Ma) and Early Proterozoic (2#300 - 11900 Ma), to Middle and Late Protero~oic-~~Keweenawan~~- (11200 - -600 Ma). Stratigraphic and structural relationships of several of the rock sequences can be observed in outcrops. 'Thuslit is possible to demonstrate the relationship of the rock sequences to each other. The general sequence of rock units in the Gogebic district is shown in Figure Z1 a diagrammatic longitudinal section along the Gogebic range.

K e w e e n a w a n

Figure 2. Idealized sketch showing generalized stratigraphic section on the eastern Gogebic range.

ARCHEA&N ROCKS

Archean rocks in the Gogebic range are variably deformed andmetamorphosed greenstones and granitoid rocks typical of Archeangreenstone-granite terranes (or belts), and, indeed, are anextension of the Wawa Greenstone belt of Canada. The district isnear the southern margin of the Superior Province of the CanadianShield. Because this fied trip is concerned primarily with theEarly Proterozoic rocks, only a brief description of the Archeanrocks is presented here.

Ramsay Formation. Archean volcanic rocks are exposed in a beltabout 5 km wide and 16 km long in the eastern Gogebic range(Prinz and others, 1975). Schmidt and Hubbard (1972) namedvolcanic and sedimentary rocks in the Ramsay-Wakefield area asthe Ramsay Formation. The Ramsay Formation consists mainly ofpillowed, fragmental, and massive basaltic rocks in the easternpart of the area; however, felsic volcanic rocks and meta-graywackes are dominant to the west, near the Wisconsin border.Most of the rocks have been metamorphosed to greenschist facies,with well-preserved pillows and other primary features. Locally,especially adjacent to the Puritan Quartz Monzonite, the rockshave been metamorphosed to amphibolite facies, and few, or no,primary features are preserved. The Ramsay Formation may be theoldest rock unit in the Gogebic range proper, however, oldergneisses (to 3,560 Ma) are exposed in the Watersmeet area, some30 km southeast (Sims and others, 1984).

Puritan Quartz Monzonite. The volcanic-sedimentary sequence ofthe Ramsay Formation has been intruded by large granitoid massesnamed the Puritan batholith (Schmidt and Hubbard, 1972). ThePuritan Quartz Monzonite exposed in the central and westernGogebic range is a weakly-deformed post-tectonic pluton thatranges in composition from granite to tonalite and has been datedto 2,735 ± 16 Ma (Sims and others, 1984).

Granitoid rocks are also extensively exposed in theMarenisco area on the eastern end of the Gogebic range. Parts ofthe granitoid has prominent gneissic banding and associatedamphibolite. The other, more extensive phase, is a weaklydeformed medium-grained granite with widely scattered peglnatitebodies (Fritts, 1969; Trent, 1973)

The Archean rocks were eroded and are unconformably overlainby Early Proterozoic rocks of the Marquette Range Supergroup.Because the Gogebic range is a north-dipping monocline, EarlyProterozoic rocks rest on Archean rocks to the south along theentire length of the range.

EARLY PROTEROZOIC ROCKS

At the ends of the Gogebic range the Archean rocks areunconformably overlain by rocks of the Chocolay Group of the

5

ARCHEAN ROCKS

Archean rocks in the Gogebic range are variably deformed and metamorphosed greenstones and granitoid rocks typical of Archean greenstone-granite terranes (or belts) and, indeedl are an extension of the Wawa Greenstone belt of Canada. The district is near the southern margin of the Superior Province of the Canadian Shield. Because this field trip is concerned primarily with the Early Proterozoic rocksl only a brief description of the Archean rocks is presented here.

Ramsay Formation. Archean volcanic rocks are exposed 'in a belt about 5 km wide and 16 km long in the eastern Gogebic range (Prinz and othersl 1975). Schmidt and Hubbard (1972) named volcanic and sedimentary rocks in the Ramsay-Wakefield area as the Ramsay Formation. The Ramsay Formation consists mainly of pillowedl fragmentall and massive basaltic rocks in the eastern part of the area; howeverl felsic volcanic rocks and meta- graywackes are dominant to the westl near the Wisconsin border. Most of the rocks have been metamorphosed to greenschist faciesI with well-preserved pillows and other primary features. Locallyl especially adjacent to the Puritan Quartz Monzonitel the rocks have been metamorphosed to amphibolite faciesl and fewl or nol primary features are preserved. The Ramsay Fomtion may be the oldest rock unit in the Gogebic range properl howeverl older gneisses (to 3#560 Ma) are exposed in the Watersmeet areal some 30 km southeast (Sims and othersl 1984) . kritan Ouartz Monzonite. The volcanic-sedimentary sequence of the Ramsay Formation has been intruded by large granitoid masses named the Puritan batholith (Schmidt and Hubbardl 1972). The Puritan Quartz Monzonite exposed in the central and western Gogebic range is a weakly-deformed post-tectonic pluton that ranges in composition from granite to tonalite and has been dated to 2#735 & 16 Ma (Sims and othersl 1984).

Granitoid rocks are also extensively exposed in the Marenisco area on the eastern end of the Gogebic range. Parts of the granitoid has prominent gneissic banding and associated amphibolite. The otherl more extensive phasel is a weakly deformed medium-grained granite with widely scattered pegmatite bodies (Frittsl 1969; Trentl 1973) .

The Archean rocks were eroded and are unconformably overlain by Early Proterozoic rocks of the Marquette Range Supergroup. Because the Gogebic range is a north-dipping monoclineI Early Proterozoic rocks rest on Archean rocks to the south along the entire length of the range.

EARLY PROTEROZOIC ROCKS

At the ends of the Gogebic range the Archean rocks are unconformably overlain by rocks of the Chocolay Group of the

6

Marquette Range Supergroup, Sunday Quartzite and Bad RiverDolomite at the eastern end and only Bad River Dolomite at thewestern end. Chocolay Group rocks are absent from the centralpart of the range, and evidently were eroded prior to depositionof overlying units.

Sunday Ouartzite. The Sunday Quartzite was depositedunconformably on Archean rocks (mostly the Ramsay Formation),however, no regolith on the Archean has been observed. The basalunit is a prominently cross-bedded reddish quartzite.Conglomerate layers with quartz and granite cobbles to about 8 cmare present within the lower part of the formation. Most of theformation is a gray vitreous quartzite in which cross-bedding iscommon and current ripple marks are well developed in places.According to Schmidt (1973) the Sunday Quartzite is present onlyon the eastern end of the range, where it has a maximum thicknessof about 46 m. One of the type localities of the SundayQuartzite has been described in the center of the district nearthe Newport Mine (Van Hise and Leith, 1911) and the problem ofthis "lost locality" has been discussed by Schmidt (1973)

Bad River Dolomite. The Sunday Quartzite grades upward into theBad River Dolomite. The transition is marked by interbeddeddolomite and quartzite; the dolomite beds are thicker and moreabundant and quartzite beds are thinner and less abundant upward.The dolomite weathers to a distinctive tan/brown color andcontains abundant layers and irregular patches of gray to blackchert. Stromatolitic layers with mounds that range in size fromabout 5-50 cm in diameter become more abundant upward in thedolomite. Stromatolitic units tend to be prominently silicified.Maximum thickness of the dolomite is about 120 m, but in mostareas it is considerably less.

A period of erosion followed deposition of the quartzite anddolomite of the Chocolay Group, removing the Sunday Quartzite andBad River Dolomite from all but the eastern and western ends ofthe Gogebic range.

Palms Formation. The Palms Formation is the basal unit of theMenomiaee Group of the Marquette Range Supergroup. It restsunconformably on the Bad River Dolomite and Sunday Quartzite onthe east and west ends of the range, and on the Puritan QuartzMonzonite and Ramsay Formation in the central part of the range.Little or no regolith is developed on the erosion surface.

The Palms Formation is about 146 meters thick and contains abasal mud-rich unit, a central interbedded mud-silt-sand unit,and an upper sand-rich unit (Ojakangas, 1983). A thin (usuallyless than 3 m thick) conglomerate occurs locally at the base ofthe formation (Aldrich, 1929). The Palms grades abruptly upwardinto the Ironwood Iron-formation. Thin (1-5 cm) beds of granulariron-formation are interbedded with quartzite (Ojakangas, 1983)across a thickness of 10 m. The abundance and thickness of bedsof iron-formation increases upward with a corresponding decreasein detrital beds. Recent studies by Ojakangas (1983) indicate

Marquette Range Supergroupl Sunday Quartzite and Bad River Dolomite at the eastern end and only Bad River Dolomite at the western end. Chocolay Group rocks are absent from the central part of the rangel and evidently were eroded prior to deposition of overlying units.

Sunday Ouartzite. The Sunday Quartzite was deposited uncon~ormably on Archean rocks (mostly the Ramsay Formation) howeverl no regolith on the Archean has been observed. The basal unit is a prominently cross-bedded reddish quartzite. Conglomerate layers with quartz and granite cobbles to about 8 cm are present within the lower part of the formation. Most of the formation is a gray vitreous quartzite in which cross-bedding is common and current ripple marks are well developed in places. According to Schmidt (1973) the Sunday Quartzite is present only on the eastern end of the rangel where it has a maximum thickness of about 46 m. One of the type localities of the Sunday Quartzite has been described in the center of the district near the Newport Mine (Van Hise and LeithI 1911) and the problem of this lllost 10cality~~ has been discussed by Schmidt (1973).

Bad River Dolomite. The Sunday Quartzite grades upward into the Bad River Dolomite. The transition is marked by interbedded dolomite and quartzite; the dolomite beds are thicker and more abundant and quartzite beds are thinner and less abundant upward. The dolomite weathers to a distinctive tanlbrown color and contains abundant layers and irregular patches of gray to black chert. Stromatolitic layers with mounds that range in size from about 5-50 cm in diameter become more abundant upward in the dolomite. Stromatolitic units tend to be prominently silicified. Maximum thickness of the dolomite is about 120 ml but in most areas it is considerably less.

A period of erosion followed deposition of the quartzite and dolomite of the Chocolay Groupl removing the Sunday Quartzite and Bad River Dolomite from all but the eastern and western ends of the Gogebic range.

Palms Formation. The Palms Formation is the basal unit of the Menominee Group of the Marquette Range Supergroup. It rests unconformably on the Bad River Dolomite and Sunday Quartzite on the east and west ends of the rangel and on the Puritan Quartz Monzonite and Ramsay Formation in the central part of the range. Little or no regolith is developed on the erosion surface.

The Palms Formation is about 146 meters thick and contains a basal mud-rich unitl a central interbedded mud-silt-sand unitl and an upper sand-rich unit (Ojakangas 1983) . A thin (usually less than 3 m thick) conglomerate occurs locally at the base of the formation (Aldrich! 1929). The Palms grades abruptly upward into the Ironwood Iron-formation. Thin (1-5 cm) beds of granular iron-formation are interbedded with quartzite (Ojakangasl 1983) across a thickness of 10 m. The abundance and thickness of beds of iron-formation increases upward with a corresponding decrease in detrital beds. Recent studies by Ojakangas (1983) indicate

6

that the Palms Formation may have been deposited by tidalcurrents in a transgressing sea (Figure 3).

Figure 3. Sedimentational model showing lateral relationships ofthe siliciclastic tidal facies, the iron-formationfacies on the shelf, and the deeper-water turbidite-mudfacies. Thicknesses of units not drawn to scale.(From Ojakangas, 1983)

Ironwood Iron-Formation. The transition from the Palms Formationinto the Ironwood Iron-Formation records an abrupt change fromdetrital to chemical sedimentation. Like other iron-formations,the Ironwood contains little detrital material even though it is150-275 m thick.

Although iron-formation has a simple composition - - chertand iron minerals - - it is extremely varied in appearance. Boththe chert and the iron minerals are varied in color. Although itis widely cited as indicating the chemistry of the depositionalenvironment (James, 1954), the mineralogy of iron-formations isthe product of the depositional, diagenetic, metamorphic, and tosome extent, the weathering environment that the rocks haveundergone. Still moderate subsequent changes to the primarymineralogy do not usually reduce our ability to infer primarymineralogy.

Iron-formations have two basic textural types. One islaminated with layers of chert about 5-10 mm thick alternatingwith layers of iron minerals of similar thickness. Thislaminated type of deposit is. commonly referred to as "slaty"iron-formation (Figure 4). It has a grain size and beddingcharacteristic of siltstones (LaBerge, 1964, Dimroth, 1968). Theother textural type of iron-formation, referred to as "cherty"iron-formation, consists of 1-10 cm thick layers containing finesand-size (<2 mm) "grains" of chert with variable amounts of ironminerals set in a chert matrix. These layers of "granular" chertare extremely variable in thickness over short distances,commonly forming lenses (Figure 5). The cherty lenses are

7

that the Palms Fomtion may have been deposited by tidal currents in a transgressing- sea (Figure 3j .

Figure 3. Sedimentational model showing lateral relationships of the siliciclastic tidal faciesI the iron-formation facies on the shelfI and the deeper-water turbidite-mud facies. Thicknesses of units not drawn to scale. (From OjakangasI 1983) .

Ironwood Iron-Formation. The transition from the Palms Formation into the Ironwood Iron-Formation records an abrupt change from detrital to chemical sedimentation. Like other iron-formationsI the Ironwood contains little detrital material even though it is 150-275 m thick.

Although iron-formation has a simple composition - - chert and iron minerals - - it is extremely varied in appearance. Both the chert and the iron minerals are varied in color. Although it is widely cited as indicating the chemistry of the depositional environment (JamesI 1954)1 the mineralogy of iron-formations is the product of the depositionalI diageneticI metamorphicI and to some extentI the weathering environment that the rocks have undergone. Still moderate subsequent changes to the primary mineralogy do not usually reduce our ability to infer primary mineralogy.

Iron-formations have two basic textural types. One is laminated with layers of chert about 5-10 nun thick alternating with layers of iron minerals of similar thickness. This laminated type of deposit is comonly referred to as 1lslatyll iron-formation (Figure 4). It has a grain size and bedding characteristic of siltstones (LaBergeI 19641 Dimroth! 1968). The other textural type of iron-formationI referred to as l1chertyI1 iron-formationI consists of 1-10 cm thick layers containing fine sand-size (c2 mm) llgrainsn of chert with variable amounts of iron minerals set in a chert matrix. These layers of llgranularll chert are extremely variable in thickness over short distancesI commonly forming lenses (Figure 5). The cherty lenses are

8

Figure 4. Photo of thin-bedded (laminated) ironformation with some thick-bedded (granular)layers. Mt. Whittlesey, nearMellen, WI.

. ;—* —,: :A. '

_

::::: • .,. '&.

-:

: • .

..— .vr.,:

.• • ..

... .-

¶ , g_:L...'

I .-

Figure 5. Photo of thick, irregularly bedded, granulariron-formation. Mt. Whittlesey, near Mellen,WI.

Figure 4. Photo of thin-bedded (laminated) iron formation with some thick-bedded (granular) layers. Mt. Whittlesey, near-Mellen, WI.

Figure 5. Photo of thick, irregularly bedded, granular iron-formation. Mt. Whittlesey, near Mellen, WI .

8

typically separated by 0.5-3 cm layers of laminated ironminerals. "Cherty" iron-formations have the grain size andbedding characteristics of sandstones (Mengel, 1963; Dimroth andChauvel, 1973). The mineralogy of the iron minerals is almostcompletely independent of the textural varieties of iron-formation. Most geologists agree that granular ("cherty") iron-formation represents shallow-water deposition and laminated iron-formation represents somewhat deeper (quieter) water deposition.

Several iron-formations in the Lake Superior region havebeen subdivided into."members" on the basis of their bedding(textural) characteristics. Hotchkiss (1919) divided theIronwood Iron-formation into five members. The -60 m thickPlymouth Member is an irregularly bedded granular cherty unitwith a basal zone containing some detrital quartz and patches ofstromatolitic jasper. The overlying 46 m thick Yale Member ismainly an even-bedded (slaty) siderite-chert unit with a basalcarbonaceous pyritic slate zone and a layer that is probablytuffaceous. The succeeding Norrie Member is a 60 m thick unit ofirregularly bedded cherty iron-formation. The Pence Member isabout 45 m of siderite-chert iron-formation, and the uppermostAnvil Member is a 15-60 m thick mixture of slaty and cherty iron-formation. These members are continuous and uniform throughoutthe 128 km length of the range. However, they are difficult todistinguish east of Wakefield, Michigan, where a thick sequenceof volcanic rocks, the Emperor Volcanic Complex (Trent, 1973), isinterbedded with the iron-formation.

Iron ores. The iron ores on the Gogebic range were produced bythe chemical removal of silica from the iron-formation.Dissolution of the silica required the removal of nearly 50percent of the original volume of the rock and produced veryporous orebodies. The ore consisted mainly of earthy goethiteand hematite; however, in open spaces produced by removal ofsilica, large botryoidal masses of crystalline hematite andgoethite were found, as well as local concentrations ofpsilomelane (romancheite), manganite, rhodochrosite, calcite,manganocalcite, barite, gypsum, and marcasite. The presence ofiron ore cobbles in Keweenawan rocks in the district suggeststhat at least some of the ore was formed prior to deposition ofrocks of the Keweenawan Supergroup. Nearly all the mines on theGogebic range were underground mines (Figures 6 and 7). Miningon the Gogebic range began in 1884 and ended in 1967, when theunderground mines could no longer compete with the large open pittaconite mines elsewhere in the world. Ore was taken by rail toAshland, Wisconsin, and Escanaba, Michigan, from where it wasshipped to steel mills in Indiana and Ohio.

Emperor Volcanic Complex. The Emperor Volcanic Complexconstitutes a thick pile of volcanic rocks and sills in thesedimentary sequence on the eastern Gogebic range where theyreach a thickness of at least 2,000 meters. Although thevolcanic rocks in the eastern Gogebic range have been known for100 years (Irving and Van Hise, 1892), few studies have beenundertaken on these rocks. Irving and Van Hise (1892) stated

9

typically separated by 0.5-3 cm layers of laminated iron minerals. llChertyll iron-formations have the grain size and bedding characteristics of sandstones (Mengel, 1963; Dimroth and Chauvel, 1973). The mineralogy of the iron minerals is almost completely independent of the textural varieties of iron- formation. Most geologists agree that granular (llchertyll) iron- formation represents shallow-water deposition and laminated iron- formation represents somewhat deeper (quieter) water deposition.

Several iron-formations in the Lake Superior region have been subdivided into."membersN on the basis of their bedding (textural) characteristics. Hotchkiss (1919) divided the Ironwood Iron-formation into five members. The -60 m thick Plymouth Member is an irregularly bedded granular cherty unit with a basal zone containing some detrital quartz and patches of stromatolitic jasper. The overlying 46 m thick Yale Member is mainly an even-bedded (slaty) siderite-chert unit with a basal carbonaceous pyritic slate zone and a layer that is probably tuffaceous. The succeeding Norrie Member is a 60 m thick unit of irregularly bedded cherty iron-formation. The Pence Member is about 45 m of siderite-chert iron-formation, and the uppermost Anvil Member is a 15-60 m thick mixture of slaty and cherty iron- formation. These members are continuous and uniform throughout the 128 km length of the range. However, they are difficult to distinguish east of Wakefield, Michigan, where a thick sequence of volcanic rocks, the Emperor Volcanic Complex (Trent, 1973), is interbedded with the iron-formation.

Iron ores. The iron ores on the Gogebic range were produced by the chemical removal of silica from the iron-formation. Dissolution of the silica required the removal of nearly 50 percent of the original volume of the rock and produced very porous orebodies. The ore consisted mainly of earthy goethite and hematite; however, in open spaces produced by removal of silica, large botryoidal masses of crystalline hematite and goethite were found, as well as local concentrations of psilomelane (romancheite), manganite, rhodochrosite, calcite, manganocalcite, barite, gypsum, and marcasite. The presence of iron ore cobbles in Keweenawan rocks in the district suggests that at least some of the ore was formed prior to deposition of rocks of the Keweenawan Supergroup. Nearly all the mines on the Gogebic range were underground mines (Figures 6 and 7). Mining on the Gogebic range began in 1884 and ended in 1967, when the underground mines could no longer compete with the large open pit taconite mines elsewhere in the world. Ore was taken by rail to Ashland, Wisconsin, and Escanaba, Michigan, from where it was shipped to steel mills in Indiana and Ohio.

Emperor Volcanic Complex. The Emperor Volcanic Complex constitutes a thick pile of volcanic rocks and sills in the sedimentary sequence on the eastern Gogebic range where they reach a thickness of at least 2,000 meters. Although the volcanic rocks in the eastern Gogebic range have been known for 100 years (Irving and Van Hise, 18921, few studies have been undertaken on these rocks. Irving and Van Hise (1892) stated

10

GOGEBIC RANGEGENERALIZED CROSS SECTION

(P. NC, WE ST

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Figure 6. Schematic diagram showing the occurrence of ironorebodies on the Gogebic range. (From Goldich andMarsden, 1956)

Figure 7. Sunday Lake mine in 1960. Note the stockpile of ironore in the upper left. Photo taken from the top ofRadio Tower Hill In Wakefield, Michigan.

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GOGEBIC RANGE GEN E R A L I Z E D CROSS SECTION

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Figure 6. Schematic diagram showing the occurrence of iron orebodies on the Gogebic range. (From Goldich and Marsden, 1956)

Figure 7. Sunday Lake mine in 1960. Note the stockpile of iron ore in the upper left. Photo taken from the top of Radio Tower Hill In Wakefield, Michigan.

that the volcanic rocks are interbedded with the Ironwood Iron-formation, which has been demonstrated convincingly by subsequentexploration drilling f or iron ore in the area. Mapping by Trent(1973) documented the distribution of igneous rocks and showedsome of the lithologies present. Dann (1978) showed that thevolcanics range in composition from basalt to dacite, and thatthey have been metamorphosed only to prehnite-pumpellyite, or tolowest greenschist facies. He also elaborated on the possiblevolcanic setting in which the rocks may have formed. Traceelement studies by Schulz (Sims and others, 1990) indicate thatthe volcanic rocks are rift-related continental tholeiites.

Reconnaissance mapping in 1991 by LaBerge and J. S. Klasnerindicates that the Emperor Volcanic Complex consists of a widevariety of mainly subaqueous volcanic rocks and sills. Bothmafic and felsic volcanics were recognized. Mafic rocks includesills (such as the Wolf Mountain sill of Trent, 1973) pillowedand massive flows, some with columnar jointing, and extensivehyaloclastites and pillow breccias (Figure 8). Felsic rocksinclude hyaloclastites, massive felsite breccias, and unitscomprised of felsite breccia with 2-10 cm clasts in a finerhyaloclastite matrix (Figure 9). The felsite breccia -hyaloclastites may be debris flows from a subaqueous felsic dome.Although there are zones in the Emperor Volcanic Complex withstrong foliation, large areas are almost undeformed. Because ofthe low metamorphic grade and minimal deformation, primaryfeatures are well-preserved.

The relationship between volcanism and iron-formation haslong been debated, with some advocates (e.g., Van Hise and Leith,1911) arguing in favor of volcanic contributions of iron, andothers (e.g., James, 1954) contending that there is no geneticrelationship between volcanism and iron-formation deposition.The presence of a thick volcanic sequence interbedded with theiron-formation in the eastern Gogebic range shows that therecertainly was volcanism contemporaneous with iron-formationdeposition in the Lake Superior region. Therefore, the Gogebicrange is similar to the Hamersley range in Western Australia andthe Proterozoic iron-formations in South Africa where numerousvolcanic ash layers are interbedded with the iron-formations(LaBerge, 1966a, 196Gb).

Tyler and Copps Formations. The Ironwood Iron-Formation, andlocally the Emperor Volcanic Complex, are overlain by a 2,000 mthick graywacke-slate sequence. The unit is thickest in thewestern and eastern ends of the range, and has been completelyremoved by erosion prior to deposition of the KeweenawanSupergroup in an area near Wakefield, Michigan. Throughout mostof the range the unit is called the Tyler Formation, however thecorrelative(?) portion at the eastern end of the range is calledthe Copps Formation. The Tyler Formation is correlated with theMichigamme Formation to the east, and with the Thomson, RabbitLake, Virginia and Rove Formations to the west and north inMinnesota and Ontario. All these units were probably depositedin the same basin.

11

that the volcanic rocks are interbedded with the Ironwood Iron- formation, which has been demonstrated convincingly by subsequent exploration drilling for iron ore in the area. Mapping by Trent (1973) documented the distribution of igneous rocks and showed some of the lithologies present. Dann (1978) showed that the volcanics range in composition from basalt to dacite, and that they have been metamorphosed only to prehnite-pumpellyite, or to lowest greenschist facies. He also elaborated on the possible volcanic setting in which the rocks may have formed. Trace element studies by Schulz (Sims and others, 1990) indicate that the volcanic rocks are rift-related continental tholeiites.

Reconnaissance mapping in 1991 by LaBerge and J. S. Klasner indicates that the Emperor Volcanic Complex consists of a wide variety of mainly subaqueous volcanic rocks and sills. Both mafic and felsic volcanics were recognized. Mafic rocks include sills (such as the Wolf Mountain sill of Trent, 1973) pillowed and massive flows, some with columnar jointing, and extensive hyaloclastites and pillow breccias (Figure 8). Felsic rocks include hyaloclastites, massive felsite breccias, and units comprised of felsite breccia with 2-10 cm clasts in a finer hyaloclastite matrix (Figure 9). The felsite breccia - hyaloclastites may be debris flows from a subaqueous felsic dome. Although there are zones in the Emperor Volcanic Complex with strong foliation, large areas are almost undeformed. Because of the low metamorphic grade and minimal deformation, primary features are well-preserved.

The relationship between volcanism and iron-formation has long been debated, with some advocates (e.g., Van Hise and Leith, 1911) arguing in favor of volcanic contributions of iron, and others (e.g., James, 1954) contending that there is no genetic relationship between volcanism and iron-formation deposition. The presence of a thick volcanic sequence interbedded with the iron-formation in the eastern Gogebic range shows that there certainly was volcanism contemporaneous with iron-formation deposition in the Lake Superior region. Therefore, the Gogebic range is similar to the Hamersley range in Western Australia and the Proterozoic iron-formations in South Africa where numerous volcanic ash layers are interbedded with the iron-formations (LaBerge, 1966a, 1966b) . Tyler and CORDS Formations. The Ironwood Iron-Formation, and locally the Emperor Volcanic Complex, are overlain by a 2,000 m thick graywacke-slate sequence. The unit is thickest in the western and eastern ends of the range, and has been completely removed by erosion prior to deposition of the Keweenawan Supergroup in an area near Wakefield, Michigan. Throughout most of the range the unit is called the Tyler Formation, however the correlative(?) portion at the eastern end of the range is called the Copps Formation. The Tyler Formation is correlated with the Michigamme Formation to the east, and with the Thomson, Rabbit Lake, Virginia and Rove Formations to the west and north in Minnesota and Ontario. All these units were probably deposited in the same basin.

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Figure 8. Pillow breccia in Emperor Volcanic Complex. Pillowsand pillow fragments in a hyaloclastite matrix.Jackknife is on a pillow.

Figure 9. Felsic phase of Emperor Volcanic Complex. Felsiteclasts in a finer hyaloclastite matrix. Jackknife is 8cm.

Figure 8. Pillow breccia in Emperor Volcanic Complex. Pillows and pillow fragments in a hyaloclastite matrix. Jackknife is on a pillow.

Figure 9. Felsic phase of Emperor Volcanic Complex. Felsite clasts in a finer hyaloclastite matrix. Jackknife is 8 cm .

The relationship of the Tyler Formation to the underlyingiron-formation is unclear. According to Hotchkiss (1919) thebasal unit of the Tyler Formation is the Pabst Member, adistinctive unit containing mainly jasper and quartz pebbles.Figure 10, from drill hole data (Atwater, 1938) shows therelationship of the conglomerate to the underlying iron-formation. Hotchkiss (1919) interprets the relationships betweenthe Tyler and the iron-formation to be unconformable. Atwater(1938) did not believe' that a major unconformity separated theIronwood and Tyler. Schmidt and Hubbard (1972) interpreted theformations to be gradational and sedimentation to have beencontinuous in the central Gogebic. However, Trent (1973) statedthat field relationships indicate an unconformity between theIronwood and Tyler on the eastern Gogebic. Schmidt (1980)summarizes the field evidence pertaining to the Ironwood-Tylercontact as well as the several alternative interpretations.

Figure 10. Longitudinal section through part of the Gogebicdistrict, showing one interpretation ofrelationship between the "Pabst Member" of theTyler Formation and the Anvil and Pence Members ofthe Ironwood Iron-Formation. The upper slate is alayer of thin-bedded carbonate iron-formation, atleast partly within the Anvil Member. Compiledfrom measured sections in mines and drill holes.Location of sections given in map of IronwoodIron-Formation in lower part of diagram. Modifiedfrom Atwater (1938, p. 163, fig. 2) (From Schmidt,1980)

13

R2E n:ii

The relationship of the Tyler Formation to the underlying iron-formation is unclear. According to Hotchkiss (1919) the basal unit of the Tyler Formation is the Pabst Member, a distinctive unit containing mainly jasper and quartz pebbles. Figure 10, from drill hole data (Atwater, 1938) shows the relationship of the conglomerate to the underlying iron- formation. Hotchkiss (1919) interprets the relationships between the Tyler and the iron-formation to be unconformable. Atwater (1938) did not believe* that a major unconformity separated the Ironwood and Tyler. Schmidt and Hubbard (1972) interpreted the formations to be gradational and sedimentation to have been continuous in the central Gocrebic. However, Trent (1973) stated that field relationships indicate an unconformity between the Ironwood and Tyler on the eastern Gogebic. Schmidt (1980) summarizes the field evidence pertaining to the Ironwood-Tyler contact as well as the several alternative interpretations.

1 ATLANTIC MINE NO 3 SHAfl D 0 HOI I 2 PLUMMER SHAF1 51H LEVEL CROSSCUI 3 PENCE NO 2 SHAFT AND 0 0 HOLE 4 MONTREAL NO 4 SHAFT 20TH I EVE1 5 MONTREAL NO 4 SHAFI BTH LEVEL I I 1 l HOLE 0 2 M R F S 6 OTTAWA 10TH LEVEL SHAFT CROSSCIII 7 GARY 19IH LEVEL NO 16 CROSSCUI 1) U HOLE 8 NORRIE 14TH AND 17TH LEVELS

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9 AURORA E SHAFT 13TH LEVEL H ~ I I: ~ N I A I XAI c 10 PABS1 G SHAD vCRTICAL SCALE

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Figure 10. Longitudinal section through part of the Gogebic district, showing one interpretation of relationship between the "Pabst Membern of the Tyler Formation and the Anvil and Pence Members of the Ironwood Iron-Formation. The upper slate is a layer of thin-bedded carbonate iron-formation, at least partly within the Anvil Member. Compiled from measured sections in mines and drill holes. Location of sections given in map of Ironwood Iron-Formation in lower part of diagram. Modified from Atwater (1938, p. 163, fig. 2) (From Schmidt, 1980)

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The Tyler Formation consists dominantly of intercalatedargillite with lesser amounts of metasiltstone and graywacke.Graded metagraywacke beds containing Bouxna sequences indicativeof deposition by turbidity currents are common, but structurelessbeds are even more abundant and a grain-flow mechanism may alsohave been operative. Sole marks and small-scale cross-beddingindicate a paleocurrent trend from east-southeast to west-northwest (Alwin, 1976).

The "average" graywacke of the Tyler consists of 28 percentmicaceous matrix and 72 percent framework grains (Alwin, 1976).Of the framework grains, 73 percent are mono- and polycrystallinequartz and chert, 10 percent are plagioclase (mostly alteredplagioclase), and 17 percent are rock fragments, mainly ofgranitic to quartz dioritic and volcanic origin, but some grainsof sedimentary and metamorphic origin also are present. Thus,the "average" graywacke of the Tyler is compositionally submaturequartzose lithic graywacke that is texturally immature as definedby angular framework grains and poor sorting.

The paleocurrent directions and the composition of thegraywacke suggest that the Tyler Formation was derived from adominantly "granitic" source to the southeast.

MIDDLE PROTEROZOIC ROCKS

Following the Penokean orogeny, about 1,860 - 1,830 Ma, theGogebic range and the rest of the Lake Superior region wassubjected to erosion for nearly 700 million years. Erosionremoved an unknown thickness of rocks in the range, but more rockwas removed from some areas than in others. For example, atleast 2,000 meters of the Tyler Formation is preserved in thewestern Gogebic, and a comparable thickness of the CoppsFormation is preserved at the eastern end of the range. However,all of the Tyler, and some of the underlying Ironwood Iron-Formation as well, was removed in the eastern Gogebic where therange experienced more folding. The erosion interval presumablyended with the onset of the Middle Proterozoic Keweenawan riftingevent.

Bessemer Ouartzite. In most of the Gogebic range the BessemerQuartzite, the basal unit of the Keweenawan Supergroup,unconformably overlies the Tyler Formation. However, locally onthe eastern part of the range the basal Keweenawan rests on theIronwood Iron-Formation or the Emperor Volcanic Complex. Thequartzite is approximately 100 m thick, has channel cross-beddingand is a relatively mature quartz sandstone (Ojakangas andMatsch, 1982). The Bessemer Quartzite evidently originated in afluvial environment in a slight sag that presaged the Keweenawanrifting that developed into the Midcontinent Rift System(Ojakangas and Matsch, 1982).

The Tyler Formation consists dominantly of intercalated argillite with lesser amounts of metasiltstone and graywacke. Graded metagraywacke beds containing B o w sequences indicative of deposition by turbidity currents are common, but structureless beds are even more abundant and a grain-flow mechanism may also have been operative. Sole marks and small-scale cross-bedding indicate a paleocurrent trend from east-southeast to west- northwest (Alwin, 1976) .

The "averagen graywacke of the Tyler consists of 28 percent micaceous matrix and 72 percent framework grains (Alwin, 1976). Of the framework grains, 73 percent are mono- and polycrystalline quartz and chert, 10 percent are plagioclase (mostly altered plagioclase), and 17 percent are rock fragments, mainly of granitic to quartz dioritic and volcanic origin, but some grains of sedimentary and metamorphic origin also are present. Thus, the "averagev graywacke of the Tyler is compositionally submature quartzose lithic graywacke that is texturally immature as defined by angular framework grains and poor sorting.

The paleocurrent directions and the composition of the graywacke suggest that the Tyler Formation was derived from a dominantly "graniticv source to the southeast.

MIDDLE PROTEROZOIC ROCKS

Following the Penokean orogeny, about 1,860 - 1,830 Ma, the Gogebic range and the rest of the Lake Superior region was subjected to erosion for nearly 700 million years. Erosion removed an unknown thickness of rocks in the range, but more rock was removed from some areas than in others. For example, at least 2,000 meters of the Tyler Formation is preserved in the western Gogebic, and a comparable thickness of the Copps Formation is preserved at the eastern end of the range. However, all of the Tyler, and some of the underlying Ironwood Iron- Formation as well, was removed in the eastern Gogebic where the range experienced more folding. The erosion interval presumably ended with the onset of the Middle Proterozoic Keweenawan rifting event.

Bessemer Ouartzite. In most of the Gogebic range the Bessemer Quartzite, the basal unit of the Keweenawan Supergroup, unconformably overlies the Tyler Formation. However, locally on the eastern part of the range the basal Keweenawan rests on the Ironwood Iron-Formation or the Emperor Volcanic Complex. The quartzite is approximately 100 m thick, has channel cross-bedding and is a relatively mature quartz sandstone (Ojakangas and Matsch, 1982). The Bessemer Quartzite evidently originated in a fluvial environment in a slight sag that presaged the Keweenawan rifting that developed into the Midcontinent Rift System (Ojakangas and Matsch, 1982).

Powder Mill Group. Overlying the Bessemer Quartzite is a thicksequence of mainly basaltic lava flows that formed during themain phase of Keweenawan rifting in this area. Volcanic activity(and rifting?) began approximately 1,200 Ma ago and producedhundreds of lava flows that have an aggregate thickness of morethan 3,000 m. The lowermost basaltic lava flows that overlie theBessemer Quartzite in the Bessemer, Michigan, area are pillowed,suggesting that they were extruded in water, either a lake or afluvial system.

Oronto Group. Keweenawan Supergroup volcanic rocks are overlainby a thick sequence of mainly red clastic rocks in the LakeSuperior region. Various representatives of this sequence arewell exposed north of the Gogebic range. The basal unit of theOronto Group is the Copper Harbor Conglomerate, which is as thickas 2,000 m. The major lithology is red arkosic sandstone andsiltstone with numerous conglomeratic horizons. The CopperHarbor Conglomerate probably represents alluvial fan depositsformed near a rugged source area along the south side of LakeSuperior.

The Copper Harbor Conglomerate is overlain by the NonesuchFormation, a 100 meter thick clastic unit, that is dominantlygray to black carbonaceous siltstone. The Nonesuch was probablydeposited in a lake developed on the subsiding pile of volcanicrocks and Copper Harbor sediments (e.g., Suszek, 1991). Inaddition to the world class copper deposits of the White Pinemine, the Nonesuch Formation also contains one of the oldestknown occurrences of petroleum.

The Freda Sandstone of the Oronto Group consists ofapproximately 3,000 m of red clastics, and conformably overliesthe Nonesuch Formation. The Freda strata is believed to consistof fluvial deposits formed during continued subsidence of theMiddle Proterozoic rift.

KEWEENAWAN TILTING

Following the onset of deposition of the volcanic andsedimentary rocks along the Keweenawan Midcontinent Rift System,the south side of the rift, including the Gogebic district, wastilted steeply (659O0) to the north.

TECTONIC SEFING

Early Proterozoic rocks in the Gogebic range unconformablyoverlie Late Archean greenstone and granite of the SuperiorProvince of the Canadian Shield. Larue (1981, 1983) suggestedthat the sedimentary rocks of the Lake Superior region (theMarquette Range Supergroup) were deposited in a number of graben-like basins in a platformal environment. Sims and others (1990)

15

Powder Mill Grouu. Overlying the Bessemer Quartzite is a thick sequence of mainly basaltic lava flows that formed during the main phase of Keweenawan rifting in this area. Volcanic activity (and rifting?) began approximately 1,200 Ma ago and produced hundreds of lava flows that have an aggregate thickness of more than 3,000 m. The lowermost basaltic lava flows that overlie the Bessemer Quartzite in the Bessemer, Michigan, area are pillowed, suggesting that they were extruded in water, either a lake or a fluvial system.

Oronto Group. Keweenawan Supergroup volcanic rocks are overlain by a thick sequence of mainly red clastic rocks in the Lake Superior region. Various representatives of this sequence are well exposed north of the Gogebic range. The basal unit of the Oronto Group is the Copper Harbor Conglomerate, which is as thick as 2,000 m. The major lithology is red arkosic sandstone and siltstone with numerous conglomeratic horizons. The Copper Harbor Conglomerate probably represents alluvial fan deposits formed near a rugged source area along the south side of Lake Superior.

The Copper Harbor Conglomerate is overlain by the Nonesuch Formation, a 100 meter thick clastic unit, that is dominantly gray to black carbonaceous siltstone. The Nonesuch was probably deposited in a lake developed on the subsiding pile of volcanic rocks and Copper Harbor sediments (e.g., Suszek, 1991). In addition to the world class copper deposits of the White Pine mine, the Nonesuch Formation also contains one of the oldest known occurrences of petroleum.

The Freda Sandstone of the Oronto Group consists of approximately 3,000 m of red clastics, and conformably overlies the Nonesuch Formation. The Freda strata is believed to consist of fluvial deposits formed during continued subsidence of the Middle Proterozoic rift.

KEWEENAWAN TILTING

Following the onset of deposition of the volcanic and sedimentary rocks along the Keweenawan Midcontinent Rift System, the south side of the rift, including the Gogebic district, was tilted steeply (65-90') to the north.

TECTONIC SETTING

Early Proterozoic rocks in the Gogebic range unconformably overlie Late Archean greenstone and granite of the Superior Province of the Canadian Shield. Lame (1981, 1983) suggested that the sedimentary rocks of the Lake Superior region (the Marquette Range Supergroup) were deposited in a number of graben- like basins in a platformal environment. Sims and others (1990)

proposed that rocks of the Marquette Range Supergroup formed on arifted continental margin. Break-up of the Archean cratonresulted in the development of oceanic crust off the southernmargin of the Superior Province. Therefore, the sedimentaryrocks were deposited during a rifting stage that developed into apassive margin. Nd isotope studies by Gerlach and others (1988)indicate that iron-formations in the Lake Superior region weredeposited about 2,100 Ma.

The concept of a rif ted continental margin in the LakeSuperior region is probably best exemplified by the easternGogebic range. As discussed earlier, the stratigraphicsuccession increases considerably in thickness east of Wakefield,Michigan, and igneous rocks (volcanic rocks and sills) become amajor component of the succession. The increase in thickness ofthe sedimentary sequence and the influx of volcanic rocks issuggestive of a half -graben structure with a hinge near Wakefieldand a boundary fault (or faults) near Lake Gogebic. Emplacementof basaltic sills (e.g., the Wolf Mountain sill of Trent, 1973)and eruption of continental tholeiites (Sims, and others, 1990)would be characteristic of a rif ted continental margin. Thepresence of the sills and volcanic rocks in the Ironwood Iron-Formation as well as dramatic eastward thickening of the iron-formation indicates that major graben formation and volcanismoccurred during deposition of the Ironwood Iron-Formation (Figure11)

Following the rifting and passive margin stages, a phase ofconvergent tectonics produced a volcanic island arc (theWisconsin magmatic terranes) that collided with the Superiorcraton. Isotopic ages in rocks of the rnagmatic terrane rangefrom 1,890 Ma to 1,840 Ma (Sims and others), and docking occurredabout 1,860 Ma (Sims and others, 1990). Barovich and others(1989) showed that Nd isotopes indicate that much of thegraywackes in the Michigainme Formation were derived from an EarlyProterozoic source. This suggests that the graywackes weredeposited in a foreland basin during docking of the island arc onthe margin of the Superior craton.

Deformation and metamorphism of rocks of the Marquette RangeSupergroup on the Gogebic range resulted from the collision ofthe island arc with the margin of the Superior craton. Thiscollision fits very well with a foreland basin model as developedby Hoffman (1987) for the Early Proterozoic rocks that surroundthe Archean Superior Province. This model was utilized bySouthwick and Morey (1991) to explain lithologic and structuralrelationships in east-central Minnesota. A similar foredeepmodel, with associated southward subduction and collision of avolcanic arc against the craton to the north was proposed byOjakangas (in press) to explain the origin of the MichigainmeFormation of the Upper Peninsula of Michigan, as well as theorigin of the Copps and Tyler formations in the vicinity of theGogebic range. In this model, the iron-formation was depositedon the peripheral bulge on the north side of the northward-migrating foreland basin, whereas the aforementioned turbidite

16

proposed that rocks of the Marquette Range Supergroup formed on a rifted continental margin. Break-up of the Archean craton resulted in the development of oceanic crust off the southern margin of the Superior Province. Therefore, the sedimentary rocks were deposited during a rifting stage that developed into a passive margin. Nd isotope studies by Gerlach and others (1988) indicate that iron-formations in the Lake Superior region were deposited about 2,100 Ma.

The concept of a rifted continental margin in the Lake Superior region is probably best exemplified by the eastern Gogebic range. As discussed earlier, the stratigraphic succession increases considerably in thickness east of Wakefield, Michigan, and igneous rocks (volcanic rocks and sills) become a major component of the succession. The increase in thickness of the sedimentary sequence and the influx of volcanic rocks is suggestive of a half-graben structure with a hinge near Wakefield and a boundary fault (or faults) near Lake Gogebic. Emplacement of basaltic sills (e.g., the Wolf Mountain sill of Trent, 1973) and eruption of continental tholeiites (Sims, and others, 1990) would be characteristic of a rifted continental margin. The presence of the sills and volcanic rocks in the Ironwood Iron- Formation as well as dramatic eastward thickening of the iron- formation indicates that major graben formation and volcanism occurred during deposition of the Ironwood Iron-Formation (Figure 11) .

Following the rifting and passive margin stages, a phase of convergent tectonics produced a volcanic island arc (the Wisconsin magmatic terranes) that collided with the Superior craton. Isotopic ages in rocks of the magmatic terrane range from 1.890 Ma to 1,840 Ma (Sims and others), and docking occurred about 1.860 Ma (Sims and others, 1990). Barovich and others (1989) showed that Nd isotopes indicate that much of the graywackes in the Michigamme Formation were derived from an Early Proterozoic source. This suggests that the graywackes were deposited in a foreland basin during docking of the island arc on the margin of the Superior craton.

Deformation and metamorphism of rocks of the Marquette Range Supergroup on the Gogebic range resulted from the collision of the island arc with the margin of the Superior craton. This collision fits very well with a foreland basin model as developed by Hoffman (1987) for the Early Proterozoic rocks that surround the Archean Superior Province. This model was utilized by Southwick and Morey (1991) to explain lithologic and structural relationships in east-central Minnesota. A similar foredeep model, with associated southward subduction and collision of a volcanic arc against the craton to the north was proposed by Ojakangas (in press) to explain the origin of the Michigamme Formation of the Upper Peninsula of Michigan, as well as the origin of the Copps and Tyler formations in the vicinity of the Gogebic range. In this model, the iron-formation was deposited on the peripheral bulge on the north side of the northward- migrating foreland basin, whereas the aforementioned turbidite

16

sequences were deposited in the deeper axial parts of the basin.This would require that the Ironwood Iron-Formation and theoverlying Tyler Formation have a gradational and conformablerelationship, and that they were deposited, at least in part,contemporaneously. In this model, axial tholeiitic volcanism(Hoffman, 1988) and hydrothermal activity in the foredeep wouldhave provided the iron and silica that were precipitated as iron-formation on the shelf, as upwelling brought the solutions to afavorable site of deposition.

Note that the foreland basin model provides an alternativeinterpretation for the Ironwood and the intertonguing EmperorVolcanic Complex compared with a rif ted continental margin model.Obviously more work is needed to resolve this problem.

Deformation during the Penokean orogeny may have resulted inreactivation of the normal faults in the graben structures. Wesuggest that the structure in the eastern Gogebic may represent ahalf-graben that has been subjected to thrusting during thePenokean orogeny (Figure 12). Perhaps significantly, erosionprior to deposition of the Keweenawan Supergroup cut deepest intothe Early Proterozoic sedimentary sequence in the centralGogebic, where the proposed thrust faults are present. Whereasover 2,000 m of the Tyler Formation remain on the western Gogebicin Wisconsin, and a similar thickness of Copps Formation ispresent at the eastern end of the range, erosion removed all ofthe Tyler and part of the iron-formation east of Wakefield.Assuming that erosion cut down to a relatively uniform level,then the central Gogebic may have been uplifted at least 2,000 mmore than areas to the east and west. An Archean cored gneissdome is present near Watersmeet about 50 km to the east. Thatarea may represent an even more pronounced uplift. Therefore,rocks on the eastern Gogebic range may record depositional anddeformational events of both the rif ted margin and convergenttectonics during the Early Proterozoic in the Lake Superiorregion.

17

sequences were deposited in the deeper axial parts of the basin. This would require that the Ironwood Iron-Formation and the overlying Tyler Formation have a gradational and conformable relationship, and that they were deposited, at least in part, contemporaneously. In this model, axial tholeiitic volcanism (Hoffman, 1988) and hydrothermal activity in the foredeep would have provided the iron and silica that were precipitated as iron- formation on the shelf, as upwelling brought the solutions to a favorable site of deposition.

Note that the foreland basin model provides an alternative interpretation for the Ironwood and the intertonguing Emperor Volcanic Complex compared with a rifted continental margin model. Obviously more work is needed to resolve this problem.

Deformation during the Penokean orogeny may have resulted in reactivation of the normal faults in the graben structures. We suggest that the structure in the eastern Gogebic may represent a half-graben that has been subjected to thrusting during the Penokean orogeny (Figure 12). Perhaps significantly, erosion prior to deposition of the Keweenawan Supergroup cut deepest into the Early Proterozoic sedimentary sequence in the central Gogebic, where the proposed thrust faults are present. Whereas over 2,000 m of the Tyler Formation remain on the western Gogebic in Wisconsin, and a similar thickness of Copps Formation is present at the eastern end of the range, erosion removed all of the Tyler and part of the iron-formation east of Wakefield. Assuming that erosion cut down to a relatively uniform level, then the central Gogebic may have been uplifted at least 2,000 m more than areas to the east and west. An Archean cored gneiss dome is present near Watersmeet about 50 km to the east. That area may represent an even more pronounced uplift. Therefore, rocks on the eastern Gogebic range may record depositional and deformational events of both the rifted margin and convergent tectonics during the Early Proterozoic in the Lake Superior region.

/

Diagram showing hypothetical pre-Penokeanrelationships on the eastern Gogebic range.

18

Figure 12. Simplified geologic map of the eastern part of theGogebic range. (Not to scale)

tRON WOOD

'---'---'——-— '- —.-.—'.--— ___A._

Water Surface-7w -- - - -

- -

- EROR ---- - -::,/_.-- —

/4 4u ,

M

A

v A

RAM SAYI

bA

y •V

V

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Figure 11.

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Figure 11. Diagram showing hypothetical pre-Penokean relationships on the eastern Gogebic range.

Figure 12. Simplified geologic map of the eastern part of the Gogebic range. (Not to scale)

STOPDESCRIPTIONS

Gogebic Field Trip

19

STOP DESCRIPTIONS

Gogebic Field Trip

20

Stop 1. Tyler Formation.

Location: Roadcuts at junction of U.S. Hwy. 2 and Hwy.51 north of Hurley, WI

Rocks exposed at this locality are representativeof the upper part of the Tyler Formation, which isabout 2,100 meters thick here. It thickens westward toabout 2,900 meters in Wisconsin and thins eastward tozero east of Ramsay, where it was entirely removed byerosion prior to deposition of Keweenawan strata. TheTyler is unconformably overlain by the BessemerQuartzite and a substantial thickness of Keweenawanvolcanic rocks.

The relationship of the Tyler to the underlyingIronwood Iron-Formation is uncertain because of pooroutcrop. Most geologists (e.g., Atwater, 1938)consider the two formations to be conformable (Schmidtand Hubbard, 1972); however, Aldrich (1929) presentsevidence for erosion of the iron-formation and a basalconglomerate at the base of the Tyler. The presence ofa graywacke-slate sequence overlying the iron-formationis characteristic throughout the Lake Superior region

As elsewhere on the western Gogebic range, thebeds here strike east-northeast, dip 60°-75° northwest,and top to the northwest, part of a large block thatwas tilted northward during Keweenawan rifting to forma large north-facing monocline. Schmidt and Hubbard(1972) and Kiasner and others (1991) pointed out thatthe cleavage/bedding relationships in these (and other)outcrops are unusual and may be of tectonicsignificance. As shown in the accompanying diagrams(Figure 13), the graywacke-slate was dipping gentlysouthward prior to Keweenawan time. Because thecleavage dips less steeply than the bedding, rotationof the rocks southward to their pre-Keweenawan sub-horizontal attitude results in a south-dippingcleavage.

Cannon and others (1990) showed that originallyvertical Early Proterozoic diabase dikes in the Archeanterrane to the south of the Gogebic range now have asouthward dip. This dip is explained by a northwardrotation along the east -northeast - trending Mareniscofault, a major listric thermal fault with perhaps 10 kmof southward thrust motion on it. This motion explainsthe present steep northward dip of the major monoclinalstructure of the Gogebic range.

Aiwin (1976) showed that the Tyler consistsdominantly of intercalcated argillite and slate withlesser amounts of siltstone and graywacke. Graded

stow 1. Wler Formation.

Location: Roadcuts at junction of U.S. Hwy. 2 and Hwy. 51 north of HurleyI WI

Rocks exposed at this locality are representative of the upper part of the Tyler Formationl which is about 21100 meters thick here. It thickens westward to about 2#900 meters in Wisconsin and thins eastward to zero east of Ramsay! where it was entirely removed by erosion prior to deposition of Keweenawan strata. The Tyler is unconformably overlain by the Bessemer Quartzite and a substantial thickness of Keweenawan volcanic rocks.

The relationship of the Tyler to the underlying Ironwood Iron-Formation is uncertain because of poor outcrop. Most geologists (e.ge1 Atwater, 1938) consider the two formations to be conformable (Schmidt and Hubbardl 1972) ; howeverl Aldrich (1929) presents evidence for erosion of the iron-formation and a basal conglomerate at the base of the Tyler. The presence of a graywacke-slate sequence overlying the iron-formation is characteristic throughout the Lake Superior region

As elsewhere on the western Gogebic rangeI the beds here strike east-northeastl dip 60'-75' northwestl and top to the northwest! part of a large block that was tilted northward during Keweenawan rifting to form a large north-facing monocline. Schmidt and Hubbard (1972) and Klasner and others (1991) pointed out that the cleavagelbedding relationships in these (and other) outcrops are unusual and may be of tectonic significance. As shown in the accompanying diagrams (Figure 13) the graywacke-slate was dipping gently southward prior to Keweenawan time. Because the cleavage dips less steewlv than the beddingl rotation of the rocks southward to their pre-Keweenawan sub- horizontal attitude results in a south-dipping cleavage.

Cannon and others (1990) showed that originally vertical Early Proterozoic diabase dikes in the Archean terrane to the south of the Gogebic range now have a southward dip. This dip is explained by a northward rotation along the east-northeast-trending Marenisco faultl a major listric thermal fault with perhaps 10 km of southward thrust motion on it. This motion explains the present steep northward dip of the major monoclinal structure of the Gogebic range.

Alwin (1976) showed that the Tyler consists dominantly of intercalcated argillite and slate with lesser amounts of siltstone and graywacke. Graded

SOUTH NORTH

METERS

6OO

METERS

SOUTH NORTH

Figure 13. Generalized block diagram and cross sectionillustrating structural relationships in Ironwood,Mich.-Hurley, Wis., area. Heavy dashed line,fault. A, Block diagram showing relationship ofpre-Keweenawan strata to rocks of Midcontinentrift. Note that S1 foliation in Tyler Formationdips less steeply than bedding. Modified fromSchmidt and Hubbard (1972). B, Reconstructedpost-Penokean, pre-Keweenawan position of thestrata shown in A. Note gently south dipping S1foliation. Modified from Schmidt and Hubbard(1972). (From Klasner and others, 1991)

21

B

SOUTH I

Rocks of

Earlv Proterozo~c

NORTH

Figure 13. Generalized block diagram and cross section illustrating structural relationships in Ironwoodl Mich.-Hurley, Wise1 area. Heavy dashed linel fault. Al Block diagram showing relationship of pre-Keweenawan strata to rocks of Midcontinent rift. Note that S' foliation in qler Formation dips less steeply than bedding. Modified from Schmidt and Hubbard (1972) . Bl ~econstructed post-Penokeanl pre-Keweenawan position of the strata shown in A. Note gently south dipping Sl foliation. Modified from Schmidt and Hubbard (1972) . (From Klasner and others! 1991)

22

graywacke beds containing Bouina sequences indicatedeposition by turbidity currents. Sole marks andsmall-scale cross-beds indicate a paleocurrent trendtoward the west-northwest. Framework grains in thegraywackes are quartz and feldspar and rock fragmentsof granitic, volcanic,and metamorphic rocks.Therefore, the source area for the Tyler sediments isinf erred to be an Archean granitic-metarnorphic(cratonic) source area to the southeast. The Tyler wasonly slightly metamorphosed to sub-greenschist faciesduring the Penokean orogeny, and the metamorphic gradehere was not changed during the Keweenawan rifting.

At this exposure, note graded graywacke bedstopping NW, mud-chips in some beds, rare sole marks,loading on some soles, flame structures, and small-scale cross-bedding. The assemblage of sedimentarystructures indicates a turbidity current mechanism, anddeposition on a submarine fan is likely but notessential. In the fan model the "packets" of graywackebeds would be channel deposits whereas the mudstoneswould be overbank deposits.

Continue east on U.S. Hwy 2 to Golf Club Road just east ofIronwood. Thrn right (south) on Golf Club Road and proceed about1.1 miles south to large brick buildings of the Newport Mine.Walk to the northeast across the small field to old railroadcuts, that are just north of the main building behind the trees.Caution: Area is now part of a gravel washing operation.

Stop 2. Old Newport mine (Archean/Early ProterozoicUnconformity).

The Palms Formation of the Menominee Group of theMarquette Range Supergroup rests upon the 2,700 m.y.old Puritan Quartz Monzonite (Schmidt, 1976). TheMarquette Range Supergroup in the Gogebic range wasdeposited on a moderately flat erosion surfacedeveloped on Archean "granite" and greenstone.

At this locality, the steeply dipping (to thenorth) unconformable contact between the PalmsFormation and Archean granitoid is exposed in the oldrailroad cut. Small depressions in the surface, as canbe seen here, contain laminated cherty material, someof which is granular and somewhat resembles iron-formation. The granules are 0.5 to 1.0 mm in diameterand consist of mixtures of chert, chlorite, andcalcite. There are also a few phosphatic argillaceouschips just above the contact. Note that there is noappreciable evidence of weathering of the granitebeneath the Palms.

graywacke beds containing Bouma sequences indicate deposition by turbidity currents. Sole marks and small-scale cross-beds indicate a paleocurrent trend toward the west-northwest. Framework grains in the graywackes are quartz and feldspar and rock fragments of graniticl volcanicland metamorphic rocks. Therefore, the source area for the Tyler sediments is inferred to be an Archean granitic-metamorphic (cratonic) source area to the southeast. The Tyler was only slightly metamorphosed to sub-greenschist facies during the Penokean orogenyl and the metamorphic grade here was not changed during the Keweenawan rifting.

At this exposurel note graded graywacke beds topping NWl mud-chips in some bedsl rare sole marksl loading on some solesl flame structuresl and small- scale cross-bedding. The assemblage of sedimentary structures indicates a turbidity current mechanisml and deposition on a submarine fan is likely but not essential. In the fan model the npacketsll of graywacke beds would be channel deposits whereas the mudstones would be overbank deposits.

Continue east on U.S. Hwy 2 to Golf Club Road just east of Ironwood. Turn right (south) on Golf Club Road and proceed about 1.1 miles south to large brick buildings of the Newport Mine. Walk to the northeast across the small field to old railroad cutsl that are just north of the main building behind the trees. Caution: Area is now part of a gravel washing operation.

S t o ~ 2. Old Nemort mine (ArcheanIEarlv Proterozoic Unconformitv) .

The Palms Formation of the Menominee Group of the Marquette Range Supergroup rests upon the 21700 m.y. old Puritan Quartz Monzonite (Schmidtl 1976). The Marquette Range Supergroup in the Gogebic range was deposited on a moderately flat erosion surface developed on Archean I1graniten and greenstone.

At this localityl the steeply dipping (to the north) unconformable contact between the Palms Formation and Archean granitoid is exposed in the old railroad cut. Small depressions in the surfacel as can be seen herel contain laminated cherty'material, some of which is granular and somewhat resembles iron- formation. The granules are 0.5 to 1.0 mm in diameter and consist of mixtures of chertI chlorite, and calcite. There are also a few phosphatic argillaceous chips just above the contact. Note that there is no appreciable evidence of weathering of the granite beneath the Palms.

Red argillaceous and silty beds of the lowermember of the Palms Formation are exposed here. Theycontain wavy bedding and small mudcracks, and have beeninterpreted as upper tidal flat deposits of atransgressing sea (Ojakangas, 1983).

Return to U.S. Hwy. 2, cross it, and continue north approximately1 mile to rock knob on right, beyond the Kangas farm.

Stop 3. Contact between Bessemer Ouartzite and Powder MillVol canics

This exposure is approximately two miles north ofStop 2, and the interval between these two stopsdemonstrates the thickness of the Early Proterozoicsequence in this area. The exposure shows the contactbetween the basal Keweenawan Bessemer Quartzite and theoverlying basalt of the Powder Mill Group. Thus theexposure illustrates the abrupt change fromsedimentation of quartzose sandstone to flood basaltvolcanism during the Middle Proterozoic in this part ofthe Lake Superior region. Only the lowermost basaltflows are pillowed, and locally, the soft, siltysediment was squeezed up between rubbly blocks andpillows. Some units of Bessemer-type quartzite dooccur higher in the flow sequence. These featuresindicate that volcanism here was initiated in asedimentary basin, overwhelmed sedimentation, andresulted in a thick sequence of subaerial flows. Flowsjust to the north are massive.

Stop 4. Ironwood Iron-Formation and Ironwood-Tylerrelationships. (Modified from Schmidt, 1972)

Location: Exposures on the east and west banks of theBlack River upstream from Hwy. 2 on the westernoutskirts of Ramsay, Michigan. (NE ', SW, Sec. 12,T.47N., R.46W.)

The Ironwood Iron-Formation is a major sedimentaryGogebic range, and is the source rock for all of theiron ores mined in the area. Unfortunately, the iron-formation is very poorly exposed on the easternGogebic. Excellent exposures of the Ironwood Iron-Formation are present on Mt. Whittlesey, southeast ofMellen, Wisconsin, however, that locality is more than30 miles away, and will not be visited on this trip.

Schmidt and Hubbard (1972) described severalexposures along the Black River at this locality thatillustrate different phases of the iron-formation aswell as possible stratigraphic relationships betweenthe Ironwood Iron-Formation and the overlying TylerFormation. They describe thin-bedded chert-carbonate

23

Red argillaceous and silty beds of the lower member of the Palms Formation are exposed here. They contain wavy bedding and small mudcracksf and have been interpreted as upper tidal flat deposits of a transgressing sea (Ojakangas 1983 1 .

Return to U.S. H w y . cross it, and continue north approximately 1 mile to rock knob on rightl beyond the Kangas farm.

S t o ~ 3. Contact between Bessemer Ouartzite and Powder Mill Volcanics

This exposure is approximately two miles north of Stop and the interval between these two stops demonstrates the thickness of the Early Proterozoic sequence in this area. The exposure shows the contact between the basal Keweenawan Bessemer Quartzite and the overlying basalt of the Powder Mill Group. Thus the exposure illustrates the abrupt change from sedimentation of quartzose sandstone to flood basalt volcanism during the Middle Proterozoic in this part of the Lake Superior region. Only the lowermost basalt flows are pillowedl and locallyI the softl silty sediment was squeezed up between rubbly blocks and pillows. Some units of Bessemer-type quartzite do occur higher in the flow sequence. These features indicate that volcanism here was initiated in a sedimentary basinl overwhelmed sedimentationl and resulted in a thick sequence of subaerial flows. Flows just to the north are massive.

S t o ~ - 4. Ironwood Iron-Formation and Ironwood-Tvler relationshi~s. (Modified from Schmidtl 1972)

Location: Exposures on the east and west banks of the Black River upstream from H w y . 2 on the western outskirts of Ramsayl Michigan. (NE XI SWAl Sec. 12, T.47N. R.46W.

The Ironwood Iron-Formation is a major sedimentary Gogebic rangel and is the source rock for all of the iron ores mined in the area. Unfortunatelyl the iron- formation is very poorly exposed on the eastern Gogebic. Excellent exposures of the Ironwood Iron- Formation are present on Mt. Whittleseyl southeast of Mellenl Wisconsinl howeverl that locality is more than 30 miles awayl and will not be visited on this trip.

Schmidt and Hubbard (1972) described several exposures along the Black River at this locality that illustrate different phases of the iron-formation as well as possible stratigraphic relationships between the Ironwood Iron-Formation and the overlying qler Formation. They describe thin-bedded chert-carbonate

23

24

iron-formation at the top of the Anvil Member of theIronwood Iron-Formation on the east bank of the river600 feet upstream from the bridge on the secondary roadsouth of Hwy. 2. Granular jasper is described fromtest pits in the SE(, SW1X, Sec. 12, T.47N., R.46W.Schmidt and Hubbard (1972) conclude that the rocks arebasically unmetamorphosed examples of the iron-formation. Time limitations will not permit us tovisit all of the localities described by Schmidt andHubbard (1972), and those interested in examiningunaltered iron-formation should consult their 1972guidebook.

Schmidt and Hubbard (1972) also discussedalternative interpretations of the relationship betweenthe Tyler and Ironwood Formations. The contact isexposed on the east bank of the Black River 180 m (600ft.) upstream from the secondary road, where Schmidtand Hubbard (1972) interpreted the contact to begradational.

We will examine only the exposure on the west bankof the Black River just south of Hwy. 2 where a lens ofcarbonate iron-formation and a sulfide unit is presentwithin the lower part of the Tyler Formation. Althoughexposure is poor, there are outcrops along the streambank consisting of highly carbonaceous slate, massiveand concretionary(?) pyrite/marcasite, and sideriticiron- formation.

Stop 5. Radio Tower Hill in Wakefield.Palms Formation of Marguette Range Supercrroup

Location: The purposes of this stop are to (1) examinean excellent exposure of the Palms Formation and (2)obtain a scenic view of this part of the Gogebic range,of the Archean terrane to the south, and of theKeweenawan terrane to the north.

From the top of the hill, several abandoned ironmines can be seen in the lowland immediately to thenorth (refer to Figure 7); the ridges in theintermediate distance are underlain by basalt of earlyKeweenawan (Middle Proterozoic) age. The lowland inthe far distance is underlain by the late KeweenawanJacobsville Sandstone.

The view to the south overlooks Archean terraneconsisting of the Ramsay Formation (mafic and felsicmetavolcanic rocks) and the Puritan Quartz Monzoniteand related rocks (—2,700 m.y.), which constitute amajor batholith more than 150 km long.

In this area, the Palms Formation unconformablyoverlies the Archean Ramsay Formation and is overlain

iron-formation at the top of the Anvil Member of the Ironwood Iron-Formation on the east bank of the river 600 feet upstream from the bridge on the secondary road south of Hwy. 2. Granular jasper is described from test pits in the SE%# SWA1 Sec. 121 T.47Na1 R.46W. Schmidt and Hubbard (1972) conclude that the rocks are basically umetamorphosed examples of the iron- formation. Time limitations will not permit us to visit all of the localities described by Schmidt and Hubbard (1972)1 and those interested in examining unaltered iron-formation should consult their 1972 guidebook.

Schmidt and Hubbard (1972) also discussed alternative interpretations of the relationship between the Tyler and Ironwood Formations. The contact is exposed on the east bank of the Black River 180 m (600 ft.) upstream from the secondary roadl where Schmidt and Hubbard (1972) interpreted the contact to be gradational.

We will examine only the exposure on the west bank of the Black River just south of Hwy. 2 where a lens of carbonate iron-formation and a sulfide unit is present within the lower part of the Tyler Formation. Although exposure is poorl there are outcrops a1 bank consisting of highly carbonaceous and concretionary ( ? ) pyrite/marcasite, iron-formation.

.ong the stream slatel massive and sideritic

Radio Tower Hill in Wakefield. Palms Formation of Marmette Ranqe Su~erqrouq

Location: The purposes of this stop are to (1) examine an excellent exposure of the Palms Formation and (2) obtain a scenic view of this part of the Gogebic rangel of the Archean terrane to the southl and of the Keweenawan terrane to the north.

From the top of the hilll several abandoned iron mines can be seen in the lowland immediately to the north (refer to Figure 7); the ridges in thi intermediate distance are underlain by basalt of early - - Keweenawan (Middle Proterozoic) age. The lowland in the far distance is underlain by the late Keweenawan Jacobsville Sandstone.

The view to the south overlooks Archean terrane consisting of the Ramsay Formation (mafic and felsic metavolcanic rocks) and the Puritan Quartz Monzonite and related rocks (-21700 rn.y.1 which constitute a major batholith more than 150 km long.

In this areal the Palms Formation unconfombly overlies the Archean Ramsay Formation and is overlain

conformably by the Ironwood Iron-Formation, whichunderlies the low area north of the hill and alsosouthwest of the hill. Low on the next hill to theeast, rare Sunday Quartzite is present. Further east,the Palms unconformably overlies the Bad RiverDolomite, and has a thin phosphatic conglomerate at its

base. The Palms dips steeply northward, in accord withthe regional dip of Proterozoic rocks in the Gogebicrange. This regional dip resulted mainly fromnorthward tilting during Keweenawan time, toward theaxis of the (Keweenawan) Lake Superior syncline. TheEarly Proterozoic rocks were not appreciably deformedor metamorphosed during the tectonic event (Penokeanorogeny) that closed Early Proterozoic deposition, buta few folds plunging to the east-northeast at 20-25°have been recognized.

Exposures of the Palms Formation at this localityare representative of the formation which extends for80 km to the west-southwest. Representative rock typeswell-exposed at the top of the hill include the upperpart of the middle member which consists of interbeddedargillite, siltstone, and sandstone, and the transitionto the massive quartzite of the upper quartzite member.The middle member is thin bedded, with beds generally2-10 cm thick. In general, beds of buff to pinksiltstone and white sandstone alternate with buff, redor green beds of more argillaceous material; coarse-grained, dark-red, hematitic sandstone beds are alsopresent. Most beds are quite continuous across theoutcrop but "pinch and swell" irregularities impart awaviness to the generally uniform bedding. Lenticularbeds of sandstone, commonly cross-bedded, are common.Flaser bedding is present locally where the mud supplywas subordinate to the sand supply. Minor cut-and-filland soft-sediment deformation structures also arepresent. Symmetrical but irregular ripple marks andmud cracks, although not visible at this locality, arepresent on the next hill to the east (Ojakangas, 1983).

Measurement of 42 cross-beds on this hill shows astrong paleocurrent trend to the west with a weakertrend to the east. Nearly 200 measurements in theformation as a whole accentuate this bimodality.Correction for plunge, if indeed the entire formationhas a plunge as well as a tilt, would rotate the majorand minor modes clockwise, thus relocating the modes tothe west-northwest and east-southeast, respectively.

General characteristics of the formation, andespecially the bimodal cross-bedding pattern, thebedding styles, and the overall lithologies are highlysuggestive of deposition in a tidal environment.Recall that the lower member interpreted as upper tidal

25

conformably by the Ironwood Iron-Formation, which underlies the low area north of the hill and also southwest of the hill. Low on the next hill to the east, rare Sunday Quartzite is present. Further east, the Palms unconformably overlies the Bad River Dolomite, and has a thin phosphatic conglomerate at its base. The Palms dips steeply northward, in accord with the regional dip of Proterozoic rocks in the Gogebic range. This regional dip resulted mainly from northward tilting during Keweenawan time, toward the axis of the (Keweenawan) Lake Superior syncline. The Early Proterozoic rocks were not appreciably deformed or metamorphosed during the tectonic event (Penokean orogeny) that closed Early Proterozoic deposition, but a few folds plunging to the east-northeast at 20-25' have been recognized.

Exposures of the Palms Formation at this locality are representative of the formation which extends for 80 km to the west-southwest. Representative rock types well-exposed at the top of the hill include the upper part of the middle member which consists of interbedded argillite, siltstone, and sandstone, and the transition to the massive quartzite of the upper quartzite member. The middle member is thin bedded, with beds generally 2-10 cm thick. In general, beds of buff to pink siltstone and white sandstone alternate with buff, red or green beds of more argillaceous material; coarse- grained, dark-red, hematitic sandstone beds are also present. Most beds are quite continuous across the outcrop but "pinch and swelln irregularities impart a waviness to the generally uniform bedding. Lenticular beds of sandstone, commonly cross-bedded, are common. Flaser bedding is present locally where the mud supply was subordinate to the sand supply. Minor cut-and-fill and soft-sediment deformation structures also are present. Symmetrical but irregular ripple marks and mud cracks, although not visible at this locality, are present on the next hill to the east (Ojakangas, 1983).

Measurement of 42 cross-beds on this hill shows a strong paleocurrent trend to the west with a weaker trend to the east. Nearly 200 measurements in the formation as a whole accentuate this bimodality. Correction for plunge, if indeed the entire formation has a plunge as well as a tilt, would rotate the major and minor modes clockwise, thus relocating the modes to the west-northwest and east-southeast, respectively.

General characteristics of the formation, and especially the bimodal cross-bedding pattern, the bedding styles, and the overall lithologies are highly suggestive of deposition in a tidal environment. Recall that the lower member interpreted as upper tidal

flat, was seen at Stop 2. Here on Radio Tower Hill,the middle member is interpreted as middle tidal flat,and the cross-bedded quartz sand upper member isinterpreted as lower tidal flat or subtidal (Ojakangas,1983)

Thin sections from this locality show that thesandstone beds are feldspathic quartzite. Well-roundedunit quartz grains and feldspar (mostly fresh K-feldspar) grains are the dominant frameworkconstituents, with chert grains a poor third. Silicacement is abundant and illitic clay is present in manysamples as minor to abundant matrix. Illite is alsothe major constituent of the argillaceous beds withchlorite locally prominent. The coarser-grained,hematitic sandstone beds are really thin beds of iron-formation, consisting of granules of hematite, chert,and iron silicates in addition to the common quartzgrains.

Deformation of the Palms Formation has not beenwell studied. Basically, it is part of a largemonocline in which folding is rarely seen except atthis locality. On the west flank of the hill, severalopen, round-crested folds exhibit a cleavage that isflatter than the bedding. The extent, origin, and ageof these folds and their associated cleavage is notknown, but the folds may have formed when the beds wereapproximately flat-lying, presumably during thePenokean orogeny. Alternatively, the folding mighthave been caused by drag along the Sunday Lake fault, anorthwest-trending fault that passes just west of thishill.

Stop 6. Archean Ramsay Formation (pillowed reenstone)

Location: Roadcut along U.S. Hwy. 2 at eastern citylimits of Wakefield, about 2.5 mi. SE of the junctionof Highways 2 and 28 in Wakefield.

The Archean Ramsay Formation forms part of theArchean basement upon which the Early Proterozoic rockswere deposited. According to Prinz and others (1975)metavolcanic rocks of Archean age extend eastward forapproximately 15 km along the range in the vicinity ofWakefield. They are intruded on the south and west bythe Puritan Quartz Monzonite, and are bounded on theeast by gneissic rocks of the Puritan Quartz Monzonite.They reported that the eastern two-thirds of the beltof Archean rocks is mainly mafic to intermediate flowsand pyroclastic rocks with sparse felsic volcanicrocks. However, intermediate to felsic rocks aredominant in the western part of the belt. The rocksgenerally strike west to northwest (roughly parallel to

26

flat, was seen at Stop 2. Here on Radio Tower Hill, the middle member is interpreted as middle tidal flat, and the cross-bedded quartz sand upper member is interpreted as lower tidal flat or subtidal (Ojakangas, 1983).

Thin sections from this locality show that the sandstone beds are feldspathic quartzite. Well-rounded unit quartz grains and feldspar (mostly fresh K- feldspar) grains are the dominant framework constituents, with chert grains a poor third. Silica cement is abundant and illitic clay is present in many samples as minor to abundant matrix. Illite is also the major constituent of the argillaceous beds with chlorite locally prominent. The coarser-grained, hematitic sandstone beds are really thin beds of iron- formation, consisting of granules of hematite, chert, and iron silicates in addition to the common quartz grains.

Deformation of the Palms Formation has not been well studied. Basically, it is part of a large monocline in which folding is rarely seen except at this locality. On the west flank of the hill, several open, round-crested folds exhibit a cleavage that is flatter than the bedding. The extent, origin, and age of these folds and their associated cleavage is not known, but the folds may have formed when the beds were approximately flat-lying, presumably during the Penokean orogeny. Alternatively, the folding might have been caused by drag along the Sunday Lake fault, a northwest-trending fault that passes just west of this hill.

Stop 6. Archean Ramsav Formation (pillowed meenstone)

Location: Roadcut along U.S. Hwy. 2 at eastern city limits of Wakefield, about 2.5 mi. SE of the junction of Highways 2 and 28 in Wakefield.

The Archean Ramsay Formation forms part of the Archean basement upon which the Early Proterozoic rocks were deposited. According to Prinz and others (1975) metavolcanic rocks of Archean age extend eastward for approximately 15 km along the range in the vicinity of Wakefield. They are intruded on the south and west by the Puritan Quartz Monzonite, and are bounded on the east by gneissic rocks of the Puritan Quartz Monzonite. They reported that the eastern two-thirds of the belt of Archean rocks is mainly mafic to intermediate flows and pyroclastic rocks with sparse felsic volcanic rocks. However, intermediate to felsic rocks are dominant in the western part of the belt. The rocks generally strike west to northwest (roughly parallel to

the strike of Early Proterozoic rocks) and dip steeplyto the south. Pillows indicate that the lava flowsgenerally face southward.

Prinz and others (1975) suggested that the maficand the felsic volcanics each have a thickness ofapproximately 3,000 meters. Metamorphic gradeincreases from greenschist facies in the west toamphibolite facies along the eastern margin.

Rocks exposed here are typical pillowed maficvolcanic rocks that have been metamorphosed togreenschist fades. They are composed of fine-grainedsodic plagioclase, quartz, epidote-zoisite, andchlorite, with some pale amphibole, carbonate andbiotite (Prinz and others, 1975). Pillows with topsfacing southward are well-exposed on the south side ofthe highway.

Take U.S. Hwy. 2 east of Wakefield city limits about two miles toGreat Lakes Road. Go to compressor station on pipeline and turnleft (north) on gravel road. About 0.3 mile in is a "Y"; keepleft and go about 1.5 miles to end of road. Walk west on loggingroad ± 270 paces, then go north through the woods to the highhill. The outcrop is an escarpment along SW side of hill in theSE (, Sec. 18, T47N., R.44W., (Wakefield NE 7� MinuteQuadrangle.)

Stop 7. Archean-Early Proterozoic unconformity.

The unconformity between the Archean Ramsaygreenstone and the Early Proterozoic Sunday Quartziteis exposed along the southwest side of this hill. Notethe schistosity in the otherwise massive greenstone,and the very thin basal conglomerate. Just westwardfrom the unconformity, the Sunday Quartzite at firstglance appears to be conglomeratic, but it is a thinskin along the face of the outcrop and is probably afault breccia. This prominent exposure is probably afault scarp.

Moving along the base of the escarpment, note thecross-bedding (some of it herringbone type, with cross-beds in successive beds oriented at 180° to eachother), the numerous layers of mudchip conglomerates,and mud-cracked horizons. These are included in aseries of stacked tidal channels to about 2 m thick,each starting with a mud-cracked horizon. These areoverlain by a cross-bedded unit that includesherringbone cross-beds, and ends with a parallel-beddedunit. At this exposure, 41 cross-beds yield a broadtrimodal (but largely bimodal-bipolar) pattern with themajor mode to the northwest and a less prominent mode

27

the strike of Early Proterozoic rocks) and dip steeply to the south. Pillows indicate that the lava flows generally face southward.

Prinz and others (1975) suggested that the mafic and the felsic volcanics each have a thickness of approximately 3,000 meters. Metamorphic grade increases from greenschist facies in the west to amphibolite facies along the eastern margin.

Rocks exposed here are typical pillowed mafic volcanic rocks that have been metamorphosed to greenschist facies. They are composed of fine-grained sodic plagioclase, quartz, epidote-zoisite, and chlorite, with some pale amphibole, carbonate and biotite (Prinz and others, 1975). Pillows with tops facing southward are well-exposed on the south side of the highway.

Take U.S. Hwy. 2 east of Wakefield city limits about two miles to Great Lakes Road. Go to compressor station on pipeline and turn left (north) on gravel road. About 0 - 3 mile in is a "Yn ; keep left and go about 1.5 miles to end of road. Walk west on logging road 2 270 paces, then go north through the woods to the high hill. The outcrop is an escarpment along SW side of hill in the SE %, Sec. 18, T47N., R.44W., (Wakefield NE 7% Minute Quadrangle. )

Archean-Earlv Proterozoic unconformitv.

The unconformity between the Archean Ramsay greenstone and the Early Proterozoic Sunday Quartzite is exposed along the southwest side of this hill. Note the schistosity in the otherwise massive greenstone, and the very thin basal conglomerate. Just westward from the unconformity, the Sunday Quartzite at first glance appears to be conglomeratic, but it is a thin skin along the face of the outcrop and is probably a fault breccia. This prominent exposure is probably a fault scarp.

Moving along the base of the escarpment, note the cross-bedding (some of it herringbone type, with cross- beds in successive beds oriented at 180' to each other), the numerous layers of mudchip conglomerates, and mud-cracked horizons. These are included in a series of stacked tidal channels to about 2 m thick, each starting with a mud-cracked horizon. These are overlain by a cross-bedded unit that includes herringbone cross-beds, and ends with a parallel-bedded unit. At this exposure, 41 cross-beds yield a broad trimodal (but largely bimodal-bipolar) pattern with the major mode to the northwest and a less prominent mode

28

to the southeast. All of these features arecharacteristic of a tidal environment of deposition forthe Sunday Quartzite. Thus a sandy tidal environmenttransgressed onto a peneplained Archean craton.

Continue along escarpment and note the nature ofthe transition from the quartzite into dolomite. Thisis the Bad River Dolomite. Note also the abundance ofwhite and black chert within the dolomite, and thepresence of stromatolites. Hanirnering Please!Because the vertical facies give the horizontal faciesrelationships, it seems that the carbonate wasdeposited seaward of the terrigenous sands that are nowthe Sunday Quartzite, and this carbonate environmentthen transgressed over the quartz sands as the seatransgressed northward.

Stop 8. Mafic phase. Emperor Volcanics (with contribution by K.Licht)

Location: NE '(, NE 14• Sec. 24. T.47N., R.43W.Forest Service Roads 523 and then 8640 approximately6.5 miles north of Marenisco, Michigan.

Rocks exposed on this hill illustrate severalvarieties of the mafic phase of the Emperor VolcanicComplex. The Emperor Formation is at least 2,000meters thick in this area but thins rapidly to the westand is not recognized 10 miles to the west in theWakefield area. The volcanics are truncated on theeast by major faults. Therefore, the volcanic rocksform an eastward-thickening wedge interbedded with theIronwood Iron-Formation and the wedge is truncated byfaults as shown by Trent's (1973) mapping.

The Emperor Volcanic Complex consists of a maficand felsic phase. Dann (1978) showed that the rocksrange in composition from basalt to dacite. Mapping byLaBerge and J. S. Kiasner in 1991 indicates that themafic phase Emperor volcanics are the products ofmainly subaqueous eruptions, and include massive andpillowed basalts, pillow breccias, hyaloclastites, anddebris flows.

Exposures near the base at the north end of thehill are pillowed basalt with little or no interpillowhyaloclastite. Higher on the hill and in the cliffsalong the western side of the hill, some exposures aremainly hyaloclastite, others are hyaloclastitecontaining pillows and pillow fragments, and someexposures are mainly pillows (Figure 14).

Figure 15 is a possible interpretation of therocks exposed here. Lava fountaining in a shallow

to the southeast. All of these features are characteristic of a tidal environment of deposition for the Sunday Quartzite. Thus a sandy tidal environment transgressed onto a peneplained Archean craton.

Continue along escarpment and note the nature of the transition from the quartzite into dolomite. This is the Bad River Dolomite. Note also the abundance of white and black chert within the dolomiteI and the presence of stromatolites. Hammering Please! Because the vertical facies give the horizontal facies relationshipsl it seems that the carbonate was deposited seaward of the terrigenous sands that are now the Sunday Quartzitel and this carbonate environment then transgressed over the quartz sands as the sea transgressed northward.

Stop 8. Mafic phase, Emperor Volcanics (with contribution by K. Licht)

Location: NE NE %, Sec. 24, T.47N., R.43W. on Forest Semice Roads 523 and then 8640 approximately 6.5 miles north of Mareniscot Michigan.

Rocks exposed on this hill illustrate several varieties of the mafic phase of the Emperor Volcanic Complex. The Emperor Formation is at least 21000 meters thick in this area but thins rapidly to the west and is not recognized 10 miles to the west in the Wakefield area. The volcanics are truncated on the east by major faults. Thereforel the volcanic rocks foxm an eastward-thickening wedge interbedded with the Ironwood Iron-Formation and the wedge is truncated by faults as shown by Trent's (1973) mapping.

The Emperor Volcanic Complex consists of a mafic and felsic phase. D a m (1978) showed that the rocks range in composition from basalt to dacite. Mapping by LaBerge and J. S. Klasner in 1991 indicates that the mafic phase Emperor volcanics are the products of mainly subaqueous eruptionsl and include massive and pillowed basaltsI pillow breccias! hyaloclastitesl and debris flows.

Exposures near the base at the north end of the hill are pillowed basalt with little or no interpillow hyaloclastite. Higher on the hill and in the cliffs along the western side of the hilll some exposures are mainly hyaloclastitel others are hyaloclastite containing pillows and pillow fragmentsI and some exposures are mainly pillows (Figure 14).

Figure 15 is a possible interpretation of the rocks exposed here. Lava fountaining in a shallow

Figure 14. Pillow breccia with pillow fragments in ahyaloclastite matrix. Jackknife is 8 cm long.

4 , Hyaloclastite formed by4 shallow-water fountarning.

ii 41 fr1-1I I_4 A.

':* 4 u

a 14I—

01 1ii iii t

Sketch showing possible environment of formationof basalts exposed at Stop 8.

29

,. - ,.t# flSJ#%/% fS ..F — 'S1 fl -

Water Surface

Ilebri S f (iw. ni p1 OW I,?It(. Iend hyalocldst.itC.

Pillows and pillow brecciaformed by flows into —_...

i:. • or onto hya1ocaSt1te

) Li/ 4 .4 -. Li • •fr,.• '

Figure 15.

Figure 14. Pillow breccia with pillow fragments in a hyaloclastite matrix. Jackknife is 8 cm long.

Figure 15. Sketch showing possible environment of formation of basalts exposed at Stop 8.

30

\

MARENISCO QUADRANGLEMICHIGAN—COGEBIC CO.

7.5 MINUTE SERIES (TOPOGRAPHIC)NW4 MARENtSOO 5' QUADRANGLE

MARENISCO QUADRANGLE MICHIGAN-GOGEBlC CO.

7.5 MINUTE S E R I E S (TOPOGRAPHIC) N W ( 4 MARE&!SCO 15' QGADRANGLE

subaqueous environment may have produced ahyaloclastite dome. Later eruptions may have resultedin emplacement of pillow lavas onto (or into) thehyaloclastite. Additional hyaloclastite may haveformed on the surface of the flow. Pillows and pillowfragments cascading off the flow front may have becamemixed with hyaloclastite. Debris flows ofhyaloclastite and pillow breccia may also have beenformed.

Rocks collected from this stop were analyzed inthin-section and by chemical analysis. Some texturesand unique findings will be described in the followingparagraphs.

Many pillows within the hyaloclastite containreaction rims around the outer margins and along quenchcracks. The thicknesses of these rims vary from a fewmillimeters to a few centimeters. The reaction rimsare characterized by a fairly uniform highly alteredbasaltic texture with some remnant tabular plagioclase.Vein fillings, within the reaction rims containchlorjte, recrystallized quartz, and epidote.

Jasper occurs sporadically and tends to be"molded" between the pillows and in fractures withinpillows. Source of Si02 for the jasper may have beengelatinous ooze on the seafloor at the time oferuption. The source of the Fe was probably theextruded basalt.

The massive basalt on the north end of the hillhas an abnormally high Si02 content of 62.48's. Inthin-section, epidote, recrystallized quartz, chlorite,and calcite are associated with the altered basalt.

The hyaloclastite varies greatly in texture, butvaries little in composition. The included basaltfragments are highly altered. The hyaloclastite matrixis composed of plagioclase, calcite, quartz, chlorite,amphibole, and epidote.

Stop 9. Felsic phase. Emperor Volcanics

Location: Outcrop in SE 1A NE 1, Sec. 20, T47N., R.44Wnear Presque Isle River on Forest Service Road 523approximately 6 miles north of Marenisco, Michigan.

This exposure contains rocks representative ofsome varieties of the felsic phase of the EmperorVolcanics. The dominant lithology at this localityconsists of relatively angular, tan, 2 to 10 cm,monolithic clasts of microcrystalline felsite in a

31

subaqueous environment may have produced a hyaloclastite dome. Later eruptions may have resulted in emplacement of pillow lavas onto (or into) the hyaloclastite. Additional hyaloclastite may have formed on the surface of the flow. Pillows and pillow fragments cascading off the flow front may have became mixed with hyaloclastite. Debris flows of hyaloclastite and pillow breccia may also have been formed.

Rocks collected from this stop were analyzed in thin-section and by chemical analysis. Some textures and unique findings will be described in the following paragraphs.

Many pillows within the hyaloclastite contain reaction rims around the outer margins and along quench cracks. The thicknesses of these rims vary from a few millimeters to a few centimeters. The reaction rims are characterized by a fairly uniform highly altered basaltic texture with some remnant tabular plagioclase. Vein fillings) within the reaction rims contain chlorite) recrystallized quartz) and epidote.

. Jasper occurs sporadically and tends to be l1mo1dedl1 between the pillows and in fractures within pillows. Source of Si02 for the jasper may have been gelatinous ooze on the seafloor at the time of eruption. The source of the Fe was probably the extruded basalt.

The massive basalt on the north end of the hill has an abnormally high Si02 content of 62.48%. In thin-section) epidote) recrystallized quartz) chlorite) and calcite are associated with the altered basalt.

The hyaloclastite varies greatly in texture) but varies little in composition. The included basalt fragments are highly altered. The hyaloclastite matrix is composed of plagioclasel calcite) quartz) chlorite) amphibolel and epidote.

Stow 9. Felsic ~hase, Em~eror Volcanics

Location: Outcrop in SE ?4 NE ?4 Sec. 20) T47N.) R.44W near Presque Isle River on Forest Service Road 523 approximately 6 miles north of MareniscoI Michigan.

This exposure contains rocks representative of some varieties of the felsic phase of the Emperor Volcanics. The dominant lithology at this locality consists of relatively angular) tan) 2 to 10 cml monolithic clasts of microcrystalline felsite in a

32

finer hyaloclastite matrix. Whereas the smallhyaloclastite fragments typically have a whitishalteration rind, the larger felsite clasts generallylack alteration rinds. Prinz and others (1975) statedthat the felsite clasts as well as the matrix have adacitic composition. In zones of deformation thehyaloclastite matrix tends to be strongly foliated,whereas the felsite clasts tend to show littledeformation (Figure 16). Some clasts contain abundantperlitic cracks, others are more massive felsite. Thislithology of the felsic phase of the Emperor extendswestward at least 5 km, where there are extensiveexposures in Secs. 14 and 23 north of Wolf Mountain.In addition to the variety of rock exposed here, Sec.14 north of Wolf Mountain contains extensive brecciatedfelsite with scattered quartz phenocrysts. Althoughthe rock is extensively brecciated, there appears tohave been little or no movement of adjacent blocks withrespect to one another. In short, the rock appears tobe a felsic dome that has been brecciated.

Surrounding the brecciated felsite are numerousexposures with breccia fragments of felsite in a felsichyaloclastite matrix (Figure 17). It is suggested thatthe felsite breccia may be a felsic dome emplaced intofelsic hyaloclastite produced by an earlier eruption.Debris flows off the dome may have resulted in morewidespread units of the felsite breccia/hyaloclastiteas shown in Figure 18.

finer hyaloclastite matrix. Whereas the small hyaloclastite fragments typically have a whitish alteration rind! the larger felsite clasts generally lack alteration rinds. Prinz and others (1975) stated that the felsite clasts as well as the matrix have a dacitic composition. In zones of deformation the hyaloclastite matrix tends to be strongly foliated! whereas the felsite clasts tend to show little deformation (Figure 16). Some clasts contain abundant perlitic cracks! others are more massive felsite. This lithology of the felsic phase of the Emperor extends westward at least 5 kml where there are extensive exposures in Secs. 14 and 23 north of Wolf Mountain. In addition to the variety of rock exposed here! Sec. 14 north of Wolf Mountain contains extensive brecciated felsite with scattered quartz phenocrysts. Although the rock is extensively brecciated, there appears to have been little or no movement of adjacent blocks with respect to one another. In short! the rock appears to be a felsic dome that has been brecciated.

Surrounding the brecciated felsite are numerous exposures with breccia fragments of felsite in a felsic hyaloclastite matrix (Figure 17). It is suggested that the felsite breccia may be a felsic dome emplaced into felsic hyaloclastite produced by an earlier eruption. Debris flows off the dome may have resulted in more widespread units of the felsite breccia/hyaloclastite as shown in Figure 18.

Figure 16.

Figure 17.

Photo of sawed slab of Emperor Volcanics from Stop9. Larger, angular felsite clasts in a finergrained, foliated hyaloclastite matrix. Slab is18 cm long.

Photomicrograph of hyaloclastite from the felsicphase of tie Emperor Volcanics. Note the perliticcracks in the oval clast on the right side ofphoto. Bar scale is 2 mm.

33

Figure 16. Photo of sawed slab of Emperor Volcanics from Stop 9. Larger, angular felsite clasts in a finer grained, foliated hyaloclastite matrix. Slab is 18 cm long.

Figure 17. Photomicrograph of hyaloclastite from the felsic phase of the Emperor Volcanics. Note the perlitic cracks in the oval clast on the right side of photo. Bar scale is 2 mm.

Figure 18.

Stop 10.

Sketch of possible relationships within the felsicphase of the Emperor Volcanics. Highly fracturedfelsite blocks become dispersed in finer grainedfelsic hyaloclastite in debris flows of f asubaqueous lava dome.

Archean metacrravwacke

34

Location: Roadcuts on Highway 2, about 10.5 mi SE ofMarenisco, SW 'A, Sec. 30, T. 46 N., R. 41 W. (Thayer,MI 15' quadrangle).

The metagraywacke in these roadcuts is part of anortheast-trending belt 4 km wide and 20 km long. Therocks are highly folded on tight north-northeast-trending axes, and the unit is probably about 1 kmthick. The graywacke is a fine-grained, thin- tothick-bedded rock with a pervasive northeast-trendingschistosity (S2) that is Penokean in age; an olderschistosity (Si) that is Archean in age is nearlyparallel to bedding. Graded beds are common, and topsat this outcrop are to the south. Reversals of gradingindicate that the fold crests are about 200 m apart.

W a t e r S u r f a c e

C a r a p a c e o f hyaloclastite formed ( Iu I -~ I I~ e a r l i e r fountaininq.

Figure 18. Sketch of possible relationships within the felsic phase of the Emperor ~olcanics. Highly fractured felsite blocks become dispersed in finer grained felsic hyaloclastite in debris flows off a subaqueous lava dome.

Stow 10. Archean metaqrawacke

Location: Roadcuts on Highway 2, about 10.5 mi SE of Marenisco, SW X , Sec. 30, T. 46 N., R. 41 W. (Thayer, MI 15' quadrangle) .

The metagraywacke in these roadcuts is part of a northeast-trending belt 4 km wide and 20 km long. The rocks are highly folded on tight north-northeast- trending axes, and the unit is probably about 1 km thick. The graywacke is a fine-grained, thin- to thick-bedded rock with a pervasive northeast-trending schistosity (S,) that is Penokean in age; an older schistosity (Si) that is Archean in age is nearly parallel to bedding. Graded beds are common, and tops at this outcrop are to the south. Reversals of grading indicate that the fold crests are about 200 m apart.

The rocks were metamorphosed to amphibolite gradeduring the Archean and are totally recrystallized; mostof the garnet was retrograded to chlorite + muscovite +

epidote and opaques during the Penokean orogeny (Simsand others, 1984). The nature of the original sedimentis unknown because of the thorough recrystallization,but CIA indices (chemical indices of alteration)calculated for the graywackes indicate an originalimmature detritus that had undergone little if anyweathering in the source area and could have beenimmature volcaniclastics. The area was mapped byFritts (1969), Trent (1973), and Prinz (1981) . Cannon(1986b) produced a structural and tectonic map of theregion.

Stor 11. Watersmeet Gneiss (3560 Ma)

Location: Roadcuts on Highway 45, about 4 mi N of thejunction of Highways 2 and 45. This junction is on theS edge of the village of Watersmeet. Roadcut locationis S edge, NE 'A, Sec. 4, T. 46 N., R. 39 W.(Watersmeet, MI, 15' quadrangle).

This exposure is located in the northern part ofthe Watersmeet dome which has an Archean core about 25km long in an E-W direction and about 8 km wide in a N-S direction. The dome is considered to be a mantledgneiss dome. Sims (1990) recognizes considerable EarlyProterozoic rocks in the core infolded with theArchean. Deformation of the Archean and adjacent EarlyProterozoic rocks occurred during the Penokean orogeny.The rocks in the core were metamorphosed to theamphibolite fades, a 15 km-wide zone of epidoteamphibolite facies surrounds the core, and this in turnpasses outward into greenschist facies. (This is theWatersmeet metamorphic node of James, 1955). The EarlyProterozoic rocks to the north of the dome have beenoverturned toward the northwest. This area was studiedby Sims and others (1984) and Peterxnan and others(1980)

The gray gneiss is a tonalitic augen gneiss about3560 Ma old (zircon data) (Sims and others, 1984); itis a medium-gray, medium- to coarse-grained, biotite-rich, irregularly layered rock with plagioclase augen.The alternating layers are feldspar-quartz-rich andbiotite-rich. A modal analysis showed 48% plagioclase,30% quartz, 8% K-feldspar, and 13% biotite. A possibleprotolith was dacitic volcanic rock (Sims and others,1984). In this exposure, biotite leucogranite veins2590 Ma old (Rb/Sr and U-Pb methods) cut the gneiss.Secondary whole-rock and mineral isochrons give ages of1750 to 1800 Ma and are thought to approximate the timespan of the Penokean event in this area (Sims and

35

The rocks were metamorphosed to amphibolite grade during the Archean and are totally recrystallized; most of the garnet was retrograded to chlorite + muscovite + epidote and opaques during the Penokean orogeny (Sims and others, 1984). . The nature of the original sediment is unknown because of the thorough recrystallization, but CIA indices (chemical indices of alteration) calculated for the graywackes indicate an original immature detritus that had undergone little if any weathering in the source area and could have been immature volcaniclastics. The area was mapped by Fritts (1969) , Trent (1973) , and Prinz (1981) . Cannon (1986b) produced a structural and tectonic map of the region.

Stoo 11. Watersmeet Gneiss (3560 Ma)

Location: Roadcuts on Highway 45, about 4 mi N of the junction of Highways 2 and 45. This junction is on the S edge of the village of Watersmeet. Roadcut location is S edge, NE 1/, Sec. 4, T. 46 N., R. 39 W. (Watersmeet, MI, 15' quadrangle) .

This exposure is located in the northern part of the Watersmeet dome which has an Archean core about 25 km long in an E-W direction and about 8 km wide in a N- S direction. The dome is considered to be a mantled gneiss dome. Sims (1990) recognizes considerable Early Proterozoic rocks in the core infolded with the Archean. Deformation of the Archean and adjacent Early Proterozoic rocks occurred during the Penokean orogeny. The rocks in the core were metamorphosed to the amphibolite facies, a 15 km-wide zone of epidote amphibolite facies surrounds the core, and this in turn passes outward into greenschist facies. (This is the Watersmeet metamorphic node of James, 1955). The Early Proterozoic rocks to the north of the dome have been overturned toward the northwest. This area was studied by Sims and others (1984) and Peterman and others (1980).

The gray gneiss is a tonalitic augen gneiss about 3560 Ma old (zircon data) (Sims and others, 1984); it is a medium-gray, medium- to coarse-grained, biotite- rich, irregularly layered rock with plagioclase augen. The alternating layers are feldspar-quartz-rich and biotite-rich. A modal analysis showed 48% plagioclase, 30% quartz, 8% K-feldspar, and 13% biotite. A possible protolith was dacitic volcanic rock (Sims and others, 1984). In this exposure, biotite leucogranite veins 2590 Ma old (Rb/Sr and U-Pb methods) cut the gneiss. Secondary whole-rock and mineral isochrons give ages of 1750 to 1800 Ma and are thought to approximate the time span of the Penokean event in this area (Sims and

36

others, 1984)

Whereas the Archean gneisses of the area may havebeen folded three times, the cross-cutting Late Archeanleucogranite shows only a single foliation, that of thePenokean event.

The gneiss is comparable in age to those of theMinnesota River Valley, about 3500 Ma old: these areparts of the ancient gneiss terrane that is separatedfrom the granite-greenstone terrane to the north by theGLTZ, the Great Lakes Tectonic Zone of Sims and others(1980)

others, 1984).

Whereas the Archean gneisses of the area may have been folded three times, the cross-cutting Late Archean leucogranite shows only a single foliation, that of the Penokean event.

The gneiss is comparable in age to those of the Minnesota River Valley, about 3500 Ma old: these are parts of the ancient gneiss terrane that is separated from the granite-greenstone terrane to the north by the GLTZ, the Great Lakes Tectonic Zone of Sims and others (1980).

References Cited

Aldrich, H. R., 1929, The Geology of the Gogebic Iron Range ofWisconsin: Wisconsin Geological and Natural History SurveyBulletin 71, 279 p.

Allen, R. C., and Barrett, L. p., 1915, Contributions to the pre-Cambrian geology of northern Michigan and Wisconsin:Michigan Geological and Biological Survey, Publication 18,Geology Series 15, pp. 65-139.

Aiwin, B., 1976, Sedimentation of the Middle Precambrian TylerFormation of north central Wisconsin and northwesternMichigan: Master's Thesis, Univ. of Minnesota-Duluth, 200

p.

Bailey, S. W. and Tyler, S. A., 1960, Clay Minerals Associatedwith the Lake Superior iron ores: Economic Geology, vol.55, pp. 150-175.

Barovich, K. M., Patchett, P. J., Peterman, Z. E., and Sims, P.K., 1989, Origin of 1.9-1.7 Ga Penokean continental crust ofthe Lake Superior region: Geological Society of AmericaBulletin, v. 101, p. 333-338.

Cannon, W. F., 1986, Bedrock geologic map of the Iron River 10 x2° quadrangle, Michigan and Wisconsin; 1:450,000: U.S.Geological Survey Map 1-1360-B.

Cannon, W. F., 1986b, Structural and tectonic map of the IronRiver 1° x 2° Quadrangle, Michigan and Wisconsin, 1:250,000:U.S. Geological Survey Map I-1360-D.

Cannon, W. F. and Gair, J. E., 1970, A Revision of stratigraphicnomenclature for Middle Precambrian rocks in northernMichigan: Geological Society of America Bulletin 81, pp.2 843-2846.

Cannon, W. F., Peterman, Z. E., and Sims, P. K., 1990, Structuraland isotopic evidence for middle Proterozoic thrust faultingof Archean and Early Proterozoic rocks near the Gogebicrange, Michigan and Wisconsin: 36th Annual Institute onLake Superior Geology, Thunder Bay, Ontario, p. 11-13.

Dann, J. C., 1978, Major-element variation within the EmperorIgneous complex and the Hemlock and Badwater Volcanicformations: unpublished M.S. thesis, Michigan TechnologicalUniversity, 199 p.

Dimroth, E., 1968, Sedimentary textures, diagenesis, andsedimentary environment of certain Precambrian ironstones:Neues Jahrbuch für Geology v. Paleontology, Abh. v. 130, pp.247-274.

37

References Cited

Aldrich, H. R., 1929, The Geology of the Gogebic Iron Range of Wisconsin: Wisconsin Geological and Natural History Survey Bulletin 71, 279 p.

Allen, R. C., and Barrett, L. P., 1915, Contributions to the pre- Cambrian geology of northern Michigan and Wisconsin: Michigan Geological and Biological Survey, Publication 18, Geology Series 15, pp. 65-139.

Alwin, B., 1976, Sedimentation of the Middle Precambrian Tyler Formation of north central Wisconsin and northwestern Michigan: Master's Thesis, Univ. of Minnesota-Duluth, 200 P-

Bailey, S. W. and Tyler, S. A., 1960, Clay Minerals Associated with the Lake Superior iron ores: Economic Geology, vol. 55, pp. 150-175.

Barovich, K. M., Patchett, P. J., Peterman, Z. E., and Sims, P. K., 1989, Origin of 1.9-1.7 Ga Penokean continental crust of the Lake Superior region: Geological Society of America Bulletin, v. 101, p. 333-338.

Cannon, W. F., 1986, Bedrock geologic map of the Iron River 1' x 2' quadrangle, Michigan and Wisconsin; 1:450,000: U.S. Geological Survey Map I-1360-B.

Cannon, W. F., 1986b, Structural and tectonic map of the Iron River 1' x 2' Quadrangle, Michigan and Wisconsin, 1:250,000: U.S. Geological Survey Map I-1360-D.

Cannon, W. F. and Gair, J. E., 1970, A Revision of stratigraphic nomenclature for Middle Precambrian rocks in northern Michigan: Geological Society of America Bulletin 81, pp. 2843-2846.

Cannon, W. F., Peterman, Z. E., and Sims, P. K., 1990, Structural and isotopic evidence for middle Proterozoic thrust faulting of Archean and Early Proterozoic rocks near the Gogebic range, Michigan and Wisconsin: 36th Annual Institute on Lake Superior Geology, Thunder Bay, Ontario, p. 11-13.

Dann, J. C., 1978, Major-element variation within the Emperor Igneous complex and the Hemlock and Badwater Volcanic formations: unpublished M.S. thesis, Michigan Technological University, 199 p.

Dimroth, E., 1968, Sedimentary textures, diagenesis, and sedimentary environment of certain Precambrian ironstones: Neues Jahrbuch fur Geology v. Paleontology, Abh. v. 130, pp. 247-274.

Dimroth, E. and Chauvel, J. J., 1973, Petrography of the SokomanIron-formation in part of the central Labrador trough,Quebec, Canada: Geological Society of America Bulletin, v.84, p. 111-134.

Fritts, C. E., 1969, Bedrock geologic map of the Marenisco-Watersmeet area, Gogebic and Ontonogan Counties, Michigan:U.S. Geological Survey Miscellaneous Geologic InvestigationsMap 1-576, scale: 1:48,000.

Gerlach, D. C., Shirey, S. B., and Carison, R. W., 1988, Ndisotopes in Proterozoic iron-formations: Evidence formixed-age provenance and depositional variability:Abstract, Am. Geophys. Union Trans., EOS, v. 69.

Goldich, S. S. and Marsden, R. W., 1956, Precambrian ofNortheastern Minnesota; Field Trip No. 1, Geological Societyof America Annual Meeting, Minneapolis, MN.

Hoffman, P. F., 1987, Early Proterozoic foredeeps, foredeepniagmatism, and Superior-type iron-formations of the CanadianShield, in Krâner, A., ed., Proterozoic lithosphericevolution: Geodynamic Series, American Geophysical Union,v. 17, pp. 85-98.

Hoffman, P. F., 1988, Animikie Group: A Penokean Foredeep?, 34thAnnual Institute on Lake Superior Geology, Marquette, MI, p.40-41.

Hotchkiss, W. 0., 1919, Geology of the Gogebic Range:Engineering and Mining Journal, vol. 108.

Irving, R. D. and Van Hise, 1892, The Penokee Iron-Bearing Seriesof Michigan and Wisconsin: U.S. Geological Survey Monograph19, 534 p.

James, H. L., 1954, Sedimentary Facies of Iron-Formation:Economic Geology, vol. 49, p. 235-293.

James, H. L., 1955, zones of regional metamorphism in thePrecambrian of northern Michigan: Geological Society ofAmerica Bulletin, v. 66, p. 1455-1487.

Kiasner, J. S., Ojakangas, R. W., Schulz, K. J., and LaBerge, G.L., 1991, Nature and Style of Deformation in the Foreland ofthe Early Proterozoic Penokean Orogen, Northern Michigan:U.S.G.S. Bulletin 1904-K, 22 p.

LaBerge, G. L., 1964, Development of Magnetite in Iron-Formationsof the Lake Superior Region: Economic Geology, vol. 59, p.1313-1342.

LaBerge, G. L., l966a, Altered Pyroclastic Rocks in Iron-Formation Hamersley Range, Western Australia: EconomicGeology, vol. 61, p. 147-161.

38

Dimroth, E. and Chauvel, J. J., 1973, Petrography of the Sokoman Iron-formation in part of the central Labrador trough, Quebec, Canada: Geological Society of America Bulletin, v. 84, p. 111-134.

Fritts, C. E., 1969, Bedrock geologic map of the Marenisco- Watersmeet area, Gogebic and Ontonogan Counties, Michigan: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-576, scale: 1:48,000.

Gerlach, D. C., Shirey, S. B., and Carlson, R. W., 1988, Nd isotopes in Proterozoic iron-formations: Evidence for mixed-age provenance and depositional variability: Abstract, Am. Geophys. Union Trans., EOS, v. 69.

Goldich, S. S. and Marsden, R. W., 1956, Precambrian of Northeastern Minnesota; Field Trip No. 1, Geological Society of America Annual Meeting, Minneapolis, MN.

Hoffman, P. F., 1987, Early Proterozoic foredeeps, foredeep magmatism, and Superior-type iron-formations of the Canadian Shield, Krnner, A., ed., Proterozoic lithosoheric evolution : Geodynamic Series, American ~eo~h~kical union, v. 17, pp. 85-98.

Hoffman, P. F., 1988, Animikie Group: A Penokean Foredeep?, 34th Annual Institute on Lake Superior Geology, Marquette, MI, p. 40-41.

Hotchkiss, W. O., 1919, Geology of the Gogebic Range: Engineering and Mining Journal, vol. 108.

Irving, R. D. and Van Hise, 1892, The Penokee Iron-Bearing Series of Michigan and Wisconsin: U.S. Geological Survey Monograph 19, 534 p.

James, H. L., 1954, Sedimentary Facies of Iron-Formation: Economic Geology, vol. 49, p. 235-293.

James, H. L., 1955, zones of regional metamorphism in the Precambrian of northern Michigan: Geological Society of America ~ulletin, v. 66, p. 1455-1487.

Klasner, J. S. , Ojakangas, R. W., Schulz, K. J., and LaBerge, G. L., 1991, Nature and Style of Deformation in the Foreland of the Early Proterozoic Penokean Orogen, Northern Michigan: U.S.G.S. Bulletin 1904-K, 22 p.

LaBerge, G. L., 1964, Development of Magnetite in Iron-Formations of the Lake Superior Region: Economic Geology, vol. 59, p. 1313-1342.

LaBerge, G. L., l966a. Altered Pyroclastic Rocks in Iron- Formation Hamersley Range, Western Australia: Economic Geology, vol. 61, p. 147-161.

38

LaBerge, G. L., 1966b, Altered Pyroclastic Rocks in South AfricanIron-Formation: Economic Geology, vol. 61, p. 572-581.

Larue, D. K., 1981, The Chocolay Group, Lake Superior region,U.S.A.: sedimentological evidence for deposition in basinaland platform settings on an early Proterozoic craton:Geological Society of America Bulletin, vol. 92, p. 417-435.

Larue, D. K., 1983, Early Proterozoic tectonics of the LakeSuperior region: Tectonostratigraphic terranes near thepurported collision zone, in Medaris, L. G., Jr., EarlyProterozoic geology of the Great Lakes region: GeologicalSociety of merica Memoir 160, p. 33-47.

Mengel, J. T., Jr., 1963, The Cherts of the Lake Superior Iron-Bearing formations: Ph.D. Thesis, University of Wisconsin.

Morey, G. B., Sims, P. K., Cannon, W. F., Mudrey, M. G., Jr., andSouthwick, D. L., 1982, Geological Map of the Lake Superiorregion: Minnesota, Wisconsin, and Northern Michigan:Minnesota Geological Survey State Map Series S-13.

Ojakangas, R. W., 1983, Tidal deposits in the Early Proterozoicbasin of the Lake Superior region - the Palms and PokegamaFormations: Evidence for sub-tidal deposition of Superior-type banded iron-formation: j Early Proterozoic Geology ofthe Great Lakes Region (edited by L. G. Medaris), GeologicalSociety of America Memoir 160, p. 49-66.

Ojakangas, R. W., in press, Sedimentology and provenance of theEarly Proterozoic Michigaxnme Formation, Michigan: U.S.Geological Survey Bulletin 1904-IL

Ojakangas, R. W. and Matsch, C. L., 1982, Minnesota Geology,Univ. of Minnesota Press, Minneapolis, 255 p.

Peterman, Z. E., Zartman, R. E., and Sims, P. K., 1980, EarlyArchean tonalitic gneiss from northern Michigan, U.S.A., jMorey, G. B., and Hanson, G. N., Selected studies of Archeanand lower Proterozoic rocks, southern Canadian Shield:Geological Society of America Special Paper 182, p. 125-134.

Prinz, W. C., 1981, Geologic map of the Gogebic Range -Watersmeet area, Gogebic and Ontonogan Counties, Michigan:U.S. Geological Survey Miscellaneous Investigations SeriesMap 1-1365, scale 1:125,000.

Prinz, W. C., Gair, J. E., and Cannon, W. F., 1975, "Greenstone:Field Trip 2", 21st Annual Institute on Lake SuperiorGeology, Marquette, Michigan, pp. 41-85.

Schmidt, R. G., 1976, Geology of the Precambrian W (LowerPrecambrian) Rocks in Western Gogebic County, Michigan:U.S. Geological Survey Bulletin 1407.

39

LaBerge! G. L.! 1966b1 Altered Pyroclastic Rocks in South African Iron-Formation: Economic GeologyI vol. 611 p. 572-581.

Larue! D. K.! 19811 The Chocolay Group! Lake Superior region! U.S.A.: sedimentological evidence for deposition in basinal and platform settings on an early Proterozoic craton: Geological Society of America Bulletin! vol. 92! p. 417-435.

Larue! D. K.! 1983! Early Proterozoic tectonics of the Lake Superior region: Tectonostratigraphic terranes near the purported collision zone! Medaris! L. G e l Jr.! Early Proterozoic geology of the Great Lakes region: Geological Society of America Memoir 1601 p. 33-47.

Mengel! J. T.! Jr.! 1963! The Cherts of the Lake Superior Iron- Bearing formations: Ph.D. Thesis! University of Wisconsin.

Morey! G. B. Sims P. K. Cannon! W. F. Mudrey! M. G. Jr. and Southwickl D. L.! 1982! Geological Map of the Lake Superior region: Minnesota! Wisconsinl and Northern Michigan: Minnesota Geological Survey State Map Series S-13.

Ojakangas! R. W.! 1983! Tidal deposits in the Early Proterozoic basin of the Lake Superior region - the Palms and Pokegama Formations: Evidence for sub-tidal deposition of Superior- type banded iron-formation: in Early Proterozoic Geology of the Great Lakes Region (editerby L. G. Medaris) Geological Society of America Memoir 1601 p. 49-66.

Ojakangas! R. W.! in press! Sedimentology and provenance of the Early Proterozoic Michigamme Formationl Michigan: U.S. Geological Survey Bulletin 1904-R.

Ojakangas! R. W. and Matsch! C. L.! 1982! Minnesota Geologyl Univ. of Minnesota Press! Minneapolisl 255 p.

Peterman! Z. Eel Zartman! R. Eel and Sims! P. K.! 19801 Early Archean tonalitic gneiss from northern Michigan! U.S.A.! Morey! G. Ba1 and Hanson! G. N.! Selected studies of Archean and lower Proterozoic rocksl southern Canadian Shield: Geological Society of America Special Paper 182! p. 125-134.

Prinz! W. C.! 19811 Geologic map of the Gogebic Range - Watersmeet area! Gogebic and Ontonogan Countiesl Michigan: U.S. Geological Survey Miscellaneous Investigations Series Map I-1365! scale 1:1251000.

Prinz! W. C.! Gairl J. E.! andcannon! W. F.! 1975! I1Greenstone: Field Trip 2If1 21st Annual Institute on Lake Superior Geologyl Marquette! Michigan! pp. 41-85.

Schmidt! R. G e l 1976! Geology of the Precambrian W (Lower Precambrian) Rocks in Western Gogebic Countyl Michigan: U.S. Geological Survey Bulletin 1407.

40

Schmidt, R. G., 1980, The Marquette Range Supergroup in theGogebic Iron District, Michigan and Wisconsin: U.S.Geological Survey Bulletin 1460, 96 p.

Schmidt, R. G and Hubbard, H. A., 1972, Penokean orogeny in thecentral and western Gogebic region, Michigan and Wisconsin:Field Trip A, 19th Annual Institute on Lake SuperiorGeology, Houghton, MI, p. A1-A27.

Sims, P. K., 1990, Geologic map of Precambrian Rock, Marenisco,Thayer, and Watersmeet 15 Minute Quadrangles Gogebic andOntonagon Counties, Michigan and Vilas County, Wisconsin:U.S. Geological Survey Miscellaneous Investigation Map1-2093.

Sims, P. K., Peterman, Z. E., 1976, Geology and Rb-Sr ages ofreactivated Precambrian gneisses in the Marenisco-Watersmeetarea, Northern Michigan: Journal of Research, U.S.Geological Survey, vol. 4, pp. 405-414.

Sims, P. K., Peterman, Z. E., Prinz, W. C., and Benedict, F. C.,1984, Geology, geochemistry and age of Archean and EarlyProterozoic rocks in the Marenisco-Watersmeet area, northernMichigan: U.S. Geological Survey Prof. Paper 1292-A, 41 p.

Sims, P. K., Van Schmus, W. R., Schulz, K. J., and Peterman, Z.E., 1990, Tectonostratigraphic evolution of the EarlyProterozoic Wisconsin magmatic terranes of the PenokeanOrogen: Can. Jour. Earth Sci.

Southwick, D. L. and Morey, G. B., 1991, Tectonic imbrication andforedeep development in the Penokean orogen, east-centralMinnesota-An interpretation based on regional geophysics andresults of test drilling: U.S. Geological Survey Bulletin1904-C, 17 p.

Suszek, T. J., 1991, Petrography and sedimentation of the MiddleProterozoic (Keweenawan) Nonesuch Formation, Western LakeSuperior region, Midcontinent Rift System: unpublished M.S.thesis, University of Minnesota, Duluth, 198 p.

Trent, V. A., 1973, Geological Map of the Marenisco and WakefieldNE quadrangles, Gogebic County, Michigan: U.S. GeologicalSurvey Open File Report, scale 1:48,000.

Van Hise, C.R. and Leith, C. K., 1911, Geology of the LakeSuperior Region: U.S. Geological Survey Monograph 52.

Schmidtl R. G., 19801 The Marquette Range Supergroup in the Gogebic Iron District, Michigan and Wisconsin: U.S. Geological Survey Bulletin 14601 96 p.

Schmidtl R. G and Hubbardl H. A v 1 19721 Penokean orogeny in the central and western Gogebic regionl Michigan and Wisconsin: Field Trip Al 19th Annual Institute on Lake Superior Geologyl Houghtonl MIl p. Al-A27.

Simsl P. Ks1 19901 Geologic map of Precambrian Rockl Mareniscol Thayerl and Watersmeet 15 Minute Quadrangles Gogebic and Ontonagon Countiesl Michigan and Vilas Countyl Wisconsin: U.S. Geological Survey Miscellaneous Investigation Map 1-2093.

Simsl P. Ke1 Petermanl Z. Eel 1976# Geology and Rb-Sr ages of reactivated Precambrian gneisses in the Marenisco-Watersmeet areal Northern Michigan: Journal of Researchl U.S. Geological Surveyl vol. pp. 405-414.

Simsl P. Ka1 Petermanl Z. E e l Prinzl W. Ce1 and Benedictl F. Ce1 1984# Geology, geochemistry and age of Archean and Early Proterozoic rocks in the Marenisco-Watersmeet areal northern Michigan: U.S. Geological Survey Prof. Paper 1292-Al 41 p.

Simsl P. Ke1 Van Schmusl W. Schulzl K. J a 1 and Peterman# Z. E e l 19901 Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Can. Jour. Earth Sci.

Southwickl D. L. and MoreyI G. B a 1 19911 Tectonic imbrication and foredeep development in the Penokean orogenl east-central Minnesota-An interpretation based on regional geophysics and results of test drilling: U.S. Geological Survey Bulletin 1904-Cl 17 p.

Suszekl T. J a 1 1991, Petrography and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formationl Western Lake Superior region, Midcontinent Rift System: unpublished M.S. thesisl University of Minnesotal Duluthl 198 p.

Trentl V. A e 1 19731 Geological Map of the Marenisco and Wakefield NE quadranglesl Gogebic Countyl Michigan: U.S. Geological Survey Open File Reportl scale 1:481000.

Van Hisel C.R. and Leithl C. K a 1 19111 Geology of the Lake Superior Region: U.S. Geological Survey Monograph 52.

KEWEENAWAN

SEDIMENTARY ROCKOF THE

SOUTH SHORE,LAKE SUPERIOR

A.B. DickasDepartment of Geology,

University of Wisconsin-Superior

M.G. Mudrey, Jr.Wisconsin Geological and

Natural History Survey,Madison, Wisconsin

OF THE

A.B. Dickas Department of Geology,

University of Wisconsin-Superior

M.G. Mudrey, Jr. Wisconsin Geological and

Natural History Sumey, Madison, Wisconsin

INTRODUCTION

The sedimentary and igneous rock composing the Precambrian (Middle Proterozoic,Keweenawan) section of the Lake Superior stratigraphic column have had a long history ofstudy. Almost six decades ago (1933), the geology of the "Lake Superior Region," constitut-ing Excursion C-4, was presented as Guidebook 27 of the XVI International GeologicalCongress. The field guide for this excursion was prepared by W. 0. Hotchldss of the Michi-gan College of Mining and Technology (now Michigan Technological University) and waspurchased for the sum of 25 cents.

In his introduction to that guidebook, C. K. Leith (University of Wisconsin) stated:

The Lake Superior region has been of special interest to students of pre-Cambriangeology because it presents the largest and most varied pre-Cambrian succession thathas been definitely worked out. Its content of valuable iron and copper ores hasmade possible more intensive and detailed studies than have been accorded toextensive pre-Cambrian areas elsewhere. The pre-Cambrian succession now known(in the Lake Superior area) represents a greater thickness of sediments and a largertime than all the post-Cambrian of North America.

It is interesting to note how Charles Leith, in a later section of this guidebook, presented theKeweenawan sequence, now known as the Midcontinent Rift sequence:

Keweenawan: Next below (the Cambrian) is the nonfossiliferous Keweenawanseries, consisting of an immense mass, possibly 5 miles (8 km) thick, of sandstone,with intercalated shales and conglomerates, containing in its lower part large quanti-ties of extrusive lavas and intrusive laccoliths and sills. In degree of metamorphismit is more like the Cambrian than the underlying Huronian series. It has characteris-tic reddish, yellowish, and purplish colors and carries various evidences that it wasessentially a continental deposit under semiarid conditions. Its lower part is tilted inmaiked unconformity to the Cambrian, but its upper part lies nearly, if not quite,parallel to the Cambrian. Obviously, it was mainly deposited in an independentbasin before the incursion of the Upper Cambrian sea. Although the Keweenawan ispre-Cambrian in the sense of preceding the Upper Cambrian transgression, havingstructural and igneous affiliations with the pre-Cambrian, and being nonfossffiferous,it may be Cambrian in the sense that it was being formed at the same time as Middleand Lower Cambrian sediments in distant Cambrian seas.

Today as we gather for this 38th Institute on Lake Superior Geology fieldtrip, comparisonswith conditions in 1933 form distinct contrasts. In 1992, with a singular exception, thecopper mines are closed and the massive iron ore fields have fallen under different economicclimates. Yet, a new era of evaluation and exploration has begun. The Lake SuperiorKeweenawan section is widely recognized as one of the fmest extant examples of continentalrifting. The advent of industrial field evaluation of the hydrocarbon potential of these riftstrata, beginning in 1983, has directly and indirectly facilitated the collection of approxi-mately 4200 km of seismic reflection data in the Lake Superior district alone, in addition tosupportive magnetic and gravity surveys.

This new period of exploration within the Lake Superior region has altered our interpretation ofits geologic development, especially during Keweenawan time. Many of these updated geologic

43

The sedimentary and igneous rock composing the Precambrian (Middle Roterozoic, Keweenawan) section of the Lake Superior stratigraphic column have had a long history of study. Almost six decades ago (19331, the geology of the "Lake Superior Region," constitut- ing Excursion C-4, was presented as Guidebook 27 of the XVI International Geological Congress. The field guide for this excursion was prepared by W. 0. Hotchlciss of the Michi- gan College of Mining and Technology (now Michigan Technological University) and was purchased for the sum of 25 cents.

In his introduction to that guidebook, C. K. k i t h (University of Wisconsin) stated:

The Lake Superior q i o n has been of special interest to students of pre-Cambrian geology because it presents the largest and most varied pm-Cambrian succession that has been definitely worked out. Its content of valuable imn and copper ores has made possible more intensive and detailed studies than have been accorded to extensive pre-Cambrian areas elsewhere. The pre-Cambrian succession now known (in the Lake Superior area) represents a mater thickness of sediments and a larger time than all the post-Cambrian of North America.

It is interesting to note how Charles k i t h , in a later section of this guidebook, presented the Keweenawan sequence, now known as the Midcontinent Rift sequence:

Keweenawan: Next below (the Cambrian) is the nonfossilifemus Keweenawan series, consisting of an immense mass, possibly 5 miles (8 km) thick, of sandstone, with intercalated shales and conglomerates, containing in its lower part large quanti- ties of extrusive lavas and intrusive laccoliths and sills. In degree of metamorphism it is more like the Cambrian than the underlying Humnian series. It has characteris- tic reddish, yellowish, and purplish colors and cames various evidences that it was essentially a continental deposit under semiarid conditions. Its lower part is tilted in marked unconformity to the Cambrian, but its upper part lies nearly, if not quite, parallel to the Cambrian. Obviously, it was mainly deposited in an independent basin before the incursion of the Upper Cambrian sea. Although the Keweenawan is pre-Cambrian in the sense of preceding the Upper Cambrian transgression, having structural and igneous affiliations with the pre-Cambrian, and being nonfossiliferous, it may be Cambrian in the sense that it was being formed at the same time as Middle and Lower Cambrian sediments in distant Cambrian seas.

Today as we gather for this 38th Institute on Lake Superior Geology fieldtrip, comparisons with conditions in 1933 form distinct contrasts. In 1992, with a singular exception, the copper mines are closed and the massive iron ore fields have fallen under different economic climates. Yet, a new era of evaluation and exploration has begun. The Lake Superior Keweenawan section is widely recognized as one of the fmest extant examples of continental rifting. The advent of industrial field evaluation of the hydrocarbon potential of these rift strata, beginning in 1983, has directly and indhctly facilitated the collection of approxi- mately 4200 km of seismic reflection data in the Lake Superior dismct alone, in addition to supportive magnetic and gravity surveys.

This new period of exploration within the Lake Superior region has altered our interpretation of its geologic development, especially during Keweenawan time. m y of these updated geologic

43

44

relationships will be examined during the process of this field trip. Our principal focus will beon the geology and geologic history of the youngest Precambrian rock in the region, theKeweenawan. This rock is a remnant of major continental rifting, initiated circa 1100 Ma. Theresultant Midcontinent Rift System trends from the Lake Superior region to as far southwest ascentral Kansas and as far southeast as southern Ohio.

As of this writing (March 2, 1992) site preparation has been underway for several weeks forthe Terra/Patrick #7-22 wildcat borehole, a test of the hydrocarbon potential of the Wiscon-sin section of the Midcontinent rift of national significance and interest. Located in 22, 47N,6W, Keystone Township, Bayfield County, Wisconsin, this well will be operated by TerraEnergy, Ltd of Traverse City, Michigan, in partnership with Patrick Petroleum Company ofJackson, Michigan, under farmout arrangements with Amoco Production Company (USA),Houston, Texas. The target depth will be 6,000 feet, permitting a test of the oil or gaspotential of the Oronto Group sequence of sedimentary rocks. The structure is a large drag-fold anticline with north closure against the Douglas reverse fault. The surface expression ofthis structure extends over an area of approximately 60 square miles. Available informationon this state record depth wildcat will be included in the 38th Institute on Lake SuperiorGeology Proceedings as an abstract (Albert B. Dickas).

The site of the Terra/Patrick #7-22 borehole is approximately 6 miles north-northeast of anOronto Group wildcat test borehole announced in 1985 by Amoco Production Company (USA).This borehole, planned for 13, 46N,7W, and targeted for 15,000 feet, was never drilled foreconomic and legislative reasons.

relationships will be examined during the process of this field mp. Our principal focus will be on the geology and geologic history of the youngest Precambrian rock in the region, the Keweenawan. This rock is a remnant of major continental rifting, initiated circa 1100 Ma. The resultant Midcontinent Rift System trends from the Lake Superior region to as far southwest as central Kansas and as far southeast as southern Ohio.

As of this writing (March 2, 1992) site preparation has been underway for several weeks for the TerraPamck #7-22 wildcat borehole, a test of the hydrocarbon potential of the Wiscon- sin section of the Midcontinent rift of national significance and interest. Located in 22,47N, 6W, Keystone Township, Bayfield County, Wisconsin, this well will be operated by Terra Energy, Ltd of Traverse City, Michigan, in partnership with Patrick Petroleum Company of Jackson, Michigan, under farmout arrangements with Ammo Production Company (USA), Houston, Texas. The target depth will be 6,000 feet, permitting a test of the oil or gas potential of the Oronto Group sequence of sedimentary rocks. The structure is a large drag- fold anticline with north closure against the Douglas reverse fault. The surface expression of this structure extends over an area of approximately 60 square miles. Available information on this state record depth wildcat will be included in the 38th Institute on Lake Superior Geology Proceedings as an abstract (Albert B. Dickas).

The site of the TerraPatrick #7-22 borehole is approximately 6 miles north-northeast of an Oronto Group wildcat test W h o l e announced in 1985 by Ammo M u c t i o n Company (USA). This b h o l e , planned for 13,46N,7W, and targeted for 15,000 feet, was never drilled for economic and legislative reasons.

GEOLOGY OF THE MIDCONTINENT RIFT SYSTEM ALONG THE SOUTHSHORE OF LAKE SUPERIOR, WISCONSIN AND MICHIGAN

Reprinted with permission from Dickas, Albert B. (editor), 1989, Lake Superior basin segmentof the Midcontinent Rjft System: Field trip guidebook T344, 28th International GeologicalCongress, 62 pages.

Introduction

The Midcontinent Rift System has been mapped over an axial length of in excess of 3,300km. Portions of this Precambrian (Ca. 1.1 Ga) extensional structure are contained in the statesof Kansas, Nebraska, Iowa, Minnesota, Wisconsin, Michigan, and Ohio. This report presentsthe geology of this rift system within the geographic region generally defined as northwest-ern Wisconsin and the adjacent upper peninsula of Michigan.

Wisconsin Section

The Midcontinent Rift sequence of Precambrian sedimentary strata in the western LakeSuperior region constitutes some of the oldest known and studied such assemblages in theUnited states. These units were initially described by Hunt (1873), who named them theKeweenaw Group after the cupriferous strata exposed along the Keweenaw Peninsula ofMichigan. Thwaites (1912) divided this section into the Oronto (lower) and Bayfield (upper)groups (Fig. 1). More recently the U. S. and Ontario Geological surveys have been infor-mally using the term Keweenawan Supergroup.

The outcrop belt of the Oronto Group is traced from northwestern Wisconsin east and north-east to the tip of Keweenaw Point, Michigan. The Oronto Group crops out principally alongthe Ashland syncine (Fig. 2), which is centrally located on the St. Croix horst. The Ashlandsyncline is asymmetrical as the steeper dips are found within the southern limb. The horst isbounded by the reverse-throw Douglas and Isle Royale faults to the north and the LakeOwen and Keweenaw faults to the south. While seismic information suggests the presenceof Oronto components in the flanking basins, deep drilling has yet to confirm this conjecture.However, on the basis of a drag fold exposure of Oronto type strata (Freda Sandstone) inDouglas County, Wisconsin, White (1966) supported the concept of Oronto strata beingfound north of the Douglas fault. Alternately, White (1966) supported the possibility ofpost-Oronto (Bayfield Group) inliers being found within the horst position as suggested byheavy mineral analyses conducted by Tyler et al. (1940).

The Oronto Group is 40 percent sandstone and 60 percent shale (Thiel, 1956) and constitutesa maximum 6,100 m thick sequence of reddish conglomerate, sandstone, and mudstone. Itshows a general upward increase in maturity, from poorly sorted arkosic to quartzose, rela-tively well-sorted sedimentary rocks. Much of this group was locally derived througherosion of Middle Keweenawan basalts and transported only a short distance to its deposi-tional basin.

45

GEOLOGY OF THE MIDCONTINENT RIFT SYSTEM ALONG THE SOUTH SHORE OF LAKE SUPERIOR, WISCONSIN AND MICHIGAN

Reprinted with permission /rom Dickas, Albert B. (editor), 1989, Lake Superior basin segment of the Midcontinent Rift System: Field trip guidebook T344.28th International Geological Congress. 62 pages.

Introduction

The Midcontinent Rift System has been mapped over an axial length of in excess of 3,300 krn. Portions of this Precambrian (ca. 1.1 Ga) extensional structure are contained in the states of Kansas, Nebraska, Iowa, Minnesota, Wisconsin, Michigan, and Ohio. This report presents the geology of this rift system within the geographic region generally defined as northwest- em Wisconsin and the adjacent upper peninsula of Michigan.

Wisconsin Section

The Midcontinent Rift sequence of Precambrian sedimentary strata in the western Lake Superior region constitutes some of the oldest known and studied such assemblages in the United states. These units were initially described by Hunt (1873), who named them the Keweenaw Group after the cupriferous strata exposed along the Keweenaw Peninsula of Michigan. Thwaites (1912) divided this section into the Oronto (lower) and Bayfield (upper) groups (Fig. 1). More recently the U. S. and Ontario Geological surveys have been infor- mally using the term Keweenawan Supergroup.

The outcrop belt of the Oronto Group is traced from northwestern Wisconsin east and north- east to the tip of Keweenaw Point, Michigan. The Oronto Group crops out principally along the Ashland syncline (Fig. 2), which is centrally located on the St. Croix horst. The Ashland syncline is asymmetrical as the steeper dips are found within the southern limb. The horst is bounded by the reverse-throw Douglas and Isle Royale faults to the north and the Lake Owen and Keweenaw faults to the south. While seismic information suggests the presence of Oronto components in the flanking basins, deep drilling has yet to confirm this conjecture. However, on the basis of a drag fold exposure of Oronto type strata (Freda Sandstone) in Douglas County, Wisconsin, White (1966) supported the concept of Oronto strata being found north of the Douglas fault. Alternately, White (1966) supported the possibility of post-Oronto (Bayfield Group) inliers being found within the horst position as suggested by heavy mineral analyses conducted by Tyler et al. (1940).

The Oronto Group is 40 percent sandstone and 60 percent shale (Thiel, 1956) and constitutes a maximum 6,100 m thick sequence of reddish conglomerate, sandstone, and mudstone. It shows a general upward increase in maturity, from poorly sorted arkosic to quartzose, rela-. tively well-sorted sedimentary rocks. Much of this group was locally derived through erosion of Middle Keweenawan basalts and transported only a short distance to its deposi- tional basin.

46

The lowest unit of the Oronto Group, the Copper Harbor Conglomerate, conformably over-lies basement lavas and locally interfingers with them (White and Wright, 1960). Whileexposures can regionally exceed 1,800 m in thickness, as much as 610 m of this column maybe basic volcanics (Halls, 1966). Overall this unit is a fining-upward accumulation of brownto red, arkosic, pebble to boulder conglomerate, dominantly composed of mafic to felsicvolcanics, with lesser amounts of rather coarse, cross-bedded, graywacke sandstone (Daniels,1982). Within the sands, stratification is crude, shallow water flow regime structures arecommon and lateral correlations for any distance are limited. Bedding plane dips range fromten degrees to overturned. The depositional regime of the conglomerate facies reflects aprograding, alluvial fan complex (Daniels, 1982), while the sandstone units are more sugges-tive of shallow water. A localized "redder" arenaceous facies, found along the KeweenawPeninsula, has been classified as a flood plain or standing-water deposit by White and Wright(1960). The same environment is elsewhere locally supported by the limited presence ofalgal stromatolites in the vicinity of Copper Harbor, Michigan, the location of the typesection of this formation.

MuD—CO TI I'T 'I' I. I F' 'I'T I M 2 'I' I. A. 'I' I C R. .A. P II Y

1 — F'1a.n12 —orst3 — tJ. P e xii xisu. 1 a.

— i... pex1ins.iia.

FIGURE 1: Strati grap hic terminology and time relationships of Upper Keweenawan sedimentaryrocks along the Midcontinent Rft System trend. After Dickas (1986), reprinted by permission of theAmerican Association of Petroleum Geologists.

The lowest unit of the Oronto Group, the Copper Harbor Conglomerate, conformably over- lies basement lavas and locally interfingers with them (White and Wright, 1960). While exposures can regionally exceed 1,800 m in thickness, as much as 610 m of this column may be basic volcanics (Halls, 1966). Overall this unit is a fining-upward accumulation of brown to red, arkosic, pebble to boulder conglomerate, dominantly composed of mafic to felsic volcanics, with lesser amounts of rather coarse, cross-bedded, graywacke sandstone (Daniels, 1982). Within the sands, stratification is crude, shallow water flow regime structures are common and lateral correlations for any distance are limited. Bedding plane dips range from ten degrees to overturned. The depositional regime of the conglomerate facies reflects a prograding, alluvial fan complex (Daniels, 1982), while the sandstone units are more sugges- tive of shallow water. A localized "redder" arenaceous facies, found along the Keweenaw Peninsula, has been classified as a flood plain or standing-water deposit by White and Wright (1960). The same environment is elsewhere locally supported by the limited presence of algal stromatolites in the vicinity of Copper Harbor, Michigan, the location of the type section of this formation.

1 p i a n i t MID-CONTINENT R I F'T z - s o r s t T I M E S T R A T I G R A F H Y 3-u.r'eninsula

4 - L peninsula - -- --

Iowa I Minn I Wisc 1 Mich

1 1 Freda. 1 Freda S s s s . .

a! None- None- Q Fm Such Such f f l

- , - , , Tnor > St croik chenkwi. l=>p rt age >

d r 7 d 7 Â ¥ 7 v , . ," ' I^^ V < " ^ 2 4 I-. , -Val s 7 > V-01 s i , ̂ >tans_> < Lake r , V A "'I- A < I v 7 \ ' A . < > J 7 < L , , 7 , 4 v i " >"4 ." > < - ' < < A VOlS',

+ * VO-1 s L

FIGURE 1: Stratigraphic terminology and time relationships of Upper Keweenawan sedimentary rocks along the Midcontinent Rift System trend. After Dickas (1986), reprinted by permission of the American Association of Petroleum Geologists.

In conformable and interfingering contact with the Copper Harbor Conglomerate is theNonesuch Formation, a hydrocarbon source rock Dickas, 1991) and the center of industrialinterest focused on the Midcontinent rift since the early 1980's. These strata are gray toblack, easily distinguishable from enveloping red to brown formations. The grain size rangesfrom medium sand to clay with shale subordinate to siltstone and sandstone. This unit is thethinnest of the Oronto Group, ranging from at least 140 m in northern Wisconsin to a maxi-mum 200 m in adjacent Michigan. Framework composition emphasizes mafic rock frag-ments, with analysis showing ranges of 15 to 30 percent (Hite, 1968). Of economic interestis the cupriferous nature of the basal 10 m of the Nonesuch Formation, especially in theWhite Pine, Michigan area where an estimated 11 million tones of copper have been found(Cox et aL, 1973), predominantly in the form of chalcocite and native copper. The variety ofsedimentary structures found in this formation have locally been employed as supportingevidences for a variety of depositional environments. The best model appears to be that ofreducing conditions within a standing body of water (Hubbard, 1975), distinguished byfrequent variations in water depths (Daniels, 1982), influenced by fluvial and deltaic associa-tions (Barghoorn et al., 1965), and marked by periodic salinity levels exceeding that ofgypsum precipitation. Recently, however, Hieshima and Pratt (1991) offer evidence sup-porting a marine embayment environment.

The copper is found in direct lithologic association with finely disseminated carbonaceousmatter which averages 0.5% of the formation by volume. Kelly and Nishioka (1985) believecopper precipitation was directly facilitated by the intimate presence of organic material.The organic matter, upon chemical extraction, shows remarkable structural features that havebeen preserved by a bituminization, rather than a carbonization, process. This material isfound both as matrix masses and as recognizable remains, such as bacteria cells, algal-likeunits and fungal hyphae, with volume distribution being concentrated in the finest grainedstrata. Overall, individual laminae are alternately rich and poor in organic components.Moore et al. (1969), after thin-section analysis of the organic rich shale layers, concludedthese units are characteristic of lake, swamp and tidal flat environments and that the majorityof preserved organic material developed in place.

The cupriferous zone is also of interest because, at the White Pine mine, associated vugularporosity and fractures often are filled with small amounts of solid and liquid hydrocarbons.Analysis by Barghoorn et al. (1965) indicates this Nonesuch crude oil contains higher con-centrations of alkanes and lower percentages of aromatic s than an average crude oil, andresembles the paraffinic crude commonly associated with Paleozoic production in the state ofPennsylvania. Further analysis has indicated the oil is indigenous to its host rock (Eglintonet aL, 1964), is of organic origin (Moore et al., 1969), and was formed under mild forma-tional temperature conditions (Barghoorn et a!., 1965). A low thermal environment is alsosupported by Brown (1971), who reported the presence of pink bornite and djurleite in theNonesuch Formation, minerals which cannot exist above respective ranges of 75 degree Cand 95 degree C (167 F and 203 F).

The Nonesuch crude presently represents one of the oldest known liquid hydrocarbon, beingapproximately 1,046 million years in age. This was determined for the Nonesuch host byusing rubidium-strontium ratios. This date is verified by the work of Ruiz et a!. (1984), whoobtained an age of 1,047 +1-35 million years by Rb-Sr dating of calcite filled veins cutting

47

In conformable and interfingering contact with the Copper Harbor Conglomerate is the Nonesuch Formation, a hydrocarbon source rock Dickas, 199 1) and the center of industrial interest focused on the Midcontinent rift since the early 1980's. These strata are gray to black, easily distinguishable from enveloping red to brown formations. The grain size ranges from medium sand to clay with shale subordinate to siltstone and sandstone. This unit is the thinnest of the Oronto Group, ranging from at least 140 m in northern Wisconsin to a maxi- mum 200 m in adjacent Michigan. Framework composition emphasizes mafic rock frag- ments, with analysis showing ranges of 15 to 30 percent (Hite, 1968). Of economic interest is the cupriferous nature of the basal 10 m of the Nonesuch Formation, especially in the White Pine, Michigan area where an estimated 11 million tones of copper have been found (Cox et al., 1973), predominantly in the form of chalcocite and native copper. The variety of sedimentary structures found in this formation have locally been employed as supporting evidences for a variety of depositional environments. The best model appears to be that of reducing conditions within a standing body of water (Hubbard, 1975), distinguished by frequent variations in water depths (Daniels, 1982), influenced by fluvial and deltaic associa- tions (Barghoom et al., 1965), and marked by periodic salinity levels exceeding that of gypsum precipitation. Recently, however, Hieshima and Pratt (1991) offer evidence sup- porting a marine embayment environment.

The copper is found in direct lithologic association with finely disseminated carbonaceous matter which averages 0.5% of the formation by volume. Kelly and Nishioka (1985) believe copper precipitation was directly facilitated by the intimate presence of organic material. The organic matter, upon chemical extraction, shows remarkable structural features that have been preserved by a bituminization, rather than a carbonization, process. This material is found both as matrix masses and as recognizable remains, such as bacteria cells, algal-like units and fungal hyphae, with volume distribution being concentrated in the finest grained strata. Overall, individual laminae are alternately rich and poor in organic components. Moore et al. (1969), after thin-section analysis of the organic rich shale layers, concluded these units arc characteristic of lake, swamp and tidal flat environments and that the majority of preserved organic material developed in place.

The cupriferous zone is also of interest because, at the White Pine mine, associated vugular porosity and fractures often arc filled with small amounts of solid and liquid hydrocarbons. Analysis by Barghoom et al. (1965) indicates this Nonesuch crude oil contains higher con- centrations of alkanes and lower percentages of aromatics than an average crude oil, and resembles the paraffinic crude commonly associated with Paleozoic production in the state of Pennsylvania. Further analysis has indicated the oil is indigenous to its host rock (Eglinton et al., 1964), is of organic origin (Moore et al., 1969), and was formed under mild forma- tional temperature conditions (Barghoom et al., 1965). A low thermal environment is also supported by Brown (197 I), who reported the presence of pink bornite and djurleite in the Nonesuch Formation, minerals which cannot exist above respective ranges of 75 degree C and 95 degree C (1 67 F and 203 F).

The Nonesuch crude presently represents one of the oldest known liquid hydrocarbon, being approximately 1,046 million years in age. This was determined for the Nonesuch host by using rubidium-strontium ratios. This date is verified by the work of Ruiz et al. (1984), who obtained an age of 1,047 +I-35 million years by Rb-Sr dating of calcite filled veins cutting

48

across the cupriferous shale of the Nonesuch Formation in the White Pine area. As thiscalcite contains primary liquid oil inclusions, Kelly and Nishioka (1985) refer to this date asthe time of oil entrapment and thus consider it as a minimum age for White Pine hydrocar-bon.

The upper unit of the Oronto Group, the Freda Sandstone, lies conformably over the None-such Formation. The nature of the Freda Sandstone upper contact is not known for nowhereis this formation found in exposed (Myers, 1971) or drilled contact with any unit of theBayfield Group or its Michigan correlative, the Jacobsville Sandstone (Fig. 1). While listedby Hite (1968) as exceeding 3,660 m in thickness, only 1,500 m is found on the north coastof the Keweenaw Peninsula (Halls, 1966), near the type section at Freda, Michigan. TheFreda Sandstone is composed of basal, subordinate conglomerate fming-upward to sandstoneand siltstone. In color, a repetition of the Copper Harbor red to brown spectrum is observed,interrupted by spotty and laminar unoxidized zones (Daniels, 1982). With an analyzed maturityindex of 0.63, a feldspar range of 10 to 24 percent (Hite, 1968), and the grain shape beingangular to sub-rounded, these sandstone units are classed as immature. Daniels (1982) considersthe average sand unit to be a fme-grained feldspathic lithic arenite. Sedimentary structuresinclude cross-bedding, mud-cracks, graded beds, ripple marks, and a wide variation inpaleocurrent indicators. The implied depositional environment ranges from fluvial (Daniels,1982) to fluvial plain or tidal flat (Hamblin, 1961).

Along the eastern shore of Lake Superior, in the Pt. Mamainse region of Ontario, a 60 msection of gray sandstone is found unconformably overlying Keweenawan basalts. Work byHamblin (1961) and Annels (1973) allows correlation of these strata, termed the Mica Baysandstone, to the Freda sandstone and suggests a fluvial environment of deposition.

Overall the Oronto Group can be viewed as a thick sequence of clastic sedimentary rocksthat are stratigraphically unified by similar heavy mineral suites, an upward and distal de-creasing in grain size, and an upward increase in lithologic maturity. The three formationsof this group were derived from a common source of Keweenawan igneous material (Tyleret at., 1940). While each formation is interpreted in light of slightly different depositionalenvironments, Elmore and Daniels (1980) view the group environment as a trangressive-regressive alluvial fan, lacustrine and fluvial system that filled the developing MidcontinentRift basin during the final stages, and after cessation, of volcanic activities.

The Bayfield Group, wherever studied, is found to be in distinct contrast to the OrontoGroup. The Bayfield sequence displays a higher degree of compositional maturity andstructurally the bedding is more commonly subhorizontal. In terms of heavy minerals thetourmaline-zircon to epidote ratio is reversed in the Bayfield Group as compared to the Fredaand Nonesuch assemblages (Tyler et a!., 1940). There is a question as to the type of contactbetween these groups. Supporting an unconformable contact is the reported change inpaleomagnetic pole position (Du Bois, 1962), the change in accessory minerals (Tyler et a!.,1940) and the immediate change from steep Oronto dips to low Bayfleld dips (Morey andOjakangas, 1982). In support of a conformable contact, or one of minimal hiatus, is the lackof clasts of Oronto age in the basal Orienta sandstone (Thwaites, 1912), and the nature of thepreviously mentioned Freda Sandstone drag fold exposure in Douglas County, Wisconsin(Tyler et at., 1940). From a tectonic point of view, Morey and Ojakangas (1982) state the

across the cupriferous shale of the Nonesuch Formation in the White Pine area. As this calcite contains primary liquid oil inclusions, Kelly and Nishioka (1985) refer to this date as the time of oil entrapment and thus consider it as a minimum age for White Pine hydrocar- bon.

The upper unit of the Oronto Group, the Freda Sandstone, lies conformably over the None- such Formation. The nature of the Freda Sandstone upper contact is not known for nowhere is this formation found in exposed (Myers, 1971) or drilled contact with any unit of the Bayfield Group or its Michigan correlative, the Jacobsville Sandstone (Fig. 1). While listed by Hite (1968) as exceeding 3,660 m in thickness, only 1,500 m is found on the north coast of the Keweenaw Peninsula (Halls, 1966), near the type section at Freda, Michigan. The Freda Sandstone is composed of basal, subordinate conglomerate fining-upward to sandstone and siltstone. In color, a repetition of the Copper Harbor red to brown spectrum is observed, interrupted by spotty and laminar unoxidized zones (Daniels, 1982). With an analyzed maturity index of 0.63, a feldspar range of 10 to 24 percent (Hite, 1968), and the grain shape being angular to sub-rounded, these sandstone units arc classed as immature. Daniels (1982) considers the average sand unit to be a fine-grained feldspathic lithic arcnite. Sedimentary structures include cross-bedding, mud-cracks, graded beds, ripple marks, and a wide variation in paleocurrent indicators. The implied depositional environment ranges from fluvial (Daniels, 1982) to fluvial plain or tidal flat (Hamblin, 1961).

Along the eastern shore of Lake Superior, in the Pt. Mamainse region of Ontario, a 60 m section of gray sandstone is found unconformably overlying Keweenawan basalts. Work by Hamblin (1961) and Annels (1973) allows correlation of these strata, termed the Mica Bay sandstone, to the Freda sandstone and suggests a fluvial environment of deposition.

Overall the Oronto Group can be viewed as a thick sequence of clastic sedimentary rocks that are stratigraphically unified by similar heavy mineral suites, an upward and distal de- creasing in grain size, and an upward increase in lithologic maturity. The three formations of this group were derived from a common source of Keweenawan igneous material (Tyler et al., 1940). While each formation is interpreted in light of slightly different depositional environments, Elmore and Daniels (1980) view the group environment as a trangressive- regressive alluvial fan, lacustrine and fluvial system that filled the developing Midcontinent Rift basin during the final stages, and after cessation, of volcanic activities.

The Bayfield Group, wherever studied, is found to be in distinct contrast to the Oronto Group. The Bayfield sequence displays a higher degree of compositional maturity and structurally the bedding is more commonly subhorizontal. In terms of heavy minerals the tourmaline-zircon to epidote ratio is reversed in the Bayfield Group as compared to the Freda and Nonesuch assemblages (Tyler et al., 1940). There is a question as to the type of contact between these groups. Supporting an unconformable contact is the reported change in paleomagnetic pole position (Du Bois, 1962), the change in accessory minerals (Tyler et al., 1940) and the immediate change from steep Oronto dips to low Bayfield dips (Morey and Ojakangas, 1982). In support of a conformable contact, or one of minimal hiatus, is the lack of clasts of Oronto age in the basal Orienta sandstone (Thwaites, 1912), and the nature of the previously mentioned Freda Sandstone drag fold exposure in Douglas County, Wisconsin (Tyler et al., 1940). From a tectonic point of view, Morey and Ojakangas (1982) state the

FIGURE 2: Midcontinent Rift System trend and geology from northwestern Minnesota into theUpper Peninsula of Michigan. Projection of Keweenawan geology under western Lake Superior isshown by hachured patterns. From Dickas (1986), reprinted by permission of the AmericanAssociation of Petroleum Geologists.

change from Oronto to Bayfield sedimentation marks a transition from an extensional regimeto one characterized by vertical tectonic processes, i.e., the activation of the St. Croix horstby uplifting of 4,600 m (Thiel, 1956).

The type section for this group is along the southwestern shore of Lake Superior from theApostle Islands to just east of Superior, Wisconsin. Reported group thicknesses range from aminimal 815 m in outcrop (Craddock, 1972), to a maximum 2,100 m in the subsurface assuggested by seismic review (Mooney et al., 1970).

Because of the relative uniformity in composition, the three Bayfield sandstones can be de-scribed as a unit. Thiel (1956) considered the overall composition to be 99% sandstone and 1%shale. Peirographic studIed by Myers (1971) showed that quartz constitutes approximately 80%of framework grains. The Orienta and Chequamegon sandstones and siltstones are commonlyred in color and feldspathic in mineralogy and contain layers of shale and conglomerate. TheDevil's Island sandstone is buff to white in color and so quartzose as to be classifiedas analmost pure orthoquartzite. An ascending fluvial-lacustrine-fluvial environment of deposition isdocumented by respectively, trough cross-bedding, ripple marks, and a return to cross-beddingin the Chequarnagon Sandstone (Morey and Ojakangas, 1982). Myers (1971), states the

49

04

Isle Royale J•)Isle .o3rale1/

ayfie 1 da.sixi

TI1iel P'a.u.lt

3rx1clir1e

Jacosiile a.si-MI ICe-we eu aw P'a.u. it

I_i. Qre n P'a.tiit

______

a.s a. it

..slilaxid Sy-ricilne L1 Cla.st].os-Iiv-er F'ails $yucline l ] Ga.bbro

RIFT: NORTHERN SECTOR OKm5O

OMi 25

.Superior

Basalt

sfiland Sync line

u

IFT : N O R T H E R N SECT-? o Mi 25

FIGURE 2: Midcontinent Rifi System trend and geology fiom northwestern Minnesota into the Upper Peninsula ofMichigan. Projection of Keweenawan geology under western Lde Superwr is shown by hachured patterns. From D i c h (19861, reprinted by permission of the American Association of Petroleum Geologists.

change from Oronto to Bayfield sedimentation marks a transition fkom an extensional regime to one characterized by vertical tectonic pracessesy i.eaY the activation of the St. Croix horst by uplifting of 4'600 m (Thiely 1956).

The type section for this p u p is along the southwestern shore of Lake Superior from the Apostle Islands to just east of Superiory Wisconsin. Reported group thicknesses range fiom a minimal 815 m in outcrop (Craddocky 1972)' to a maximum 2y100 m in the subsurface as suggested by seismic review (Mmney et al.' 1970).

Because of the relative uniformity in composition, the three Bayfield sandstones can be de- scribed as a unit. Thiel(l956) considered the overall composition to be 99% sandstone and 1% shale. Petrographic studied by Myers (1971) showed that q m constitutes approximately 80% of framework grains. The Orienta and Chequamegon sandstones and siltstones are commonly d in color and feldspathic in mineralogy and contain layers of shale and conglomerate. The Devil's Island sandstone is buff to white in color and so quartzose as to be classified as an almost p m o r thoqd te . An ascending fluvial-lacustrine-fluvial environment of deposition is documented by respectivelyy trough mss-bedding' ripple marks' and a return to mss-bedding in the Chequamagon Sandstone (Morey and Ojakangas, 1982). Myers (1971)' states the

50

Bayfleld Group sedimentary rocks were derived mainly from older Keweenawan sediments andprobably represent the reworking of Oronto Group units. Ojakangas and Morey (1982), how-ever, believe the Bayfield Group clastics were derived from an older granitic terrane.

West of the Thiel fault (Fig. 2), the basic structure of Lake Superior is a continuation ofnorthwestern Wisconsin Midcontinent Rift geology. The Ashland syncline is projectedoffshore as the Lake Superior syncline. Evidence is lent by northwest dips of Oronto Groupsedimentary rocks along the Keweenaw Peninsula (Daniels, 1982) and the southeast dips insedimentary rock of the same age which outcrop on the southwest shore of Isle Royale(Wolff and Huber, 1973). This major syncline was later disrupted by reverse faulting,forming the offshore segment of the St. Croix horst (Fig. 2). Both the Bayfield basin and theSt. Croix horst offshore extension are considered to be floored by Middle Keweenawanclastics of the Oronto and/or Bayfield groups (Green, 1982). Thus, the western portion ofLake Superior is generally portrayed to be immediately underlain by KeweenawanSupergroup clastics up to 10 km in thickness (Ilinze et al., 1982).

Michigan Section

Keweenawan-age outcrops form the entirety of the Keweenaw Peninsula jutting into south-central Lake Superior (Fig. 2). The keel of this peninsula is formed of Portage Lakevolcanics, upon which all Upper Keweenawan sedimentary rocks in the area rest (Fig. 1).Those outcrops north and northwest of the Portage Lake Volcanics form the classic OrontoGroup. South and southeast of the Portage Lake volcanic outcrop belt and in contact with theKeweenaw fault is found a thick sequence of northerly dipping clastics named theJacobsville Sandstone by Lane and Seaman (1907) from outcrops near Jacobsville, locatednear the southeast base of the Keweenaw Peninsula. By geologic position these outcropsappear to be an easterly extension of the River Falls syncline. However, because the RiverFalls syncline and the Jacobsvile sandstone outcrops are not in direct contact due to theexposure of intervening Archean granitoid rocks, this area of Jacobsville sandstone deposition isherein referred to as the Jacobsville basin (Fig. 2).

In this basin the Jacobsville sandstone has a maximum thickness of 867 m by drill holemeasurement (Kalliokoski, 1982), but on the Keweenaw Peninsula proper its maximumthickness has been determined by geophysics to be approximately 3,000 m. The formation ispredominately quartzose and is composed of conglomerates, sandstones, siltstones, andshales, generally found in repetitive upward-fining sequences. The conglomerates aregenerally basal, up to 100 m in thickness and contain clasts composed of iron formation, veinquartz, volcanics, and metamorphic rocks. The sandstones are fine to coarse grained, lightreddish to purple to mottled cream-white in color and contain structures ranging from cross-bedding to oscillation and current ripple-marking. Shales and mudstones appear to be in theminority in terms of thickness and are found interbedded with laminated sands. The overallsection varies from sub-arkose to a quartz arenite, with plagioclase found more commonly inthe younger layers (Kalliokoski, 1982).

The Jacobsville sandstone was derived from highlands situated to the southwest and south-east of the depositional basin. The determined environments of deposition include fluvial,

Bayfield Group sedimentary rocks were derived mainly from older Keweenawan sediments and probably represent the reworking of Omnto Group units. O j h g a s and Morey (19821, how- ever, believe the Bayfleld Group clastics we= derived from an older granitic terrane.

West of the Thiel fault (Fig. 21, the basic structure of Lake Superior is a continuation of northwestern Wisconsin Midcontinent Rift geology. The Ashland syncline is projected offshore as the Lake Superior syncline. Evidence is lent by northwest dips of Oronto Group sedimentary rocks along the Keweenaw Peninsula (Daniels, 1982) and the southeast dips in sedimentary rock of the same age which outcrop on the southwest shore of Isle Royale (Wolff and Huber, 1973). This major syncline was later disrupted by reverse faulting, forming the offshore segment of the St. Croix horst (Fig. 2). Both the Ba*eld basin and the St. Croix horst offshore extension are considered to be floored by Middle Keweenawan clastics of the Oronto and/or Bayfield groups (Green, 1982). Thus, the western portion of Lake Superior is generally portrayed to be immediately underlain by Keweenawan Supergroup clastics up to 10 km in thickness (Hinze et al., 1982).

Michigan Section

Keweenawan-age outcrops form the entirety of the Keweenaw Peninsula jutting into south- central Lake Superior (Fig. 2). The keel of this peninsula is formed of Portage Lake volcanics, upon which all Upper Keweenawan sedimenmy rocks in the area rest (Fig. 1). Those outcrops north and northwest of the Portage Lake Volcanics form the classic Oronto Group. South and southeast of the Portage Lake volcanic outcrop belt and in contact with the Keweenaw fault is found a thick sequence of northerly dipping clastics named the Jacobsville Sandstone by Lane and Seaman (1907) from outcrops near Jacobsville, located near the southeast base of the Keweenaw Peninsula. By geologic position these outcrops appear to be an easterly extension of the River Falls syncline. However, because the River Falls syncline and the Jacobsville sandstone outcrops axt not in direct contact due to the exposure of intervening Archean granitoid rocks, this area of Jacobsville sandstone deposition is herein referred to as the Jacobsville basin (Fig. 2).

In this basin the Jacobsville sandstone has a maximum thickness of 867 m by drill hole measurement (Kalliokoski, 19821, but on the Keweenaw Peninsula proper its maximum thickness has been determined by geophysics to be approximately 3,000 m. The formation is p~dominately quartzose and is composed of conglomerates, sandstones, siltstones, and shales, generally found in repetitive upward-fining sequences. The conglomerates are generally basal, up to 100 m in thickness and contain clasts composed of iron formation, vein quartz, volcanics, and metamorphic rocks. The sandstones are fine to coarse grained, light reddish to purple to mottled cream-white in color and contain structures ranging from cross- bedding to oscillation and current ripple-marking. Shales and mudstones appear to be in the minority in terms of thickness and are found interbedded with laminated sands. The overall section varies from sub-arkose to a quartz arenite, with plagioclase found more commonly in the younger layers (Kalliokoski, 1982).

The Jacobsville sandstone was derived from highlands situated to the southwest and south- east of the depositional basin. The determined environments of deposition include fluvial,

lacustrine, and alluvial fans. Because the Jacobsville sandstone is nowhere found in directcontact with either the Oronto or Bayfield Group formations, its exact relationship within theKeweenawan Supergroup is in some doubt. As it overlies Portage Lake volcanics andunderlies the Late Cambrian Munising sandstone there seems little doubt regarding itsKeweenawan age. Considering that Tyler et al. (1940) noted a strong similarity between theJacobsvile Sandstone and the Orienta sandstone heavy mineral suites and that Kalliokoski(1982) related the Jacobsville Sandstone to the Fond du Lac Formation of Minnesota on thebasis of sediment, mineral, and clay matrix comparison, most workers today consider theJacobsville Sandstone to be a Michigan equivalent to the Bayfield Group of neighboringWisconsin (Fig. 1).

References

Annells, R. N., 1973, Proterozoic flood basalts of eastern Lake Superior; the Keweenawanvolcanic rocks of the Mamainse Point area, Ontario: Geological Survey of Canada Paper72-10, 51 p.

Barghoorn, E. S., W. G. Meinschein and J. W. Schopf, 1965, Paleobiology of a Precambrianshale: Science, v. 148, p. 461-472.

Brown, A. C., 1971, Zoning in the White Pine copper deposit, Ontonagon County, Michigan:Economic Geology, v. 66, p. 543-573.

Cox, D. P., R. G. Schmidt, J. D. Vine, H. Kirkemo, E. B. Tourtelot and M. Fleisher, 1973,Copper, in Brobst, D. A. and W. P. Pratt (eds.), United States mineral resources: U. S.Geological Survey Professional Paper 820, p. 163-195.

Craddock, C., 1972, Regional geologic setting, in Sims, P. K. and G. B. Morey (eds.):Geology of Minnesota: A centennial volume, Minnesota Geological Survey, p. 281-291.

Daniels, P. A., 1982, Upper Precambrian sedimentary rocks, Oronto Group, Michigan-Wisconsin, in Wold, R. J., and W. J. Hinze (eds.), Geology and tectonics of the LakeSuperior basin: Geological Society of America Memoir 156, p. 107-133.

Dickas, A. B., 1987, Hydrocarbon potential of the Midcontinent Rift System, proceedings ofMidcontinent Rift System Scientific Drilling Workshop, Duluth, Minn, Sept. 24-25,1987, p. 22-27.

DuBois, P. M., 1962, Paleomagnetism and correlation of Keweenawan rocks: GeologicalSurvey of Canada Bulletin, v. 71, 75 p.

Eglinton, G., P. M. Scott, T. Beisky, A. L. Burlengame and M. Calvin, 1964, Hydrocarbonsof biological origin from a one-billion year old sediment: Science, v. 145, p. 263-264.

Elmore, R. D. and P. A. Daniels, Jr., 1980, Depositional system model for UpperKeweenawan Oronto Group sediments, Northern Peninsula, Michigan (abs): Eos, v. 61,

p.1195. 51

lacustrine, and alluvial fans. Because the Jacobsville sandstone is nowhere found in d k c t contact with either the Oronto or Bayfield Group formations, its exact relationship within the Keweenawan Supergroup is in some doubt. As it overlies Portage Lake volcanics and underlies the Late Cambrian Munising sandstone there seems little doubt regarding its Keweenawan age. Considering that Tyler et al. (1940) noted a strong similarity between the Jacobsville Sandstone and the Orienta sandstone heavy mineral suites and that Kalliokoski (1982) related the Jacobsville Sandstone to the Fond du Lac Formation of Minnesota on the basis of sediment, mineral, and clay matrix comparison, most workers today consider the Jacobsville Sandstone to be a Michigan equivalent to the Bayfield Group of neighboring Wisconsin (Fig. 1).

References

Annells, R. N., 1973, Proterozoic flood basalts of eastern Lake Superior the Keweenawan volcanic rocks of the Mamainse Point area, Ontario: Geological Survey of Canada Paper 72- 10,5 1 p.

Barghmm, E. S., W. G. Meinschein and J. W. Schopf, 1965, Paleobiology of a bcambrian shale: Science, v. 148, p. 461-472.

Brown, A. C., 1971, Zoning in the White Pine copper deposit, Ontonagon County, Michigan: Economic Geology, v. 66, p. 543-573.

Cox, D. P., R. G. Schmidt, J. D. Vine, H. Kirkemo, E. B. Tomelot and M. Fleisher, 1973, Copper, in Brobst, D. A. and W. P. Pratt (eds.), United States mineral resources: U. S. Geological Survey hfessional Paper 820, p. 163-195.

Craddock, C., 1972, Regional geologic setting, in Sims, P. K. and G. B. Morey (eds.): Geology of Minnesota: A centennial volume, Minnesota Geological Survey, p. 28 1-29 1.

Daniels, P. A., 1982, Upper Precambrian sedimentary rocks, Oronto Group, Michigan- Wisconsin, in Wold, R. J., and W. J. Hinze (eds.), Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 107-133.

Dickas, A. B., 1987, Hydrocarbon potential of the Midcontinent Rift System, proceedings of Midcontinent Rift System Scientific Drilling Workshop, Duluth, Minn, Sept. 24-25, 1987, p. 22-27.

DuBois, P. M., 1962, Paleomagnetism and correlation of Keweenawan rocks: Geological Survey of Canada Bulletin, v. 7 1,75 p.

Eglinton, G., P. M. Scott, T. Belsky, A. L. Burlengame and M. Calvin, 1964, Hydrocarbons of biological origin from a one-billion year old sediment: Science, v. 145, p. 263-264.

Elmore, R. D. and P. A. Daniels, Jr., 1980, Depositional system model for Upper Keweenawan Oronto Group sediments, Northern Peninsula, Michigan (abs): Eos, v. 61, p. 1195. 5 1

Green, J. C., 1982, Geology of the Keweenawan extrusive rocks, in Wold, R. J. and W. J.Hinze (eds.), Geology and tectonics of the Lake Superior region: Geological Society ofAmerica Memoir 156, p. 47-55.

Halls, H. C., 1966, A review of the Keweenawan geology of the Lake Superior region, in J.S. Steinhart and T. J. Smith (eds.), The Earth beneath the continents: American Geo-physical Union Geophysical Monograph 10, p. 5-27.

Hamblin, W. K., 1961, Micro-cross-lamination in Upper Keweenawan sediments of northernMichigan: Journal of Sedimentary Petrology, v. 31, p. 390-401.

Hieshima, 0. B. and L. M. Pratt, 1991, Sulfur/carbon ratios and extractable organic matterof the Middle Proterozoic Nonesuch Formation, North American Midcontinent Rift:Precambrian Research, v. 54, p. 65-79.

Hinze, W. J., R. J. Wold and N. W. O'Hara, 1982, Gravity and magnetic anomaly study ofLake Superior, in Wold, R. J. and W. J. Hinze (eds.), Geology and tectonics of the LakeSuperior basin: Geological Society of America Memoir 156, p. 203-22 1.

Hite, D. M., 1968, Sedimentology of the Upper Keweenawan sequence of northern Wiscon-sin and adjacent Michigan: Ph. D. dissertation, University of Wisconsin-Madison, 217 p.

Hubbard, H. A., 1975, Geology of Porcupine Mountains in Carp River and White Pinequadrangles, Michigan: U. S. Geological Survey Journal of Research, v. 3, p. 519-528.

Hunt, T. S., 1873, The geognostical history of metals: American Institute of Mining andEngineering Transactions, v. 1, P. 33 1-345.

Kalliokoski, J., 1982, Jacobsville Sandstone, in Wold, R. J. and W. J. Hinze (eds.), Geologyand tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p.147-155.

Kelly, W. C., and 0. K. Nishioka, 1985, Precambrian oil inclusions in late veins and the roleof hydrocarbons on copper mineralization at White Pine, Michigan: Geology, v. 13, p.334-337.

Lane, A. C. and A. E. Seaman, 1907, Notes on the geological section of Michigan, part 1, the Pre-Ordovician: Journal of Geology, v. 15, p. 680-695.

Mooney, H. M., P. R. Famham. S. H. Johnson, G. Volz and C. Craddock, 1970, Seismic studiesover the Midcontinent Gravity High in Minnesota and northwestern Wisconsin: MinnesotaGeological Survey Report of Investigations 11, 191 p.

Moore, L. R., J. R. Moore and E. Spinner, 1969, A geomicrobiological study of the PrecambrianNonesuch Shale: Proceedings of the Yorkshire Geological Society, v. 37, p. 35 1-394.

Morey, G. B. and R. W. Ojakangas, 1982, Keweenawan sedimentary rocks of eastern Minnesota andnorthwestern Wisconsin, in R. J. Wold and W. J. Hinze (eds.),Geology and tectonics of the Lake

52 Superior basin: Geological Society of America Memoir 156, p. 135-146.

Green, J. C., 1982, Geology of the Keweenawan extrusive rocks, in Wold, R. J. and W. J. Hinze (eds.), Geology and tectonics of the Lake Superior region: Geological Society of America Memoir 156, p. 47-55.

Halls, H. C., 1966, A review of the Keweenawan geology of the Lake superior region, in J. S. Steinhart and T. J. Smith (eds.), The Earth beneath the continents: American Geo- physical Union Geophysical Monograph 10, p. 5-27.

Hamblin, W. K., 1961, Micro-cross-lamination in Upper Keweenawan sediments of northern Michigan: Journal of Sedimentary Petrology, v. 3 1, p. 390-401.

Hieshima, G. B. and L. M. Pratt, 1991, Sulfur/carbon ratios and extractable organic matter of the Middle Proterozoic Nonesuch Formation, North American Midcontinent Rift: Precambrian Research, v. 54, p. 65-79.

Hinze, W. J., R. J. Wold and N. W. O'Hara, 1982, Gravity and magnetic anomaly study of Lake Superior, in Wold, R. J. and W. J. Hinze (eds.), Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 203-221.

Hite, D. M., 1968, Sedimentology of the Upper Keweenawan sequence of northern Wiscon- sin and adjacent Michigan: Ph. D. dissertation, University of Wisconsin-Madison, 217 p.

Hubbard, H. A., 1975, Geology of Porcupine Mountains in Carp River and White Pine quadrangles, Michigan: U. S. Geological Survey Joumal of Research, v. 3, p. 519-528.

Hunt, T. S., 1873, The geognostical history of metals: American Institute of Mining and Engineering Transactions, v. I, P. 33 1-345.

Kalliokoslci, J., 1982, Jacobsville Sandstone, in Wold, R. J. and W. J. Hinze (eds.), Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 147-155.

Kelly, W. C., and G. K. Nishioka, 1985, Recambrim oil inclusions in late veins and the role of hydrocarbons on copper mineralization at White Pine, Michigan: Geology, v. 13, p. 334-337.

Lane, A. C. and A. E. Seaman, 1907, Notes on the geological section of Michigan, part 1, the Pre- Ordovician: Journal of Geology, v. 15, p. 680-695.

Mooney, H. M., P. R. Famham. S. H. Johnson, G. Volz and C. Craddock, 1970, Seismic studies over the Midcontinent Gravity High in Minnesota and northwestern Wisconsin: Minnesota Geological Survey Report of Investigations 1 1,19 1 p.

Moore, L. R., J. R. Moore and E. Spinner, 1969, A geomicrobiological study of the Precambrian Nonesuch Shale: Proceedings of the Yorhhire Geological Society, v. 37, p. 351-394.

Morey, G. B. and R. W. Ojakangas, 1982, Keweenawan sedimentary rocks of eastern Minnesota and northwestern Wisconsin, in R. J. Wold and W. J. Hime (eds.),Geology and tectonics of the Lake

52 Superior basin: Geological Society of America Memoir 156, p. 135- 146.

Myers, W. D., 1971, The sedimentology and tectonic significance of the Bayfield Group (UpperKeweenawan?), Wisconsin and Minnesota: Ph. D. dissertation, University of Wisconsin-Madi-son, 269 p.

Ojakangas. R. W. and G. B. Morey, 1982, Keweenawan pm-volcanic quartz sandstones and relatedrocks of the Lake Superior region, in Wold, R. J. and W. J. Hinze (eds.), Geology and tectonicsof the Lake Superior basin: Geological Society of America Memoir 156, p. 85-96.

Ruiz, J., L. M. Jones and W. C. Kelly, 1984, Rubidium-strontium dating of ore deposits hosted byRn-rich rocks, using calcite and other common Sr-bearing minerals: Geology, v. 12, p. 259-262.

Thiel, E. C., 1956, Correlation of gravity anomalies with the Keweenawan geology of Wisconsin andMinnesota: Geological Society of America Bulletin, v. 67, p. 1079-1100.

Thwaites, F. T., 1912, Sandstones of the Wisconsin shore of Lake Superior: Wisconsin Geologicaland Natural History Survey Bulletin, 25, 117p.

Tyler, S. A., R. W. Marsden, F. F. Grout and G. A. Thiel, 1940, Studies of the Lake Superior Pre-cambrian by accesory-mineral methods: Geological Society of America Bulletin, v. 51, p. 1429-1538.

White, W. S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region: U. S.Geological Survey Professional Paper 524-E, p. E-l to E-23.

White, W. S., and J. C. Wright, 1960, Lithofacies of the Copper Harbor Conglomerate, northernMichigan: U. S. Geological Survey Professional Paper 400-B, p. B-5 to B-8.

Wolff, R. G. and N. K Huber, 1973, The Copper Harbor Conglomerate (Middle Keweenawan) onIsle Royale, Michigan, and its regional implications: U. S. Geological Survey Professional Paper754-B, p. B-i to B-15.

53

Myen, W. D., 1971, The sedimentology and tectonic significance of the Bayfield Group (Upper Keweenawan?), Wisconsin and Minnesota: Ph. D. dissertation, University of Wisconsin-Madi- son, 269 p.

Ojakangas, R. W. and G. B. Morey, 1982, Keweenawan pre-volcanic quartz sandstones and related rocks of the Lake Superior region, in Wold, R. J. and W. J. Hinze (eds.), Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 85-96.

Ruiz, J., L. M. Jones and W. C. Kelly, 1984, Rubidium-strontium dating of ore deposits hosted by Rn-rich rocks, using calcite and other common Sr-bearing minerals: Geology, v. 12, p. 259-262.

Thiel, E. C., 1956, Correlation of gravity anomalies with the Keweenawan geology of Wisconsin and Ivhnesota: Geological Society of America Bulletin, v. 67, p. 1079-1 100.

Thwaites, F. T., 1912, Sandstones of the Wisconsin shore of Lake Superior: Wisconsin Geological and Natural History Survey Bulletin, 25,117 p.

Tyler, S. A., R. W. Marsden, F. F. Gmut and G. A. Thiel, 1940, Studies of the Lake Superior Pre- cambrian by accesory-mineral methods: Geological Society of America Bulletin, v. 51, p. 1429- 1538.

White, W. S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region: U. S. Geological Survey Professional Paper 524-E, p. E-1 to E-23.

White, W. S., and J. C. Wright, 1960, Lithofacies of the Copper Harbor Conglomerate, noxthem Michigan: U. S. Geological Survey Pmfessional Paper W B , p. B-5 to B-8.

Wolff, R. G. and N. K Huber, 1973, The Copper Harbor Conglomerate (Middle Keweenawan) on Isle Royale, Michigan, and its regional implications: U. S. Geological Survey mfessional Paper 754-B, p. B-1 to B-15.

CENTRAL NORTH AMERICAN CASE FOR SEGMENTED RWf DEVELOPMENT

Reprinted with permission from Dickas, A. B. and M. G. Mudrey, Jr. 1989, Abstracts, 28thInternational Geological Congress, vol. 1, p. 1-396 to 1-397.

The Midcontinent Rift System (MRS) is a major Middle Proterozoic intracontinental,thermotectonic structure that has been traced by regional gravity and magnetic data, subsur-face drilling, and outcrop control over a length of 3,300 km in the central United States. Inthe Lake Superior region, the only area in which MRS outcrop is known, this structure wasinfihled by plateau lavas and sedimentary rock comprising the Keweenawan Supergroup.About 1140 Ma, basalt was extruded along the rift. After cessation of volcanic activity,generally maturing upward, elastic, sedimentary rock was deposited. Geophysical analysis inthe 1950's (Thiel, 1956) suggested a broad graben with a central horst separating wedge-shaped thick accumulations of low-density sedimentary rocks. These sedimentary sectionswere identified by negative gravity anomalies. Since then, models and geologic maps of therift have been consistent in demonstrating an early developed symmetrical extensionalcrustal basin filled with extrusive and volcaniclastic rock, subsequently modified by late-phase compressional faulting forming the presently identified central horst (Fig. 2, previouspaper).

Recent acquisition of over 4200km of onshore and offshorereflection seismic data and analy-sis of deep mineral explorationdrill holes from northern Wiscon-sin suggest a different geometryfor the MRS. This geometry, asproposed here, is similar to struc-tures attributed to the Gregory riftin East Africa. Bosworth et al.(1986) suggest the Gregory riftstructure formed through thedevelopment of deep, crustal,opposing detachment systems. Asthese detachments intersect atdepth, one side locks, resulting inhalf-graben development. Withlateral rift propagation, a subre-gional series of taphrogeosynclines(haif-graben) are formed, with

____ _____

adjacent grabens displayingigneous and sedimentary infillpackages of opposing geometry.The initial rift is thus unable toextend laterally in uniform struc-tural patterns but rather forms alinear pattern of individualized, enechelon, asymmetric basins, each

% 14 rN

w

NE IL

MO

KANSAS

OH

KS

o ioo#11 I

o io

Gravity High Gravity Low

r///,2 Accommodation Structure

FIGURE 3: Gravity expression of the MidcontinentR4ft System, showing the location and extent offlrst-order rift zones.

54

CENTRAL NORTH AMERICAN CASE FOR SEGMENTED RIFT DEVELOPMENT

Reprinted with permission from Dickas, A. B. and M. G. Mudrey, Jr, 1989, Abstracts, 28th International Geological Congress, vol. 1, p. 1-396 to 1-397.

The Midcontinent Rift System (MRS) is a major Middle Proterozoic intracontinental, therrnotectonic structure that has been traced by regional gravity and magnetic data, subsur- face drilling, and outcrop control over a length of 3,300 km in the central United States. In the Lake Superior region, the only area in which MRS outcrop is known, this structure was infilled by plateau lavas and sedimentary rock comprising the Keweenawan Supergroup. About 1140 Ma, basalt was extruded along the rift. After cessation of volcanic activity, generally maturing upward, clastic, sedimentary rock was deposited. Geophysical analysis in the 1950's (Thiel, 1956) suggested a broad graben with a central horst separating wedge- shaped thick accumulations of low-density sedimentary rocks. These sedimentary sections were identified by negative gravity anomalies. Since then, models and geologic maps of the rift have been consistent in demonstrating an early developed symmetrical extensional crustal basin filled with extrusive and volcaniclastic rock, subsequently modified by late- phase compressional faulting forming the presently identified central horst (Fig. 2, previous

- 1 @ Gravity High Gravity Low

Accommodation Structure

FIGURE 3: Gravity expression of the Midcontinent Rift System, showing the location and extent offirst- order rift zones.

Recent acquisition of over 4200 km of onshore and offshore reflection seismic data and analy- sis of deep mineral exploration drill holes from northern Wiscon- sin suggest a different geometry for the MRS. This geometry, as proposed here, is similar to struc- tures attributed to the Gregory rift in East Africa. Bosworth et al. (1986) suggest the Gregory rift structure formed through the development of deep, crustal, opposing detachment systems. As these detachments intersect at depth, one side locks, resulting in half-graben development. With lateral rift propagation, a subre- gional series of taphrogeosynclines (half-graben) are formed, with adjacent grabens displaying igneous and sedimentary infill packages of opposing geometry. The initial rift is thus unable to extend laterally in uniform struc- tural patterns but rather forms a linear pattern of individualized, en echelon, asymmetric basins, each

with its own infihling history and geometry and each separated by a type of cross-structure,generally referenced as "accommodation" faulting.

On a regional basis, the fmal structural configuration of a rift may also be dependent upon totalextension developed at right angles to rift propagation. Schuepbach and Vail (1980) suggest thatwith such differential rift development, and even though structural extension along rift strikemay be synchronous, basin formation and fifing could be diachronous. Quenneil (1984) sup-ported this evolution by mapping the Dead Sea rift as divided into three unique segments, eachoperating independently in geologic development. Viewed as an entirety, a rift may thus displayfirst-order, regional-size rift segments (Dickas, 1986), representing coeval stages of differing riftmaturity. In turn, these segments might be divided into subregional-sized, second-order sub-basins separated by "accommodation" faulting.

Viewed from the perspective of this geometry, the extent of the MRS is here interpreted tobe composed of five first-order rift segments, termed "zones" by Rosendahi (1987). Clock-wise from the southwestern end of the MRS. these zones are here named Kansas, Iowa,Superior, Mackinaw, and Maumee (Fig. 3), and are identified on the basis of major interrup-tions in gravity and magnetic patterns, seismic review, terrane compositions, and boreholeanalysis.

Locally, and developing as a function of overall rift extension, zones may be further dividedinto a series of structurally independent basins. These second-order sub-basins, termed"units" by Rosendahi (1987), are separated by "accommodation" faulting. Within the Supe-rior zone of the MRS, four units are now recognized on the basis of seismic interpretation,interrupted geopotential trends, and core analysis. These sub-basins are here named theChisago, Brule, Ontonagon, and Manitou units (Fig. 4). These units appear to have under-gone a similar history of structural development during the early phase of MRS evolution.On the basis of differential listric movements along axial-oriented, rift fault systems, igneousand sedimentation wedges of alternating isopach patterns distinguish juxtaposed zones.Isopach patterns of these wedge geometries differ by being symmetrical parallel, but alter-nating asymmetrical perpendicular, to the MRS axis.

In light of this suggested model, future stratigraphic and seismic correlations can no longer beinterpreted within the framework of a symmetric rift model existing along the entirety of theMRS axis. Instead, such correlations must be dependent upon consideration of isolated igneousand sedimentary packages and isopach geometries created in response to first- and second-orderdegree of tectonic development. Exploration philosophies, whether they be directed to base-metal or hydrocarbon evaluation, must be constrained so as to recognize related regional andsubregional alterations in structural style.

55

with its own infilling history and geometry and each separated by a type of cross-structure, generally referenced as "accommodation" faulting.

On a regional basis, the final structural configuration of a rift may also be dependent upon total extension developed at right angles to rift propagation. Schuepbach and Vail(1980) suggest that with such differential rift development, and even though structural extension along rift strike may be synchronous, basin formation and filling could be diachronous. Quennell(1984) sup- ported this evolution by mapping the Dead Sea rift as divided into three unique segments, each operating independently in geologic development. Viewed as an entirety, a rift may thus display first-order, regional-size rift segments (Dickas, 1986), representing coeval stages of differing rift maturity. In turn, these segments might be divided into subregional-sized, second-order sub- basins separated by "accommodation" faulting.

Viewed from the perspective of this geometry, the extent of the MRS is here interpreted to be composed of five first-order rift segments, termed "zones" by Rosendahl(1987). Clock- wise from the southwestern end of the MRS, these zones are here named Kansas, Iowa, Superior, Mackinaw, and Maumee (Fig. 3), and are identified on the basis of major interrup- tions in gravity and magnetic patterns, seismic review, terrane compositions, and borehole analysis.

Locally, and developing as a function of overall rift extension, zones may be further divided into a series of structurally independent basins. These second-order sub-basins, termed "units" by Rosendahl(1987), are separated by "accommodation" faulting. Within the Supe- rior zone of the MRS, four units are now recognized on the basis of seismic interpretation, interrupted geopotential trends, and core analysis. These sub-basins are here named the Chisago, Brule, Ontonagon, and Manitou units (Fig. 4). These units appear to have under- gone a similar history of structural development during the early phase of MRS evolution. On the basis of differential listric movements along axial-oriented, rift fault systems, igneous and sedimentation wedges of alternating isopach patterns distinguish juxtaposed zones. Isopach patterns of these wedge geometries differ by being symmetrical parallel, but alter- nating asymmetrical perpendicular, to the MRS axis.

In light of this suggested model, future stratigraphic and seismic correlations can no longer be interpreted within the framework of a symmetric rift model existing along the entirety of the MRS axis. Instead, such correlations must be dependent upon consideration of isolated igneous and sedimentary packages and isopach geometries created in response to first- and second-order degree of tectonic development. Exploration philosophies, whether they be directed to base- metal or hydrocarbon evaluation, must be constrained so as to recognize related regional and subregional alterations in structural style.

References

Bosworth, W., J. Lambiase, and R. Keisler, 1986, A new look at Gregory's Rift: the struc-tural style of continental rifting: Eos, v. 67, p. 557, 582-583.

Dickas, A.B., 1986, Seismologic analysis of arrested stage development of the MidcontinentRift, in M.G. Mudrey, Jr. (ed), Precambrian petroleum potential, Wisconsin and Michi-gan: Geoscience Wisconsin, Wisconsin Geological and Natural History Survey, v. 11, p.45-52.

Quennell, A.M., 1984, The western Arabia rift-system, in J.F. Dixon and A.H.F. Robertson(eds.), The geological evolution of the eastern Mediterranean: Geological Society SpecialPublication 17, p. 775-788.

Rosendahi, B.R.,11987, Architecture of continental rifts with special reference to East Africa:Annual Review Earth and Planetary Science Letters, v. 15, p. 445-503.

Schuepbach, M.A. and P.R. Vail, 1980, Evolution of outer highs on divergent continentalmargins, in Continental tectonics: National Academy of Science, 197 p.

Thiel, E., 1956, Correlation of gravity anomalies with the Keweenawan geology of Wiscon-

56sin and Minnesota: Geological Society of America Bulletin, v. 67, p. 1079-1100.

FIGURE 4: Structural rift units composing the Superior rift zone, Midcontinent Rift System, asdefined by accomodation structures (A.S.) and isopach thickening trend,

Central Structure (St. Croix Horst)

SCHEMATIC SECTION

FIGURE 4: Structural rift units composing the Superior rift zone, Midcontinent Rift System, as defined by accomodation structures (AS.) and isopach thickening trend,

References

Bosworth, W., J. Lambiase, and R. Keisler, 1986, A new look at Gregory's Rift: the struc- tural style of continental rifting: Eos, v. 67, p. 557,582-583.

Dickas, A.B., 1986, Seismologic analysis of arrested stage development of the Midcontinent Rift, in M.G. Mudrey, Jr. (ed), Precambrian petroleum potential, Wisconsin and Michi- gan: Geoscience Wisconsin, Wisconsin Geological and Natural History Survey, v. 11, p. 45-52.

Quennell, A.M., 1984, The western Arabia rift-system, in J.F. Dixon and A.H.F. Robertson (eds.), The geological evolution of the eastern Mediterranean: Geological Society Special Publication 17, p. 775-788.

Rosendahl, B.R.,11987, Architecture of continental rifts with special reference to East Africa: Annual Review Earth and Planetary Science Letters, v. 15, p. 445-503.

Schuepbach, M.A. and P.R. Vail, 1980, Evolution of outer highs on divergent continental margins, in Continental tectonics: National Academy of Science, 197 p.

Thiel, E., 1956, Correlation of gravity anomalies with the Keweenawan geology of Wiscon- sin and Minnesota: Geological Society of America Bulletin, v. 67, p. 1079-1 100.

56

LAKE SUPERIOR BASIN AS A HYDROCARBON FRONTIER

Mod(fied from a series of requested articles written by Albert Dickas for the daily print mediaof northwest Wisconsin, January, 1992, during site preparations for the Terra/Patrick #7-22,Bayjield County, Wisconsin exploratory wildcat.

Since the initial discovery of crude oil at a depth of 21.2 m beneath the rolling hills ofTitusville, Pennsylvania in 1859, hundreds of thousands of boreholes have sought thisnatural resource in the United States. Wisconsin has played a small role in this 133 year old"hydrocarbon revolution." Between 1865 and 1992, 49 boreholes have sought economicvolumes of oil or gas in the Badger state. Not one of these wells however, has found any-thing other than fresh and salt water.

The majority of exploration in Wisconsin has taken place close to the Lake Michigan shore-line between Milwaukee and Door County (Fig. 5). Here the rocks form the western edge ofthe Michigan basin, one of the most prolific producers of hydrocarbon in the United States.Unfortunately, this productivity is basically confined to the deeper portions of the basin, andnot along its edges. The remaining area of Wisconsin has long been considered non-produc-tive because of the great geologic age and physical nature of much of its bedrock.

In the early 1980s, pessimism turned to guarded optimism. Petroleum geologists began torecognize that rocks deposited during early chapters of earth history--the so termed Precam-brian Eon--possesses, contrary to conventional wisdom, those seven to eight characteristicsnecessary for the development of an oil or gas field. Studies suggested these characteristicswere often contained within rift structures, formed by the dissection of pre-existent conti-nents, much as Africa is presently being torn into portions along the African Rift Zone.

It has been known for 35 years that northwestern Wisconsin occupies a portion of the world-class Midcontinent Rift System, extending 3300 km from Kansas to Ohio by way of LakeSuperior. With the recognized association of ancient rift rocks with economic hydrocarbonreserves, that portion of the Midcontinent rift lying between the upper peninsula of Michiganand Kansas was quietly invaded by the United States oil and gas industry. Initial interestsincluded Mobil, Texaco, Standard of California, and Amoco, among others. Field crewsbegan operating in the early 1980's and special "frontier region" analysis teams of geolo-gists, geophysicists, paleontologists, and geochemists were formed. Studies ranged fromhand investigation of rock samples to sophisticated computer reviews of newly collected datapertaining to the seismic, magnetic and gravity character of the rift.

In 1984 Texaco made the first move by drilling, in northeast Kansas, their #1 Noel Poerschwell to a record state depth of 3444 m. In certain aspects this was a guinea-pig borehole.Because the rift had never been drilled beyond normal water-well depths of several hundredsof feet, industry was probing into the geologic darkness. Expectations were based uponcomparison of Kansas data to the near-surface geology of Douglas and Bayfield Counties,Wisconsin, a region where rift rocks are exposed at the surface and thus easy to study. TheTexaco drilling plan envisioned several thousands of feet of sandstone and organic shaleoverlying an even thicker sequence of layered lava. Under ideal conditions either oil or gaswould have formed within the organic shales and subsequently moved upward into thesandstones until trapped and concentrated into an economic "pool."

57

LAKE SUPERIOR BASIN AS A HYDROCARBON FRONTIER

Modified from a series of requested articles written by Albert Dickas for the daily print media of northwest Wisconsin, January, 1992, during site preparations for the TerrdPatrick #7-22, Bayjield County, Wisconsin exploratory wildcat.

Since the initial discovery of crude oil at a depth of 21.2 m beneath the rolling hills of Titusville, Pennsylvania in 1859, hundreds of thousands of boreholes have sought this natural resource in the United States. Wisconsin has played a small role in this 133 year old "hydrocarbon revolution." Between 1865 and 1992,49 boreholes have sought economic volumes of oil or gas in the Badger state. Not one of these wells however, has found any- thing other than fresh and salt water.

The majority of exploration in Wisconsin has taken place close to the Lake Michigan shore- line between Milwaukee and Door County (Fig. 5). Here the rocks form the western edge of the Michigan basin, one of the most prolific producers of hydrocarbon in the United States. Unfortunately, this productivity is basically confined to the deeper portions of the basin, and not along its edges. The remaining area of Wisconsin has long been considered non-produc- tive because of the great geologic age and physical nature of much of its bedrock.

In the early 1980s, pessimism turned to guarded optimism. Petroleum geologists began to recognize that rocks deposited during early chapters of earth history--the so termed Precam- brian Eon--possesses, contrary to conventional wisdom, those seven to eight characteristics necessary for the development of an oil or gas field. Studies suggested these characteristics were often contained within rift structures, formed by the dissection of pre-existent conti- nents, much as Africa is presently being tom into portions along the African Rift Zone.

It has been known for 35 years that northwestern Wisconsin occupies a portion of the world- class Midcontinent Rift System, extending 3300 krn from Kansas to Ohio by way of Lake Superior. With the recognized association of ancient rift rocks with economic hydrocarbon reserves, that portion of the Midcontinent rift lying between the upper peninsula of Michigan and Kansas was quietly invaded by the United States oil and gas industry. Initial interests included Mobil, Texaco, Standard of California, and Amoco, among others. Field crews began operating in the early 1980's and special "frontier region" analysis teams of geolo- gists, geophysicists, paleontologists, and geochemists were formed. Studies ranged from hand investigation of rock samples to sophisticated computer reviews of newly collected data pertaining to the seismic, magnetic and gravity character of the rift.

In 1984 Texaco made the first move by drilling, in northeast Kansas, their #1 Noel Poersch well to a record state depth of 3444 m. In certain aspects this was a guinea-pig borehole. Because the rift had never been drilled beyond normal water-well depths of several hundreds of feet, industry was probing into the geologic darkness. Expectations were based upon comparison of Kansas data to the near-surface geology of Douglas and Bayfield Counties, Wisconsin, a region where rift rocks are exposed at the surface and thus easy to study. The Texaco drilling plan envisioned several thousands of feet of sandstone and organic shale overlying an even thicker sequence of layered lava. Under ideal conditions either oil or gas would have formed within the organic shales and subsequently moved upward into the sandstones until trapped and concentrated into an economic "pool."

58

Drilling ceased in early 1985 but detailed results were not released until mid-1988. Thisthree year delay made sense when information became public. The drill sequence appearedreversed when compared to expectations, with lava and related igneous rock overlying thicksandstone layers. Shale was found only in trace amounts and that shale present was notorganic. Without organic shale an oil or gas field is not possible. The Texaco well wasclassed as "dry and abandoned" and exploration geologists began to reinterpret the geologichistory of the Midcontinent rift.

Amoco Production Company was the second major player to enter the search for Precam-brian-age rift hydrocarbon. Disenchanted by the Kansas results, Amoco geologists chosewest-central Iowa as their preferred drilling site. In March of 1987 their #1 Eischeid bore-hole was set into operation and seven months later reached a state depth record of 5,441 m.This borehole was shrouded in secrecy, protected by fence and guard. Under favorableweather conditions a nearby hill was covered with curious observers and industry scouts andskimpy reports were issued daiiy by the media. Even after the projected depth was reachedAmoco, following the lead established by Texaco in Kansas, maintained secrecy. With therelease of information in the spring of 1990 encouragement was given by the fact a normalsequence of rock colunm was drilled--shales were found and they were organic.

The high risks associated with the oil and gas business is largely attributed to the fact that whilethe proper conditions for an economic reservoir are often found, the sought after product is oftenabsent. The Iowa venture was high risk in planning and execution and the end result waslabeled "dry and abandoned."

The third borehole to test the Midcontinent rift was initiated in late 1987 and completed twomonths later. The operator was again Amoco Production Company, but now the geographic sitewas near the upper peninsula community of Munising, Michigan. The fmal total depth was2210 m. While complete information has not yet been released, it has been reported that allthree of the formations composing the target Oronto Group were drilled--but not in the normalorder as known from Wisconsin geology.

It is interesting to note this borehole is located in a region of historic, little known, explora-tion for crude oil. In 1865 the Ontonagon Petroleum Company, financed by a capital stockoffering of $500,000, was formed for the purpose of "engaging in and carrying on the busi-ness of mining---petroleum." By December of that year the first and only venture of thiscompany was underway, fueled by a rapid increase in the price of oil stock and an outbreakof "oil fever." The final drilling depth was 38 m. While microfilm records indicate "twobarrels (of oil) have been hoisted up," there is no record of the Ontonagon Petroleum Com-pany beyond the year of its incorporation.

Since 1929 active underground oil seeps have been reported from the White Pine mine inOntonagon County, Michigan. Analyses shows this crude to be very similar chemically tohigh grade oil of Phanerozoic age produced in the state of Pennsylvania. Radiometric age-dating techniques suggest the White Pine product may be the oldest known crude oil in theworld. While these volumes are modest, their presence keeps alive the possibility of "blackgold" production in northern Wisconsin.

Drilling ceased in early 1985 but detailed results were not released until mid-1988. This three year delay made sense when information became public. The drill sequence appeared reversed when compared to expectations, with lava and related igneous rock overlying thick sandstone layers. Shale was found only in trace amounts and that shale present was not organic. Without organic shale an oil or gas field is not possible. The Texaco well was classed as "dry and abandoned" and exploration geologists began to reinterpret the geologic history of the Midcontinent rift.

Amoco Production Company was the second major player to enter the search for Precam- brian-age rift hydrocarbon. Disenchanted by the Kansas results, Amoco geologists chose west-central Iowa as their preferred drilling site. In March of 1987 their #1 Eischeid bore- hole was set into operation and seven months later reached a state depth record of 5,441 m. This borehole was shrouded in secrecy, protected by fence and guard. Under favorable weather conditions a nearby hill was covered with curious observers and industry scouts and skimpy reports were issued daily by the media. Even after the projected depth was reached Amoco, following the lead established by Texaco in Kansas, maintained secrecy. With the release of information in the spring of 1990 encouragement was given by the fact a normal sequence of rock column was drilled--shales were found and they were organic.

The high risks associated with the oil and gas business is largely attributed to the fact that while the proper conditions for an economic reservoir are often found, the sought after product is often absent. The Iowa venture was high risk in planning and execution and the end result was labeled "dry and abandoned."

The third borehole to test the Midcontinent rift was initiated in late 1987 and completed two months later. The operator was again Amoco Production Company, but now the geographic site was near the upper peninsula community of Munising, Michigan. The final total depth was 2210 m. While complete information has not yet been released, it has been reported that all three of the formations composing the target Oronto Group were drilled--but not in the normal order as known from Wisconsin geology.

It is interesting to note this borehole is located in a region of historic, little known, explora- tion for crude oil. In 1865 the Ontonagon Petroleum Company, financed by a capital stock offering of $500,000, was formed for the purpose of "engaging in and carrying on the busi- ness of mining~petroleum." By December of that year the first and only venture of this company was underway, fueled by a rapid increase in the price of oil stock and an outbreak of "oil fever." The final drilling depth was 38 m. While microfilm records indicate "two barrels (of oil) have been hoisted up," there is no record of the Ontonagon Petroleum Com- pany beyond the year of its incorporation.

Since 1929 active underground oil seeps have been reported from the White Pine mine in Ontonagon County, Michigan. Analyses shows this crude to be very similar chemically to high grade oil of Phanerozoic age produced in the state of Pennsylvania. Radiometric age- dating techniques suggest the White Pine product may be the oldest known crude oil in the world. While these volumes are modest, their presence keeps alive the possibility of "black gold" production in northern Wisconsin.

As of early March, 1992, site preparation was underway for the fourth test of the hydrocar-bon potential of this rift. Located in Bayfield County, Wisconsin, this wildcat has beenprogrammed as a 1825 m (6000 ft.) test of the Oronto Group in its position overlying thecentral St. Croix horst. Available information on this Terra/Patrick #7-22 borehole will bepresented in the Proceedings volume of this 38th meeting of the Institute on Lake SuperiorGeology.

59

As of early March, 1992, site preparation was underway for the fourth test of the hydrocar- bon potential of this rift. Located in Bayfield County, Wisconsin, this wildcat has been programmed as a 1825 m (6000 ft.) test of the Oronto Group in its position overlying the central St. Croix horst. Available information on this TerraPatrick #7-22 borehole will be presented in the Proceedings volume of this 38th meeting of the Institute on Lake Superior Geology.

Title: Presque Isle State Park — Nonesuch Formation

Location: Mouth of the Presque Isle River, Sec. 19 and 30, T. 50 N.,

R. 45 W., Gogebic County, Michigan (Thomason 15—minute topographic quadrangle, 1956).

Authors: M.G. Mudrey, Jr. and P.A. Daniels, Jr.

and Daniels, 1982)

(modified from Hite, 1968

60

Description: The Nonesuch is more drab in color and generally of greater tex- tural and compositional maturity than the underlying or overlying red bed se-

quences. Generally, the Nonesuch is a medium gray to black shale to medium sandstone. The finest grained rock is the darkest in color. The bedding

ranges from massive—appearing to evenly laminated with many laminae less than

1 mm thick. Thin, rhythmic grain—size alternations are common in microscale,

and many show graded bedding and dewatering features such as clastic dikes.

The rock is rich in organic material and is petroliferous in the White Pine

area. The Nonesuch averages about 180 m in thickness, and generally thins to the southwest. In the White Pine area about 40 km east of Presque Isle State

Park, the Nonesuch thickens basinward at more than 4 rn/km. Structural atti- tude varies from overturned in the Porcupine Mountain area to about 10 de-

grees in the White Pine and Presque Isle River areas, and 75 degrees in

Wisconsin.

The Nonesuch possesses a great variety of sedimentary features. Bothhorizontal stratification and cross stratification occur. Laminated stratifi-cation is the most conmion; lenticular, wavy or irregular, and ripple stratifi-cation also occur. Units rarely exceed 60 cm in thickness, and the thickerbeds are coarser sandstone and conglomerate. Alternating laminae of fine

gray sandstone and black shaley ailtstone less then 0.5 cm thick are common.The cross—stratification is of two types, planar cross stratification and riband furrow. Both of these types are abundant on the Presque Isle River inMichigan. Shrinkage cracks and disturbed bedding occur locally and sometimes

abundantly. Sediment and current transport data support a dominant flowregime to the west—southwest during deposition of the Nonesuch in the PresqueIsle area.

Discussion: The sedimentary structures indicate that the depositional envi-ronment of the Nonesuch Formation was that of a standing body of water, withperhaps significant variation in water depth. Salinity was at least highenough to precipitate gypsum.

The initial formation of this water body could have occurred either dueto subsidence along the rift, creating a closed topographic low that wouldthen act as local base level, or perhaps, more likely, the disruption of re-gional drainage pattern by some type of damming.

61

The Nonesuch possesses a great variety of sedimentary features. Both horizontal stratification and cross stratification occur. Laminated stratifi- cation is the most common; lenticular, wavy or irregular, and ripple stratifi- cation also occur. Units rarely exceed 60 cm in thickness, and the thicker beds are coarser sandstone and conglomerate. Alternating laminae of fine gray sandstone and black shaley siltstone less then 0.5 can thick are common. The cross-stratification is of two types, planar cross stratification and rib and furrow. Both of these types are abundant on the Presque Isle River in Michigan. Shrinkage cracks and disturbed bedding occur locally and sometimes abundantly. Sediment and current transport data support a dominant flow regime to the west-southwest during deposition of the Nonesuch in the Presque Isle area.

Discussion: The sedimentary structures indicate that the depositional envi- ronment of the Nonesuch Formation was that of a standing body of water, with perhaps significant variation in water depth. Salinity was at least high enough to precipitate gypsum.

- The initial formation of this water body could have occurred either due

to subsidence along the rift, creating a closed topographic low that would then act as local base level, or perhaps, more likely, the disruption of re- gional drainage pattern by some type of damming.

Title: Parker Creek — Oronto Group IR 47/O1E/30

Location: Approximately 300 a upstream to the southeast from the northwestcorner of section 30. NWl/4, Sec. 30, T. 47 N., H. 1 F., IronCounty, Wisconsin (Oronto Bay 7 1/2—minute topographic quadrangle,1980).

Author: M.G. Mudrey, Jr. (modified from Myers, 1971 and Rosenberry, 1924)

Description: Approximately 600 a of continuous exposure from the last Kewee—nawan lava flow, through the Copper Harbor and Nonesuch Formations into thelower 300 in of the Freda area exposed along Parker Creek (also known as DavisCreek). Because the upper reaches of the creek tend to run parallel to thebedding, the Copper Harbor appears to be much thicker than normal.

units trend N. 60 degrees F., 85 degrees NW. There are aflexures and faults in the wall of the valley, but the sec—be continuous with no repetition by faulting.

m of Copper Harbor Formation are exposed in the upperThe unit appears to consist of five main conglomeriticin thickness, interbedded with coarse, sandy

All of thefew, very minortion appears to

Approximately 200reaches of the creek.zones about 30 to 60 a

62

Title: Parker Creek - Oronto Group IR 47/01E/30

Location: Approximately 300 m upstream to the southeast from the northwest corner of section 30. NW1/4, Sec. 30, T. 47 N., R. 1 E., Iron County, Wisconsin (Oronto Bay 7 1/2-minute topographic quadrangle, 1980).

Author: M.G. Mudrey, Jr. (modified from Myers, 1971 and Rosenberry, 1924)

Description: Approximately 600 m of continuous exposure from the last Kewee- nawan lava flow, through the Copper Harbor and Nonesuch Formations into the lower 300 m of the Freda area exposed along Parker Creek (also known as Davis Creek). Because the upper reaches of the creek tend to run parallel to the bedding, the Copper Harbor appears to be much thicker than normal.

All of the units trend N. 60 degrees E., 85 degrees NW. There are a few, very minor flexures and faults in the wall of the valley, but the sec- tion appears to be continuous with no repetition by faulting.

Approximately 200 m of Copper Harbor Formation are exposed in the upper reaches of the creek. The unit appears to consist of five main conglomeritic zones about 30 to 60 m in thickness, interbedded with coarse, sandy

conglomerate. Both lithologies are clast—supported conglomerate. The CopperHarbor is relatively typical, consisting of rounded cobbles of Keweenawan vol-canic rock and granite and iron formation from the underlying Early Protero—zoic and Archean succession.

About 310 in of Nonesuch Formation is exposed. The Nonesuch consists ofseveral dark colored slate units, interbedded with sandy slate. The lowercontact of the Nonesuch and the Copper Harbor is gradational in that thenonconglomerate interbeds become finer grained and more abundant as the upper

contact with the Nonesuch is approached. Bedding is clearly defined in theNonesuch, and local cross bedding and deformation structures can be recog-nized. The upper contact of the Nonesuch with the Freda is also gradationalin that sand beds become more common and slate beds less so.

The lower member of the Freda Formation is a thick bedded, fine— tomedium—grained sandstone that is strongly stained with iron. Local pebble

zones are common. The middle Member of the Freda is seen only at the

northernmost outcrop in the creek. It is much better exposed in Spoon Creek

to the west. This member consists of siltstone and very fine—grained sand-stone with occasional shale and cross—stratified fine—grained sandstonebeds. Bedding is more pronounced in the middle member compared to the lower

member. The upper member of the Freda is best seen at the mouth of theMontreal River to the east, particularly at Superior Falls in Michigan. The

upper member consists of thick bedding units of micaceous siltstone, veryfine—grained, laminated sandstone with abundant scours and and current direc-tion indicators, and slightly conglomeritic, fine—grained sandstone with bed-ding poorly developed. The White River location is probably in the upper mem-ber.

Discussion: Clearly, the Oronto Group represents a single depositional epi-sode, the individual formations representing various environments within thatepisode. All of the units appear to be gradational. The Freda appears tothicken from its type locality near Freda, Michigan, to the Montreal Riverand west. The Nonesuch, conversely, appears to thin toward the west fromPresque Isle State Park. At Copper Falls State Park the dominant shale unitappears to be less than 20 in thick; however, the shale-bearing interval isabout 140 in thick.

All along the Gogebic Range, the Early and Middle Proterozoic units dipnorthwest at a steep angle. Reconstruction of the thickness of the sectionresults in an estimate that is too thick, 20 km of section from Hurley toLake Superior. In Michigan this is easy to understand as the Keweenaw and

associated faults clearly repeat parts of the section; however, in Wisconsinbecause of poor exposure, the faults are not readily recognized. As will beseen and discussed at White River, there is at least one major anticlineknown in outcrop at Marble Point, and at least one major syncline with theFreda. A more reasonable estimate for the total thickness of the OrontoGroup would be 5 km.

63

conglomerate. Both lithologies are clast-supported conglomerate. The Copper Harbor is relatively typical, consisting of rounded cobbles of Keweenawan vol- canic rock and granite and iron formation from the underlying Early Protero- zoic and Archean succession.

About 310 m of Nonesuch Formation is exposed. The Nonesuch consists of several dark colored slate units, interbedded with sandy slate. The lower contact of the Nonesuch and the Copper Harbor is gradational in that the nonconglomerate interbeds become finer grained and more abundant as the upper contact with the Nonesuch is approached. Bedding is clearly defined in the Nonesuch, and local cross bedding and deformation structures can be recog- nized. The upper contact of the Nonesuch with the Freda is also gradational in that sand beds become more common and slate beds less so.

The lower member of the Freda Formation is a thick bedded, fine- to mediwgrained sandstone that is strongly stained with iron. Local pebble zones are common. The middle Member of the Freda is seen only at the northernmost outcrop in the creek. It is much better exposed in Spoon Creek to the west. This member consists of siltstone and very fine-grained sand- stone with occasional shale and cross-stratified fine-grained sandstone beds. Bedding is more pronounced in the middle member compared to the lower member. The upper member of the Freda is best seen at the mouth of the Montreal River to the east, particularly at Superior Falls in Michigan. The upper member consists of thick bedding units of micaceous siltstone, very fine-grained, laminated sandstone with abundant scours and and current direc- tion indicators, and slightly conglomeritic, fine-grained sandstone with bed- ding poorly developed. The White River location is probably in the upper mem- ber.

Discussion: Clearly, the Oronto Group represents a single depositional epi- sode, the individual formations representing various environments within that episode. All of the units appear to be gradational. The Freda appears to thicken from its type locality near Freda, Michigan, to the Montreal River and west. The Nonesuch, conversely, appears to thin toward the west from Presque Isle State Park. At Copper Falls State Park the dominant shale unit appears to be less than 20 m thick; however, the shale-bearing interval is about 140 m thick.

All along the Gogebic Range, the Early and Middle Proterozoic units dip northwest at a steep angle. Reconstruction of the thickness of the section results in an estimate that is too thick, 20 km of section from Hurley to Lake Superior. In Michigan this is easy to understand as the Keweenaw and associated faults clearly repeat parts of the section; however, in Wisconsin because of poor exposure, the faults are not readily recognized. As will be seen and discussed at White River, there is at least one major anticline known in outcrop at Marble Point, and at least one major syncline with the Freda. A more reasonable estimate for the total thickness of the Oronto Group would be 5 km.

64

NE 250

conglomerate

Distance from NE25 to El/4 25= 1/2 mile

Lo.QI conglomeratesandy conglomerate

•:.:1 sandstonesandstone and shale

shale

slate

andyconglomerate

ale

glomerate;hale

late

on glomerate

0E '/425

Sketch of geology in Parker Creek from Rosenberry (1924).

D i s t a n c e f r o m N E 2 5 to E 114 25 = 112 m i l e

ml c o n g l o m e r a t e

sandy c o n g l o m e r a t e

s a n d s t o n e ... s a n d s t o n e a n d s h a l e

-.A.

a t e

Sketch o f geology in Parker Creek from Rosenberry (1924).

::.'.) Ss, some OsUnit 2(UNIT 4) 65

:=2-:- SttsIns.alternating, some

—--=. Unit I.

(UNIT 3).;)5ac.irflobIW.

61— sm.— to c.- grain—

sole-morks

Stratigraphic section showing 100 feet of typical Nonesuch Formation (figure20 of Rite, 1968).

Stratigraphic section showing 100 feet of typical Lower Freda Formation(figure 30 of Rite, 1968).

65

70

Silistone onashole,grOy toblock, alter-noting and 40

-- lTt.oi::_l alternating andlaminated, witho few frne, thin-bedded sand----tones

laminated tovery thin bed-ded (1/4— I");occasionalmedium- to •35

coorse-grainedsandstone units3 to 6" thick

(UNIT I)

tUN IT90

7)

85

30

Sandstone, blactcmedium— to coarse-groined. someirregular bosalcontocis, generollythick—bedded

(UNIT 8)Ss. blk., vT-to c.—groinedthin to thickbedded(UNIT 6

Sandstone, block medium- to coorse- groined. some irregular basal contocfs, generally thick-bedded

( U N I T 8 )

Stratigraphic section showing 100 feet of typical Nonesuch Formation (figure 20 of Hite, 1968).

Stratigraphic section showing 100 feet of typical Lower Freda Formation (figure 30 of Hite, 1968).

66

Stratigraphic section showing 100 feet of typical Upper Freda Formation(figure 42 of Hite, 1968).

Stratigraphic section showing 100 feet of typical Middle Freda Formation(figure 33 of Hite, 1968).

21 70

U.

95

Siltstone, redmicoceous andlaminated, 1ter-bedded withsandstone, white togreen, finegroinedcross—bedded

(UN I T 8)

Siltstone, redrnicoceous,lOm-irioted withsholes ondcross-beddedsiltstones, inter—stratified(UNIT 3)

40

6iltstone, red:.— micaceous,

- irregular lam—motions withmicro (ripple)

• cross—stabin upper part.... interbedded St

—.....- red, fine-grained(UNIT 2)

— _30Sandstone, red-:;.-: fmne—groined,

.: laminated,rib and furrowStruClurC

(UNIT I)

:

Sandstone, redvery fine— to tinegroined, 10mm—oted,w,th 65micro cross-bedding whichbecomes moreabundant upwordin the sectiondistorted beddingrn central 60part of theunit.

(UN IT 4)

90

Sandstone,same asC\ Unit4,butlarger scale

I Cross—strata.—U.LNLL6)-.

Shale andfine silistone,brick red,micaceous,laminated. 85interbeddedcoorse—grounedcross—beddedsandstone ocair6 feet belowthe top of theunit

(UNIT 5) 80

Sandstone redfine-groined,bedding poorlydevelopedshale pebblescommon inupper part(UNIT 7)

55

Stratigraphic section showing 100 feet of typical Middle Freda Formation (figure 33 of Hite, 1968).

Stratigraphic section showing 100 feet of typical Upper Freda Formation (figure 42 of Hite, 1968).

Title: Copper Falls State Park — Oronto Group AS 45/02E/17B

Location: Northwest of Mellen on Highway 169. SE1/4, Sec. 17, T. 45 N.,B. 2 E., Ashland County, Wisconsin. (Mellen and High Bridge7 1/2—minute topographic quadrangles, 1967 and 1984, respectively).

Author: M.G. Mudrey, Jr.

Description: Copper Falls is formed where the Bad River cuts through the re-sistant ridge of Keweenawan lava flows. Downstream, 0.6 km from Copper Fallsat the contact between basalt to the south and rhyolite to the north, theTyler Forks River joins the Bad River at Brownstone Falls, which occurs atthe contact between basalt to the south and rhyolite to the north. The con-glomerate of the Copper Harbor Formation is exposed in the lower gorge atDevils Gate about 200 m downstream from Brownstone Falls. The conglomerateis 129 m thick; at the Montreal River the conglomerate is 515 m thick. North-west of the conglomerate is the black shale of the Nonesuch Formation; north-west of the Nonesuch is the sandstone of the Freda Formation. There is blackshale distributed through 141 in of section, but only 20 in are consideredNonesuch Formation. Above the interval with black shale, the sediment is redshaley arkose.

67

Title: Copper Falls State Park - Oronto Group AS 45/02E/17B

Location: Northwest. of Mellen on Highway 169. SE1/4, Sec. 17, T. 45 N., R. 2 E., Ashland County, Wisconsin. (Mellen and High Bridge 7 1/2-minute topographic quadrangles, 1967 and 1984, respectively).

Author: M.G. Mudrey, Jr.

Description: Copper Falls is formed where the Bad River cuts through the re- sistant ridge of Keweenawan lava flows. Downstream, 0.6 km from Copper Falls at the contact between basalt to the south and rhyolite to the north, the Tyler Forks River joins the Bad River at Brownstone Falls, which occurs at the contact between basalt to the south and rhyolite to the north. The con- glomerate of the Copper Harbor Formation is exposed in the lower gorge at Devils Gate about 200 m downstream from Brownstone Falls. The conglomerate is 129 m thick; at the Montreal River the conglomerate is 515 m thick. North- west of the conglomerate is the black shale of the Nonesuch Formation; north- west of the Nonesuch is the sandstone of the Freda Formation. There is black shale distributed through 141 m of section, but only 20 m are considered Nonesuch Formation. Above the interval with black shale, the sediment is red shaky arkose.

68

The name Copper Falls comes from a small copper prospect in a small ra-vine about 200 m south of the concession building. The working was begun bythe Ashland Mining Company in August 1864 and closed in February 1866. Cop-per in a quartz vein in diabase was prospected. Apparently the property wasreopened at the turn of the century.

Discussion: Although faults complicate the local geology, the essentiallyconformable relationship between the Oronto Group and Keweenawan flows can beseen. The lower parts of the Oronto Group, the Copper Harbor and theNonesuch, are significantly thinner here than at localities to the east.

The name Copper Falls comes from a small copper prospmt in a small ra- vine about 200 m south of the concession building. The working was begun by the Ashland Mining Company in August 1864 and closed in February 1866. Cop- per in a quartz vein in diabase was prospected. Apparently the property was reopened at the turn of the century.

Discussion: Although faults complicate the local geology, the essentially conformable relationship between the Oronto Group and Keweenawan flows can be seen. The lower parts of the Oronto Group, the Copper Harbor and the Nonesuch, are significantly thinner here than at localities to the east.

Title: South Fish Creek

Location: Exposures in banks of South Fish Creek beneath bridge on north—southsecondary road 1.2 miles south of U. S. Highway 2 on the east line of the SE-,

SE, NE*, Sec. 20, T.47N., R.5W., Bayfield County (Moquah 7.5 minute topographicquadrangle, 1964).

Author: M. E. Ostrom (modified from Myers, 1971)

Description: Exposures of steeply—dipping Freda Sandstone exhibit the lithologicand mineralogic character of the formation. A description of the strata down-stream from the bridge is:

PRECAMBR IAN SYSTEM

Keweenawan Series

Oronto Group

Freda Sandstone Formation (11.0 feet)

11.0' Sandstone, grayish red to reddish brown, uniformly fine—grained,hard, cross—bedded with parting lineation. Much leaching.Penecontenporaneous deformation. Current ripple marks found infloat.

69

T i t l e : South Fish Creek

Location: Exposures i n banks of South Fish Creek beneath bridge on north-south secondary road 1.2 m i l e s south of U. S. Highway 2 on the e a s t l i n e of the SE*, S E ~ , NE*, Sec. 20, T.47N., R.5W., Bayfield County (Moquah 7.5 minute topographic quadrangle, 1964).

Author: M. E. Ostrom (modified from Myers, 1971)

Description: Exposures of steeply-dipping Freda Sandstone e x h i b i t t h e l i t h o l o g i c and mineralogic cha rac te r of the formation. A d e s c r i p t i o n of t h e s t r a t a down- stream from t h e br idge is:

PRECAMBRIAN SYSTEM

Keweenawan Ser ie s

Oronto Group

Freda Sandstone Formation (11.0 f e e t )

11.0' Sandstone, grayish red t o reddish brown, uniformly f ine-grained, hard, cross-bedded with p a r t i n g l i n e a t i o n . Much leaching. Penecontenporaneous deformation. Current r i p p l e marks found i n f l o a t .

70

BASE OF EXPOSURE

Significance: Provides evidence of environmental, geologic and structuralhistory. Examine lithology and mineralogy. What do they signify? What direc-tion is the top of the beds? Measure dip and strike of beds. What do thesemean in terms of structural history? From what direction did the sand come?What is the origin of the red color?

References: Thwaites, 1912; Myers, 1971.

BASE OF EXPOSURE

Significance: Provides evidence of environmental, geologic and structural history. Examine lithology and mineralogy. What do they signify? What direc- tion is the top of the beds? Measure dip and strike of beds. What do these mean in terms of structural history? From what direction did the sand come? What is the origin of the red color?

References: Thwaites, 1912; Myers, 1971.

Title: Washburn Harbor -- Chequamegon Formation BA 48104W/33

Location: Outcrop on northside of Washburn Harbor, SW1/4, SEll4, Sec. 33, T48N, R4W, Bayfield County, Wisconsin (Washburn7.5—minute quadrangle, topographic, 1975).

Description: This low outcrop of Chequamegon sandstone of theBayfield Group consists of thick, massively bedded, fine — tomedium-grained, grayish—red, quartzose sandstone. The grainsare subangular to subrounded. The bedding surfaces are grittyto coarse grained, but are thin and discontinuous. Somebedding planes display scattered small quartz pebbles. Here,bedding dips gently to the southeast; however, at Big RockWayside Park at the east quarter corner of Section 24, T48N,R5W, crossbedded sandstone dips 15 degress to the northwest,suggesting that small, low-amplitude folds with anortheasterly trend may occur in the region.

Discussion: The Chequamegon sandstone is similar to theunderlying Orienta sandstone, which is found in fault contact

Author: M.G. Nudrey, Jr. (1992)

U l e : Washburn Harbor -- Chequamegon Formation BA 48/04W/33

J,ocatton~ Outcrop on northside of Washburn Harbor! SW1/4, SEl/ 4! Sec. 33, T48Nr R4Wr Bayfield Countyf Wisconsin (Washburn 7.5-minute quadrangle! topographic! 1975).

thor: M.G. Mudrey, Jr. (1992)

Description: This low outcrop of Chequamegon sandstone of the Bayfield Group consists of thick, massively bedded! fine - to medium-grained! grayish-red! quartzose sandstone. The grains are subangular to subrounded. The bedding surfaces are gritty to coarse grained, but are thin and discontinuous. Some bedding planes display scattered small quartz pebbles. Here! bedding dips gently to the southeast; however, at Big Rock Wayside Park at the east quarter corner of Section 24! T48Nf R5Wf crossbedded sandstone dips 15 degress to the northwest! suggesting that small! low-amplitude folds with a northeasterly trend may occur in the region.

Qiscussion~ The Chequamegon sandstone is similar to the underlying Orienta sandstone! which is found in fault contact

72

with Keweenawan volcanics and Oronto group strata. BayfieldGroup strata are not known to have high dips, and on thisbasis have been distinguished from the upper parts of theunderlying Freda sandstone. The Douglas Fault intervenesbetween this locality and South Fish Creek where the OrontoGroup dips south at a high angle to the south. A petroleumtest well has been drilled between these two locationsimmediately adjacent to the Douglas Fault. At the time ofthis writing, no further information was available.

with Keweenawan volcanics and Oronto group strata. Bayfield Group strata are not known to have high dipsI and on this basis have been distinguished from the upper parts of the underlying Freda sandstone. The Douglas Fault intervenes between this locality and South Fish Creek where the Oronto Group dips south at a high angle to the south. A petroleum test well has been drilled between these two locations immediately adjacent to the Douglas Fault. At the time of this writingI no further information was available.

REFERENCES

Aldrich, H.R., 1925, VI. General geology, in Wisconsin Geological andNatural History Survey Township Report T. 45 N., R. 2 W. (1924), p.36—69.

Daniels, P.A., Jr., 1982, Upper Precambrian sedimentary rocks: Oronto Group,Michigan—Wisconsin, in Wold, R.J., and Hinze, W.F., eds., Geology andtectonics of the Lake Superior basin: Geological Society of AmericaMemoir 156, p. 107—133.

Dickas, A.B., 1986, Comparative Precambrian stratigraphy and structure alongthe Mid—continent rift: American Association of Petroleum GeologistsBulletin, v. 70, no, 3, p. 227.

Eckert, K.B., 1982, The sandstone architecture of the Lake Superior region:Michigan State University, Lansing, unpublished Ph.D. dissertation,

504 p.

Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region,in Bara,dr, W.R.A., ed., Volcanic regimes in Canada: GeologicalAssociation of Canada Special Paper .16, p. 407—422.

Hainblin, W.K., 1965, Basement control of Keweenawan and Cambriansedimentation in the Lake Superior region: American Association ofPetroleum Geologists Bulletin, v. 49, p. 950—959.

Hatch, J.R., and Morey, G.B., 1985, Hydrocarbon source rock evaluation ofMiddle Proterozoic Solor Church Formation, North American Mid—ContinentRift System, Rice County, Minnesota: American Association of PetroleumGeologists Bulletin, v. 69, no. 8, p. 1208—1216.

Hite, D.M., 1968, Sedimentology of the Upper Keweenawan sequence of northernWisconsin and adjacent Michigan: University of Wisconsin, Madison,unpublished Ph.D. thesis, 217 p.

Hubbard, H. A., 1975, Keweenawan geology of the North Ironwood, Ironwood andLittle Girls Point quadrangles, Gogebic County, Michigan: U.S.

Geological Survey Open File Report OF 75—152, 23 p.

Irving, R.D., 1880, The Keweenawan or copper—bearing system, in Geology ofWisconsin, v. III., p. 185, 205—206.

Lee, C.K., and Kerr, S.D., 1984, Midcontinent—a frontier oil province: Oiland Gas Journal, August 13, 1984, p. 144—150.

Morey, G. B., 1978, Metamorphism in the Lake Superior region, U.S. A., and itsrelation to crustal evolution, in Fraser, J.A., and Heywood, W.W., eds.,Metamorphism in the Canadian Shield: Geological Survey of Canada Paper78—10, P. 283—314.

Morey, G. B., and Ojakangas, R. W., 1982, Keweenawan sedimentary rocks ofeastern Minnesota and northwestern Wisconsin, in Wold, R.J., and Hinze,W.F., eds., Geology and tectonics of the Lake Superior Basin: GeologicalSociety of America Memoir 156, p. 135—146.

73

Aldrich, H.R., 1925, VI. General geology, & Wisconsin Geological and Natural History Survey Township Report T. 45 N., R. 2 W. (19241, p. 36-69.

Daniels, P.A., Jr., 1982, Upper Precambrian sedimentary rocks: Oronto Group, Michigan-Wisconsiny Wold, R.J., and Hinze, W.F., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 107-133.

Dickasy A.B., 1986, Comparative Precambrian stratigraphy and structure along the Mid-continent rift: American Association of Petroleum Geologists Bulletin, v. 70, no, 3, p. 227.

Eckert, K.B., 1982, The sandstone architecture of the Lake Superior region: Michigan State University, Lansing, unpublished Ph.D. dissertation, 504 p.

Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Bara&dr, W.R.A., ed., Volcanic regimes in Canada: Geological - Association of Canada Special Paper.16, p. 407422.

Hamblin, W.K., 1965, Basement control of Keweenawan and Cambrian sedimentation in the Lake Superior region: American Association of Petroleum Geologists Bulletin, v., 49, p. 950-959.

Hatchy J.R., and Morey, G.B., 1985, Hydrocarbon source rock evaluation of Middle Proterozoic Solor Church Formation, North American Mid-Continent Rift System, Rice County, Minnesota: American Association of Petroleum Geologists Bulletin, v. 69, no. 8, p. 1208-1216.

Hite, D.M., 1968, Sedimentology of the Upper Keweenawan sequence of northern Wisconsin and adjacent Michigan: University of Wisconsin, Madison, unpublished Ph.D. thesis, 217 p.

Hubbard, H.A., lW5, Keweenawan geology of the North Ironwood, Ironwood and Little Girls Point quadrangles, Gogebic County, Michigan: U.S. Geological Survey Open File Report OF 75-152, 23 p.

Irving, R.D., 1880, The Keweenawan or copper-bearing system, $J Geology of Wisconsin, v. III., p. 185, 205-206.

Lee, C.K., and Kerr, S.D., 1984, Midcontinent-a frontier oil province: Oil and Gas Journal, August 13, 1984, p. 144-150.

Morey, G.B., 1978, Metamorphism in the Lake Superior regions U.S.A., and its relation to crustal evolution, Fraser, J.A., and Heywood, W.W., edseS Metamorphism in the Canadian Shield: Geological Survey of Canada Paper 78-10, p. 283-314.

Morey, G. I3 . , and 0 jakangas , R. W. , 1982, Keweenawan sedhentary rocks of eastern Minnesota and northwestern Wisconsin, $J Wold, R.JeS and Hinzes W.F., eds., Geology and tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 135-146.

Mudrey, M.G., Jr., 1979, Geologic summary of the Ashland 2° quadrangle:Wisconsin Geological and Natural History Survey Open—file Report WOFR79—1, 39 p.

Myers, W.D., II, 1971, The sedimentology and tectonic significance of theBayfield Group (Upper Keweenawan?), Wisconsin and Minnesota: Universityof Wisconsin, Madison, unpublished Ph.D. thesis, 269 p.

Ostrom, M.E., 1967, Paleozoic stratigraphic nomenclature of Wisconsin:Wisconsin Geological and Natural History Survey Information Circular 8,chart and text.

Ostrom, ME., and Slaughter, A.E., 1967, Correlation problems of the Cambrianand OrdOvician outcrop areas of the Northern Peninsular fsicJ ofMichigan: Annual Field Excursion, Michigan Basin Geological Society,p. 1—5.

Paull, R.K., and Paull, R.A., 1980, Field Guide to Wisconsin and UpperMichigan: Kendall/Hunt Publishing Co., Dubuque, Iowa, 260 p.

Rosenberry, S.C., 1924, VI. General geology, in: Wisconsin Geological andNatural History Survey Township Report T. 47 N., R. 1 E. (1924),p. 17—28.

Thwaites, F.T., 1912, Sandstones of the Wisconsin coast of Lake Superior:Wisconsin Geological and Natural History Survey Bulletin 25, 117 p.

Weiblen, P.W., and Morey, G.B., 1980, A summary of the stratigraphy,petrology and structure of the Duluth Complex: American Journal ofScience, v. 280—A, pt. 1, p. 88—133.

Wold, R.J., and Hinze, W.F., eds., 1982, Geology and teconics of the LakeSuperior Basin: Geological Society of America Memoir 156, 280 p.

74

Mudrey, M.G., Jr., 1979, Geologic summary of the Ashland 20 quadrangle: Wisconsin Geological and Natural History Survey Open-file Report WOFR 79-1, 39 p.

Myers, W.D., 11, 1971, The sedimentology and tectonic significance of the Bayfield Group (Upper Keweenawan?), Wisconsin and Minnesota: University of Wisconsin, Madison, unpublished Ph-D. thesis, 269 p.

Ostrom, M.E., 1967, Paleozoic stratigraphic nomenclature of Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 8, chart and text.

Ostrom, M.E., and Slaughter, A.E., 1967, Correlation problems of the Cambrian and Ordovician outcrop areas of the Northern Peninsular [sic] of Michigan: Annual Field Excursion, Michigan Basin Geological Society, p. 1-5.

Paull, R.K., and Paull, R.A., 1980, Field Guide to Wisconsin and Upper Michigan: Kendall/Hunt Publishing Co., Dubuque, Iowa, 260 p.

Rosenberry, S.C., 1924, VI. General geology, &: Wisconsin Geological and Natural History Survey Township Report T. 47 N., R. 1 E. (1924), p. 17-28.

Thwaites, F.T., 1912, Sandstones of the Wisconsin coast of Lake Superior: Wisconsin Geological and Natural History Survey Bulletin 25, 117 p.

Weiblen, P.W., and Morey, G.B., 1980, A summary of the stratigraphy, petrology and structure of the Duluth Complex: American Journal of Science, v. 280-A, pt. 1, p. 88-133.

Wold, R.J., and Hinze, W.F., eds., 1982, Geology and teconics of the Lake Superior Basin: Geological Society of America Memoir 156, 280 p.

GEOLOGY OFKEWEENAWAN

SUPERGROUP ROCKS

NEAR THEPORCUPINE MOUNTAINS,ONTONAGON & GOGEBIC

COUNTIES, MICHIGAN

William F. Cannon,Suzanne W. Nicholson,

Cheryl A. Hedgman,Laurel G. Woodruff,and Klaus J. Schulz

U.S. Geological Survey,Reston, Virginia

GEOLOGY OF KEWEENAWAN

SUPERGROUP ROCKS NEAR THE

PORCUPINE MOUNTAINS, ONTONAGON & GOGEBIC

COUNTIES, MICHIGAN

William F. Cannon, Suzanne W. Nicholson,

Cheryl A. Hedgman, Laurel G. Woodruff, and Klaus J. Schulz

U.S . Geological Survey, Reston, Virginia

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GEOLOGY OF KEWEENA WAN SUPERGROUP ROCKS NEAR THE PORCUPINE MOUNTAINS,ONTONA GON AND GOGEBIC COUNTIES, MICHIGAN

William F. Cannon, Suzanne W. Nicholson, Cheryl A. Hedgman, Laurel G. Woodruff, andKlaus J. SchulzU.S. Geological Survey, Reston, VA

INTRODUCTION

This field trip examines the geology of rocks of the Keweenawan Supergroup (1 .1 Ga) andrelated intrusive rocks of the Midcontinent rift system (MRS) in the western part of the northernpeninsula of Michigan. The combination of stops includes all formations of the KeweenawanSupergroup in this region. Examination of all described localities requires more than a single dayand participants are encouraged to use this guidebook on their own to supplement the localities thatwill be visited on our one-day trip. Because of uncertainties of weather, road conditions, andremaining snow pack in early May in this region of very heavy snowfall, the stops that we will visitwill not be known until the date of the trip. Stops are numbered in stratigraphic order, from oldestto youngest, not in the order in which they will be visited.

GENERAL GEOLOGY

The Keweenawan Supergroup is a very thick sequence of volcanic and sedimentary rocksthat was deposited during and shortly after an episode of continental rifting at about 1 .1 Ga, whenthe MRS formed within the Proterozoic craton. The Keweenawan Supergroup was deposited in andmarginal to rift graben and in a post-rift thermal basin more or less centered on the axis of theformer rift. In the area of the field trip (figs. 1 and 2), the Keweenawan stratigraphic section (fig. 3)consists of a thin basal quartzite (Bessemer Quartzite) that is overlain by a great thickness ofsubaerial flood basalt flows and lesser andesite and felsite flows (Powder Mill Group and PortageLake Volcanics). The volcanic section becomes progressively thicker toward the rift axis andexceeds 20 km in places beneath Lake Superior (Behrendt and others, 1 988; Cannon and others,1990).

In the vicinity of the Porcupine Mountains, the volcanic sequence includes, at the top, astratovolcano composed mostly of andesite and rhyolite flows and sub-volcanic felsic intrusions(Porcupine Volcanics). These late intermediate and felsic volcanic rocks are atypical of the MRS asa whole and are limited to a few felsic volcanic centers.

Conformably overlying the volcanic rocks are fluvial sedimentary rocks and lesser lacustrinesedimentary rocks (Oronto Group), which are as much as 8 km thick beneath Lake Superior (Cannonand others, 1 990) and at least 5 km thick on shore in the field trip area.

Post-rift reverse faulting has caused block rotation on a very large scale so that theKeweenawan Supergroup section is steeply to vertically dipping. More than 13 km of volcanicrocks and 5 km of sedimentary rocks are exposed in a generally north-facing section. Thisenormously thick section constituted at least half of the crustal thickness by the close of rifting andthus our field trip provides a traverse through the upper half of the Middle Proterozoic crust.

The initial subsidence of the MRS is recorded by deposition of the Bessemer Quartzite (stop1), a blanket of relatively pure, fluvial quartzite as much as 100 m thick (Ojakangas and Morey,1982). It formed in a broad basin, more or less centered on the Site of the future deep rift. Theage of deposition has an upper bound of about 1109 Ma, the age of the oldest overlying basaltflows (Davis and Sutcliffe, 1985). Following deposition of the Bessemer, the area became volcani-cally active and a great thickness (as much as 15 km) of basalts and lesser andesites and rhyolitesaccumulated in only about 1 5 m.y. The earliest basalts constitute the Siemens Creek Formation ofthe Powder Mill Group (Hubbard, 1 975a) (stops 1 and 2), which lies conformably on the BessemerQuartzite. Soft sediment deformation of the Bessemer by the overriding basal basalt flow (seen atstop 1) indicates a very short interval between the units. Locally, as at Stop 1, the basal flow ispillowed, but all succeeding flows are subaerial.

77

GEOLOGY OF KEWEENA WAN SUPERGROUP ROCKS NEAR THE PORCUPINE MOUNTAINS, ONTONAGON AND GOGEBIC COUNTIES. MICHIGAN

William F. Cannon, Suzanne W. Nicholson, Cheryl A. Hedgman, Laurel G. Woodruff, and Klaus J. Schulz U.S. Geological Survey, Reston, VA

INTRODUCTION

This field trip examines the geology of rocks of the Keweenawan Supergroup (1 .I Gal and related intrusive rocks of the Midcontinent rift system (MRS) in the western part of the northern peninsula of Michigan. The combination of stops includes all formations of the Keweenawan Supergroup in this region. Examination of all described localities requires more than a single day and participants are encouraged to use this guidebook on their own to supplement the localities that will be visited on our one-day trip. Because of uncertainties of weather, road conditions, and remaining snow pack in early May in this region of very heavy snowfall, the stops that we will visit will not be known until the date of the trip. Stops are numbered in stratigraphic order, from oldest to youngest, not in the order in which they will be visited.

GENERAL GEOLOGY

The Keweenawan Supergroup is a very thick sequence of volcanic and sedimentary rocks that was deposited during and shortly after an episode of continental rifting at about 1 .I Ga, when the MRS formed within the Proterozoic craton. The Keweenawan Supergroup was deposited in and marginal to rift graben and in a post-rift thermal basin more or less centered on the axis of the former rift. In the area of the field trip (figs. 1 and 21, the Keweenawan stratigraphic section (fig. 3) consists of a thin basal quartzite (Bessemer Quartzite) that is overlain by a great thickness of subaerial flood basalt flows and lesser andesite and felsite flows (Powder Mill Group and Portage Lake Volcanics). The volcanic section becomes progressively thicker toward the rift axis and exceeds 20 km in places beneath Lake Superior (Behrendt and others, 1988; Cannon and others, 1 990).

In the vicinity of the Porcupine Mountains, the volcanic sequence includes, at the top, a stratovolcano composed mostly of andesite and rhyolite flows and sub-volcanic felsic intrusions (Porcupine Volcanics). These late intermediate and felsic volcanic rocks are atypical of the MRS as a whole and are limited to a few felsic volcanic centers.

Conformably overlying the volcanic rocks are fluvial sedimentary rocks and lesser lacustrine sedimentary rocks (Oronto Group), which are as much as 8 km thick beneath Lake Superior (Cannon and others, 1990) and at least 5 km thick on shore in the field trip area.

Post-rift reverse faulting has caused block rotation on a very large scale so that the Keweenawan Supergroup section is steeply to vertically dipping. More than 13 km of volcanic rocks and 5 km of sedimentary rocks are exposed in a generally north-facing section. This enormously thick section constituted at least half of the crustal thickness by the close of rifting and thus our field trip provides a traverse through the upper half of the Middle Proterozoic crust.

The initial subsidence of the MRS is recorded by deposition of the Bessemer Quartzite (stop 1 ), a blanket of relatively pure, fluvial quartzite as much as 100 m thick (Ojakangas and Morey, 1982). It formed in a broad basin, more or less centered on the site of the future deep rift. The age of deposition has an upper bound of about 1109 Ma, the age of the oldest overlying basalt flows (Davis and Sutcliffe, 1985). Following deposition of the Bessemer, the area became volcani- cally active and a great thickness (as much as 15 km) of basalts and lesser andesites and rhyolites accumulated in only about 15 m.y. The earliest basalts constitute the Siemens Creek Formation of the Powder Mill Group (Hubbard, 1975a) (stops 1 and 2). which lies conformably on the Bessemer Quartzite. Soft sediment deformation of the Bessemer by the overriding basal basalt flow (seen at stop 1) indicates a very short interval between the units. Locally, as at stop 1, the basal flow is pillowed, but all succeeding flows are subaerial.

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O.bbrc or dleb...—grancphyn. ifitrunin. cOrgia000

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Volcanic and ..dimsntery rock.

Jccob.vllla S.nd.ton.— •d end bran, f.ld.p.thln and aantoo..scndstone. all (atcn.. nunglanut.

SAUGL— Ornate Group

Fr.do Fcnn.tlc,,— r.d .cndst.n. end nu.,dsl,n.

Noc.aoh Shale— gre—bl eat .llt.tone. shela. hoe needol000

Capper Herbs, Cnnqlansn.te— tears. nod—bins ..nglnr.te sad

Ccpp.r Harbor Ccnglanr.te basalt ,m,H.r

Bergland Grasp

Perapie. Vclo.nlo. rhyollt. m..H.r— .phyr a to gaunt. and f.ldoperpitynlo rhy.lit* in abner 1.1 don... fleas end d.brl. floe.

________

Poroopin. Voloeni.. basalt cod end., it. .e.,*.r thin ft... of and.. It.°F and miner tholelillo ba.clt

ParIng. 1.1.. Vnl.noloe—nmotly nphltlo IlicleIltI. bdeelt floe..nil,,, naHasIt.. nhy.llt. utid intart lee t.ngla,arnt.

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Figure 2. Generalized geologic map and section of the Porcupine Mountains area showingthe location of field trip stops.

79

EXPLANATION Intrusive r o c k s

Oronlo Group

Figure 2, Generalized geologic map and section of the the location of field trip stops.

Porcupine Mountains area showing

80

STOP j

12

LjJocob,ville Sandstone

Freda Sandstone

Nonesuch ShaleCopper Harbor Conglanerate(bosa; t rnerther)

Porcupine Volcanic,

1o

I Portage Lake Volconic,

4

5

Kal lander Creek Formation

3

-2 Sernena Creek Formation

1 0 Bessner QuartziteKM

Early Preterozoic rocks

Figure 3. Stratigraphic section of Keweenawan Supergroup rocks in the field trip areashowing the stratigraphic position of stops. Thicknesses are approximate averagethicknesses for the area. Some formation thicknesses vary significantly across the region.

STOP #

12a Jocobsville Sandstone

Freda Sandstone

Nonesuch Shale Copper Horbor Conglamrote (bosoi k rnmber)

Porcupine Volcanics

Portage Lake Volcanics

Kollander Creek Formation

Siemens Creek Formation

Bess-r Quartzite

Eorly Proterozoic rocks

Figure 3. Stratigraphic section of Keweenawan Supergroup rocks in the field trip area showing the stratigraphic position of stops. Thicknesses are approximate average thicknesses for the area. Some formation thicknesses vary significantly across the region.

Overlying the Siemens Creek Formation is a thick section of more felsic rocks (stop 3)composed of rhyodacite, trachyandesite, andesite, and basalt. These rocks constitute the KallanderCreek Formation of the Powder Mill Group (Hubbard, 1 975a). The Powder Mill volcanic rocks aredistinguished from overlying basalts of similar character largely on the basis of their reversedmagnetic polarity as opposed to the normal polarity of younger units such as the Portage LakeVolcanics. In Michigan, the Powder Mill Group crops out in a belt that is separated from theoutcrop belt of the overlying Portage Lake Volcanics by the Jacobsville Sandstone, which lies withangular unconformity on Powder Mill rocks. In the subsurface, however, the Powder Mill andPortage Lake probably form a continuous depositional sequence.

Volcanic rocks of the Keweenawan Supergroup range in composition from olivine tholeiite torhyolite. By far the dominant rock type is high-Al olivine tholeiite (Al2 03 =15-19 wt%) followed bylesser high-Fe tholeiite and rocks of intermediate and felsic composition (Green, 1982; Brannon,1984; Paces, 1988). The olivine basalts commonly are ophitic in texture and the dominantphenocryst is plagioclase. The most primitive Keweenawan basalts are geochemically similar toprimitive midocean ridge basalts. However, incompatible trace elements in most Keweenawanbasalts are enriched compared to depleted or primitive mantle. Radiogenic isotope analyses (Sr. Nd,and Pb) of the main stage high-Al olivine tholeiites suggest that a likely source of the voluminousbasalts is a mantle plume (Paces and Bell, 1989; Nicholson and Shirey, 1990).

Some of the oldest flows, such as those in part of the Siemens Creek Formation, havedistinctly different chemical compositions than the younger basalts. These basal rocks are transi-tional to weakly alkaline olivine basalts characterized by low A1203 content and clinopyroxenephenocrysts. Locally, in other parts of the rift, basal flows are picritic but such flows have not beenidentified in this area.

Flows near the base of the exposed Portage Lake Volcanics north of the Keweenaw faultwere erupted at about 1096 Ma and those near the top of the formation at about 1094 Ma (Davisand Paces, 1 990). Thus, the great thickness of Portage Lake Volcanics, at least 8 km in this area,was erupted in only a few million years. The estimated eruption rate for Portage Lake Volcanics inthe western Lake Superior region must have approximated the present eruption rate of the Hawaiianhot spot, the most vigorous volcanic center of the modern earth (Cannon, in press). Becausesynchronous volcanism occurred along the entire trend of the rift, the rift system as a whole wasproducing basalt at a rate unrivalled by any modern analog.

The area of the Porcupine Mountains contains a felsic to andesitic volcanic center thatbecame active late in the volcanic history of the region, at about 1090 Ma. The PorcupineVolcanics (previously called the "unnamed formation") (Cannon and Nicholson, 1992) were eruptedfrom that center and accumulated as much as 5 km of andesite, rhyolite, and basalt in a large shielddeposited on top of the Portage Lake Volcanics lava field and centered near the PorcupineMountains. The present arcuate shape of the mountains and the unusual hook-shaped map patternof the Porcupine Volcanics is partly a reflection of the original shape of the volcanic shield.

The Porcupine Volcanics consists of a sequence of subaerially deposited andesite, basalt,felsite, and quartz-porphyry lava flows, and minor interbedded volcaniclastic lithic sandstone,siltstone, and conglomerate (Hubbard, 1 975b). The abundance of felsic rocks and the predomi-nance of andesite over basalt clearly distinguish the Porcupine Volcanics from the underlyingPortage Lake Volcanics. Felsite is most common near the top of the formation where it occurs asboth flows and domes (stops 5 and 6).

The major element chemistry of basalt, basaltic andesite, and andesite of the PorcupineVolcanics and the Portage Lake Volcanics are very similar, but the Porcupine Volcanics are distinctlyenriched in rare earth elements (REE) and Th compared to the Portage Lake Volcanics. The twoformations differ more profoundly in their rhyolite chemistry. Rhyolite that occurs most commonlyin the Portage Lake Volcanics is aphyric or may contain sparse quartz phenocrysts. Rhyolites of thePortage Lake Volcanics on Keweenaw Peninsula have very low abundance of incompatible traceelements (such as REE, Zr, Y, Hf, and Th), whereas the rhyolite body near Bergland (stop 4), one ofthe few rhyolite bodies with the Portage Lake Volcanics in the Porcupine Mountains area, hasmoderate abundances of incompatible trace elements. In contrast, the numerous rhyolite bodies inthe Porcupine Volcanics range from rhyolites that are aphyric to those with abundant quartz and/or

81

Overlying the Siemens Creek Formation is a thick section of more felsic rocks (stop 3) composed of rhyodacite, tra~hyandesite~ andesite, and basalt. These rocks constitute the Kallander Creek Formation of the Powder Mill Group (Hubbardl 1975a). The Powder Mill volcanic rocks are distinguished from overlying basalts of similar character largely on the basis of their reversed magnetic polarity as opposed to the normal polarity of younger units such as the Portage Lake Volcanics. In Michiganl the Powder Mill Group crops out in a belt that is separated from the outcrop belt of the overlying Portage Lake Volcanics by the Jacobsville Sandstone, which lies with angular unconformity on Powder Mill rocks. In the subsurfacef however, the Powder Mill and Portage Lake probably form a continuous depositional sequence.

Volcanic rocks of the Keweenawan Supergroup range in composition from olivine tholeiite to rhyolite. By far the dominant rock type is high-A1 olivine tholeiite (A1203 = 1 5-1 9 wt%) followed by lesser high-Fe tholeiite and rocks of intermediate and felsic composition (Green, 1982; Brannon, 1984; Paces, 1988). The olivine basalts commonly are ophitic in texture and the dominant phenocryst is plagioclase. The most primitive Keweenawan basalts are geochemically similar to primitive midocean ridge basalts. However, incompatible trace elements in most Keweenawan basalts are enriched compared to depleted or primitive mantle. Radiogenic isotope analyses (Sr, Nd, and Pb) of the main stage high41 olivine tholeiites suggest that a likely source of the voluminous basalts is a mantle plume (Paces and Bell, 1989; Nicholson and Shirey, 1990).

Some of the oldest flowsl such as those in part of the Siemens Creek Formation, have distinctly different chemical compositions than the younger basalts. These basal rocks are transi- tional to weakly alkaline olivine basalts characterized by low A1203 content and clinopyroxene phenocrysts. Locally, in other parts of the rift, basal flows are picritic but such flows have not been identified in this area.

Flows near the base of the exposed Portage Lake Volcanics north of the Keweenaw fault were erupted at about 1096 Ma and those near the top of the formation at about 1094 Ma (Davis and Paces, 1990). Thusl the great thickness of Portage Lake Volcanics, at least 8 km in this areaf was erupted in only a few million years. The estimated eruption rate for Portage Lake Volcanics in the western Lake Superior region must have approximated the present eruption rate of the Hawaiian hot spotl the most vigorous volcanic center of the modern earth (Cannonf in press). Because synchronous volcanism occurred along the entire trend of the rift, the rift system as a whole was producing basalt at a rate unrivalled by any modern analog.

The area of the Porcupine Mountains contains a felsic to andesitic volcanic center that became active late in the volcanic history of the regionl at about 1090 Ma. The Porcupine Volcanics (previously called the "unnamed formation") (Cannon and Nicholson, 1992) were erupted from that center and accumulated as much as 5 km of andesite, rhyolite, and basalt in a large shield deposited on top of the Portage Lake Volcanics lava field and centered near the Porcupine Mountains. The present arcuate shape of the mountains and the unusual hook-shaped map pattern of the Porcupine Volcanics is partly a reflection of the original shape of the volcanic shield.

The Porcupine Volcanics consists of a sequence of subaerially deposited andesite, basalt, felsite, and quartz-porphyry lava flows, and minor interbedded volcaniclastic lithic sandstone, siltstonel and conglomerate (Hubbardl 1975b). The abundance of felsic rocks and the predomi- nance of andesite over basalt clearly distinguish the Porcupine Volcanics from the underlying Portage Lake Volcanics. Felsite is most common near the top of the formation where it occurs as both flows and domes (stops 5 and 6).

The major element chemistry of basalt, basaltic andesite, and andesite of the Porcupine Volcanics and the Portage Lake Volcanics are very similarl but the Porcupine Volcanics are distinctly enriched in rare earth elements WEE) and Th compared to the Portage Lake Volcanics. The two formations differ more profoundly in their rhyolite chemistry. Rhyolite that occurs most commonly in the Portage Lake Volcanics is aphyric or may contain sparse quartz phenocrysts. Rhyolites of the Portage Lake Volcanics on Keweenaw Peninsula have very low abundance of incompatible trace elements (such as REE, Zr, Y, Hff and ThIl whereas the rhyolite body near Bergland (stop 41, one of the few rhyolite bodies with the Portage Lake Volcanics in the Porcupine Mountains area, has moderate abundances of incompatible trace elements. In contrast, the numerous rhyolite bodies in the Porcupine Volcanics range from rhyolites that are aphyric to those with abundant quartz andlor

8 1

The thrust faulting and folding can be indirectly dated in the interval of approximately 1060to 1040 Ma (Ruiz and others, 1984; Bornhorst and others, 1988; Cannon and others, 1990). Thecause of the regional compression is not known with certainty but is likely linked in some mannerwith the Grenville orogeny, which was in progress at that time.

FIELD TRIP STOPS

Stop 1 .--Bessemer Quartzite and basalt at the base of the Powder Mill GroupA low rock knob in a pasture east of the county road (see fig. 4) shows quartzite of the

upper part of the Bessemer Quartzite in contact with the overlying basal basalt flow of the PowderMill Group. The basalt is the lowermost flow of the Siemens Creek Formation (Hubbard, 1 975a).The Siemens Creek is composed predominantly of thin flows of basalt and minor andesite. Averagethickness of flows is about 3 m, but the basal flow is about 50 m thick at this locality. Here, thebasal flow is nonporphyritic and holocrystalline. Unlike some of the basal flows along strike to thewest, these flows do not contain clinopyroxene phenocrysts. Chemical analyses (table 1) of thebasal flows are similar to the younger high-alumina main-stage basalts of the Portage LakeVolcanics. The basal Siemens Creek flow is pillowed, which is very unusual for basalts of theKeweenawan Supergroup, nearly all of which are subaerial. In places, stringers of the quartzitehave been injected up into rubbly material at the base of the flow, indicating that the Bessemer wasunconsolidated at the time of eruption.

Note that dips here are about 65° N. Contrast these with steeper to slightly overturneddips at stops 2 and 3, higher in the Powder Mill section. This updip fanning relationship commonlyshown by the Powder Mill flows led Hubbard (1 975a) to propose that they were deposited in abasin that was centered south (updip) of the present outcrop belt and the volcanics originallythickened in that direction. If that interpretation is correct, there must have been a rapid change indepocenters at the outset of volcanism because most paleocurrent directions in the Bessemer arenortherly (Ojakangas and Morey, 1982).

Stop 2.--Basalt flows of Siemens Creek FormationRoadcuts along Powderhorn Road (fig. 4), which follows the valley of Powder Mill Creek,

provide a convenient cross section of the Powder Mill Group in its type area as defined by Hubbard(1 975a). At stop 2 typical basalt flows of the Siemens Creek Formation are exposed and at stop 3the Kallander Creek Formation can be seen. Steeply north-dipping basalt flows of the SiemensCreek Formation are well exposed in outcrops on the hill west of the road and in low roadcuts. Thebasalt flows are thin, typically a few meters or less thick. Commonly these fine-grained flows haveamygdaloidal and rubbly flow tops, and pipe vesicles can be seen locally at the base of some flows.The basalts show patchy alteration to chlorite and have abundant chlorite in the matrix. This rela-tively high degree of metamorphism is typical of basalts of the Siemens Creek Formation and is onedistinction between them and less metamorphosed basalt of the overlying Portage Lake Volcanics.Some flows are porphyritic with small plagioclase phenocrytsts.

Stop 3.--Kallander Creek FormationAbout 2 km north of stop 2, a road improvement project has produced freshly blasted out-

crops in the Kallander Creek Formation. Because all flows are dipping nearly 90° in this area, thedrive between stops passes up through about 2 km of section, an impressive pile of basalt, yet thisis only 10% of the total thickness of basalt erupted into the deepest parts of the rift. The flows inthe Kallander Creek Formation are basaltic to andesitic, more rarely rhyolitic, and generally a fewmeters thick. Overall, the Kallander Creek is more felsic than the Siemens Creek. At this stop theexposed flows include chloritized amygdular basalt, fine-grained basalt with plagioclase phenocryts(locally a TMdaisy stone"), non-porphyritic andesite, andesite with plagioclase phenocrysts, and tuffbreccias containing both basalt and andesite fragments. Although rhyolite occurs higher in theformation, none is exposed here. The Kallander Creek flows are generally less metamorphosed thanthe Siemens Creek flows.

83

The thrust faulting and folding can be indirectly dated in the interval of approximately 1060 to 1040 Ma (Ruiz and othersl 1984; Bornhorst and others, 1988; Cannon and others, 1 990). The cause of the regional compression is not known with certainty but is likely linked in some manner with the Grenville orogeny, which was in progress at that time.

FIELD TRIP STOPS

Stop 1 .--Bessemer Quartzite and basalt at the base of the Powder Mill Group A low rock knob in a pasture east of the county road (see fig. 4) shows quartzite of the

upper part of the Bessemer Quartzite in contact with the overlying basal basalt flow of the Powder Mill Group. The basalt is the lowermost flow of the Siemens Creek Formation (Hubbard, 1975a). The Siemens Creek is composed predominantly of thin flows of basalt and minor andesite. Average thickness of flows is about 3 m, but the basal flow is about 50 m thick at this locality. Here, the basal flow is nonporphyritic and holocrystalline. Unlike some of the basal flows along strike to the west, these flows do not contain clinopyroxene phenocrysts. Chemical analyses (table 1) of the basal flows are similar to the younger high-alumina main-stage basalts of the Portage Lake Volcanics. The basal Siemens Creek flow is pillowedl which is very unusual for basalts of the Keweenawan Supergroupl nearly all of which are subaerial. In placesl stringers of the quartzite have been injected up into rubbly material at the base of the flow, indicating that the Bessemer was unconsolidated at the time of eruption.

Note that dips here are about 65O N. Contrast these with steeper to slightly overturned dips at stops 2 and 3, higher in the Powder Mill section. This updip fanning relationship commonly shown by the Powder Mill flows led Hubbard (1 975a) to propose that they were deposited in a basin that was centered south (updip) of the present outcrop belt and the volcanics originally thickened in that direction. If that interpretation is correctD there must have been a rapid change in depocenters at the outset of volcanism because most paleocurrent directions in the Bessemer are northerly (Ojakangas and Morey, 1982).

Stop 2.--Basalt flows of Siemens Creek Formation Roadcuts along Powderhorn Road (fig. 41, which follows the valley of Powder Mill Creek,

provide a convenient cross section of the Powder Mill Group in its type area as defined by Hubbard (1 975a). At stop 2 typical basalt flows of the Siemens Creek Formation are exposed and at stop 3 the Kallander Creek Formation can be seen. Steeply north-dipping basalt flows of the Siemens Creek Formation are well exposed in outcrops on the hill west of the road and in low roadcuts. The basalt flows are thin, typically a few meters or less thick. Commonly these fine-grained flows have amygdaloidal and rubbly flow topsl and pipe vesicles can be seen locally at the base of some flows. The basalts show patchy alteration to chlorite and have abundant chlorite in the matrix, This rela- tively high degree of metamorphism is typical of basalts of the Siemens Creek Formation and is one distinction between them and less metamorphosed basalt of the overlying Portage Lake Volcanics. Some flows are porphyritic with small plagioclase phenocrytsts.

Stop 3.--Kallander Creek Formation About 2 km north of stop 2, a road improvement project has produced freshly blasted out-

crops in the Kallander Creek Formation. Because all flows are dipping nearly 90Â in this area, the drive between stops passes up through about 2 km of section, an impressive pile of basaltl yet this is only 10% of the total thickness of basalt erupted into the deepest parts of the rift. The flows in the Kallander Creek Formation are basaltic to andesiticD more rarely rhyoliticl and generally a few meters thick. Overall, the Kallander Creek is more felsic than the Siemens Creek. At this stop the exposed flows include chloritized amygdular basalt, fine-grained basalt with plagioclase phenocryts (locally a -daisy stonew), non-porphyritic andesite, andesite with pla~ioclase phenocrysts, and tuff breccias containing both basalt and andesite fragments. Although rhyolite occurs higher in the formation, none is exposed here. The Kallander Creek flows are generally less metamorphosed than the Siemens Creek flows.

84

Figure 4. Location and geologic setting for stops 1, 2, and 3. Geology generalized andmodified from Hubbard (1 975a). Note change in scale between the two adjacentquadrangles.

48N.47N.

asvc'RokFor

Falls

° fPuritSchFigure 4. Location and geologic setting foi stops 1, 2, and 3. Geology generalized and modified from Hubbard (1 975a). Note change in scale between the two adjacent quadrangles.

TABLE 1: CHEMICAL ANALYSES FOR SOME VOLCANIC UNITS IN THE KEWEENAWAN SUPERGROUP

1 2 3 4 5 6 7ELEMENT N=2 N=4 N=5 N=1 N=2 N=1 1 N=8

Si02 52.02 77.23 48.66 71.79 47.60 48.29 50.68Ti02 1.75 0.12 1.69 0.42 2.26 1.70 2.47A1203 14.32 12.91 16.51 12.84 17.03 16.99 14.36Fe203 1.85 0.60 1.83 1.74 2.04 1.65 2.11FeO 9.42 1.19 10.38 3.49 12.30 10.90 11.94Ph0 0.17 0.03 0.18 0.10 0.19 0.22 0.23MgO 6.93 0.30 7.37 1.18 7.60 8.16 5.32CeO 8.18 1.43 10.59 0.23 5.11 6.50 6.46Na20 3.57 1.57 2.29 0.76 4.09 2.86 2.70K20 1.57 4.62 0.32 7.38 1.26 2.53 2.89P205 0.23 0.01 0.18 0.06 0.53 0.20 0.86SUM 99.99 100.00 100.00 99.99 100.00 100.00 100.00

Cr 233 2.9 188 2.5 168 159 64Ni 94 1 204 <2 95.5 188 42Nb 20 50 8 32 17 8.7 18Rb 35 212 7 219 61 62Sr 470 18 252 53 490 407Zr 123 281 100 1130 126 294V 16 112 23 103 26 56La 19.0 76.9 11.6 273 37.2 11.9 55.7Sm 4.9 12.9 4.0 27.3 7.1 4.0 11.1

Yb 1.67 10.00 2.08 10.4 3.55 2.12 5.11

Hf 3.12 10.24 2.65 22.2 4.475 2.65 7.10Ta 1.04 3.92 0.58 2.96 0.86 0.59 1.32

Th 1.94 39.48 1.04 35.70 2.63 1.20 5.94

1 Average analysis of Siemens Creek basalts2 Average analysis of rhyolite from quarry near town of Bergland3 Average analysis of basalts of the Portage Lake Volcanics north of crossroads at Merriweather near Bergland4 Analysis of rhyolite at quarry near White Pine mine5 Average analysis of basalts at Lake of the Clouds overlook in Porcupine Mountains Wilderness State Park6 Average analysis of basalts of the Portage Lake Volcanics on Keweenaw Peninsula7 Average analysis of basalts from the Porcupine Volcanics

TABLE 1: CHEMICAL ANALYSES FOR SOME VOLCANIC UNITS IN THE KEWEENAWAN SUPERGROUP

ELEMENT

Si02 T i 0 2 A1203 Fe203 FeO MnO MgO Cao Na20 K20 P205 SUM

Cr Ni Nb Rb Sr Zr Y La Sm Yb Hf Ta Th

1 Average analysis of Siemens Creek basalts 2 Average analysis of rhyolite from quarry near town of Bergland 3 Average analysis of basalts of the Portage Lake Volcanics north of crossroads at Merriweather near Bergland 4 Analysis of rhyolite at quarry near White Pine mine 5 Average analysis of basalts at Lake of the Clouds overlook in Porcupine Mountains Wilderness State Park 6 Average analysis of basalts of the Portage Lake Vofcanics on Keweenaw Peninsula 7 Average analysis of basalts from the Porcupine Volcanics

86

Stop 4.--Portage Lake VolcanicsThis stop illustrates rhyolite, basalt, and interfiow sandstone of the Portage Lake Volcanics.

Exposures are in a roadcut and quarry along the north side of Michigan Highway 28 (fig. 5). Theprominent south-facing scarp that runs parallel to the highway usually is the geomorphic expressionof the Keweenaw fault, but here the fault lies more than a mile to the south, and the Portage LakeVolcanics underlie the lowlands surrounding the north end of Lake Gogebic as well as the highlandsnorth of the scarp.

The Portage Lake Volcanics are a thick pile of dominantly basaltic, subaerial flows, withlesser rhyolites and interflow sedimentary rocks. At this locality, however, rhyolite is the dominantrock type, although good examples of basalt are in the roadcut. Another unusual feature here is thelocal development of pillows along the toe of a flow where it apparently entered a body of water inwhich sandstone was being deposited.

The stratigraphy in the quarry area consists of an ophitic basalt flow overlain by a thin inter-flow sandstone to siltstone which in turn is overlain by an amygdaloidal basalt. The amygdaloidalbasalt exhibits well developed pipe vesicles at road level. Rhyolite overlies the amygdaloidal basaltand is well exposed in the quarry floor and walls and is overlain by ophitic basalt flows on thehillside above the quarry. The basalts here are identical compositionally to low Ti02 basalts onKeweenaw Peninsula (table 1).

It is not clear whether the rhyolite is a single shallow intrusive body or a series of flows. Itcontains several lenticular masses of basalt and an unusual greenish sandstone. Are these xenolithsin an intrusion or discontinuous interbeds? Several features suggestive of extrusive origin can benoted. Folds are present near the contact of the rhyolite with the underlying basalt. In the upperpart of the quarry, the rhyolite shows well-developed columnar jointing, some of which is bent,following a ramp structure in the rhyolite. The possibility of a series of flows is enhanced by thedistribution of spherulitic rhyolite. Spherulites are common throughout the quarry but are concen-trated near the base of the rhyolite, near the contact with a sedimentary lens, and near the top ofthe quarry. More massive or stony rhyolite occurs between the spherulitic zones.

Alteration related to copper mineralization is widespread in the quarry. Epidote is dissemi-nated in much of the rhyolite, giving it an unusual greenish color. Secondary copper minerals areabundant on joint surfaces.

Stop 5.-Porcupine Volcanics: Rhyolite at Summit PeakSummit Peak is the highest point in Porcupine Mountains Park and one of the highest points

in the state. The observation tower at the summit provides a panoramic view of the field trip area(fig. 6). To the south, the highlands are underlain by the Portage Lake Volcanics and PorcupineVolcanics along the main monocline of volcanic rocks of the Keweenawan Supergroup. Thelowlands immediately to the southeast are underlain by rocks of the Oronto Group in the east-plunging Iron River syncline. Looking east along strike, the smelter at White Pine can be seen in thedistance. To the north, the interior of the park extends over the rugged topography in the fore-ground to Lake Superior in the distance. The interior of the park is maintained as a wilderness areaand access is only by hiking. It contains the largest stand of virgin timber in the state.

Most of the park interior is underlain by a thick unit of rhyolite composed of a series offlows and domes typified by the rocks seen at Summit peak. Good exposures of coarse rhyolitebreccia are along the trail leading to the summit and at the overlook platform west of the summit.This breccia is probably the carapace of a rhyolite dome.

Excellent exposures of typical intermediate and felsic units of the Porcupine Volcanics canbe seen on the north side of the hill (549 m elevation) near the center of Sec. 31. Take the BeaverCreek Trail about 0.5 mi from the Summit Peak parking lot. The units dip to the south and include,in stratigraphic order, sparse outcrops of intermediate to mafic rocks in the creek bed, overlain by avesicular siliceous andesite, which is in turn overlain by a coarse rhyolite breccia or debris flow.The breccia contains clasts ranging in size from nearly a meter to less than 1 cm. The breccia isclast supported and some clasts are subrounded. Overlying the breccia is a medium-grained basaltflow. Capping the hill, and overlying the basalt, is an aphanitic massive rhyolite that ismicrospherulitic.

Stop 4.--Portage Lake Volcanics This stop illustrates rhyolite, basalt, and interflow sandstone of the Portage Lake Volcanics.

Exposures are in a roadcut and quarry along the north side of Michigan Highway 28 (fig. 5). The prominent south-facing scarp that runs parallel to the highway usually is the geomorphic expression of the Keweenaw fault, but here the fault lies more than a mile to the south, and the Portage Lake Volcanics underlie the lowlands surrounding the north end of Lake Gogebic as well as the highlands north of the scarp.

The Portage Lake Volcanics are a thick pile of dominantly basaltic, subaerial flows, with lesser rhyolites and interflow sedimentary rocks. At this locality, however, rhyolite is the dominant rock type, although good examples of basalt are in the roadcut. Another unusual feature here is the local development of pillows along the toe of a flow where it apparently entered a body of water in which sandstone was being deposited.

The stratigraphy in the quarry area consists of an ophitic basalt flow overlain by a thin inter- flow sandstone to siltstone which in turn is overlain by an amygdaloidal basalt. The amygdaloidal basalt exhibits well developed pipe vesicles at road level. Rhyolite overlies the amygdaloidal basalt and is well exposed in the quarry floor and walls and is overlain by ophitic basalt flows on the hillside above the quarry. The basalts here are identical compositionally to low Ti02 basalts on Keweenaw Peninsula (table 1).

It is not clear whether the rhyolite is a single shallow intrusive body or a series of flows. It contains several lenticular masses of basalt and an unusual greenish sandstone. Are these xenoliths in an intrusion or discontinuous interbeds? Several features suggestive of extrusive origin can be noted. Folds are present near the contact of the rhyolite with the underlying basalt. In the upper part of the quarry, the rhyolite shows well-developed columnar jointing, some of which is bent, following a ramp structure in the rhyolite. The possibility of a series of flows is enhanced by the distribution of spherulitic rhyolite. Spherulites are common throughout the quarry but are concen- trated near the base of the rhyolite, near the contact with a sedimentary lens, and near the top of the quarry. More massive or stony rhyolite occurs between the spherulitic zones.

Alteration related to copper mineralization is widespread in the quarry. Epidote is dissemi- nated in much of the rhyolite, giving it an unusual greenish color. Secondary copper minerals are abundant on joint surfaces.

Stop 5.-Porcupine Volcanics: Rhyolite at Summit Peak Summit Peak is the highest point in Porcupine Mountains Park and one of the highest points

in the state. The observation tower at the summit provides a panoramic view of the field trip area (fig. 6). To the south, the highlands are underlain by the Portage Lake Volcanics and Porcupine Volcanics along the main monocline of volcanic rocks of the Keweenawan Supergroup. The lowlands immediately to the southeast are underlain by rocks of the Oronto Group in the east- plunging Iron River syncline. Looking east along strike, the smelter at White Pine can be seen in the distance. To the north, the interior of the park extends over the rugged topography in the fore- ground to Lake Superior in the distance. The interior of the park is maintained as a wilderness area and access is only by hiking. It contains the largest stand of virgin timber in the state.

Most of the park interior is underlain by a thick unit of rhyolite composed of a series of flows and domes typified by the rocks seen at Summit peak. Good exposures of coarse rhyolite breccia are along the trail leading to the summit and at the overlook platform west of the summit. This breccia is probably the carapace of a rhyolite dome.

Excellent exposures of typical intermediate and felsic units of the Porcupine Volcanics can be seen on the north side of the hill (549 m elevation) near the center of Sec. 31. Take the Beaver Creek Trail about 0.5 mi from the Summit Peak parking lot. The units dip to the south and include, in stratigraphic order, sparse outcrops of intermediate to mafic rocks in the creek bed, overlain by a vesicular siliceous andesite, which is in turn overlain by a coarse rhyolite breccia or debris flow. The breccia contains clasts ranging in size from nearly a meter to less than 1 cm. The breccia is clast supported and some clasts are subrounded. Overlying the breccia is a medium-grained basalt flow. Capping the hill, and overlying the basalt, is an aphanitic massive rhyolite that is microspherulitic.

0 1000 2000

CONTOUR INTERVAL 5 METERS

Figure 5. Location and geologic setting of stop 4. Geology generalized from unpublishedcompilation by Walter White.

87

—H-- -j

/1—.

I,

1000 — 0

1 .5

SCALE 1:25 000

1000

KILOMETERS

METERS -0

2000

itt: I

MILES

3000 4000 5000 6000 1000 8000 9000 10000

SCALE 1:25 OW 1 I-- A A 0 KILOM€TE 1 2

1000 0 METERS 1000 2000

1 7 - .5 - - 0 1

MILES

1000 0 1000 .?OM- 3000- 4000 - 5000 6000 7004 8000 WOO 10!00 + - - ------ -

FEET

CONTOUR INTERVAL 5 METERS

Figure 5. Location and geologic setting of stop 4. Geology generalized from unpublished compilation by Walter White.

1000

Oa0 0 1000 2000 3000

2000

51N.

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4000 5000 6000 7000 8000 9000 10000

FEET

88

CONTOUR INTERVAL 5 METERS

Figure 6. Location and geologic setting of stop 5. Recent road and trail improvements, and observation platform and tower on Summit peak are not shown, but all are well marked.

Geology generalized from Copper Range Corp./Michigan Geological Survey cooperative mapping (unpublished).

R.44W.

1 .5 0 KILOMETERS 1

0 METERS 1001)

Stop 6.--Porcupine Volcanics: Rhyolite quarry near White PineThe rocks exposed here are part of a small rhyolite body near the top of the Porcupine

Volcanics. The body probably does not extend much beyond the hill into which the quarry is cut(fig. 7). The quarry provides a cross section through part of a subaerial agglutinate deposit.Agglutinate deposits form at vents by spatter of erupting magma and buildup of mounds of hot,viscous material. The mound of erupted material eventually flows outward under its own weightresulting in large flow folds such as seen in this quarry. The near-vent nature of this deposit isdeduced from the presence of lithic fragments within the rhyolite, large flow folds and stratificationof rhyolite (light and dark units). At the edges of agglutinate deposits, flowage typically hashomogenized the magma and such stratification and folding are generally not preserved.

The rhyolite here contains feldspar phenocrysts, which are typically aligned parallel to thestratification and foliation. It is enriched substantially in such incompatible trace elements as Zr, Ba,Hf, and the light rare earth elements compared to the rhyolite in the Portage Lake Volcanics nearBergland (table 1). These differences suggest different petrogenesis and sources for the tworhyolites in spite of their physical proximity.

Stop 7.--Basalt flows within the Copper Harbor Conglomerate (Lake of the Clouds overlook)Along the road leading to Lake of the Clouds overlook are several exposures of conglomer-

ate of the Copper Harbor Conglomerate. To the north is a good view of Lake Superior and the low-lands underlain by sedimentary rocks of the Oronto Group. From the overlook parking lot, a shorthike leads to the overlook and a spectacular view of Lake of the Clouds and the Porcupine Moun-tains Wilderness State Park.

The overlook is along the south escarpment of a high ridge supported by a series of north-dipping lava flows within the Copper Harbor Conglomerate (fig. 8). The low area south of the ridge,including Lake of the Clouds, is underlain by sandstone and siltstone and a few basalt flows whichconstitute the lower part of the Copper Harbor Conglomerate. The higher regions farther south areunderlain by volcanic rocks, mostly rhyolite of the Porcupine Volcanics.

Toward the east end of the overlook area, a large glaciated surface shows a series of thinbasalt flows, which average a few meters thick. Individual flows can be readily identified by chilledvesicular bases, in places containing inclusions of older flows, and by rubbly or vesicular tops.Abundant epidote alteration and vesicle fillings also impart a distinctive greenish cast to flowmargins. Hubbard (1 975b) described the flows in the Copper Harbor as mostly andesite with minorbasalt, but chemical analyses of two samples from this locality (table 1) indicate that they are basaltsimilar to an average basalt from the Porcupine Volcanics. Compared to Portage Lake Volcanics,these basalts are enriched in incompatible trace elements.

Stop 8.--Upper part of Copper Harbor Conglomerate at Union Bay CampgroundGood exposures of reddish sandstone containing thin conglomerate beds are abundant along

the shore of Lake Superior at this locality (see fig. 9). The Copper Harbor Conglomerate is noted forcoarse volcanogenic conglomerates, which form most of the lower part of the section throughoutmuch of its outcrop belt and which grade up into finer grained sandstones. But, north of thePorcupine Mountains there is a different facies relationship. The lower part of the formation ismostly sandstone, siltstone and lava flows; conglomerate is very subordinate. These rocks underliethe high hills immediately south of the highway. The coarse conglomerate facies is less abundantand is higher in the section. The exposures here at Union Bay are near the base of the upper unitand are probably about 1,000 m above the base of the formation. The sandstones at Union Bay dip10-20° to the north. They are volcanogenic and quartz-poor. They show excellent examples oftrough cross bedding, generally indicating a northeastward current vector, and a variety of othersedimentary features including desiccation cracks, rip-up clasts, oscillation and current ripples, andswash marks.

The exposures of Copper Harbor Conglomerate north of the Porcupine Mountains are thefarthest removed from the source highlands to the south of any part of the unit on land. Therelative scarcity of thick units of coarse conglomerate compared to exposures farther south probably

89

Stop 6.--Porcupine Volcanics: Rhyolite quarry near White Pine The rocks exposed here are part of a small rhyolite body near the top of the Porcupine

Volcanics. The body probably does not extend much beyond the hill into which the quarry is cut (fig. 7). The quarry provides a cross section through part of a subaerial agglutinate deposit. Agglutinate deposits form at vents by spatter of erupting magma and buildup of mounds of hotl viscous material. The mound of erupted material eventually flows outward under its own weight resulting in large flow folds such as seen in this quarry. The near-vent nature of this deposit is deduced from the presence of lithic fragments within the rhyolite, large flow folds and stratification of rhyolite (light and dark units). At the edges of agglutinate deposits, flowage typically has homogenized the magma and such stratification and folding are generally not preserved.

The rhyolite here contains feldspar phenocrystsl which are typically aligned parallel to the stratification and foliation. It is enriched substantially in such incompatible trace elements as Zr, Ba, Hfl and the light rare earth elements compared to the rhyolite in the Portage Lake Volcanics near Bergland (table 1). These differences suggest different petrogenesis and sources for the two rhyolites in spite of their physical proximity.

Stop 7.--Basalt flows within the Copper Harbor Conglomerate (Lake of the Clouds overlook) Along the road leading to Lake of the Clouds overlook are several exposures of conglomer-

ate of the Copper Harbor Conglomerate. To the north is a good view of Lake Superior and the low- lands underlain by sedimentary rocks of the Oronto Group. From the overlook parking lotl a short hike leads to the overlook and a spectacular view of Lake of the Clouds and the Porcupine Moun- tains Wilderness State Park.

The overlook is along the south escarpment of a high ridge supported by a series of north- dipping lava flows within the Copper Harbor Conglomerate (fig. 8) . The low area south of the ridgel including Lake of the Cloudsl is underlain by sandstone and siltstone and a few basalt flows which constitute the lower part of the Copper Harbor Conglomerate. The higher regions farther south are underlain by volcanic rocksl mostly rhyolite of the Porcupine Volcanics.

Toward the east end of the overlook areal a large glaciated surface shows a series of thin basalt flowsl which average a few meters thick. Individual flows can be readily identified by chilled vesicular basesl in places containing inclusions of older flows, and by rubbly or vesicular tops. Abundant epidote alteration and vesicle fillings also impart a distinctive greenish cast to flow margins. Hubbard (1 975b) described the flows in the Copper Harbor as mostly andesite with minor basaltl but chemical analyses of two samples from this locality (table 1) indicate that they are basalt similar to an average basalt from the Porcupine Volcanics. Compared to Portage Lake Volcanics, these basalts are enriched in incompatible trace elements.

Stop 8.--Upper part of Copper Harbor Conglomerate at Union Bay Campground Good exposures of reddish sandstone containing thin conglomerate beds are abundant along

the shore of Lake Superior at this locality (see fig. 91. The Copper Harbor Conglomerate is noted for coarse volcanogenic conglomerates, which form most of the lower part of the section throughout much of its outcrop belt and which grade up into finer grained sandstones. Butl north of the Porcupine Mountains there is a different facies relationship. The lower part of the formation is mostly sandstonel siltstone and lava flows; conglomerate is very subordinate. These rocks underlie the high hills immediately south of the highway. The coarse conglomerate facies is less abundant and is higher in the section. The exposures here at Union Bay are near the base of the upper unit and are probably about 1,000 m above the base of the formation. The sandstones at Union Bay dip 10-20Â to the north. They are volcanogenic and quartz-poor. They show excellent examples of trough cross bedding, generally indicating a northeastward current vectorl and a variety of other sedimentary features including desiccation cracksl rip-up clasts, oscillation and current ripplesl and swash marks.

The exposures of Copper Harbor Conglomerate north of the Porcupine Mountains are the farthest removed from the source highlands to the south of any part of the unit on land. The relative scarcity of thick units of coarse conglomerate compared to exposures farther south probably

89

ft 41 W.

2

/4 - '— —---- _\__ \—I --\ //— ° 0 ' -- (Cdn9$,Wc3i:T2

k1>r : /

SCALE 1:24000

FEET

5 0 1 KILOMETER— — I

CONTOUR INTERVAL 20 FEET

Figure 7. Location and geologic setting of stop 6. Geology generalized from Johnson andWhite (1969).

1. 50 N.

1 MILE

90

SCALE 1:24000 0 1 MILE 1 i - - - w , I

1 OCO 0 1000 2003 3000 4000 5030 6000 7000 FEET H U H , I

1 w - - - - *

5 0 1 KILOMETER 4

CONTOUR INTERVAL 20 FEET

Figure 7. Location and geologic setting of stop 6. Geology generalized from Johnson and White (1 969).

51 N.

KILOMETERS 1 .5 0 1

_______

2

H—

1

MILES 1 .5 0

SCALE 1:250001 CENTIMETER ON THE MAP REPIIESENTS 250 METERS ON THE GROUND

CONTOUR INTERVAL 5 METERS

Figure 8. Location and geologic setting of stop 7. Geology generalized from Hubbard(1 975b).

91

R. 44 W.

1 2 KILOMETERS 1 .5 0 I 3

k [ - - - -

H H H W - 0 1 MILES 1 .5

SCALE 1:25 000 1 (2ENllMElE.R ON T I E MAP REPRESENTS 2% MEIERS ON THE GROUND

CONTOUR INTERVAL 5 METERS

Figure 8. Location and geologic setting of stop 7 . Geology generalized from Hubbard (1 975b).

92

R. 43 W.

51N.

SCALE 1:24 000

— .5 — KILOMETERS 1 2

1000 0 METERS 1000 2000

1 .5 0

MILES

1000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

FEET

CONTOUR INTERVAL 5 METERS

Figure 9. Location and geologic setting of stop 8. Geology generalized from Hubbard

(1 975b).

Figure 9. Location and geologic setting of stop 8. Geology generalized from Hubbard (1 975b).

reflects the distal nature of these rocks and may be a good representative of much of the CopperHarbor beneath Lake Superior. The unit here shows a crude coarsening-upward trend as opposedto the fining-upward trend typical of the more proximal parts of the unit. This relationship is consis-tent with northward prograding alluvial plain deposition.

Several large boulders are distributed along the beach and are composed of conglomeratetypical of the lower part of the formation elsewhere. These conglomerates contain almost exclu-sively clasts of Keweenawan Supergroup volcanic rocks types common in the region. An interest-ing question is the source of these boulders. Although the Copper Harbor does contain someconglomerate beds of this aspect nearby, none of the streams entering Lake Superior near hereseem capable of transporting such large boulders. Most have low gradients, especially near thelakeshore, and the streambeds do not contain such large boulders. No outcrops of coarse conglom-erate would be expected nearby to the north beneath the lake; so if the boulders are glacial erratics,they must have been transported for a long distance, perhaps from the vicinity of Isle Royale.

Stop 9.--Nonesuch Shale at Bonanza FallsThe most complete exposure of the Nonesuch Shale in the region is along the Big Iron River

near Bonanza Falls, although access to parts of it is difficult and dangerous at times of high water(see fig. 10). The Nonesuch is exposed nearly continuously in a gently southeast-dipping sectionfrom just upstream of Bonanza Falls to the sharp bend in the river near the northeast corner ofsection 13. A detailed measured section is presented by Suszek (1991) and fig. 11 is generalizedfrom that section. The exposed rocks total 226 m of section, which includes nearly all of theNonesuch, although neither the upper nor lower contact is directly exposed.

The Nonesuch Shale is distinguished from other sedimentary units of the KeweenawanSupergroup by the predominance of gray, green or black, fine-grained sediments. The Big Iron Riversection has a predominance of siltstones and fine sandstones over true shales. Many rocks showtrough cross bedding, symmetrical and asymmetrical ripples, rib and furrow structures, and partinglineations. The finer-grained rocks include well-laminated shales, which are most abundant lower inthe section. The shaley units commonly have ball and pillow structures and calcareous concretions.

The Nonesuch displays coarsening-upward sequences at scales ranging from a few metersto the entire thickness of the unit. On a smaller scale, normally graded sequences are common inunits from a few centimeters to a few meters thick.

In the lower 10 m of the section copper mineralization can be seen as concentrations ofchalcocite, bornite and malachite along bedding planes. The mineralization is cogenetic with themajor copper mineralization at the White Pine Mine where the downdip extension of this unit ismined just to the south and east.

A good exposure of the mineralized base of the Nonesuch Shale and the top of the CopperHarbor Conglomerate is along the Little Iron River near the center of the SW 1/4, Sec. 13, butrequires a walk of about 1 mi south from Highway 107. It is an easy walk along an unmaintainedtrail on the east bank of the river for those who can spend more time in the area. Remains of earlymining efforts for native silver can be seen there, as well as "ore" specimens from old dumps.

White Pine MineThe trip will not visit the White Pine Mine; but because of the importance of this orebody,

the following brief summary is provided.White Pine recovers copper and silver from a very large strataform orebody in the base of

the Nonesuch Shale and in places from the upper few meters of the Copper Harbor Conglomerate.Excellent summaries of the geology and origin of the orebody are provided by White and Wright(1966), Ensign and others (1968), White (1971), and Brown (1971). Ore, composed mostly ofchalcocite and lesser native copper, grades about 1 .1 % Cu and 9 g/t Ag over a mining heighttypically about 5 m. Some beds within that interval consistently contain greater than 3% Cu.Reserves are about 200,000,000 t of extractable ore. Mining of the gently dipping orebody is byroom and pillar method. The mine workings underlie about 35 km2 of the low country south andeast of the town of White Pine.

93

reflects the distal nature of these rocks and may be a good representative of much of the Copper Harbor beneath Lake Superior. The unit here shows a crude coarsening-upward trend as opposed to the fining-upward trend typical of the more proximal parts of the unit. This relationship is consis- tent with northward prograding alluvial plain deposition.

Several large boulders are distributed along the beach and are composed of conglomerate typical of the lower part of the formation elsewhere. These conglomerates contain almost exclu- sively clasts of Keweenawan Supergroup volcanic rocks types common in the region. An interest- ing question is the source of these boulders. Although the Copper Harbor does contain some conglomerate beds of this aspect nearby, none of the streams entering Lake Superior near here seem capable of transporting such large boulders. Most have low gradients, especially near the lakeshore, and the streambeds do not contain such large boulders. No outcrops of coarse conglom- erate would be expected nearby to the north beneath the lake; so if the boulders are glacial erratics, they must have been transported for a long distance, perhaps from the vicinity of Isle Royale.

Stop 9.--Nonesuch Shale at Bonanza Falls The most complete exposure of the Nonesuch Shale in the region is along the Big Iron River

near Bonanza Falls, although access to parts of it is difficult and dangerous at times of high water (see fig. 10). The Nonesuch is exposed nearly continuously in a gently southeast-dipping section from just upstream of Bonanza Falls to the sharp bend in the river near the northeast corner of section 13. A detailed measured section is presented by Suszek (1 991 ) and fig. 1 1 is generalized from that section. The exposed rocks total 226 m of section, which includes nearly all of the Nonesuch, although neither the upper nor lower contact is directly exposed.

The Nonesuch Shale is distinguished from other sedimentary units of the Keweenawan Supergroup by the predominance of gray, green or black, fine-grained sediments. The Big Iron River section has a predominance of siltstones and fine sandstones over true shales. Many rocks show trough cross bedding, symmetrical and asymmetrical ripples, rib and furrow structures, and parting lineations. The finer-grained rocks include well-laminated shales, which are most abundant lower in the section. The shaley units commonly have ball and pillow structures and calcareous concretions.

The Nonesuch displays coarsening-upward sequences at scales ranging from a few meters to the entire thickness of the unit. On a smaller scale, normally graded sequences are common in units from a few centimeters to a few meters thick.

In the lower 10 m of the section copper mineralization can be seen as concentrations of chalcocite, bornite and malachite along bedding planes. The mineralization is cogenetic with the major copper mineralization at the White Pine Mine where the downdip extension of this unit is mined just to the south and east.

A good exposure of the mineralized base of the Nonesuch Shale and the top of the Copper Harbor Conglomerate is along the Little Iron River near the center of the SW 114, Sec. 13, but requires a walk of about 1 mi south from Highway 107. It is an easy walk along an unmaintained trail on the east bank of the river for those who can spend more time in the area. Remains of early mining efforts for native silver can be seen there, as well as "ore" specimens from old dumps.

White Pine Mine The trip will not visit the White Pine Mine; but because of the importance of this orebody,

the following brief summary is provided. White Pine recovers copper and silver from a very large strataform orebody in the base of

the Nonesuch Shale and in places from the upper few meters of the Copper Harbor Conglomerate. Excellent summaries of the geology and origin of the orebody are provided by White and Wright (1 966), Ensign and others (1 9681, White (1 971 1, and Brown (1 971 1. Ore, composed mostly of chalcocite and lesser native copper, grades about 1.1 % Cu and 9 glt Ag over a mining height typically about 5 m. Some beds within that interval consistently contain greater than 3% Cu. Reserves are about 200,000,000 t of extractable ore. Mining of the gently dipping orebody is by room and pillar method. The mine workings underlie about 35 km2 of the low country south and east of the town of White Pine.

5 0

CONTOUR INTERVAL 20 FEET

Figure 10. Location and geologic setting of stop 9. Geology generalized from CopperRange Corp./Michigan Geological Survey cooperative mapping (unpublished).

4 0

94

MILE

I KILOMETER

1 1 w - 7

7 0 1 MILE

7 I i

I 5 0 ; KILOMETER + 4 - - -

CONTOUR INTERVAL 20 FEET

Figure 10. Location and geologic setting of stop 9. Geology generalized from Copper Range CorpJMichigan Geological Survey cooperative mapping (unpublished).

STRATICRAPHIC SECTION OF NONESUCH SHALE AT BONANZA FALLSGeneralized from Susiek (1991)

Sandstone, siltstone, rnudstone. Fine— tocoarse—grai ned. coarsening—upward trend.Reddish—brown units increase in top 50 m

200— of unit.

SiItstone, sandstone. mustone. Fine— tomedi.rn—grained, general coarsening upward trend.

Sndstcne. shale. Coarse— to fine— grained.

Sandstone. shale. Fine— tornediun—grained.Contains some red—brown units.

Shale, sandstone. mudstone. Black to dark gray.Lominites. trough cross bedS, and undulatorybedding. Chioritic and henatitic beds and

oc lenses. Calcareous concretions.

Sandstone, siltstone. mudstone. Light to dcrkgray with some beds of red hematitic mudstoneand siltstone. Fine—grained. Containscarbonate lominite units, ball and pillowstructures, and colcareous concretions.

Fissil. shcie, mudstone, siltstone. Black todark gray. Carbonate lanintes with interbedsof siltstone and sandstone.

Siltstone, sandstone, shale. Fine—grained.Dessication cracks, ball and pillowstructures, floating clasts.

Figure 11. Measured stratigraphic section of Nonesuch Shale along Big Iron River nearBonanza Falls, generalized from Suszek (1 991).

95

STRATIGRAPHIC SECTION OF NONESUCH SHALE AT BONANZA FALLS Generalized from Suszek (1991)

Sandstone, siltstone. mudstone. Fine- to coarse-grained, coarsening-upward trend. Reddish-brown units increase in top 50 rn of unit.

8 3

Siltstone. sandstone. muastone. Fine- to mediim-grained, general ccarsening upward trend

Sandstone, shale. Coarse- to fine- grained.

Sandstone, shale. Fine- to mediim-grained. Contains some red-brown units.

Shale, sandstone, mudstone. Black to dark gray. Lcminites. trough cross beds. end undulatory bedding. Chloritic and hematitic beds and lenses. Calcareous concretions.

Sandstone, siltstone. mudstone. Light to dark gray with some beds of red hematitic mudstone and siltstone. Fine-grained. Contains carbonate iominita units, boll and pillow structures, and calcareous concretjons.

Fissile shale. mudstone, siltstone. Block to dark gray. Carbonate laninites with interbeds of siltstone and sandstone.

Siltstone, sandstone. shale. Fine-grained. Dessication cracks, ball and pillow structures, floating clasts.

Figure 11. Measured stratigraphic section of Nonesuch Shale along Big Iron River near Bonanza Falls, generalized from Suszek (1 991 1.

96

Copper was introduced to the Nonesuch Shale mostly during early diagenesis, probably byupward circulating connate water which dissolved copper from the underlying redbeds. Chalcocite,largely of submicroscopic size, formed by the replacement of diagenetic pyrite. A later phase ofcopper mineralization, documented by Mauk and others (1989) and Mauk and others (in press),introduced native copper, mostly in structurally disturbed zones. This second stage mineralization isprobably cogenetic with the classic native copper mineralization of the Keweenawan Supergroupbasalts.

Stop 1 O.--Nonesuch Shale and Freda Sandstone at Presque Isle RiverThe upper portion of the Nonesuch Shale and the base of the Freda Sandstone are well

exposed in the gorge of the Presque Isle River near its mouth and along the shore of Lake Superiorwest of the river (fig. 12). Continuous exposures along the picturesque gorge of the river extendfrom just upstream of Nawadaha Falls to the lakeshore. Exposures continue in bluffs along thelakeshore for about half a mile west of the river mouth. In the interest of time, we will limit ourexamination to exposures near the river mouth and a short distance to the west along the shore.This requires a round trip hike of nearly a mile, mostly on well-maintained trails and stairways.

The rocks exposed here are on the northeast limb of the Presque Isle syncllne, a gentlenorthwest-plunging fold. Dips range from nearly flat to about 100 SW. The Nonesuch Shale here isvery similar to that described for stop 9 on the Big Iron River about 30 km to the east. The lowerpart of the section in this area (not exposed) is also strongly mineralized with copper and contains afine-scale stratigraphy directly correlatable with the stratigraphy at the white Pine mine, indicatingthat sedimentary conditions were very uniform over the entire region surrounding the PorcupineMountains during Nonesuch deposition.

The upper part of the Nonesuch exposed here, as described in more detail by Suszek(1991), consists of fining-upward sequences and laminites interbedded with graded sandstones andsiltstones. Most rocks are dark gray with a few reddish lenses and beds.

The Nonesuch grades upward to the Freda Sandstone through a zone in which dark graylaminated and small-scale cross-bedded siltstone and sandy mudstone is interbedded with medium-to coarse-grained reddish brown sandstone. The upper contact of the Nonesuch is placed wherethe reddish sandstone becomes dominant.

Stop 11.-- Freda Sandstone along Presque Isle RiverThis stop, near the axis of the Presque Isle syncline (fig. 12), shows reddish cross bedded

sandstone typical of the lower part of the Freda Sandstone. The gently southwest dipping bedsexposed here are probably 100-200 m above the base of the formation and slightly higher strati-graphically than those seen at stop 10. They are mostly lithic sandstone and are somewhat mica-ceous in places. The Freda marks the return to fluvial redbed deposition following the lacustrinedeposition of the underlying Nonesuch Shale.

The Freda is a very thick unit in much of the rift in the Lake Superior region and is volumet-rically the dominant unit of the post-rift sedimentary fill. Along the Montreal River, about 30 km tothe west, about 4,000 m of Freda are exposed. Seismic sections indicate that it is even thickerbeneath the Lake.

The Freda Sandstone generally becomes finer-grained and more mature upward. Excellentexposures of the upper part of the Freda can be seen near the mouth of the Montreal River, in bluffsalong the lakeshore.

Stop 12.-- Jacobsville SandstoneThe roadcut along the west side of the road is a fairly typical exposure of the conglomerate

found abundantly in the Jacobsville Sandstone in this region. Although exposures are rare, west ofLake Gogebic the Jacobsville appears to be dominated by conglomerate and coarse sandstone incontrast to areas farther east where it is a submature to mature quartzose, feldspathic, and lithicarenite with minor shale, siltstone, and conglomerate layers (Kalliokoski, 1982).

At this stop the conglomerate strikes east and dips 9° to the north, but only about 500 mto the north it dips steeply to the south (62°) as a result of drag along the Keweenaw fault. The

Copper was introduced to the Nonesuch Shale mostly during early diagenesis, probably by upward circulating connate water which dissolved copper from the underlying redbeds. Chalcocite, largely of submicroscopic size, formed by the replacement of diagenetic pyrite. A later phase of copper mineralization, documented by Mauk and others (1 989) and Mauk and others (in press), introduced native copper, mostly in structurally disturbed zones. This second stage mineralization is probably cogenetic with the classic native copper mineralization of the Keweenawan Supergroup basalts.

Stop 10.-Nonesuch Shale and Freda Sandstone at Presque Isle River The upper portion of the Nonesuch Shale and the base of the Freda Sandstone are well

exposed in the gorge of the Presque Isle River near its mouth and along the shore of Lake Superior west of the river (fig. 12). Continuous exposures along the picturesque gorge of the river extend from just upstream of Nawadaha Falls to the lakeshore. Exposures continue in bluffs along the lakeshore for about half a mile west of the river mouth. In the interest of time, we will limit our examination to exposures near the river mouth and a short distance to the west along the shore. This requires a round trip hike of nearly a mile, mostly on well-maintained trails and stairways.

The rocks exposed here are on the northeast limb of the Presque Isle syncline, a gentle northwest-plunging fold. Dips range from nearly flat to about 10Â SW. The Nonesuch Shale here is very similar to that described for stop 9 on the Big Iron River about 30 km to the east. The lower part of the section in this area (not exposed) is also strongly mineralized with copper and contains a fine-scale stratigraphy directly correlatable with the stratigraphy at the White Pine mine, indicating that sedimentary conditions were very uniform over the entire region surrounding the Porcupine Mountains during Nonesuch deposition.

The upper part of the Nonesuch exposed here, as described in more detail by Suszek (1 991 1, consists of fining-upward sequences and laminites interbedded with graded sandstones and siltstones. Most rocks are dark gray with a few reddish lenses and beds.

The Nonesuch grades upward to the Freda Sandstone through a zone in which dark gray laminated and small-scale cross-bedded siltstone and sandy mudstone is interbedded with medium- to coarse-grained reddish brown sandstone. The upper contact of the Nonesuch is placed where the reddish sandstone becomes dominant.

Stop 11 .-- Freda Sandstone along Presque Isle River This stop, near the axis of the Presque Isle syncline (fig. 121, shows reddish cross bedded

sandstone typical of the lower part of the Freda Sandstone. The gently southwest dipping beds exposed here are probably 100-200 m above the base of the formation and slightly higher strati- graphically than those seen at stop 10. They are mostly lithic sandstone and are somewhat mica- ceous in places. The Freda marks the return to fluvial redbed deposition following the lacustrine deposition of the underlying Nonesuch Shale.

The Freda is a very thick unit in much of the rift in the Lake Superior region and is volumet- rically the dominant unit of the post-rift sedimentary fill. Along the Montreal River, about 30 km to the west, about 4,000 m of Freda are exposed. Seismic sections indicate that it is even thicker beneath the Lake.

The Freda Sandstone generally becomes finer-grained and more mature upward. Excellent exposures of the upper part of the Freda can be seen near the mouth of the Montreal River, in bluffs along the lakeshore.

Stop 12.- Jacobsville Sandstone The roadcut along the west side of the road is a fairly typical exposure of the conglomerate

found abundantly in the Jacobsville Sandstone in this region. Although exposures are rare, west of Lake Gogebic the Jacobsville appears to be dominated by conglomerate and coarse sandstone in contrast to areas farther east where it is a submature to mature quartzose, feldspathic, and lithic arenite with minor shale, siltstone, and conglomerate layers (Kalliokoski, 1982).

At this stop the conglomerate strikes east and dips 9O to the north, but only about 500 m to the north it dips steeply to the south (62O) as a result of drag along the Keweenaw fault. The

96

17

R. 45W. 1

/'12 20/ — —

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- y:.__i . Il'\ '% 25

J ____KILOMETERS 1 .5 0

1— — — — —— — — — — —1

MILESI .5 0

SCALE 1:25 0001 CENW4ETER ON ThE MAP REPRESENTS 250 METERS ON ThE GROUND

CONTOUR INTERVAL 5 METERS

Figure 12. Location and geologic setting of stops 10 and 11. Geology generalized fromCopper Range Corp./Michigan Geological Survey cooperative mapping (unpublished).

97

KILOMETERS 1 .5 0 I * - - - -

1

r - - - - - 2

MILES 1 .5 0 I 1

SCALE 1:25 000 1 CENTIMETER ON THE MAP REPRESENTS 250 METERS ON THE GROUND

CONTOUR INTERVAL 5 METERS

Figure 12. Location and geologic setting of stops 10 and 1 1. Geology generalized from Copper Range CorpJMichigan Geological Survey cooperative mapping (unpublished).

R. 45 W.

'405 :

.fflN

21 r_) 2- Portage Lake V canics -

E AWALK

12- - T. 48 N.

Gravel Pit Jacob lie Sandston —

28 (Hr NH

410 --

H H

. H) '1000 0 2000 4000 6000 FEET

98

SCALE 1:25 0001 CENTIMETER ON THE MAP REPRESENTS 250 METERS ON ThE GROUND

CONTOUR INTERVAL 5 METERS

Figure 13. Location and geologic setting of stop 12. Geology generalized from CopperRange Corp./Michigan Geological Survey cooperative mapping (unpublished).

omaston500 0 1000

/

2000 METERS500 H +-- H

0 1000 2000 METERS 1 1

H H H ! , 1 ! !

1000 0 2000 4000 6000 FEET

SCALE 1:25 000 1 CENTIMETER ON THE MAP REPRESENTS 250 METERS ON THE GROUND

CONTOUR INTERVAL 5 METERS

Figure 13. Location and geologic setting of stop 1 2. Geology generalized from Copper Range CorpJMichigan Geological Survey cooperative mapping (unpublished).

conglomerate is poorly consolidated and weathers to a material that superficially resembles glacialtill or outwash, which covers most of the roadcut. From time to time parts of this material slumpaway to expose less weathered conglomerate, and small exposures of fresh conglomerate areusually exposed in the drainage ditch beside the road.

The conglomerate is clast-supported. Coarse sandy matrix surrounds pebbles. Whereweathered, it is generally unconsolidated but on fresher surfaces is fairly well cemented. Clay fillsmost interstices and coats all grains, It is uncertain whether the clay is a product of diageneticcement or is an early mechanical infiltration resulting from seepage of muddy water through coarsealluvium.

Clasts are subrounded to well rounded, range in size from 3-10 cm, and consist mostly ofiron-formation (63-77%) and quartzite (8-20%) derived from Early Proterozoic rocks such as thosenow exposed in the Gogebic Range. Small amounts of altered rhyolite (1-5%) are believed to bederived from Keweenawan Supergroup rocks. Vein quartz (9-10%), chert (1%), and other litholo-gies (1 % or less) derived from Archean rocks are minor. Several other conglomerate outcrops tothe west have similar compositions, but outcrops less than 2 km to the southeast along JacksonCreek (shown by X on the map) are dominated by vein quartz (73-82%) with lesser amounts ofquartzite (11-13%) and iron-formation (9-12%). These clast lithologies suggest that most of thesource area to the south had been stripped of Keweenawan volcanics by the time of Jacobsvilledeposition. Although the surface consisted mostly of Early Proterozoic sedimentary rocks, thepresence of large quantities of vein quartz pebbles indicates at least some drainages were erodeddown into Archean crystalline rocks.

In a few places clasts found in place appear to be imbricated but otherwise bedding withinthe conglomerate units is not apparent. Clay and iron coatings cover nearly all clasts so that thevarious lithologic types are not apparent unless clasts are broken open. Many clasts have a highlyshiny surface or patina that looks remarkably like desert varnish and some have distinctly striatedsurfaces of unknown origin. A few clasts have been found that have shapes and surface texturesresembling ventifacts.

In the streambanks east of the road, slightly higher in the section, the conglomerate is inter-layered with coarse sandstone. The sandstone layers are poorly sorted, discontinuous irregularlayers approximately 5-30 cm thick. The sandstone consists of subangular to subrounded, mediumto coarse sand (.25-2 mm) and gravel (2-30 mm) with larger cobbles scattered throughout. In thinsection the sand is framework supported and composed predominantly of quartz and feldspar withminor lithic fragments, biotite, and muscovite. Pores are filled with clay and many grains are coatedwith iron minerals.

99

conglomerate is poorly consolidated and weathers to a material that superficially resembles glacial till or outwash, which covers most of the roadcut. From time to time parts of this material slump away to expose less weathered conglomerate, and small exposures of fresh conglomerate are usually exposed in the drainage ditch beside the road.

The conglomerate is clast-supported. Coarse sandy matrix surrounds pebbles. Where weathered, it is generally unconsolidated but on fresher surfaces is fairly well cemented. Clay fills most interstices and coats all grains. It is uncertain whether the clay is a product of diagenetic cement or is an early mechanical infiltration resulting from seepage of muddy water through coarse alluvium.

Clasts are subrounded to well rounded, range in size from 3-1 0 cm, and consist mostly of iron-formation (63-77%) and quartzite (8-20%) derived from Early Proterozoic rocks such as those now exposed in the Gogebic Range. Small amounts of altered rhyolite (1-5%) are believed to be derived from Keweenawan Supergroup rocks. Vein quartz (9-1 0%), chert (1 %I, and other litholo- gies (1 % or less) derived from Archean rocks are minor. Several other conglomerate outcrops to the west have similar compositions, but outcrops less than 2 km to the southeast along Jackson Creek (shown by X on the map) are dominated by vein quartz (73-82%) with lesser amounts of quartzite (1 1-1 3%) and iron-formation (9-1 2%). These clast lithologies suggest that most of the source area to the south had been stripped of Keweenawan volcanics by the time of Jacobsville deposition. Although the surface consisted mostly of Early Proterozoic sedimentary rocks, the presence of large quantities of vein quartz pebbles indicates at least some drainages were eroded down into Archean crystalline rocks.

In a few places clasts found in place appear to be imbricated but otherwise bedding within the conglomerate units is not apparent. Clay and iron coatings cover nearly all clasts so that the various lithologic types are not apparent unless clasts are broken open. Many clasts have a highly shiny surface or patina that looks remarkably like desert varnish and some have distinctly striated surfaces of unknown origin. A few clasts have been found that have shapes and surface textures resembling ventifacts.

In the streambanks east of the road, slightly higher in the section, the conglomerate is inter- layered with coarse sandstone. The sandstone layers are poorly sorted, discontinuous irregular layers approximately 5-30 cm thick. The sandstone consists of subangular to subrounded, medium to coarse sand (.25-2 mm) and gravel (2-30 mm) with larger cobbles scattered throughout. In thin section the sand is framework supported and composed predominantly of quartz and feldspar with minor lithic fragments, biotite, and muscovite. Pores are filled with clay and many grains are coated with iron minerals.

100

REFERENCES CITED

Behrendt, J.C., Green, A.G., Cannon, W.F., Hutchinson, D.R., Lee, M., Milkereit, B., Agena,W.F., and Spencer, C., 1988, Crustal structure of the Midcontinent Rift System--results from GLIMPCE deep seismic reflection profiles: Geology, v. 16, p. 81-85.

Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K., 1988, Age ofnative copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v.83, p. 619-625.

Brannon, J.C., 1984, Geochemistry of successive lava flows of the Keweenawan NorthShore Volcanic Group: St. Louis, Washington University, Ph.D. dissertation, 312 p.

Brown, A.C., 1971, Zoning in the White Pine Copper deposit, Ontonagon County, Michigan:Economic Geology, v. 66, P. 543-573.

Cannon, W.F., in press, The Midcontinent Rift in the Lake Superior region with emphasis onits geodynamic evolution: Tectonophysics.

Cannon, W.F., Green, A.G., Hutchinson, D.R. Lee, M., Milkereit, B., Behrendt, J.C., Halls,H.C., Green, J.C., Morey, G.B., Sutcliffe, R., and Spencer, C., 1989, The NorthAmerican Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflectionprofiling: Tectonics, v. 8, p. 305-322.

Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1990, Structural and isotopic evidence forMiddle Proterozoic thrust faulting of Archean and Early Proterozoic rocks near theGogebic Range, Michigan and Wisconsin (abs): 36th Institute on Lake SuperiorGeology, Thunder Bay, Ontario Proceedings, Part 1, Abstracts, p. 11-13.

Cannon, W.F., and Nicholson, S.W., 1992, Contributions to the geology and mineralresources of the Midcontinent rift system, A--Revisions of stratigraphic nomenclaturewithin the Keweenawan Supergroup of northern Michigan: U.S. Geological SurveyBulletin 1970, p. A1-A8.

Copper Range Corp./Michigan Geological Survey cooperative mapping program, unpublishedmaps on file at Geological Survey Division, Michigan Department of NaturalResources, Lansing, Mich.

Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the KeweenawPeninsula and implications for development of the Midcontinent rift system: Earthand Planetary Science Letters, v. 97, p. 54-64.

Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern LakeSuperior: Geological Society of America Bulletin, v. 96, p. 1 572-1 579.

Ensign, C.O., White, W.S., Wright, J.C., Patrick, J.L., Leone, R.J., Hathaway, D.J., Tramell,J.W., Fritts, J.J., and Wright, T.L., 1968, Copper deposits in the Nonesuch Shale,White Pine, Michigan, in Ridge, J.D., ed., Ore deposits of the United States, 1 933-1967; the Graton-Sales Volume: American Institute of Mining, Metallurgical, andPetroleum Engineers, Inc., New York, p. 460-488.

Green, J.C., 1982, Geology of Keweenawan extrusive rocks, in Wold, R.J., and Hinze,W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society ofAmerica Memoir 156, p. 47-55.

REFERENCES CITED

Behrendt, J.C., Green, A.G., Cannon, W.F., Hutchinson, D.R., Lee, M., Milkereit, B., Agena, W.F., and Spencer, C., 1988, Crustal structure of the Midcontinent Ri f t System-- results from GLIMPCE deep seismic reflection profiles: Geology, v. 16, p. 81 -85.

Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K., 1988, Age of native copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v. 83, p. 61 9-625.

Brannon, J.C., 1984, Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group: St. Louis, Washington University, Ph.D. dissertation, 31 2 p.

Brown, A.C., 1971, Zoning in the White Pine Copper deposit, Ontonagon County, Michigan: Economic Geology, v. 66, p. 543-573.

Cannon, W.F., in press, The Midcontinent Rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics.

Cannon, W.F., Green, A.G., Hutchinson, D.R.., Lee, M., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Morey, G.B., Sutcliffe, R., and Spencer, C., 1989, The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p. 305-322.

Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1990, Structural and isotopic evidence for Middle Proterozoic thrust faulting of Archean and Early Proterozoic rocks near the Gogebic Range, Michigan and Wisconsin (abs): 36th Institute on Lake Superior Geology, Thunder Bay, Ontario Proceedings, Part 1, Abstracts, p. 1 1-1 3.

Cannon, W.F., and Nicholson, S.W., 1992, Contributions to the geology and mineral resources of the Midcontinent rift system, A--Revisions of stratigraphic nomenclature within the Keweenawan Supergroup of northern Michigan: U.S. Geological Survey Bulletin 1970, p. A1 -A8.

Copper Range CorpJMichigan Geological Survey cooperative mapping program, unpublished maps on file at Geological Survey Division, Michigan Department of Natural Resources, Lansing, Mich.

Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent rift system: Earth and Planetary Science Letters, v. 97, p. 54-64.

Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior: Geological Society of America Bulletin, v. 96, p. 1572-1 579.

Ensign, C.O., White, WS., Wright, J.C., Patrick, J.L., Leone, R.J., Hathaway, D.J., Tramell, J.W., Fritts, J.J., and Wright, T.L., 1968, Copper deposits in the Nonesuch Shale, White Pine, Michigan, in Ridge, J.D., ed., Ore deposits of the United States, 1933- 1967; the Graton-Sales Volume: American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., New York, p. 460-488.

Green, J.C., 1982, Geology of Keweenawan extrusive rocks, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 47-55.

Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile of the southernmargin of the Midcontinent Rift System in western Lake Superior: Tectonics, v. 9,p. 303-310.

Hubbard, H.A., 1 975a, Lower Keweenawan volcanic rocks of Michigan and Wisconsin:U.S. Geological Survey Journal of Research, v. 3, p. 529-541.

______

1 975b, Geology of Porcupine Mountains in Carp River and White Pine quadrangles,Michigan: U.S Geological Survey Journal of Research, v. 3, P. 519-528.

Johnson, R.F., and White, W.S., 1969, Preliminary report on the bedrock geology andcopper deposits of the Matchwood quadrangle, Ontonagon County, Michigan: U.S.Geological Survey Open-File Report, 31 p.

Kalliokoski, J., 1982, Jacobsville Sandstone, in Wold, R.J., and Hinze, W.J., eds., Geologyand tectonics of the Lake Superior basin: Geological Society of America Memoir156, p. 147-1 56.

King, E.R., 1987, Aeromagnetic map of the Iron River 1 °x2° quadrangle, Michigan andWisconsin: U.S. Geological Survey Miscellaneous Investigations Map l-1360F, scale1:250,000.

Klasner, J.S., 1989, Bouguer gravity anomaly map and geologic interpretation of the IronRiver 1 °x2° quadrangle, Michigan and Wisconsin: U.S. Geological SurveyMiscellaneous Investigations Map I-i 360E, scale 1:250,000.

Mauk, J.L., Kelly, W.C., van der Pluijm, B.A., and Seasor, R.W., in press, Relations betweendeformation and sediment-hosted copper mineralization: evidence from the WhitePine portion of the Midcontinent rift system: Geology.

Mauk, J.L., Seasor, R.W., Kelly, W.C., Andrews, R.A., and Nelson, W.S., 1989, The WhitePine stratiform copper deposit, in Margeson, G.B., ed., Precambrian geology andmetal occurrences, Michigan's upper peninsula: Society of Economic Geologistsfield conference, p. 143-153.

Nicholson, S.W., and Shirey, S.B., 1990, Midcontinent rift volcanism in the Lake Superiorregion--Sr. Nd and Pb isotopic evidence for a mantle plume origin: Journal ofGeophysical Research, v. 95, p. 10581-10868.

Ojakangas, R.W., and Morey, G.B., 1982, Keweenawan pre-volcanic quartz sandstones andrelated rocks of the Lake Superior region, in Wold, R.J., and Hinze, W.J., eds.,Geology and tectonics of the Lake Superior basin: Geological Society of AmericaMemoir 156, p. 85-96.

Paces, J.B., 1988, Magmatic processes, evolution and mantle source characteristicscontributing to the petrogenesis of Midcontinent rift basalts--Portage Lake basalts,Keweenaw Peninsula, Michigan: Houghton, Michigan Technological University,Ph.D. dissertation, 413 p.

Paces, J.B., and Bell, K., 1989, Non-depleted sub-continental mantle beneath the SuperiorProvince of the Canadian Shield: Nd-Sr isotopic and trace element evidence fromMidcontinent rift basalts: Geochimica et Cosmochimica Acta, v. 53, p. 2023-2035.

101

Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile of the southern margin of the Midcontinent Rift System in western Lake Superior: Tectonics, v. 9, p. 303-31 0.

Hubbard, H.A., 1975a, Lower Keweenawan volcanic rocks of Michigan and Wisconsin: U.S. Geological Survey Journal of Research, v. 3, p. 529-541.

1975b, Geology of Porcupine Mountains in Carp River and White Pine quadrangles, Michigan: U.S Geological Survey Journal of Research, v. 3, p. 51 9-528.

Johnson, R.F., and White, W.S., 1969, Preliminary report on the bedrock geology and copper deposits of the Matchwood quadrangle, Ontonagon County, Michigan: U.S. Geological Survey Open-File Report, 31 p.

Kalliokoski, J., 1982, Jacobsville Sandstone, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 147-1 56.

King, E.R., 1987, Aeromagnetic map of the Iron River 1 Ox2O quadrangle, Michigan and Wisconsin: U.S. Geological Survey Miscellaneous Investigations Map 1-1 360F, scale 1 :250,000.

Klasner, J.S., 1989, Bouguer gravity anomaly map and geologic interpretation of the Iron River 1 Ox2O quadrangle, Michigan and Wisconsin: U.S. Geological Survey Miscellaneous Investigations Map 1-1 360E, scale 1 :250,000.

Mauk, J.L., Kelly, W.C., van der Pluijm, B.A., and Seasor, R.W., in press, Relations between deformation and sediment-hosted copper mineralization: evidence from the White Pine portion of the Midcontinent rift system: Geology.

Mauk, J.L., Seasor, R.W., Kelly, W.C., Andrews, R.A., and Nelson, W.S., 1989, The White Pine stratiform copper deposit, in Margeson, G.B., ed., Precambrian geology and metal occurrences, Michigan's upper peninsula: Society of Economic Geologists field conference, p. 143-1 53.

Nicholson, S.W., and Shirey, S.B., 1990, Midcontinent rift volcanism in the Lake Superior region--Sr, Nd and Pb isotopic evidence for a mantle plume origin: Journal of Geophysical Research, v. 95, p. 10581 -1 0868.

Ojakangas, R.W., and Morey, G.B., 1982, Keweenawan pre-volcanic quartz sandstones and related rocks of the Lake Superior region, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 85-96.

Paces, J.B., 1988, Magmatic processes, evolution and mantle source characteristics contributing to the petrogenesis of Midcontinent rift basalts-Portage Lake basalts, Keweenaw Peninsula, Michigan: Houghton, Michigan Technological University, Ph.D. dissertation, 413 p.

Paces, J.B., and Bell, K., 1989, Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent rift basalts: Geochimica et Cosmochimica Acta, v. 53, p. 2023-2035.

102

Ruiz, J., Jones, L.M., and Kelly, W.C., 1984, Rubidium-strontium dating of ore depositshosted by Rb-rich rocks using calcite and other common Sr-bearing minerals:Geology, v. 12, p. 259-262.

Suszek, T.J., 1991, Petrography and sedimentation of the Middle Proterozoic(Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent riftsystem: Duluth, University of Minnesota-Duluth, M.S. thesis, 1 98 p.

White, W.S., 1971, A paleohydrologic model for mineralization of the White Pine copperdeposit, northern Michigan: Economic Geology, v. 66, p. 1-13.

White, W.S., and Wright, J.C., 1 966, Sulfide mineral zoning in the basal Nonesuch Shale,northern Michigan: Economic Geology, v. 61, p. 1171-11 90.

Ruiz, J., Jones, L.M., and Kelly, W.C., 1984, Rubidium-strontium dating of ore deposits hosted by Rb-rich rocks using calcite and other common Sr-bearing minerals: Geology, v. 12, p. 259-262.

Suszek, T.J., 1991, Petrography and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent rift system: Duluth, University of Minnesota-Duluth, M.S. thesis, 198 p.

White, W.S., 1971, A paleohydrologic model for mineralization of the White Pine copper deposit, northern Michigan: Economic Geology, v. 66, p. 1-13.

White, W.S., and Wright, J.C., 1966, Sulfide mineral zoning in the basal Nonesuch Shale, northern Michigan: Economic Geology, v. 61, p. 1 171 -1 1 90.

GEOLOGY OF ThEGREAT LAKES

TECTONIC ZONEINTHE

MARQUE TIE AREA,MICHIGAN

A LATE ARCHEANPALEOSUTURE

P.K. Sims and Z.E. PetermanU.S. Geological Survey, Federal Center,

Denver, Colorado

TECTONIC ZONE IN - THE

MARQUETTE AREA, MCHIGAN-

A LATE ARCHEAN PALEOSUTURE

P.K. Sims and Z.E. Peterrnan U.S . Geological Survey, Federal Center,

Denver, Colorado

Guide to the geology of the Great Lakes tectonic zone in theMarquette area, Michigan--A Late Archean paleosuture

P.K. Sims and Z.E. Peterman

SUMMARY

The Great Lakes tectonic zone (GLTZ) is an Archean crustal boundary of subcontinental lengththat separates a greenstone-granite terrane (southern part of Superior province of CanadianShield) on the north from a partly older gneiss terrane on the south. The GLTZ is generallyinterpreted as a paleosuture resulting from continent-continent collision. The tectonic zone iscovered in most of the Lake Superior region by Proterozoic rocks or Pleistocene glacial deposits,and its position and characteristics were previously determined mainly by geophysical data.Geologic mapping in the Marquette, Michigan area provides for the first time directobservations of the structure.

In the Marquette area, the GL1'Z is characterized by a zone of mylonite (orthomylonite) thatformed at brittle-ductile transition conditions; this was superposed on previously deformed rocksof both the Archean greenstone-granite terrane and the Archean gneiss terrane. Theboundaries of the GLTZ trend about N. 600 W. Foliation in the mylonite strikes aboutN. 70° W. and dips steeply southwest. A pronounced stretching lineation and tight fold hingesplunge about 42° S. 430 E. The attitude of the stretching lineation (line of tectonic transport),together with asymmetric structures indicative of movement sense, indicate that collision at thislocality was oblique. This resulted in dextral-thrust shear along the boundary, northwestwardvergence, and probable overriding of the greenstone-granite terrane by the gneiss terrane.Transmittal of the dextral shear stress across a large area of the greenstone-granite crust(Superior province) to the north may have formed the nearly east-west foliation, upright folds,and northwest- to east-west-trending dextral faults and shear zones at least as far north as theQuetico fault, in southern Ontario, a distance of about 250 km. These structures in Superiorprovince rocks are superposed on older recumbent folds.

As a whole, the GL1'Z is characterized by systematic angular bends that alternately trend west-northwestward, as in the Marquette area, and northeastward. This zigzag pattern probablyreflects relict irregularities in the continental margin (Superior province) composed ofgreenstone-granite crust. Late Archean convergence along this margin resulted in a variabletrajectory of stress into the greenstone-granite crust and probably in along-strike diachroneity oforogeny. The major deformation resulted from oblique compression at promontories, whichacted as buttresses against which compressive stress was directed into the crust. In addition tothe dominant foliation, major brittle-ductile to brittle strike-slip faults also formed, such as theVermilion fault system in northern Minnesota and the Quetico and Rainy Lake-Seine Riverfaults in southern Ontario; these resulted from a more brittle continuum of the transcurrentshear caused by collision along the GLTZ.

INTRODUCTION

The Great Lakes tectonic zone (GLTZ) is an Archean crustal boundary more than 1,000 kmlong that separates a greenstone-granite terrane (southern part of Superior province) on thenorth from a gneiss terrane on the south (Sims and others, 1980; Sims and Peterman, 1981;Peterman, 1979). The GLTZ is covered throughout most of the Great Lakes region by younger

105

Guide to the geology of the Great Lakes tectonic zone in the Marquette area, Michigan--A Late Archean paleosuture

P.K. Sims and Z.E. Peterman

SUMMARY

The Great Lakes tectonic zone (GLTZ) is an Archean crustal boundary of subcontinental length that separates a greenstone-granite terrane (southern part of Superior province of Canadian Shield) on the north from a partly older gneiss terrane on the south., The GLTZ is generally interpreted as a paleosuture resulting from continent-continent collision. The tectonic zone is covered in most of the Lake Superior region by Proterozoic rocks or Pleistocene glacial deposits, and its position and characteristics were previously determined mainly by geophysical data. Geologic mapping in the Marquette, Michigan area provides for the first time direct observations of the structure.

In the Marquette area, the GLTZ is characterized by a zone of mylonite (orthomylonite) that formed at brittle-ductile transition conditions; this was superposed on previously deformed rocks of both the Archean greenstone-granite terrane and the Archean gneiss terrane. The boundaries of the GLTZ trend about N. 60' W. Foliation in the mylonite strikes about N. 70' W. and dips steeply southwest. A pronounced stretching lineation and tight fold hinges plunge about 42' S. 43O E. The attitude of the stretching lineation (line of tectonic transport), together with asymmetric structures indicative of movement sense, indicate that collision at this locality was oblique. This resulted in dextral-thrust shear along the boundary, northwestward vergence, and probable overriding of the greenstone-granite terrane by the gneiss terrane. Transmittal of the dextral shear stress across a large area of the greenstone-granite crust (Superior province) to the north may have formed the nearly east-west foliation, upright folds, and northwest- to east-west-trending dextral faults and shear zones at least as far north as the Quetico fault, in southern Ontario, a distance of about 250 krn. These structures in Superior province rocks are superposed on older recumbent folds.

As a whole, the GLTZ is characterized by systematic angular bends that alternately trend west- northwestward, as in the Marquette area, and northeastward. This zigzag pattern probably reflects relict irregularities in the continental margin (Superior province) composed of greenstone-granite crust. Late Archean convergence along this margin resulted in a variable trajectory of stress into the greenstone-granite crust and probably in along-strike diachroneity of orogeny. The major deformation resulted from oblique compression at promontories, which acted as buttresses against which compressive stress was directed into the crust. In addition to the dominant foliation, major brittle-ductile to brittle strike-slip faults also formed, such as the Vermilion fault system in northern Minnesota and the Quetico and Rainy Lake-Seine River faults in southern Ontario; these resulted from a more brittle continuum of the transcurrent shear caused by collision along the GLTZ.

INTRODUCTION

The Great Lakes tectonic zone (GLTZ) is an Archean crustal boundary more than 1,000 krn long that separates a greenstone-granite terrane (southern part of Superior province) on the north from a gneiss terrane on the south (Sims and others, 1980; Sims and Peterman, 1981; Peterman, 1979). The GLTZ is covered throughout most of the Great Lakes region by younger

Proterozoic rocks or Pleistocene glacial deposits, but recently it has been delineated and studiedin outcrop in an area south of Marquette, Michigan (fig. 1).

The boundary was first recognized in Minnesota (Sims and Morey, 1973; Morey and Sims, 1976)from regional geologic relations, which indicated that the two adjoining basement terranes haddifferent geologic histories and probably had evolved separately. Regional magnetic and gravitydata were utilized to determine the position of the boundary. Later (Sims, 1980), the boundarywas approximately delineated in the western part of Upper Michigan (Sims, 1980; Sims andothers, 1984) and northwestern Wisconsin (Sims and others, 1985), east of the MiddleProterozoic Midcontinent rift system, and it was inferred by indirect evidence to extendeastward through the Sudbury structure, where it is truncated by the Middle ProterozoicGrenville tectonic zone (Sims and others, 1980).

Recent geologic mapping in the Sands and Palmer 7½-minute quadrangles, Michigan (fig. 1),previously mapped by Gair and Thaden (1968) and Gair (1975), has delineated this Archeanboundary in outcrop for the first time (Sims, 1991). It is exposed on the south side of the EarlyProterozoic Marquette synclinorium, and its northwestern projection into the trough coincideswith a major Early Proterozoic fault, the Richmond fault. In the Marquette area, the GLTZ isa mylonite zone about 2 km wide that is overprinted on rocks of both the greenstone-graniteterrane and the Archean gneiss terrane. In this area, the GL1'Z is interpreted as a continent-continent collision zone. The collision was oblique, resulting in dextral wrench shear on theN. 60° W.-trending boundary and northwestward vergence of the gneiss terrane against thegreenstone-granite terrane.

The purpose of this field guide is to examine the exposed GLTZ in the context of the regionalgeology, to discuss genetic relationships between convergence along the boundary and structuralfeatures in the Archean rocks to the north, and to present a new interpretation of the evolutionof the GLTZ throughout the Lake Superior region.

GEOLOGIC SETI1NG

The Great Lakes tectonic zone (GLTZ) is moderately well exposed in the Sands and Palmer7½-minute quadrangles in Upper Michigan (fig. 1). It separates the two distinctive Archeanterranes in the area. The northern greenstone-granite terrane is composed largely of LateArchean granitoid rocks, with subordinate approximately coeval metavolcanic andmetasedimentary rocks of greenstone affinity. The layered rocks and most of the granitoidrocks were metamorphosed (mainly to greenschist facies) and deformed during Late Archeanorogeny. The southern Archean gneiss terrane is composed mainly of layered gneiss, migmatite,and amphibolite--rocks that are distinct from those in the greenstone-granite terrane. Exceptfor late-tectonic to post-tectonic, generally small(?) granitoid bodies, the rocks of the terrane aremetamorphosed, mainly to amphibolite facies. The rocks exposed within the two terranes inUpper Michigan are closely similar to those in Minnesota (Morey and Sims, 1976; Sims andothers, 1980), thus establishing the identity of the GL1'Z in the Marquette area.

The Archean rocks in Michigan are overlain in the Marquette synclinorium, the Republictrough, and the Dead River, Clark Creek, and Baraga basins by shelf and foredeep deposits ofthe Early Proterozoic Marquette Range Supergroup (fig. 1; Cannon and Gair, 1970; Barovichand others, 1989).

106

Proterozoic rocks or Pleistocene glacial deposits, but recently it has been delineated and studied in outcrop in an area south of Marquette, Michigan (fig. 1).

The boundary was first recognized in Minnesota (Sims and Morey, 1973; Morey and Sirns, 1976) from regional geologic relations, which indicated that the two adjoining basement terranes had different geologic histories and probably had evolved separately. Regional magnetic and gravity data were utilized to determine the position of the boundary. Later (Sims, 1980), the boundary was approximately delineated in the western part of Upper Michigan (Sims, 1980; Sims and others, 1984) and northwestern Wisconsin (Sims and others, 1985), east of the Middle Proterozoic Midcontinent rift system, and it was inferred by indirect evidence to extend eastward through the Sudbury structure, where it is truncated by the Middle Proterozoic Grenville tectonic zone (Sims and others, 1980).

Recent geologic mapping in the Sands and Palmer 7%-minute quadrangles, Michigan (fig. I), previously mapped by Gair and Thaden (1968) and Gair (1975), has delineated this Archean boundary in outcrop for the first time (Sims, 1991). It is exposed on the south side of the Early Proterozoic Marquette synclinorium, and its northwestern projection into the trough coincides with a major Early Proterozoic fault, the Richmond fault. In the Marquette area, the GLTZ is a mylonite zone about 2 krn wide that is overprinted on rocks of both the greenstone-granite terrane and the Archean gneiss terrane. In this area, the GLTZ is interpreted as a continent- continent collision zone. The collision was oblique, resulting in dextral wrench shear on the N. 60Â W.-trending boundary and northwestward vergence of the gneiss terrane against the greenstone-granite terrane.

The purpose of this field guide is to examine the exposed GLTZ in the context of the regional geology, to discuss genetic relationships between convergence along the boundary and structural features in the Archean rocks to the north, and to present a new interpretation of the evolution of the GLTZ throughout the Lake Superior region.

GEOLOGIC SETTING

The Great Lakes tectonic zone (GLTZ) is moderately well exposed in the Sands and Palmer 7%-minute quadrangles in Upper Michigan (fig. 1). It separates the two distinctive Archean terranes in the area. The northern greenstone-granite terrane is composed largely of Late Archean granitoid rocks, with subordinate approximately coeval metavolcanic and metasedimentary rocks of greenstone affinity. The layered rocks and most of the granitoid rocks were metamorphosed (mainly to greenschist facies) and deformed during Late Archean orogeny. The southern Archean gneiss terrane is composed mainly of layered gneiss, migmatite, and amphibolite--rocks that are distinct from those in the greenstone-granite terrane. Except for late-tectonic to post-tectonic, generally small(?) granitoid bodies, the rocks of the terrane are metamorphosed, mainly to amphibolite facies. The rocks exposed within the two terranes in Upper Michigan are closely similar to those in Minnesota (Morey and Sims, 1976; Sims and others, 1980), thus establishing the identity of the GLTZ in the Marquette area.

The Archean rocks in Michigan are overlain in the Marquette synclinorium, the Republic trough, and the Dead River, Clark Creek, and Baraga basins by shelf and foredeep deposits of the Early Proterozoic Marquette Range Supergroup (fig. 1; Cannon and Gair, 1970; Barovich and others, 1989).

Van Hise and Bayley (1897) introduced the names northern complef for the Archean rocksnorth of the Marquette synclinorium and southern complef for the Archean rocks south of thesyndilnorium.

A Late Archean age for the GL1'Z is now established by regional geologic relationships innorth-central United States. The Archean rocks in the greenstone-granite terrane in northernMinnesota (Hudleston and others, 1988) and northernmost Michigan (fig. 1; Sims, 1991) arecharacterized mainly by ductile and brittle structures formed in response to dextral shear, whichaccords with the deformation pattern observed in exposures of the GLTZ south of Marquette.These structures include a generally west-trending steep foliation and upright folds, widespreadZ-shaped folds, and northwest- to west-trending dextral faults, indicative of dextral shear; thestructures are superposed on recumbent folds (Johnson and Bornhorst, 1991). In contrast, EarlyProterozoic Penokean deformation in the Marquette area had little effect on the Archeanbasement (Cambray, 1984). Cambray (1984) proposed that the Penokean deformation wasproduced by horizontal compression that was transmitted from the basement to the folded coverrocks by narrow ductile shears in the basement. Readjustment of rigid basement blocks alongthese shears utilized old weaknesses, which resulted in some shortening but not folding of thebasement rocks. For the Early Proterozoic Marquette syndlinorium, Cambray (1984) proposed anearly north-south compressional axis, which initially produced reverse dip slip on faultsbounding the syncline and compressed the Early Proterozoic sedimentary rocks within thesyncline into west-trending folds. Subsequently, resistance to this movement within the troughresulted in sinistral strike slip motion and the development of F2 folds with northwest-trendingaxial surfaces and variable plunge.

Gair and Thaden (1968) applied the name "Compeau Creek Gneis? to both the foliatedgranitoid rocks of the Archean greenstone-granite terrane (northern complex) and the layeredgneisses and massive intrusions of the gneiss terrane (southern complex), and subsequentinvestigators extended this terminology to the western parts of the southern complex (Cannonand Simmons, 1973). To apply this name to rocks in both Archean terranes, however, isinappropriate, because the two terranes consist of distinctive rock types of different origins.Accordingly, in this guide informal lithologic names are used to describe the crystalline rocks inthe two terranes.

ARCHEAN GREENSTONE-GRANITE TERRANE

The greenstone-granite terrane in the Marquette area (fig. 1) consists of a several thousand-meter-thick succession of metamorphosed subaqueous mafic to felsic flows, pyroclastic rocks,and volcanogenic sedimentary rocks that is intruded by small bodies of gabbro and ultramaficrocks and by large plutons of granitoid rocks (Bornhorst, 1988; Johnson and Bornhorst, 1991).The metamorphosed volcanic rocks have been named the Ishpeming greenstone belt (Morganand DeCristoforo, 1980); they represent the southwestern extension of the Wawa(Shebandowan) subprovince of the Superior province of Canada (Card and Ciesielski, 1986).Felsic volcanic rocks adjacent to an ultramafic body north of Ishpeming are host to the golddeposits of the Ropes mine (Brozdowski and others, 1986; Brozdowski, 1988, 1989), and theultramauic body hosts additional mineral prospects (Bodwell, 1972). Foliated tonalite from thenorthern complex (Hammond, 1978) in the greenstone-granite terrane, has a U-Pb zircon age of2,703±16 Ma (recalculated by Zell E. Peterman), and associated rhyolite has a U-Pb zircon ageof 2,780 ± 69 Ma (recalculated by Zell E. Peterman). These ages are consistent with moreprecise U-Pb ages in the Wawa subprovince in adjacent Canada (Corfu and Stott, 1986).

107

Van Hise and Bayley (1897) introduced the names "northern complex" for the Archean rocks north of the Marquette synclinorium and "southern complex" for the Archean rocks south of the synclinorium.

A Late Archean age for the GLTZ is now established by regional geologic relationships in north-central United States. The Archean rocks in the greenstone-granite terrane in northern Minnesota (Hudleston and others, 1988) and northernmost Michigan (fig. 1; Sirns, 1991) are characterized mainly by ductile and brittle structures formed in response to dextral shear, which accords with the deformation pattern observed in exposures of the GLTZ south of Marquette. These structures include a generally west-trending steep foliation and upright folds, widespread Z-shaped folds, and northwest- to west-trending dextral faults, indicative of dextral shear; the structures are superposed on recumbent folds (Johnson and Bomhorst, 1991). In contrast, Early Proterozoic Penokean deformation in the Marquette area had little effect on the Archean basement (Cambray, 1984). Cambray (1984) proposed that the Penokean deformation was produced by horizontal compression that was transmitted from the basement to the folded cover rocks by narrow ductile shears in the basement. Readjustment of rigid basement blocks along these shears utilized old weaknesses, which resulted in some shortening but not folding of the basement rocks. For the Early Proterozoic Marquette synclinorium, Cambray (1984) proposed a nearly north-south compressional axis, which initially produced reverse dip slip on faults bounding the syncline and compressed the Early Proterozoic sedimentary rocks within the syncline into west-trending folds. Subsequently, resistance to this movement within the trough resulted in sinistral strike slip motion and the development of F, folds with northwest-trending axial surfaces and variable plunge.

Gair and Thaden (1968) applied the name "Compeau Creek Gneiss" to both the foliated granitoid rocks of the Archean greenstone-granite terrane (northern complex) and the layered gneisses and massive intrusions of the gneiss terrane (southern complex), and subsequent investigators extended this terminology to the western parts of the southern complex (Cannon and Simmons, 1973). To apply this name to rocks in both Archean terranes, however, is inappropriate, because the two terranes consist of distinctive rock types of different origins. Accordingly, in this guide informal lithologic names are used to describe the crystalline rocks in the two terranes.

ARCHEAN GREENSTONE-GRANITE TERRANE

The greenstone-granite terrane in the Marquette area (fig. 1) consists of a several thousand- meter-thick succession of metamorphosed subaqueous mafic to felsic flows, pyroclastic rocks, and volcanogenic sedimentary rocks that is intruded by small bodies of gabbro and ultramafic rocks and by large plutons of granitoid rocks (Bornhorst, 1988; Johnson and Bomhorst, 1991). The metamorphosed volcanic rocks have been named the Ishpeming greenstone belt (Morgan and DeCristoforo, 1980); they represent the southwestern extension of the Wawa (Shebandowan) subprovince of the Superior province of Canada (Card and Ciesielski, 1986). Felsic volcanic rocks adjacent to an ultramafic body north of Ishpeming are host to the gold deposits of the Ropes mine (Brozdowski and others, 1986; Brozdowski, 1988, 1989), and the ultramafic body hosts additional mineral prospects (Bodwell, 1972). Foliated tonalite from the northern complex (Hammond, 1978) in the greenstone-granite terrane, has a U-Pb zircon age of 2,7032 16 Ma (recalculated by Zell E. Peterman), and associated rhyolite has a U-Pb zircon age of 2,780269 Ma (recalculated by Zeil E. Peterman). These ages are consistent with more precise U-Pb ages in the Wawa subprovince in adjacent Canada (Corfu and Stott, 1986).

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CORRELATION OF MAP UNITS

Unconformity

Penokean orogeny deformation and metamorphism

xm1L..... 1

p

Group

\\\\'' )Unconformity

Xmn Menominee

JGroup

—

Unconformity

I ChocolayXc j Group

Continent-continent collision (=2,690 Ma)North of GLTZ South of GLTZ

Greenstone-granite terrane Gnelss terrane

UnconformityI

!'zá'_1 e:w;aJ wuI- - ".1 , 4 1 - + _±I

DESCRIPTION OF MAP UNITS

________

Middle ProterozoicJacobsvllie Sandstone

Early ProterozoicAlkali granite (1.7 33 ±25 Ma)

Marquette Range SupergroupMichigamme Formation—Dominantly siaty rocks In lower part

Ciarksburg Volcanica Member of Michigamme Formation

Goodrich Quartzite

Menominee Group, undivided, in Republic trough

Negaunee Iron-formation

Siamo Slate and Ajibik Quartzite, undivided

Cisocolay Group, undivided

ArcheanGreenstone-granite terrane

Reany Creek Formation

Granitoid rocks— Dominantly granodiorite and tonal lie butIncluding granite; generally foliated

Mafic to felsic flows and pyrodastic rocks andvolcanogenic sedimentary rocks

Ultramafic rocks

_________

Gneiss terraneGranite near Tilden (Hammond, 1978)

Gneiss, migmatite, and amphibolite—lncludes foliatedand massive granite

Contact

High-angie fault—Bar and ball on downthrown side

Transcurrent fault— Showing relative horizontal movement

A Thrust fault—Sawteeth on upthrown side

50Strike and dip of foliation

Inclined

4— Vertical—15 Bearing and plunge of lineation— May be combined with

foliation symbol

Direction of stratigraphic tops

15 Bearing and plunge of minor fold

Mylonlte— Dominantly orthomyionite

CRSZ Carp River shear zone

CRFSZ Carp River Falls shear zone

DRSZ Dead River shear zone

GLTZ Great Lakes tectonic zone

GGT Greenstone-granite terrane

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\ MiddleJ Proterozotc

EarlyProterozolc

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Late

J

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Edge of L_eozoicNçcover

INDEX MAP

o 4OKIWMETERS

o 4OMLIS

CORREIATION OF MAP UNITS

Unconforrnity

Penokean orogeny deformation and metamor~ism

Baraga Group

Unconforrnily 1 Marquette Range W} Mzgy 1 suwrgrOup

unconformity I

Cmtinentantinent colliskm ( ~ 2 , 6 9 0 Ma) North of GLTZ 1 South of GLTZ

Greenstone-granite temne Gneiss terrane I I

DE!XRPTION OF MAP UNnS Mlddle Proierowic .

Jacobsville Sandstone

Early Proterozoic

F j y \ . ; ; 3 , 7 7 , ,, Alkali granite (1,733 *25 Ma)

, , Marquette Range SUPWSTOUP Mkhlgamme Fonnatbn-Domlmntly slaty rocks In lower part

Clahburg Volcnnlcs Member of Mkhigamm Fonnatbn

Godrich Quartzite

>

a

Menominee Group, undlvlded, i n Republic tmugh

h d y Proterozoic

1 Xmn 1 Negaunee Iron-formatbn w S i m o Slate and Ajibik Quartzite, undivided

Archean Greenstone-granite terrane

Reany Creek Formation

Granitoid rocks-bminantly grandiodte and tonalite but including granite; generalb foliated

1-1 Mafic to felsic flows and pyrodastic r o c k and wkanogenic sedimentaq rocks mi Utramafii rocks

Gneiss terrane \ .\I.

,-!t~yt 3 Granite near Tilden (Hammond, 1978)

Gneiss, migmatite, and amphibolide-Includes foliated and massive granite

Contact

7 Wih-angle fault-& and ball on downthrown side J - Transcurrent fault- Showing relath horizontal m-mt - Thrust fault-Swteeh on upthrown side

Strike and dip of foliation A Inclined

+ Vertical

. -15 Bearing and plunge of lineation- May be combined with foliation symbol - Direction of stratigraphic tops

-- CRSZ

CRFSZ

DRSZ

GLTZ

GGT

217

k f 5 Bearins and plunge of minor fold -- Mylonite- Dominantly orthornylonite

Carp River shear zone

Carp River Falls shear zone

Dead River shear zone

Great Lakes tectonk zone

Greenstone-granite terrane

Ropes mhe

1 Xch 1 Chocolay Group, undivided INDEX MAP

Granitoid Rocks

The Late Archean granitoid rocks in the greenstone-granite terrane (fig. 1) are dominantly pinkto grayish pink, generally medium grained, porphyritic, foliated, and homogeneous tonalite orgranodiorite (Gair and Thaden, 1968, P. 18-23). They contain xenoliths and schlieren of biotiteschist and amphibolite. In the Sands quadrangle (fig. 1), weakly foliated granite is also acommon rock type (table 1), but its age relation to the tonalite-granodiorite was not determined.The bimodal composition of the granitoid rocks suggests, however, that the granite represents adiscrete magmatic event.

The granitoid rocks of the greenstone-granite terrane in the Marquette area exposed on bothsides of the Marquette synclinorium are strongly altered (table 1). Plagioclase (oligoclase) issaussuritized and albite twinning is largely obscured, and biotite is largely changed to chloriteand opaque oxides (locally rutile). The rocks are highly fractured and fracture surfaces arecoated by chlorite and other propylitic minerals. Along the southern margin of the Marquettesyndilnorium, the granitoid rocks are exceptionally rich in quartz (table 1). The granitoid rocksin the Sands quadrangle also are protomylonites. The protomylonite is characterized byrecrystallization of quartz to fine grain sizes and localized shear-induced recrystallization ofplagioclase and potassium feldspar to fine-grained polycrystalline aggregates (type 1P, liP, 1M,and 1 1M structures of Hanmer, 1982).

The relative homogeneity of the foliated granitoid rocks, the occurrence of sharp-walledxenoliths, and the general absence of a lithologic layering that could represent relict sedimentaryor volcanic layering suggest that the granitoid rocks of the greenstone-granite terrane are ofmagmatic origin. Their pervasive foliation is attributed to deformation subsequent to primarycrystallization, as discussed following.

Structure

The rocks in the greenstone-granite terrane (northern complex), on the north side of the GLTZ(fig. 1), record early recumbent folding (F1) of metavolcanic rocks of the Ishpeming greenstonebelt. Superposed deformation (D2) produced northwest- to west-trending upright, upward- anddownward-facing folds (F2) that are Z-shaped in plan view (Johnson and Bornhorst, 1991). Anaxial plane foliation was developed during F2. The Z-symmetry of the F2 folds is consistent withtheir development in a deformation regime with a dextral shear component. Associatedgranitoid rocks also were deformed by D2. Younger, northwest- to west-trending faults, some ofwhich have demonstrable dextral movement, transect and offset the folded rocks. Commonly,these faults separate domains of volcanic rock that have opposite stratigraphic facing directions.

Foliation and lithologic layering in the vicinity of the Ropes mine (R, near center of fig. 1) ispuzzling with respect to the dominant regional structure. Here, foliation and lithologic layeringstrike about N. 700 E. and are nearly vertical (Brozdowski, 1988, p. A-44). The publishedgeologic map that includes the Ropes mine area (Negaunee SW quadrangle; Clark and others,1975) also shows a steep (stretch?) lineation that plunges southeastward in unit Wkf of the LateArchean Kitchi Schist (included in unit Wv on fig. 1). Possibly, the northeast-trending foliationin the Ropes mine area represents the east-northeast-trending limb of a large-scale D2 Z-fold.

110

Granitoid Rocks

The Late Archean granitoid rocks in the greenstone-granite terrane (fig. 1) are dominantly pink to grayish pink, generally medium grained, porphyritic, foliated, and homogeneous tonalite or granodiorite (Gau and Thaden, 1968, p. 18-23). They contain xenoliths and schlieren of biotite schist and amphibolite. In the Sands quadrangle (fig. I), weakly foliated granite is also a common rock type (table I), but its age relation to the tonalite-granodiorite was not determined. The bimodal composition of the granitoid rocks suggests, however, that the granite represents a discrete magmatic event.

The granitoid rocks of the greenstone-granite terrane in the Marquette area exposed on both sides of the Marquette synclinorium are strongly altered (table 1). Plagioclase (oligoclase) is saussuritized and albite twinning is largely obscured, and biotite is largely changed to chlorite and opaque oxides (locally rutile). The rocks are highly fractured and fracture surfaces are coated by chlorite and other propylitic minerals. Along the southem margin of the Marquette synclinorium, the granitoid rocks are exceptionally rich in quartz (table 1). The granitoid rocks in the Sands quadrangle also are protomylonites. The protomylonite is characterized by recrystallization of quartz to fine grain shes and localized shear-induced recrystallization of plagioclase and potassium feldspar to fie-grained polycrystalline aggregates (type IP, l lP, lM, and 11M structures of Hanmer, 1982).

The relative homogeneity of the foliated granitoid rocks, the occurrence of sharp-walled xenoliths, and the general absence of a lithologic layering that could represent relict sedimentary or volcanic layering suggest that the granitoid rocks of the greenstone-granite terrane are of magmatic origin. Their pervasive foliation is attributed to deformation subsequent to primary crystallization, as discussed following.

Structure

The rocks in the greenstone-granite terrane (northern complex), on the north side of the GLTZ (fig. I), record early recumbent folding (FJ of metavolcanic rocks of the Ishpeming greenstone belt. Superposed deformation (D2) produced northwest- to west-trending upright, upward- and downward-facing folds (F2) that are Z-shaped in plan view (Johnson and Bomhorst, 1991). An axial plane foliation was developed during Fp The Z-symmetry of the F2 folds is consistent with their development in a deformation regime with a dextral shear component. Associated granitoid rocks also were deformed by D2. Younger, northwest- to west-trending faults, some of which have demonstrable dextral movement, transect and offset the folded rocks. Commonly, these faults separate domains of volcanic rock that have opposite stratigraphic facing directions.

Foliation and lithologic layering in the vicinity of the Ropes mine (R, near center of fig. 1) is puzzling with respect to the dominant regional structure. Here, foliation and lithologic layering strike about N. 70' E. and are nearly vertical (Brozdowski, 1988, p. A-44). The published geologic map that includes the Ropes mine area (Negaunee SW quadrangle; Clark and others, 1975) also shows a steep (stretch?) lineation that plunges southeastward in unit Wkf of the h t e Archean Kitchi Schist (included in unit Wv on fig. 1). Possibly, the northeast-trending foliation in the Ropes mine area represents the east-northeast-trending limb of a large-scale D2 Z-fold.

Table 1. Approximate modal content of granitoid rocks in Archean greenstone-granite terrane of the Marquette area

['Fr., trace; blank, absentj

RockConstituent

Sample number

136R lilA 17Th 179A 179B 182A 182B 183A 186A 199 200 202 208 21LA 136-1 203 1 2 3

Plagioclase 37 35 60 66 54.5 48 52.5 38 54.5 36 56.5 60 39 41 273 33 61.2 58.5 50.6

Quartz 30 32.5 24 21.5 28.5 45 36 34 24 27.5 37.5 28 30 27 39 23 26 22.2 373

Potassiumfeldspar 30 31 10 5.5 2.5 0 0.5 24.5 14.5 31 3 1 30 24 32 373 3.8 12.8 1.4

Biotite Ti. Tr. Ti. Tr. 10 Ti. Tr. 2.5 7 5 Ti. 10 0 Ti. Tr. Ti. 0.2 Ti. 1

Chlorite 3 1.3 6 7 Ti. 6 63 Ti. Ti. Ti. 3 Ti. Tr. 8 13 03 3.6 23 1.8

Epidote 4 Tr. Ti. Tr. 0.6 Ti. Ti.

Sphene Ti. 0 0

Opaque oxides Tr. Ti. Tr. Tr. Ti. Ti. Ti. Tr. Tr. Ti. Tr. Tr. 0.2 Ti. Tr.

Accessoryminerals Tr. 0.2 Ti. Ti. 0.5 0.5 1 Tr. 0.5 Tr. 1 1 Ti. Tr. 1 'Fr. Tr. Ti.

Muscovite 4 Tr. Tr. 23 1.3 7

SAMPLE DESCRIPTIONS

136R. Pink, fine- to medium-grained, slightly porphyritic, altered granite; contains wispy inclusions of biotite schist. Sands quadrangle, 1,800 ft W., 675 ft N. of the SE. corner of

sec. 3, T. 47 N., R. 25 W.

lilA. Pink, medium-grained, porphyritic, massive, altered granite; cut by chlorite-coated fractures. Sands quadrangle, 775 ft 5., 2,150 ft E. of the NW. corner of sec. 16,T. 47 N.,

R. 25W.

17Th. Pink, fine-grained, porphyritic, massive, altered granite. Sands quadrangle. Same locality as 177A.

179A. Pinkish-gray, medium-grained, porphyritic, altered tonalite; contains oriented layers of biotite schist. Palmer quadrangle, 650 ft S., 250 ft W. of the NE. corner of sec. 26,

T. 47 N., R. 26 W.

179B. Gray, medium-grained, foliated, altered tonalite. Biotite is fresh. Palmer quadrangle. Same locality as 179A.

182A. Pinkish-gray, medium-grained altered tonalite gneiss. Sands quadrangle, 1,350 ft E., 100 ft N. of the SW. coiner of sec. 31, T. 47 N., R. 25 W.

Table 1. Approximate modal content of granitoid rock in Archean greenstone-granite terrane of the Marquette area

pr., trace; blank, absent]

Rock Sample number Constituent

l36R 177A 177B 179A 179B 18% 182B 183A 186A 199 200 202 208 21lA 136-1 203 1 2 3

Plagioclase 37 35 60 54.5 48 52.5 38 54.5 36 56.5 60 39 41 27.5 33 61.2 58.5 50.6

Quartz 30 32.5 24 21.5 28.5 45 36 34 24 27.5 37.5 28 30 27 39 28 26 22.2 37.5

Potassium feldspar 30 31 10 5.5 2.5 0 0.5 24.5 14.5 31 3 1 30 24 32 37.5 3.8 12.8 1.4

Biotite Tr. Tr. Tr. Tr. 10 Tr. Tr. 2.5 7 5 Tr. 10 0 Tr. Tr. Tr. 0.2 Tr. 1

Chlorite 3 1.3 6 7 Tr. 6 6.5 Tr. Tr. Tr. 1.5 0.5 3 Tr. Tr. 8 3.6 2.5 1.8

Epidote

Sphene

4 Tr. Tr. Tr.

Tr.

0.6 Tr. Tr.

0 0

Opaque oxides Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. 0.2 Tr. Tr.

Accessory minerals Tr. 0.2 Tr. Tr. 0.5 0.5 1 Tr. 0.5 Tr. 1 1 Tr. Tr. 1 Tr. Tr. Tr.

Muscovite 4 Tr. Tr. 2.5 1.3 7

SAMPLE DESCRIF'TXONS

Pink, f ie - to medium-grained, slightly porphyritic, altered granite; contains wispy inclusions of biotite schist. Sands quadrangle, 1,800 ft W., 675 ft N. of the SE. corner of

sec.3,T.47N.,R.ZW.

Pink, medium-grained, porphyritic, massive, altered granite; cut by cldorite-coated fractures. Sands quadrangle, 775 ft S., 2,l50 f t E. of the NW. corner of sec. 16, T. 47 N.,

R. 25 w. P i i fie-grained, porphyritic, massive, altered granite. Sands quadrangle. Same locality as 177A.

Pinkish-gray, medium-grained, porphyritic, altered tonalite; contains oriented layers of biotite schist. Palmer quadrangle, 650 ft S., 250 ft W. of the NJ2. corner of see 26,

T. 47 N., R. 26 w. Gray, medium-grained, foliated, altered tonalite. Biotite is fresh. Palmer quadrangle. Same locality as 179A.

Pinkish-gray, medium-grained altered tonalite gneiss. Sands quadrangle, 1,350 ft E., 100 ft N. of the SW. corner of sec. 31, T. 47 N., R. 25 W.

182B. Pinkish-gray to medium-gray, altered tonalite gneiss. Sands quadrangle. Same location as 182A.

183A. Pinkish-gray, fine- to medium-grained granite gneiss. Sands quadrangle, 675 ft N., 300 ft E.of the SW. corner of sec. 31, T. 47 N., R. 25 W.

186A. Pinkish-gray, medium-grained, altered granodiorite gneiss. Palmer quadrangle, 2,500 ft N., 2,200 ft E. of the SW. corner of sec. 36, T. 47 N., R. 26 W.

199. Pinkish-gray, medium-grained granite gneiss. Sands quadrangle, 1,000 ft S., 2,000 ft W. of the NE. corner of sec. 16, T. 46 N., R. 25 W.

200. Pinkish-gray, fine- to medium-grained altered tonalite gneiss. Sands quadrangle, 775 ft N., 2,075 ft E. of the SW. corner of sec. 9, T. 46 N.,R. 25 W.

202. Gray, fine-grained biotite tonalite gneiss cut by chlorite-coated fractures. Sands quadrangle, 850 ft N., 2,350 E. of the SW. corner of sec. 5, T. 46 N., R. 25 W.

208. Pink, medium-grained, leucocratic granite gneiss. Sands quadrangle, 2,150 ft N., 2,000 ft W. of the SE. corner of sec. 8, T. 46 N., R. 25 W.

211A. Pink, medium-grained, altered granite gneiss. Sands quadrangle, 2,550 ft S., 150 ft W. of the SE. corner of sec. 7, T. 46 N., R. 25 W.

136-1. Pink, medium- to coarse-grained, foliated granite gneiss. Fractures coated by chlorite. Sands quadrangle. Same location as 136R.

203. Pink, medium-grained, leucocratic granite gneiss. Sands quadrangle, 200 ft N., 2,300 ft W. of the SE. corner of sec. 5, T. 46 N., R. 25 W.

1. Average of 14 samples of foliated tonalite. Marquette quadrangle, north of the Marquette syndlinorium (Gair and Thaden, 1968, table 6, no. 1).

2. Average of five samples of foliated granodiorite. Marquette quadrangle, north of the Marquette synclinorium (Gair and Thaden, 1968, table 6, no. 9).

3. Average of 11 samples of quartz-rich, foliated tonalite. Sands quadrangle, south of the Marquette synclinorium (Gair and Thaden, 1968, table 6, no. 6).

Piish-gray to medium-gray, altered tonalite gneiss. Sands quadrangle. Same location as 182.4.

Pinkish-gray, fme- to medium-grained granite gneiss. Sands quadrangle, 675 f t N., 300 ft E.of the SW. wrner of sec. 31, T. 47 N., R. 25 W.

Pinkish-gray, medium-grained, altered grandorite gneiss. Palmer quadrangle, 2,333 ft N., 2,200 ft E. of the SW. wrner of sec. 36, T. 47 N., R. 26 W.

Pinkish-gray, medium-grained granite gneiss. Sands quadrangle, 1,000 ft S., 2,000 ft W. of the NE. mrner of sec. 16, T. 46 N., R. 25 W.

Piish-gray, fme- to medium-grained altered tonalite gneiss. Sands quadrangle, 775 ft N., 2,075 ft E. of the SW. corner of sec. 9, T. 46 N., R. 25 W.

Gray, tine-grained biotite tonalite gneiss cut by chlorite-mated fractures. Sands quadrangle, 850 ft N., 2,350 E. of the SW. wrner of sec. 5, T. 46 N., R. 25 W.

Pink, medium-grained, leucocratic granite gneiss. Sands quadrangle, 2,150 ft N., 2,WO ft W. of the SE. wrner of sec. 8, T. 46 N., R. 25 W.

Pink, medium-grained, altered granite gneiss. Sands quadrangle, 2,550 ft S., 150 ft W. of the SE. wrner of sec. 7, T. 46 N., R. 25 W.

Pink, medium- to coarse-grained, foliated granite gneiss. Fractures mated by chlorite. Sands quadrangle. Same location as l36R.

Pink, medium-grained, leucwratic granite gneiss. Sands quadrangle, 200 ft N., 2,300 ft W. of the SE. wrner of sec. 5, T. 46 N., R. 25 W.

Average of 14 samples of foliated tonalite. Marquette quadrangle, north of the Marquette syncborium (Gau and Thaden, 1968, table 6, no. 1).

Average of five samples of foliated grandorite. Marquette quadrangle, north of the Marquette synclinorium (G& and Thaden, 1968, table 6, no. 9).

Average of 11 samples of quartz-rich, foliated tonalite. Sands quadrangle, south of the Marquette syncborium (G& and Thaden, 1968, table 6, no. 6).

The steep, northwest- to west-trending shear zones and faults in the greenstone-granite terrane,as exposed in the northern complex, are tens of meters to a few hundred meters wide. They arehighly schistose zones characterized by an intense, close-spaced foliation, a steep stretching(?)ilneation, and strong retrograde alteration (Johnson and Bornhorst, 1991). The faults are ofboth ductile and brittle-ductile types. Major structures include the Dead River shear zone(fig. 1, DRSZ) (Puffett, 1974), which forms the northern boundaty of the Dead River basin; theCarp River shear zone (CRSZ), east-northeast of the Ropes mine, which separates two differentblocks of Archean volcanic rocks; and the Carp River Falls shear zone (CRFSZ), which formsthe northern margin of the Marquette synclinorium in this area. The Carp River Falls shearzone is reported to be cut by relatively undeformed mafic dikes of presumed Late Archean age,indicating an Archean age for the shear zone (Baxter and Bornhorst, 1988).

In the area north of the Palmer fault (fig. 1), foliated and fractured Late Archean granitoidrocks form partly fault-bounded domes surrounded by Early Proterozoic rocks of the MarquetteRange Supergroup. The large Archean granitoid body in the Sugarloaf Mountain area, 8 kmnorth of Marquette, is also a dome; foliation in adjacent metavolcanic rocks (Puffett, 1974) dipsgently to moderately away from the granite contact.

Late-Tectonic Conglomerate (Archean)

The youngest Archean unit in the greenstone-granite terrane is the Reany Creek Formation(Puffett, 1969; 1974). On the basis of a reinterpretation of age and structural relationships, theReany Creek Formation is no longer included as part of the Chocolay Group or the MarquetteRange Supergroup (Sims, 1991). It is a heterogeneous body of conglomerate, arkose, chloriticslate, graywacke, and boulder-bearing slate. It transects structures in the older volcanic rocks, isless deformed than the metamorphosed volcanic rocks, and is bounded along its southernmargin by the northwest-trending Dead River shear zone (fig. 1). Its age has been uncertain(see Puffett, 1974), but its penetrative foliation, asymmetry of basin fill, and relationship to theDead River shear zone strongly suggest that it developed concurrently with dextral shear alongthe Dead River shear zone. It is similar to "Timiskaming-type" sequences, such as the SeineGroup, in northern Ontario, now commonly interpreted as forming in Archean analogs tomodern pull-apart basins (Poulsen, 1986).

ARCHEAN GNEISS TERRANE

The Archean gneiss terrane in the Marquette area (fig. 1) constitutes the greater part of thesouthern complex, as defined by Van Hise and Bayley (1897). It consists of gneiss, migmatite,and amphibolite, substantial amounts of deformed and undeformed granite pegmatite, andmassive to weakly foliated granite plutons. Cannon and Simmons (1973) have described thegeneral rock types in much of the southern complex. One rock has been dated as Late Archean.A sample of gray gneiss (called Compeau Creek Gneiss by Gair, 1975) collected in theSE¼SWV4 of section 36, T. 47 N., R. 27 W. (Hammond, 1978) has a U-Pb zircon age of2,779 ±21 Ma and a lower intercept age of 802±76 Ma (recalculated by Zell E. Peterman).

Gneiss and Associated Granitoid Rocks

Compositionally layered, medium-grained gneiss and migmatite are the dominant rock types inthe gneiss terrane in the Marquette area. Layered felsic gneisses ranging in composition fromtonalite to granite predominate. Smaller amounts of massive to layered amphIbolite are

113

The steep, northwest- to west-trending shear zones and faults in the greenstone-granite terrane, as exposed in the northern complex, are tens of meters to a few hundred meters wide. They are highly schistose zones characterized by an intense, close-spaced foliation, a steep stretching(?) lineation, and strong retrograde alteration (Johnson and Bornhorst, 1991). The faults are of both ductile and brittle-ductile types. Major structures include the Dead River shear zone (fig. 1, DRSZ) (Puffett, 19741, which forms the northern boundary of the Dead River basin; the Carp River shear zone (CRSZ), east-northeast of the Ropes mine, which separates two different blocks of Archean volcanic rocks; and the Carp River Falls shear zone (CRFSZ), which forms the northern margin of the Marquette synclinorium in this area. The Carp River Falls shear zone is reported to be cut by relatively undeformed mafic dikes of presumed Late Archean age, indicating an Archean age for the shear zone (Baxter and Bornhorst, 1988).

In the area north of the Palmer fault (fig. I), foliated and fractured Late Archean granitoid rocks form partly fault-bounded domes surrounded by Early Proterozoic rocks of the Marquette Range Supergroup. The large Archean granitoid body in the Sugarloaf Mountain area, 8 km north of Marquette, is also a dome; foliation in adjacent metavolcanic rocks (Puffett, 1974) dips gently to moderately away from the granite contact.

Late-Tectonic Conglomerate (Archean)

The youngest Archean unit in the greenstone-granite terrane is the Reany Creek Formation (Puffett, 1969; 1974). On the basis of a reinterpretation of age and structural relationships, the Reany Creek Formation is no longer included as part of the Chwlay Group or the Marquette Range Supergroup (Sims, 1991). It is a heterogeneous body of conglomerate, arkose, chloritic slate, graywacke, and boulder-bearing slate. It transects structures in the older volcanic rocks, is less deformed than the metamorphosed volcanic rocks, and is bounded along its southern margin by the northwest-trending Dead River shear zone (fig. 1). Its age has been uncertain (see Puffett, 19741, but its penetrative foliation, asymmetry of basin fa and relationship to the Dead River shear zone strongly suggest that it developed concurrently with dextral shear along the Dead River shear zone. It is similar to "Timiskaming-typen sequences, such as the Seine Group, in northern Ontario, now commonly interpreted as forming in Archean analogs to modem pull-apart basins (Poulsen, 1986).

A R C H W GNE3SS TERRANE

The Archean gneiss terrane in the Marquette area (fig. 1) constitutes the greater part of the southern complex, as defined by Van Hise and Bayley (1897). It consists of gneiss, migmatite, and amphibolite, substantial amounts of deformed and undeformed granite pegmatite, and massive to weakly foliated granite plutons. Cannon and Simmons (1973) have described the general rock types in much of the southern complex. One rock has been dated as Late Archean. A sample of gray gneiss (called Compeau Creek Gneiss by Gair, 1975) collected in the SEY&W% of section 36, T. 47 N., R. 27 W. (Hammond, 1978) has a U-Pb zircon age of 2,779521 Ma and a lower intercept age of 802576 Ma (recalculated by Zell E. Peterman).

Gneiss and Associated Granitoid Rocks

Compositiondly layered, medium-grained gneiss and migmatite are the dominant rock types in the gneiss terrane in the Marquette area. Layered felsic gneisses ranging in composition from tonalite to granite predominate. Smaller amounts of massive to layered amphibolite are

intercalated with the felsic gneiss, but amphibolite constitutes layers several tens of meters thickat places, as can be seen on the geologic map of the Palmer 7½-minute quadrangle (Gair, 1975,p1. 1), which is immediately west of the Sands quadrangle. The felsic gneisses are gray topinkish gray; typically, compositional layering is expressed by different proportions of the majorsilicate minerals, as for example, (1) plagioclase-quartz-biotite-microperthite, (2) microperthite-quartz-plagioclase-biotite, and (3) biotite-quartz-plagioclase-microperthite. Textural differencesat places emphasize the compositional layering. Pink aplitic granite and granite pegmatitecommonly transect the gneiss and amphibolite and locally form migmatite. Metasedimentaryrocks such as iron-formation, form layers in the felsic gneisses in the vicinity of the Republictrough (Cannon and Simmons, 1973); they were not observed in the Palmer and Sandsquadrangles, in the Marquette area.

Pinkish-gray to pink, medium-grained, massive to weakly foliated, homogeneous granite (table 2)intrudes the gneisses in several places. Hammond (1978) delineated a body of massive graniteabout 4 km2 in areal extent south of Ishpeming (unit Wgt, fig. 1), which he informally called the"Tilden granite." It is a gray to pink, medium-grained, locally porphyritic, and massive granitethat locally contains oriented xenoliths of mafic gneiss. It is cut by pink pegInatite and is highlyfractured. The fractures have slickensided surfaces and a thin coating of chlorite and otherpropylitic alteration minerals. U-Pb isotopic data on a sample of the granite near Tilden(Hammond, 1978) indicate an age of 2,585±15 Ma (R.E. Zartman, oral commun., 1991).

A nearly circular body of alkali granite (unit Xga, fig. 1) about 2 km in diameter occurs about 3km south of Humboldt (Schulz and others, 1988). The granite is light red to brick red, generallymassive, fine- to medium-grained, and equigranular to hypidiomorphic granular. The granite issimilar compositionally to Sn-W mineralized alkali feldspar granites of the Arabian Shield(Jackson and Ramsay, 1986) and the Nigerian younger granite province (Kinnaird and others,1985). The granite has a Rb-Sr whole-rock age of 1,733 ±25 Ma, which is interpreted as acrystallization age (Zell E. Peterman, written commun., 1988); it is a post-tectonic intrusion.

Structure

Archean gneisses in the gneiss terrane (southern complex) form a northwest-trending antiformalstructure that closes to the west and is overlapped by Paleozoic rocks to the east (Sims, 1992).An infolded belt of Early Proterozoic (Marquette Range Supergroup) rocks indents the Archeanfold nose in the Republic trough (fig. 1). In the area west of the Republic trough, Taylor (1967)determined two principal phases of deformation: (1) early, probably flat-lying folds with axialplanes trending northeastward, and (2) younger upright folds with steep northwest-trending axialsurfaces. The younger folds mainly control the distribution of the rock units.

In the Marquette area, early gently inclined to recumbent folds that trend northwestward andplunge gently northward (fig. 2) are the dominant structure in the gneisses. These folds deforman older foliation (Si). Boudinage accompanied the folding; the boudins plunge subparallel togently inclined fold hinges. A later steep, northwest-oriented foliation, that is axial planar tonorthwest-trending folds, occurs at least locally, but it does not significantly affect thedistribution of the rock units. These younger structures will be examined at stop 3.

114

intercalated with the felsic gneiss, but amphibolite constitutes layers several tens of meters thick at places, as can be seen on the geologic map of the Palmer 7%-minute quadrangle (Gair, 1975, pi. I), which is immediately west of the Sands quadrangle. The felsic gneisses are gray to pinkish gray; typically, compositional layering is expressed by different proportions of the major silicate minerals, as for example, (1) plagioclase-quartz-biotite-microperthite, (2) microperthite- quartz-plagioclase-biotite, and (3) biotite-quartz-plagioclase-microperthite. Textural differences at places emphasize the compositional layering. Pink aplitic granite and granite pegmatite commonly transect the gneiss and amphibolite and locally form migmatite. Metasedimentary rocks such as iron-formation, form layers in the felsic gneisses in the vicinity of the Republic trough (Cannon and Simmons, 1973); they were not observed in the Palmer and Sands quadrangles, in the Marquette area.

Pinkish-gray to pink, medium-grained, massive to weakly foliated, homogeneous granite (table 2) intrudes the gneisses in several places. Hammond (1978) delineated a body of massive granite about 4 km2 in areal extent south of Ishpeming (unit Wgt, fig. I), which he informally called the "Tilden granite." It is a gray to pink, medium-grained, locally porphyritic, and massive granite that locally contains oriented xenoliths of mafic gneiss. It is cut by pink pegmatite and is highly fractured. The fractures have slickensided surfaces and a thin coating of chlorite and other propylitic alteration minerals. U-Pb isotopic data on a sample of the granite near Tilden (Hammond, 1978) indicate an age of 2,5855 15 Ma (R.E. Zartman, oral commun., 1991).

A nearly circular body of alkali granite (unit Xga, fig. 1) about 2 km in diameter occurs about 3 km south of Humboldt (Schuiz and others, 1988). The granite is light red to brick red, generally massive, fine- to medium-grained, and equigranular to hypidiomorphic granular. The granite is similar compositionally to Sn-W mineralized alkali feldspar granites of the Arabian Shield (Jackson and Ramsay, 1986) and the Nigerian younger granite province (Kinnaird and others, 1985). The granite has a Rb-Sr whole-rock age of 1,733 5 25 Ma, which is interpreted as a crystallization age (Zell E. Peterman, written commun., 1988); it is a post-tectonic intrusion.

Structure

Archean gneisses in the gneiss terrane (southern complex) form a northwest-trending antiformal structure that closes to the west and is overlapped by Paleozoic rocks to the east (Sims, 1992). An infolded belt of Early Proterozoic (Marquette Range Supergroup) rocks indents the Archean fold nose in the Republic trough (fig. 1). In the area west of the Republic trough, Taylor (1967) determined two principal phases of deformation: (1) early, probably flat-lying folds with axial planes trending northeastward, and (2) younger upright folds with steep northwest-trending axial surfaces. The younger folds mainly control the distribution of the rock units.

In the Marquette area, early gently inclined to recumbent folds that trend northwestward and plunge gently northward (fig. 2) are the dominant structure in the gneisses. These folds deform an older foliation (S,). Boudinage accompanied the folding; the boudins plunge subparallel to gently inclined fold hinges. A later steep, northwest-oriented foliation, that is axial planar to northwest-trending folds, occurs at least locally, but it does not significantly affect the distribution of the rock units. These younger structures will be examined at stop 3.

Table 2. Approximate modal content of granitoid rocks in the Archean gneiss terraneof the Marquette area

[Tr., trace; blank, absenti

RockConstituent

Sample No.

153A 146-88 1 226A 226B

Plagioclase 34.7 33.5 25 263 393

Quartz 28 23.8 25 38.2 32.7

Potassiumfeldspar 28 34.2 40 33.0 25.0

Biotite 9 8.0 3 2.2 3

Chlorite Tr. Tr. Tr. Tr.

Muscovite Tr. Tr. 2 Tr.

Epidote Tr.

Sphene Tr.

Opaqueoxides Tr. Tr.

Accessoryminerals 03 0.5 Tr. 0.3 Tr.

SAMPLE DESCRU'TIONS

153A. Pinkish-gray, medium-grained, foliated granite, SE¼NE¼ of section 7, T. 46 N.,

R. 26 W. Biotite is weakly altered.

146-88. "Tilden granite" of Hammond (1978). Quartz and biotite are recrystallized in

shears; biotite is slightly altered to chlorite.

1. "Tilden granite" of Hammond (1978, p. 63). Potassium-feldspar is microperthite.

Plagioclase has concentric zoning. Highly fractured.

226A. Light-gray, mediuni-grained, foliated granite. Cut by fractures. Biotite is highly

altered to chlorite. Quarry, SE¼SW½ of section 21, T. 46 N., R. 26 W.

226B. Pale-reddish-brown, mediuin-grained foliated granite. Cut by shears, some with

mylonite. Biotite is highly altered to chlorite and calcite. Same locality as 226A.

115

Table 2. Approximate modal content of granitoid rocks in the Archean gneiss terrane of the Marquette area

[Tr., trace; blank, absent]

Rock Sample No. Constituent 153A 146-88 1 226A 226B

Plagioclase

Quartz

Potassium feldspar

Biotite

Chlorite

Muscovite

Epidote

Sphene

Opaque oxides

Accessory minerals

28 34.2

9 8.0

Tr.

Tr. Tr.

Tr.

Tr.

40 33.0 25.0

3 2.2 3

Tr. Tr. Tr.

2 Tr.

Tr.

Tr.

Tr. 0.3 Tr.

SAMPLE DESCRIPTIONS

Pinkish-gray, medium-grained, foliated granite, SE%NE!4 of section 7, T. 46 N.,

R. 26 W. Biotite is weakly altered.

Tilden granite" of Hammond (1978). Quartz and biotite are recrystallized in

shears; biotite is slightly altered to chlorite.

Tilden granite" of Hammond (1978, p. 63). Potassium-feldspar is microperthite.

Plagioclase has concentric zoning. Highly fractured.

Light-gray, medium-grained, foliated granite. Cut by fractures. Biotite is highly

altered to chlorite. Quarry, SE%SW?h of section 21, T. 46 N., R. 26 W.

Pale-reddish-brown, medium-grained foliated granite. Cut by shears, some with

mylonite. Biotite is highly altered to chlorite and calcite. Same locality as 226A.

116

GREAT LAKES TECTONIC ZONE

The GLITZ is characterized in the Marquette area by a mylonite zone about 2 km wide that hasbeen superposed on previously deformed layered rocks and granitoid rocks of the northernArchean greenstone-granite terrane and previously deformed rocks of the southern Archeangneiss terrane. Therefore, the mylonite overprints rocks of the two terranes. The great width ofthis shear zone and the preponderance of mylonite distinguish it from the other, much narrowershear zones and faults in the region. The mylonite abruptly grades northward intoprotomylonite and highly fractured and altered rocks along the northeast wall of the GLTZ(fig. 3).

Boundaries (walls) of the GLTZ are subparallel and trend N. 55°-60° W., subparallel to thecontact between the Archean greenstone-granite and gneiss terranes (fig. 3). Foliation in themylonite has an average attitude of N. 7Q0 W., 750 SW (fig. 4). Thus, the foliation strikes 10°-15° more westward than the trend of the shear zone walls. The angular relationship between theorientation of the foliation and shear zone boundaries suggests dextral wrench shear along thewalls (Simpson, 1986, fig. 2b). A pronounced rodding (stretching) lineation in the mylonite hasan average plunge of 42° and an average bearing of S. 43° E.

Hinges of tight to open folds plunge subparallel to the stretching lineation (fig. 4). The foldshave both symmetrical and asymmetrical patterns. Z-shaped folds are most common, but S-shaped folds coexist and both may occur in the same outcrop area, as on the west shore ofPowell Lake (fig. 3). Nearly pervasive lichen covers outcrops and prevented determination of

+++

+++ ++

Figure 2. Equal-area projection of poles to foliation, lineations, and fold axes in gneisses ofArchean gneiss terrane.

0

0

0 •t

0

+++ +

++ +

+

Lower hemisphere

GREAT LAKES TECTONIC ZONE

The GLTZ is characterized in the Marquette area by a mylonite zone about 2 km wide that has been superposed on previously deformed layered rocks and granitoid rocks of the northern Archean greenstone-granite terrane and previously deformed rocks of the southern Archean gneiss terrane. Therefore, the mylonite overprints rocks of the two terranes. The great width of this shear zone and the preponderance of mylonite distinguish it from the other, much narrower shear zones and faults in the region. The mylonite abruptly grades northward into protomylonite and highly fractured and altered rocks along the northeast wall of the GLTZ (fig. 3).

Boundaries (walls) of the GLTZ are subparallel and trend N. 55O-600 W., subparallel to the contact between the Archean greenstone-granite and gneiss terranes (fig. 3). Foliation in the mylonite has an average attitude of N. 700 W., 75O SW (fig. 4). Thus, the foliation strikes 100- 15O more westward than the trend of the shear zone walls. The angular relationship between the orientation of the foliation and shear zone boundaries suggests dextral wrench shear along the walls (Simpson, 1986, fig. 2b). A pronounced rodding (stretching) lineation in the mylonite has an average plunge of 42' and an average bearing of S. 43O E.

Hinges of tight to open folds plunge subparallel to the stretching lineation (fig. 4). The folds have both symmetrical and asymmetrical patterns. Z-shaped folds are most common, but S- shaped folds coexist and both may occur in the same outcrop area, as on the west shore of Powell Lake (fig. 3). Nearly pervasive lichen covers outcrops and prevented determination of

Lower hemisphere

Figure 2. Equal-area projection of poles to foliation, lineations, and fold axes in gneisses of Archean gneiss terrane.

fold symmetries over large parts of the GLTZ. Ridley and Casey (1989) have demonstrated thatsymmetrical folds, with axes subparallel to the maximum extension direction (X finite strainaxis), are produced by wrench shear, whereas asymmetric folds, with axes close to the extensiondirection, form in a regime of combined wrench and thrust shear. In wrench shear, because thefold axis does not lie initially within the slip plane of shear, the axis rotates toward the directionof shear displacement and maximum extension with the addition of progressive strain. Incombined wrench and thrust shear, at high strain the fold axial plane and both limbs approachparallelism with the shear plane. Ridley (1986) has further shown that dextral wrench shear andsinistral wrench shear can result from strain inhomogeneity in the same shear-parallel extensionwithin a single deformation regime.

Mylonite

The mylonite in the GLTZ is mainly orthomylonite, as defined by Wise and others (1984),inasmuch as surviving megacrysts compose 10-20 percent of the rock. In the terminolor ofHanmer (1987), the mylonites are mainly "heteroclastic" because the porphyroclasts havevariable size ranges, but include "homoclastic" mylonite having more uniform texturalcharacteristics. Ultramylonite is absent except on a scale of a few centimeters.

The mylonite typically has a pronounced planar foliation that differs markedly from the moreirregular foliation in rocks outside the GLTZ. In this respect, the mylonite fits the pattern ofmost mylonite zones in comprising a "straight zone." Quartz typically is recrystallized into"ribbon quartz," yielding a pronounced rodding (stretch) lineation. The relatively stiff minerals,plagioclase and potassium feldspar, are generally recrystallized to finer grain sizes, with thedevelopment of core-mantle structures (White, 1976) or type 1P and 1M structures (Hanmer,1982); these structures form oriented aggregates of quartz and feldspar that produce aprominent stretch lineation. Accompanying biotite is mainly recrystallized in planar or irregularshears. The quartz-poor amphibolite in the GLTZ has a conspicuous lineation given by elongateaggregates of actinolite and chlorite. The mafic mineral assemblages in rocks in the GLTZ areupper greenschist facies, indicating retrogressive alteration of amphibolite to actinolite schist.

Interpretation

The Great Lakes tectonic zone has been interpreted (Gibbs and others, 1984) as a paleosutureresulting from continent-continent coffision that juxtaposed the Archean gneiss and greenstone-granite terranes. In the Marquette area, the stretching lineation, which represents the line oftectonic transport (Schackleton and Ries, 1984), indicates that collision was oblique, resulting indextral wrench-thrust shear along the N. 60° W.-trending boundary (paleosuture). Asymmetricstructures, mainly microscopic, indicate northwestward vergence and probable overriding of theArchean greenstone-granite terrane by the Archean gneiss terrane.

The oblique collision would be expected to produce dextral shear across a large region north ofthe GLTZ (fig. 5). The extent of the area affected by this dextral transcurrent shear is notdefinitely known, however, because dextral shear was the dominant mechanism of deformationthroughout most of the Superior province (Card, in press).

Structures in the greenstone-granite rocks of northern Minnesota (Wawa subprovince) areremarkably similar to those in northern Michigan. In northern Minnesota, deformed andmetamorphosed volcanic and sedimentary rocks of the Vermilion district (Sims, 1976; Sims and

117

fold symmetries over large parts of the GLTZ. Ridley and Casey (1989) have demonstrated that symmetrical folds, with axes subparallel to the maximum extension direction (X finite strain axis), are produced by wrench shear, whereas asymmetric folds, with axes close to the extension direction, form in a regime of combined wrench and thrust shear. In wrench shear, because the fold axis does not lie initially within the slip plane of shear, the axis rotates toward the direction of shear displacement and maximum extension with the addition of progressive strain. In combined wrench and thrust shear, at high strain the fold axial plane and both limbs approach parallelism with the shear plane. Ridley (1986) has further shown that dextral wrench shear and sinistral wrench shear can result from strain inhomogeneity in the same shear-parallel extension within a single deformation regime.

Mylonite

The mylonite in the GLTZ is mainly orthomylonite, as defined by Wise and others (1984), inasmuch as surviving megacrysts compose 10-20 percent of the rock. In the terminology of Hanmer (1987), the mylonites are mainly "heteroclastic" because the porphyroclasts have variable size ranges, but include "homoclastic" mylonite having more uniform textural characteristics. Ultramylonite is absent except on a scale of a few centimeters.

The mylonite typically has a pronounced planar foliation that differs markedly from the more irregular foliation in rocks outside the GLTZ. In this respect, the mylonite fits the pattern of most mylonite zones in comprising a "straight zone." Quartz typically is recrystallized into "ribbon quartz," yielding a pronounced rodding (stretch) lineation. The relatively stiff minerals, plagioclase and potassium feldspar, are generally recrystallized to finer grain sizes, with the development of core-mantle structures (White, 1976) or type IP and 1M structures (Hanmer,

. 1982); these structures form oriented aggregates of quartz and feldspar that produce a prominent stretch lineation. Accompanying biotite is mainly recrystallized in planar or irregular shears. The quartz-poor amphibolite in the GLTZ has a conspicuous lineation given by elongate aggregates of actinolite and chlorite. The mafic mineral assemblages in rocks in the GLTZ are upper greenschist fades, indicating retrogressive alteration of amphibolite to actinolite schist.

Interpretation

The Great Lakes tectonic zone has been interpreted (Gibbs and others, 1984) as a paleosuture resulting from continent-continent collision that juxtaposed the Archean gneiss and greenstone- granite terranes. In the Marquette area, the stretching lineation, which represents the line of tectonic transport (Schackleton and Ries, 1984), indicates that collision was oblique, resulting in dextral wrench-thrust shear along the N. 60Â W.-trending boundary (paleosuture). Asymmetric structures, mainly microscopic, indicate northwestward vergence and probable overriding of the Archean greenstone-granite terrane by the Archean gneiss terrane.

The oblique collision would be expected to produce dextral shear across a large region north of the GLTZ (fig. 5). The extent of the area affected by this dextral transcurrent shear is not definitely known, however, because dextral shear was the dominant mechanism of deformation throughout most of the Superior province (Card, in press).

Structures in the greenstone-granite rocks of northern Minnesota (Wawa subprovince) are remarkably similar to those in northern Michigan. In northern Minnesota, deformed and metamorphosed volcanic and sedimentary rocks of the Vermilion district (Sims, 1976; Sims and

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Figure 3. Structure map of southwestern part of Sands 7½-minute quadrangle showing theGreat Lakes tectonic zone. Modified from Sims (1991).

118

I 0

I 1 KILOMETER

Figure 3. Structure map of southwestern part of Sands 7%-minute quadrangle showing the Great Lakes tectonic zone. Modified from Sims (1991).

Archean

_______

Greenstone-granite tenaneBiotite schist and granitoid rocks

Mylonlte—Protollth dominantly granttoid rocks but includesblotite sdilst and amphlbolite

________

Gneiss terraneGneiss, migmatite, and amphibolite-includes foliated and

_________

massive graniteMylonfte—Oomlnantly mylonitic quartzofeldspathic gneiss

— — — Boundary between rocks of greenstone-granite terraneand gneiss terrane wIthin the Great Lakes tectoniczone (GLTZ)

Approximate outer limit of orthomylonite in Great Lakestectonic zone

25Strike and dip of foliation

Indined

Vertical

—ts Bearing and plunge of minor fold

20 Bearing and plunge of lineation—May be combined withIoliatlon symbols

Figure 3--Continued D(PLANATION

Wsg

Wggm

[wgn

wgmj

:.'I... Silkified rocks

+ +++ + + +-

+

0 ..

•.

Lower hemisphere

Figure 4. Equal-area projection of poles to foliation (crosses), stretching lineation (dots), andfold hinges (open circles) in mylonite of the Great Lakes tectonic zone, Sands andPalmer 7½-minute quadrangles, Marquette area, Michigan.

119

Figure 3--Continued EXPLANATION Archean

Greenstone-granite terrane

1 Biotite schist and granitoid rocks

[el MylonitÑProtolit dominantly granitoid rocks but includes biotite schist and amphibolite

Gneiss terrane

[I Gneiss, migrnatite, and amphibolitdncludei foliated and massive granite

[Â¥Wgm Mylonite-Dominantly mylonitic quarlzofeldspathic gneiss

. Silicified rocks .. . . . . --- Boundary between rocks of greenstone-granite terrane and gneiss terrane within the Great Lakes tectonic zone (GLTZ )

------ Approximate outer limit of orthomylonite in Great Lakes tectonic zone

Strike and dip of foliation - Inclined

+ Vertical

F45 Bearing and plunge of minor fold - 20 Bearing and plunge of lineation-May be combined with foliatton symbols

Lower hemisphere

Figure 4. Equal-area projection of poles to foliation (crosses), stretching lineation (dots), and fold hinges (open circles) in mylonite of the Great Lakes tectonic zone, Sands and Palmer 7%-minute quadrangles, Marquette area, Michigan.

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Southwick, 1985) compose an east-trending belt between higher grade rocks of the Late ArcheanVermilion Granitic Complex (Quetico subprovince, fig. 5) (Southwick, 1972) and the GiantsRange batholith to the south. The measured strain, a cleavage, upright folds, and a minerallineation in this belt have been attributed to the main" phase of deformation (D2) that followedan early nappe-forming event (D1) (Bauer, 1985). The nappes show little evidence of apenetrative fabric (Hudleston, 1976). Hudleston and others (1988) attributed the (D2)deformation to regional dextral transpression, as the strain pattern requires a northeast-southwest component of shortening in addition to shear. They further proposed that majordextral faults, such as the Vermilion fault (fig. 5), are later, more brittle, expressions of thisshear regime. They concluded that the D2 transpressive deformation resulted from obliquecompression between the two more rigid crustal blocks to the north (Quetico subprovince) andsouth (Giants Range batholith). A similar tectonic regime has been recognized in the RainyLake area (fig. 5; Poulsen and others, 1980; Day and Sims, 1984; Wood, 1980), where earlyrecumbent folding was followed by upright folding and dextral strike-slip faulting.

Recent precise isotopic analyses of zircon, titanite, and rutile from the Rainy Lake area, Canada(fig. 5; Davis and others, 1989), which lies between the Quetico and Wabigoon subprovinces,have provided time constraints on these structural events. The major deformation, includingnappe emplacement, thrusting, and local doming, took place between 2,696 Ma and 2,692 Ma;this deformation was followed shortly by transcurrent faulting and simultaneous deposition ofconglomerate/arenite (Timiskaming-type rocks, as in Seine Group), which occurred in theinterval 2,692-2,686 Ma. Late (Algoman) granitic plutons were emplaced about 2,686 Ma,although some are older. In the Wawa subprovince, west of Thunder Bay (fig. 5), Corfu andStott (1986) found that the D1 deformation occurred during or before the intrusion of theShebandowan Lake pluton at 2,696±2 Ma. Deformation D2 in this area occurred between2,689 +3/-2 Ma and 2,684+6/-3 Ma, similar to the age suggested by Davis and others (1989) forD2 in the Rainy Lake wrench fault zone. These ages are compatible with the less preciseisotopic ages on rocks in northern Minnesota and Michigan (Peterman, 1979), and it seemsprobable that the rocks and structures throughout the Wawa and Quetico subprovinces areapproximately coeval (Percival, 1989). Although convergence along the GLTZ undoubtedly wasdiachronous, collision probably occurred in the approximate interval 2,692-2,686 Ma (Davis andothers, 1989), to yield the transcurrent faults and Timiskiming-type rocks.

Kinematic Analysis

The attitude of the stretching lineation (line of tectonic transport) in the mylonite exposed southof Marquette together with asymmetric meso- and micro-structures that reveal sense ofmovement, indicate that the oblique collision resulted in: (1) dextral-thrust shear along theGLTZ, and (2) northwestward vergence and probable overriding of the Archean greenstone-granite terrane by the Archean gneiss terrane. Kinematic indicators--rotated mica grains withinnarrow compositional layers, asymmetric porphyroclasts with tails (a type; Simpson, 1986), andasymmetric microfolds in mylonitic layering--demonstrate northwestward vergence. Thisinformation implies southward subduction of the Archean greenstone-granite terrane (Wawasubprovince) beneath the Archean gneiss terrane (fig. 1).

121

Southwick, 1985) compose an east-trending belt between higher grade rocks of the Late Archean Vermilion Granitic Complex (Quetico subprovince, fig. 5) (Southwick, 1972) and the Giants Range batholith to the south. The measured strain, a cleavage, upright folds, and a mineral lineation in this belt have been attributed to the "main" phase of deformation (D,) that followed an early nappe-forming event (Dl) (Bauer, 1985). The nappes show little evidence of a penetrative fabric (Hudleston, 1976). Hudleston and others (1988) attributed the (D,) deformation to regional dextral transpression, as the strain pattern requires a northeast- southwest component of shortening in addition to shear. They further proposed that major dextral faults, such as the Vermilion fault (fig. 5), are later, more brittle, expressions of this shear regime. They concluded that the D2 transpressive deformation resulted from oblique compression between the two more rigid crustal blocks to the north (Quetico subprovince) and south (Giants Range batholith). A similar tectonic regime has been recognized in the Rainy Lake area (fig. 5; Poulsen and others, 1980; Day and Sims, 1984; Wood, 1980), where early recumbent folding was followed by upright folding and dextral strike-slip faulting.

Recent precise isotopic analyses of zircon, titanite, and rutile from the Rainy Lake area, Canada (fig. 5; Davis and others, 1989), which lies between the Quetico and Wabigoon subprovinces, have provided time constraints on these structural events. The major deformation, including nappe emplacement, thrusting, and local doming, took place between 2,696 Ma and 2,692 Ma; this deformation was followed shortly by transcurrent faulting and simultaneous deposition of conglomerate/arenite (Tirniskaming-type rocks, as in Seine Group), which occurred in the interval 2,692-2,686 Ma. Late (Algoman) granitic plutons were emplaced about 2,686 Ma, although some are older. In the Wawa subprovince, west of Thunder Bay (fig. 5), Corfu and Stott (1986) found that the Dl deformation occurred during or before the intrusion of the Shebandowan Lake pluton at 2,696± Ma. Deformation D2 in this area occurred between 2,689 +3/-2 Ma and 2,684+ 61-3 Ma, similar to the age suggested by Davis and others (1989) for D, in the Rainy Lake wrench fault zone. These ages are compatible with the less precise isotopic ages on rocks in northern Minnesota and Michigan (Peterman, 1979), and it seems probable that the rocks and structures throughout the Wawa and Quetico subprovinces are approximately coeval (Percival, 1989). Although convergence along the GLTZ undoubtedly was diachronous, collision probably occurred in the approximate interval 2,692-2,686 Ma (Davis and others, 1989), to yield the transcurrent faults and Timiskiming-type rocks.

Kinematic Analysis

The attitude of the stretching lineation (line of tectonic transport) in the mylonite exposed south of Marquette together with asymmetric meso- and micro-structures that reveal sense of movement, indicate that the oblique collision resulted in: (1) dextral-thrust shear along the GLTZ, and (2) northwestward vergence and probable overriding of the Archean greenstone- granite terrane by the Archean gneiss terrane. Kinematic indicators--rotated mica grains within narrow compositional layers, asymmetric porphyroclasts with tails (o type; Simpson, 1986), and asymmetric microfolds in mylonitic layering-demonstrate northwestward vergence. This information implies southward subduction of the Archean greenstone-granite terrane (Wawa subprovince) beneath the Archean gneiss terrane (fig. 1).

Evolution

The northwest direction of tectonic transport during suturing of the Archean terranes, asascertained from the Marquette area, provides a means for determining the evolution of theGL1'Z and the variable trajectory of stress into the Superior province crust.

The GL1'Z in the Lake Superior region is characterized by systematic angular bends thatalternately trend northeastward and west-northwestward (fig. 5). Presumably this zigzag patternreflects relict irregularities in the margin of the Archean greenstone-granite terrane (or Superiorprovince) crust, which was a continental margin before the convergence and coffision with thesouthern Archean gneiss terrane.

The northeast-trending and west-northwest-trending segments of the GLTZ have differentstructural styles. Deformation along the northwest-trending segments of the GLTZ, asparticularly shown by data from the Marquette segment, was principally caused by dextraltranspression resulting from oblique coffision. Transmittal of this transcurrent shear into rocksnorth of the GLTZ yielded a widespread, pervasive west-northwest- to west-striking foliation,subparallel upright folds, and northwest- to west-trending dextral faults and shear zones in theArchean greenstone-granite terrane.

The similarly-oriented northwest-trending segment of the GLTZ in northwestern Wisconsin hasmany structural features in common with the Marquette segment. Foliation and upright folds inlow amphibolite-facies rocks of the Archean greenstone-granite terrane (unit Wga, fig. 4, Simsand others, 1985) strike west-northwest and mineral lineations and fold hinges mainly plungegently southeast. The boundary between the two terranes is not exposed because of a glacialcover, but is presumed to lie along the south edge of unit Wga. Numerous northwest-trendingdextral faults, some of which reactivated in Early Proterozoic time, have been mapped in thearea (Sims and others, 1985; fig. 1).

Coffision along the northeast-trending segments of the GLTZ, on the other hand, producednortheast-trending structures of apparently more restricted areal extent. In the northeast-trending Marenisco segment (fig. 5; Sims and others, 1984), the boundary is covered by EarlyProterozoic sedimentary and volcanic rocks, however, lithologic layering and foliation in rocks ofthe adjacent Archean greenstone-granite terrane near the boundary trend northeastward and aredeformed into upright, moderately tight northeast-trending folds that plunge 45°-50° SW. Thesestructures are presumably subparallel to the covered Archean GLTZ boundary. Archeanmetamorphism has been overprinted by Early Proterozoic Penokean nodal metamorphismcentered on the Watersmeet dome (Sims and others, 1985; Sims, 1990); the presence of relictgarnet at a few places in the Archean rocks of the greenstone-granite terrane near the boundarysuggests that these rocks were metamorphosed to at least upper greenschist facies in Archeantime. Similarly, north-verging Penokean deformation in the boundary zone overprinted Archeanstructures (Sims and others, 1984). An axial plane 2 (Penokean) penetrative cleavage, strikingnortheast and dipping 45°-70° SE., was superposed on the previously folded rocks. Apparentlythe Archean rocks were not refolded, however, as a result of the Penokean deformation.

In the northeast-trending Minnesota segment of the GLTZ (fig. 5), neither the terrane boundarynor adjacent Archean rocks on either side are exposed. These rocks are covered in west-centralMinnesota by thick Quaternary glacial deposits and in central Minnesota by Early Proterozoicsedimentary and volcanic rocks of the Animikie basin (Southwick and others, 1988). The GLTZ

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Evolution

The northwest direction of tectonic transport during suturing of the Archean terranes, as ascertained from the Marquette area, provides a means for determining the evolution of the GLTZ and the variable trajectory of stress into the Superior province crust.

The GLTZ in the Lake Superior region is characterized by systematic angular bends that alternately trend northeastward and west-northwestward (fig. 5). Presumably this zigzag pattern reflects relict irregularities in the margin of the Archean greenstone-granite terrane (or Superior province) crust, which was a continental margin before the convergence and collision with the southern Archean gneiss terrane.

The northeast-trending and west-northwest-trending segments of the GLTZ have different structural styles. Deformation along the northwest-trending segments of the GLTZ, as particularly shown by data from the Marquette segment, was principally caused by dextral transpression resulting from oblique collision. Transmittal of this transcurrent shear into rocks north of the GLTZ yielded a widespread, pervasive west-northwest- to west-striking foliation, subparallel upright folds, and northwest- to west-trending dextral faults and shear zones in the Archean greenstone-granite terrane.

The similarly-oriented northwest-trending segment of the GLTZ in northwestern Wisconsin has many structural features in common with the Marquette segment. Foliation and upright folds in low amphibolite-facies rocks of the Archean greenstone-granite terrane (unit Wga, fig. 4, Sims and others, 1985) strike west-northwest and mineral lineations and fold hinges mainly plunge gently southeast. The boundary between the two terranes is not exposed because of a glacial cover, but is presumed to lie along the south edge of unit Wga. Numerous northwest-trending dextral faults, some of which reactivated in Early Proterozoic time, have been mapped in the area (Sims and others, 1985; fig. 1).

Collision along the northeast-trending segments of the GLTZ, on the other hand, produced northeast-trending structures of apparently more restricted areal extent. In the northeast- trending Marenisco segment (fig. 5; Sims and others, 1984), the boundary is covered by Early Proterozoic sedimentary and volcanic rocks, however, lithologic layering and foliation in rocks of the adjacent Archean greenstone-granite terrane near the boundary trend northeastward and are deformed into upright, moderately tight northeast-trending folds that plunge 45'-50' SW. These structures are presumably subparallel to the covered Archean GLTZ boundary. Archean metamorphism has been overprinted by Early Proterozoic Penokean nodal metamorphism centered on the Watersmeet dome (Sims and others, 1985; Sims, 1990); the presence of relict garnet at a few places in the Archean rocks of the greenstone-granite terrane near the boundary suggests that these rocks were metamorphosed to at least upper greenschist facies in Archean time. Similarly, north-verging Penokean deformation in the boundary zone overprinted Archean structures (Sims and others, 1984). An axial plane S, (Penokean) penetrative cleavage, striking northeast and dipping 45'-70' SE., was superposed on the previously folded rocks. Apparently the Archean rocks were not refolded, however, as a result of the Penokean deformation.

In the northeast-trending Minnesota segment of the GLTZ (fig. 5), neither the terrane boundary nor adjacent Archean rocks on either side are exposed. These rocks are covered in west-central Minnesota by thick Quaternary glacial deposits and in central Minnesota by Early Proterozoic sedimentary and volcanic rocks of the Animikie basin (Southwick and others, 1988). The GLTZ

has been investigated, however, by a detailed aeromagnetic survey, by computer-generatedmapping of the second vertical derivative of the gravity field, and by shallow test drilling (seeSouthwick and Sims, in press); the boundary has been located rather accurately on the basis ofthese data. The drilling has shown that the rocks on the northwest side are volcanogenicsedimentary and mafic to intermediate volcanic rocks, which are metamorphosed to uppergreenschist facies and intruded by Archean tonalite (Southwick and Chandler, 1983). Theserocks are typical of the Archean greenstone-granite terrane in exposed parts of the LakeSuperior region. A seismic reflection profile in central Minnesota acquired by COCORP(Consortium for Continental Reflection Profiling) has been interpreted to indicate that theGL1'Z in this area is a shallow (—30°) north-dipping tectonic feature (Gibbs and others, 1984).In east-central Minnesota, the GLTZ is covered by Proterozoic rocks of the Animikie basin.The structural style in the Proterozoic cover indicates north-verging tectonism (Southwick andothers, 1988), and as in the Marenisco segment, the Archean crustal boundary had a role indefining Penokean deformation.

Deformation along both of the northeast-trending (Minnesota and Marenisco) segments of theGL1'Z resulted mainly from northwest-southeast crustal shortening, probably dominantly byflattening strain. The direction of tectonic transport during convergence was virtuallyperpendicular to the juncture of the two terranes at these localities.

The origin of the zigzag pattern of the south edge of the Superior province, now marked by theGL1'Z, is uncertain. The prevailing thought is that the Wawa subprovince is one of a sequenceof stacked island arcs that formed progressively from north to south above north-dippingsubduction zones as the continental mass to the south of the GLTZ (i.e., Archean gneissterrane) migrated to the north (Card, 1990). With this interpretation, possible modern analogsof the Superior province are the convergent-plate boundaries of the western Pacific, as forexample those of the Indonesian region (Hamilton, 1979).

The physical resemblance of the southern margin of the Superior province to the Appalachian-Ouachita Paleozoic orogenic belt (Thomas, 1977), however, suggests a possible alternativeinterpretation for the origin of the Superior margin. In this interpretation, the Superior marginwas a rifted continental margin. Two interpretations have been made for the origin of thePaleozoic continental margin: (1) rift segments were offset by transform faults, as suggested byThomas (1977, 1983), or (2) intersections between active rift arms at triple junctions (Rankin,1976). Of these two suggestions, the rift-transform mechanism seems the more likely, with theMinnesota and Marenisco segments being the rifted segments (fig. 5) and the northwesternWisconsin and Marquette segments being highly modified transform faults. Regardless of themechanism by which the zigzag Archean continental margin originated, the subsequent trace ofthe orogenic belt was probably inherited from the shape of the earlier margin.

CONCLUDING REMARKS

Convergence along the irregularly shaped margin of the Archean greenstone-granite terrane(GLTZ) resulted in a variable trajectory of stress into the continental crust and probably inalong-strike diachroneity of orogeny. Structural data from the Marquette area, in particular, aswell as elsewhere along the GLTZ, suggest that the major direction of tectonic transport wasnorthwestward. Accordingly, promontories such as those along the concave parts of theMarquette and Wisconsin segments of the GLTZ (fig. 5), where the zone bends from anortheast orientation to a northwest one, must have projected as buttresses against which

123

has been investigated, however, by a detailed aeromagnetic survey, by computer-generated mapping of the second vertical derivative of the gravity field, and by shallow test drilling (see Southwick and Sims, in press); the boundary has been located rather accurately on the basis of these data. The drilling has shown that the rocks on the northwest side are volcanogenic sedimentary and mafic to intermediate volcanic rocks, which are metamorphosed to upper greenschist facies and intruded by Archean tonalite (Southwick and Chandler, 1983). These rocks are typical of the Archean greenstone-granite terrane in exposed parts of the Lake Superior region. A seismic reflection profile in central Minnesota acquired by COCORP (Consortium for Continental Reflection Profiling) has been interpreted to indicate that the GLTZ in this area is a shallow (-30° north-dipping tectonic feature (Gibbs and others, 1984). In east-central Minnesota, the GLTZ is covered by Proterozoic rocks of the Animikie basin. The structural style in the Proterozoic cover indicates north-verging tectonism (Southwick and others, 1988), and as in the Marenisco segment, the Archean crustal boundary had a role in defining Penokean deformation.

Deformation along both of the northeast-trending (Minnesota and Marenisco) segments of the GLTZ resulted mainly from northwest-southeast crustal shortening, probably dominantly by flattening strain. The direction of tectonic transport during convergence was virtually perpendicular to the juncture of the two terranes at these localities.

The origin of the zigzag pattern of the south edge of the Superior province, now marked by the GLTZ, is uncertain. The prevailing thought is that the Wawa subprovince is one of a sequence of stacked island arcs that formed progressively from north to south above north-dipping subduction zones as the continental mass to the south of the GLTZ (i.e., Archean gneiss terrane) migrated to the north (Card, 1990). With this interpretation, possible modem analogs of the Superior province are the convergent-plate boundaries of the western Pacific, as for example those of the Indonesian region (Hamilton, 1979).

The physical resemblance of the southern margin of the Superior province to the Appalachian- Ouachita Paleozoic orogenic belt (Thomas, 1977), however, suggests a possible alternative interpretation for the origin of the Superior margin. In this interpretation, the Superior margin was a rifted continental margin. Two interpretations have been made for the origin of the Paleozoic continental margin: (1) rift segments were offset by transform faults, as suggested by Thomas (1977, 1983), or (2) intersections between active rift arms at triple junctions (Rankin, 1976). Of these two suggestions, the rift-transform mechanism seems the more likely, with the Minnesota and Marenisco segments being the rifted segments (fig. 5) and the northwestern Wisconsin and Marquette segments being highly modified transform faults. Regardless of the mechanism by which the zigzag Archean continental margin originated, the subsequent trace of the orogenic belt was probably inherited from the shape of the earlier margin.

CONCLUDING REMARKS

Convergence along the irregularly shaped margin of the Archean greenstone-granite terrane (GLTZ) resulted in a variable trajectory of stress into the continental crust and probably in along-strike diachroneity of orogeny. Structural data from the Marquette area, in particular, as well as elsewhere along the GLTZ, suggest that the major direction of tectonic transport was northwestward. Accordingly, promontories such as those along the concave parts of the Marquette and Wisconsin segments of the GLTZ (fig. 5), where the zone bends from a northeast orientation to a northwest one, must have projected as buttresses against which

compressive stress was directed into the continental crust. Oblique compression at these pointsproduced dextral shear across the region north of the suture, probably at least as far northwardas the Quetico fault (fig. 5), a distance of about 250 km. This shear imposed a roughly east-west, steep structural fabric on the rocks and, also as a late, more brittle expression of the shearregime (Hudleston and others, 1988), the northwest- to west-trending dextral transcurrent faults.

Along the Marenisco and Minnesota segments of the GLTZ (fig. 5), where convergence wasmore perpendicular to the ancient continental margin, a northeast-trending structural fabric wasimposed on the rocks immediately cratonward from the suture.

We suggest that the main structural fabric (D2) in rocks of the Archean greenstone-graniteterrane in the north-central United States (Wawa and Quetico subprovinces; fig. 5) resultedfrom the coffision along the GL1'Z. The predominance of orthomylonite rather thanultramylonite and the nearly pervasive retrogressive alteration (greenschist facies) in rocks ofthe greenstone-granite terrane suggest that the exposed collision zone was developed at amoderately shallow crustal level (at brittle-ductile transition conditions). As discussed in aprevious report (Sims and others, 1980), the Archean structures in this regime also played astrong role in subsequent tectonism, especially in the Early Proterozoic north-vergingdeformation.

We further suggest that the late-tectonic granite bodies in the Archean gneiss terrane arepossibly related to the collision along the GLTZ and presumed southward subduction. Theavailable age data on these granites are compatible with a presumed 2.69 Ga age for thecoffision. The "Tilden granite" of Hammond (1978) in Michigan has a Late Archean age(2,585±15 Ma), although both the U-Pb and Rb-Sr systems are disturbed. In the MinnesotaRiver valley, in southwestern Minnesota, a large pluton of late-tectonic granite (Sacred HeartGranite) has a Pb-Pb age of about 2,605 Ma (Doe and Delevaux, 1980) and a Rb-Sr age ofabout 2.7 Ga (Goldich and others, 1970). Doe and Delevaux (1980) have shown that 207Pb-204Pbvalues in the Sacred Heart Granite are characteristic of ensialic environments, as contrastedwith the ensimatic (arc) granitoid bodies in the Superior province (greenstone-granite terrane).The ensialic environment indicates that the Archean gneiss terrane had been cratonized prior toemplacement of the Sacred Heart Granite. Precise ages are required to test the hypothesis thatthe Late Archean granites south of the GLTZ were indeed formed during continent-continentcollision.

Cumulative data on the Archean Superior province (see Hoffman, 1989, for review) indicatethat it consists of generally east trending belts of island arc and related rocks that wereassembled progressively from north to south (Card, in press), before fmally coffiding with theArchean gneiss terrane (continent) on the south at about 2,690 Ma. This pattern of accretion aswell as the tectonic style, is not unlike that in modern plate-tectonic regimes, indicating thatplate-tectonic mechanisms existed in the Archean as well as in the Proterozoic and Phanerozoic.

124

compressive stress was directed into the continental crust. Oblique compression at these points produced dextral shear across the region north of the suture, probably at least as far northward as the Quetico fault (fig. 5), a distance of about 250 km. This shear imposed a roughly east- west, steep structural fabric on the rocks and, also as a late, more brittle expression of the shear regime (Hudleston and others, 1988), the northwest- to west-trending dextral transcurrent faults.

Along the Marenisco and Minnesota segments of the GLTZ (fig. 5), where convergence was more perpendicular to the ancient continental margin, a northeast-trending structural fabric was imposed on the rocks immediately cratonward from the suture.

We suggest that the main structural fabric (D2) in rocks of the Archean greenstone-granite terrane in the north-central United States (Wawa and Quetico subprovinces; fig. 5) resulted from the collision along the GLTZ. The predominance of orthomylonite rather than ultramylonite and the nearly pervasive retrogressive alteration (greenschist fades) in rocks of the greenstone-granite terrane suggest that the exposed collision zone was developed at a moderately shallow crustal level (at brittle-ductile transition conditions). As discussed in a previous report (Sims and others, 1980), the Archean structures in this regime also played a strong role in subsequent tectonism, especially in the Early Proterozoic north-verging deformation.

We further suggest that the late-tectonic granite bodies in the Archean gneiss terrane are possibly related to the collision along the GLTZ and presumed southward subduction. The available age data on these granites are compatible with a presumed 2.69 Ga age for the collision. The "Tilden granite" of Hammond (1978) in Michigan has a Late Archean age (2,5852 15 Ma), although both the U-Pb and Rb-Sr systems are disturbed. In the Minnesota River valley, in southwestern Minnesota, a large pluton of late-tectonic granite (Sacred Heart Granite) has a Pb-Pb age of about 2,605 Ma (Doe and Delevaux, 1980) and a Rb-Sr age of about 2.7 Ga (Goldich and others, 1970). Doe and Delevaux (1980) have shown that ^Pb-^Pb values in the Sacred Heart Granite are characteristic of ensialic environments, as contrasted with the ensirnatic (arc) granitoid bodies in the Superior province (greenstone-granite terrane). The ensialic environment indicates that the Archean gneiss terrane had been cratonized prior to emplacement of the Sacred Heart Granite. Precise ages are required to test the hypothesis that the Late Archean granites south of the GLTZ were indeed formed during continent-continent collision.

Cumulative data on the Archean Superior province (see Hoffman, 1989, for review) indicate that it consists of generally east trending belts of island arc and related rocks that were assembled progressively from north to south (Card, in press), before finally colliding with the Archean gneiss terrane (continent) on the south at about 2,690 Ma. This pattern of accretion as well as the tectonic style, is not unlike that in modem plate-tectonic regimes, indicating that plate-tectonic mechanisms existed in the Archean as well as in the Proterozoic and Phanerozoic.

REFERENCES CITED

Barovich, K.M., Patchett, PJ., Peterman, Z.E., and Sims, P.K., 1989, Nd isotopes and the originof 1.9-1.7 Ga Penokean continental crust of the Lake Superior region: GeologicalSociety of America Bulletin, v. 101, p. 333-338.

Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across theboundary between an Archean gneiss belt and greenstone terrane, northeasternMinnesota: Geology, v. 13, p. 657-660.

Baxter, DA., and Bornhorst, TJ., 1988, Multiple discrete mafic intrusions of Archean toKeweenawan age, western Upper Peninsula, Michigan: 34th Annual Institute on LakeSuperior Geology, Marquette, Michigan, Proceedings and Abstracts, p. 6-8.

Bodwell, WA., 1972, Geologic compilation and non-ferrous metal potential, Precambriansection, northern Michigan: Houghton, Mich., Michigan Technological University M.S.thesis, 106 p.

Bornhorst, T.J., 1988, Geological overview of the Marquette greenstone belt, Michigan: Instituteon Lake Superior Geology Field Trip Guidebook, v. 34, pt. 2, p. A1-A18.

Brozdowski, RA., 1988, Geology of the Ropes gold mine: Institute on Lake Superior GeologyField Trip Guidebook, v. 34, pt. 2, p. A32-A53.

1989, Geology of the Ropes gold deposit, j Margeson, G.B., ed., Precambrian geologyand metal occurrences, Michigan's Upper Peninsula, Field Conference: Society ofEconomic Geologists, p. 38-75.

Brozdowski, RA., Gleason, RJ., and Scott, G.W., 1986, The Ropes mine--A pyritic gold depositin Archean volcaniclastic rock, Ishpeming, Michigan, U.S.A., j MacDonald, AJ., ed.,Proceedings of Gold 86, an International Symposium on the Geology of Gold, Toronto:p. 228-242.

Cambray, F.W., 1984, Proterozoic geology, Lake Superior, south shore: Geological Associationof Canada-Mineralogical Association of Canada, London, Ontario, Field trip 5, 55 p.

Cannon, W.F., and Gair, J.E., 1970, A revision of stratigraphic nomenclature of middlePrecambrian rocks in northern Michigan: Geological Society of America Bulletin, v. 81,p. 2843-2846.

Cannon, W.F., and Simmons, G.C., 1973, Geology of part of the southern complex, Marquettedistrict, Michigan: U.S. Geological Survey Journal of Research, v. 1, no. 2, p. 165-172.

Card, K.D., 1990, A review of the Superior province of the Canadian Shield, a product ofArchean accretion: Precambrian Research, v. 48, p. 99-156.

Card, K.D., and Ciesielski, Andre, 1986, DNAG No. 1 Subdivision of the Superior Province ofthe Canadian Shield: Geoscience Canada, v. 13, no. 1, p. 5-13.

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REFERENCES CITED

Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sirns, P.K., 1989, Nd isotopes and the origin of 1.9-1.7 Ga Penokean continental crust of the Lake Superior region: Geological Society of America Bulletin, v. 101, p. 333-338.

Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660.

Baxter, DA., and Bornhorst, T.J., 1988, Multiple discrete mafic intrusions of Archean to Keweenawan age, western Upper Peninsula, Michigan: 34th Annual Institute on Lake Superior Geology, Marquette, Michigan, Proceedings and Abstracts, p. 6-8.

Bodwell, WA., 1972, Geologic compilation and non-ferrous metal potential, Precambrian section, northern Michigan: Houghton, Mich., Michigan Technological University M.S. thesis, 106 p.

Bomhorst, T.J., 1988, Geological overview of the Marquette greenstone belt, Michigan: Institute on Lake Superior Geology Field Trip Guidebook, v. 34, pt. 2, p. A1-A18.

Brozdowski, RA., 1988, Geology of the Ropes gold mine: Institute on Lake Superior Geology Field Trip Guidebook, v, 34, pt. 2, p. A32-A53.

1989, Geology of the Ropes gold deposit, rn Margeson, G.B., ed., Precambrian geology and metal occurrences, Michigan's Upper Peninsula, Field Conference: Society of Economic Geologists, p. 38-75.

Brozdowski, RA., Gleason, R.J., and Scott, G.W., 1986, The Ropes mine--A pyritic gold deposit in Archean volcaniclastic rock, Ishpeming, Michigan, USA., rn MacDonald, A.J., ed., Proceedings of Gold 86, an International Symposium on the Geology of Gold, Toronto: p. 228-242.

Cambray, F.W., 1984, Proterozoic geology, Lake Superior, south shore: Geological Association of Canada-Mineralogical Association of Canada, London, Ontario, Field trip 5, 55 p.

Cannon, W.F., and Gair, J.E., 1970, A revision of stratigraphic nomenclature of middle Precambrian rocks in northern Michigan: Geological Society of America Bulletin, v. 81, p. 2843-2846.

Cannon, W.F., and Simmons, G.C., 1973, Geology of part of the southern complex, Marquette district, Michigan: U.S. Geological Survey Journal of Research, v. 1, no. 2, p. 165-172.

Card, K.D., 1990, A review of the Superior province of the Canadian Shield, a product of Archean accretion: Precambrian Research, v. 48, p. 99-156.

Card, K.D., and Ciesielski, Andre, 1986, DNAG No. 1 Subdivision of the Superior Province of the Canadian Shield: Geoscience Canada, v. 13, no. 1, p. 5-13.

Clark, L.D., Cannon, W.R., and Kiasner, J.S., 1975, Bedrock geologic map of the Negaunee SWquadrangle, Marquette County, Michigan: U.S. Geological Survey Geologic QuadrangleMap GQ-1226, scale 1:24,000.

Corfu, F., and Stott, G.M., 1986, U-Pb ages for late magmatism and regional deformation in theShebandowan belt, Superior province, Canada: Canadian Journal of Earth Sciences,v. 23, p. 1075-1082.

Davis, D.W., Poulsen, K.H., and Kamb, S.L., 1989, New insights into Archean crustaldevelopment from geochronology in the Rainy Lake area, Superior Province, Canada:Journal of Geology, v. 97, p. 379-398.

Day, W.C., and Sims, P.K., 1984, Tectonic evolution of the Rainy Lake area, northernMinnesota: Geological Association of Canada, the Mineralogical Association of Canada,London, Ontario, Canada, Program with Abstracts, v. 9, p. 57.

Doe, B.R., and Delevaux, M.H., 1980, Lead-isotope investigations in the Minnesota RiverValley--Late-tectonic and posttectonic granites, j Morey, G.B., and Hanson, G.N., eds.,Selected studies of Archean gneisses and lower Proterozoic rocks, southern CanadianShield: Geological Society of America Special Paper 182, p. 105-112.

Gair, J.E., 1975, Bedrock geology and ore deposits of the Palmer quadrangle, Marquette County,Michigan: U.S. Geological Survey Professional Paper 769, 159 p., scale 1:24,000.

Gair, J.E., and Thaden, R.E., 1968, Geology of the Marquette and Sands quadrangles,Marquette County, Michigan: U.S. Geological Survey Professional Paper 397, 77 p.,scale 1:24,000.

Gibbs, A.K., Payne, B., Setzer, T., Brown, L.D., Oliver, J.E., and Kaufman, S., 1984, Seismic-reflection study of the Precambrian crust of central Minnesota: Geological Society ofAmerica Bulletin, v. 95, p. 280-294.

Goldich, S.S., Hedge, C.E., and Stern, T.W., 1970, Age of the Morton and Montevideo Gneissesand related rocks, southwestern Minnesota: Geological Society of America Bulletin,v. 81, p. 3671-3696.

Hamilton, W.B., 1979, Tectonics of the Indonesian region: U.S. Geological Survey ProfessionalPaper 1078, 345 p.

Hammond, R.D., 1978, Geochronology and origin of Archean rocks in Marquette County,Upper Michigan: Lawrence, Kans., University of Kansas M.S. thesis, 108 p.

Hanmer, Simon, 1982, Microstructure and geochemistry of plagioclase and microcline innaturally deformed granite: Journal of Structural Geology, v. 4, p. 197-2 13.

______1987,

Textural map units in quarto-feldspathic mylonitic rocks: Canadian Journal ofEarth Sciences, v. 24, p. 2065-2073.

126

dark, L.D., Cannon, W.R., and Klasner, J.S., 1975, Bedrock geologic map of the Negaunee SW quadrangle, Marquette County, Michigan: U.S. Geological Survey Geologic Quadrangle Map GQ-1226, scale 1:24,000.

Corfu, F., and Stott, G.M., 1986, U-Pb ages for late magmatism and regional deformation in the Shebandowan belt, Superior province, Canada: Canadian Journal of Earth Sciences, V. 23, p. 1075-1082.

Davis, D.W., Poulsen, K.H., and Kamb, S.L., 1989, New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada: Journal of Geology, v. 97, p. 379-398.

Day, W.C., and Sims, P.K., 1984, Tectonic evolution of the Rainy Lake area, northern Minnesota: Geological Association of Canada, the Mineralogical Association of Canada, London, Ontario, Canada, Program with Abstracts, v. 9, p. 57.

Doe, B.R., and Delevaux, M.H., 1980, Lead-isotope investigations in the Minnesota River Valley--Late-tectonic and posttectonic granites, Morey, G.B., and Hanson, G.N., eds., Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper 182, p. 105-112.

Gair, J.E., 1975, Bedrock geology and ore deposits of the Palmer quadrangle, Marquette County, Michigan: U.S. Geological Survey Professional Paper 769, 159 p., scale 1:24,000.

Gair, J.E., and Thaden, R.E., 1968, Geology of the Marquette and Sands quadrangles, Marquette County, Michigan: U.S. Geological Survey Professional Paper 397, 77 p., scale 1:24,000.

Gibbs, A.K., Payne, B., Setzer, T., Brown, L.D., Oliver, J.E., and Kaufman, S., 1984, Seismic- reflection study of the Precambrian crust of central Minnesota: Geological Society of America Bulletin, v. 95, p. 280-294.

Goldich, S.S., Hedge, C.E., and Stem, T.W., 1970, Age of the Morton and Montevideo Gneisses and related rocks, southwestern Minnesota: Geological Society of America Bulletin, v. 81, p. 3671-3696.

Hamilton, W.B., 1979, Tectonics of the Indonesian region: U.S. Geological Survey Professional Paper 1078,345 p.

Hammond, R.D., 1978, Geochronology and origin of Archean rocks in Marquette County, Upper Michigan: Lawrence, Kans., University of Kansas M.S. thesis, 108 p.

Hanmer, Simon, 1982, Microstructure and geochemistry of plagioclase and microcline in naturally deformed granite: Journal of Structural Geology, v. 4, p. 197-213.

1987, Textural map units in quarto-feldspathic mylonitic rocks: Canadian Journal of Earth Sciences, v. 24, p. 2065-2073.

Hoffman, P.F., 1989, Precambrian geology and tectonic history of North America, in Bally, A.W.,and Palmer, A.R., eds., The geology of North America--An overview: Boulder,Colorado, Geological Society of America, The Geology of North America, v. A,p. 447-5 12.

Hudleston, P.J., 1976, Early deformational history of Archean rocks in the Vermilion district,northeastern Minnesota: Canadian Journal of Earth Sciences, v. 13, P. 579-592.

Hudleston, P.J., Schultz-Ela, D., and Southwick, D.L., 1988, Transpression in an Archeangreenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25,p. 1060-1068.

Jackson, NJ., and Ramsay, C.R., 1986, Post-orogenic felsic plutonism, mineralization, andchemical specialization in the Arabian Shield: Transactions of the Institute of Miningand Metallurgy (Sec. B, Applied Earth Sciences), v. 95, p. B83-B93.

Johnson R.C., and Bornhorst, T.J., 1991, Archean geology of the northern block of theIshpeming greenstone belt, Marquette County, Michigan, in Sims, P.K., and Carter,L.M.H., eds., Contributions to Precambrian geology of Lake Superior region: U.S.Geological Survey Bulletin 1904-F, 20 p.

Kinnaird, J.A., Batchelor, RA., Whitley, J.E., and MacKenzie, A.B., 1985, Geochemistry,mineralization, and hydrothermal alteration of the Nigerian high heat producing granites,in High heat production granites, hydrothermal circulation and ore genesis: Institute ofMining and Metallurgy, P. 169-195.

Morey, G.B., and Sims, P.K., 1976, Boundary between two Precambrian W terranes inMinnesota and its geologic significance: Geological Society of America Bulletin, v. 87,p. 141-152.

Morgan, J.P., and DeCristoforo, D.T., 1980, Geological evolution of the Ishpeming GreenstoneBelt, Michigan, U.S.A.: Precambrian Research, v. 11, p. 23-41.

Percival, J.A., 1989, A regional perspective of the Quetico metasedimentary belt, SuperiorProvince, Canada: Canadian Journal of Earth Sciences, v. 26, p. 677-693.

Peterman, Z.E., 1979, Geochronology and the Archean of the United States: EconomicGeology, v. 74, p. 1544-1562.

Poulsen, K.H., 1986, Rainy Lake wrench zone--An example of an Archean subprovince boundaryin northwestern Ontario, in deWit, MJ., and Ashwal, L.D., eds., Tectonic evolution ofgreenstone belts: Lunar and Planetary Institute, Houston, Texas, Technical Report86-10, P. 177-179.

Poulsen, K.H., Borradaile, GJ., and Kehlenbeck, M.M., 1980, An inverted succession at RainyLake, Ontario: Canadian Journal of Earth Sciences, v. 17, p. 1358-1369.

Puffett, W.P., 1969, The Reany Creek Formation, Marquette County, Michigan: U.S. GeologicalSurvey Bulletin 1274-F, 25 p.

127

Hoffman, P.F., 1989, Precambrian geology and tectonic history of North America, rn Bally, A.W., and Palmer, A.R., eds., The geology of North America--An overview: Boulder, Colorado, Geological Society of America, The Geology of North America, v. A, p. 447-512.

Hudleston, P.J., 1976, Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592.

Hudleston, P.J., Schultz-Ela, D., and Southwick, D.L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1060-1068.

Jackson, N.J., and Ramsay, C.R., 1986, Post-orogenic felsic plutonism, mineralization, and chemical specialization in the Arabian Shield: Transactions of the Institute of Mining and Metallurgy (Sec. B, Applied Earth Sciences), v. 95, p. B83-B93.

Johnson R.C., and Bornhorst, T.J., 1991, Archean geology of the northern block of the Ishpeming greenstone belt, Marquette County, Michigan, rn Sims, P.K., and Carter, L.M.H., eds., Contributions to Precambrian geology of Lake Superior region: U.S. Geological Survey Bulletin 1904-F, 20 p.

Kinnaird, J.A., Batchelor, RA., Whitley, J.E., and MacKenzie, A.B., 1985, Geochemistry, mineralization, and hydrothermal alteration of the Nigerian high heat producing granites, in High heat production granites, hydrothermal circulation and ore genesis: Institute of - Mining and Metallurgy, p. 169-195.

Morey, G.B., and Sims, P.K., 1976, Boundary between two Precambrian W terranes in Minnesota and its geologic significance: Geological Society of America Bulletin, v. 87, p. 141-152.

Morgan, J.P., and DeCristoforo, D.T., 1980, Geological evolution of the Ishpeming Greenstone Belt, Michigan, U.S.A.: Precambrian Research, v. 11, p. 23-41.

Percival, J.A., 1989, A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada: Canadian Journal of Earth Sciences, v. 26, p. 677-693.

Peterman, Z.E., 1979, Geochronology and the Archean of the United States: Economic Geology, v. 74, p. 1544-1562.

Poulsen, K.H., 1986, Rainy Lake wrench zone--An example of an Archean subprovince boundary in northwestern Ontario, in dewit, M.J., and Ashwal, L.D., eds., Tectonic evolution of greenstone belts: Lunar and Planetary Institute, Houston, Texas, Technical Report 86-10, p. 177-179.

Poulsen, K.H., Borradaile, G.J., and Kehlenbeck, M.M., 1980, An inverted succession at Rainy Lake, Ontario: Canadian Journal of Earth Sciences, v. 17, p. 1358-1369.

Puffett, W.P., 1969, The Reany Creek Formation, Marquette County, Michigan: U.S. Geological Survey Bulletin 1274-F, 25 p.

1974, Geology of the Negaunee quadrangle, Marquette County, Michigan: U.S.Geological Survey Professional Paper 788, 53 p., scale 1:24,000.

Rankin, D.W., 1976, Appalachian salients and recesses--Late Precambrian continental breakupand opening of the lapetus Ocean: Journal of Geophysical Research, v. 81, p. 5605-56 19.

Ridley, John, 1986, Parallel stretching lineations and fold axes oblique to a shear displacementdirection--A model and observations: Journal of Structural Geology, v. 8, p. 647-653.

Ridley, John, and Casey, M., 1989, Numerical modeling of folding in rotational strain histories--Strain regimes expected in thrust belts and shear zones: Geology, v. 17, p. 875-878.

Schackleton, R.M., and Pies, A.C., 1984, The relation between regionally consistent stretchinglineations and plate motions: Journal of Structural Geology, v. 6, p. 111-117.

Schulz, K.J., Sims, P.K., and Peterman, Z.E., 1988, A post-tectonic rare-metal-rich granite in thesouthern complex, Upper Peninsula, Michigan: 34th Annual Institute on Lake SuperiorGeology, Marquette, Michigan, Proceedings and Abstracts, p. 95-96.

Simpson, Carol, 1986, Determination of movement sense in mylonites: Journal of GeologicalEducation, v. 34, p. 246-26 1.

Sims, P.K., 1976, Early Precambrian tectonic-igneous evolution in the Vermilion district,northeastern Minnesota: Geological Society of America Bulletin, v. 87, p. 379-389.

1980, Boundary between Archean greenstone and gneiss terranes in northern Wisconsinand Michigan, in Morey, G.B., and Hanson, G.N., eds., Selected studies of Archeangneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Society ofAmerica Special Paper 182, p. 113-124.

1990, Geologic map of Precambrian rocks, Marenisco, Thayer, and Watersmeet 15-minute quadrangles, Gogebic and Ontonagon Counties, Michigan, and Vilas County,Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series Map 1-2093, scale1:62,500.

1991, Great Lakes tectonic zone in Marquette area, Michigan--Implications for Archeantectonics in north-central United States, j Sims, P.K., and Carter, L.M.H., eds.,Contributions to Precambrian geology of Lake Superior region: U.S. Geological SurveyBulletin 1904-E, l7p.

1992, Geologic map of Precambrian rocks, southern Lake Superior region, Wisconsin andnorthern Michigan: U.S. Geological Survey Miscellaneous Investigations Series Map1-2185, scale 1:500,000.

Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, The Great Lakes tectonic zone--A major crustal structure in central North America: Geological Society of AmericaBulletin, pt. 1, p. 690-698.

128

1974, Geology of the Negaunee quadrangle, Marquette County, Michigan: U.S. Geological Survey Professional Paper 788, 53 p., scale 1:24,000.

Rankin, D.W., 1976, Appalachian salients and recesses-Late Precambrian continental breakup and opening of the Iapetus Ocean: Journal of Geophysical Research, v. 81, p. 5605-5619.

Ridley, John, 1986, Parallel stretching lineations and fold axes oblique to a shear displacement direction--A model and observations: Journal of Structural Geology, v. 8, p. 647-653.

Ridley, John, and Casey, M., 1989, Numerical modeling of folding in rotational strain histories- Strain regimes expected in thrust belts and shear zones: Geology, v. 17, p. 875-878.

Schackleton, R.M., and Ries, A.C., 1984, The relation between regionally consistent stretching lineations and plate motions: Journal of Structural Geology, v. 6, p. 111-117.

Schuiz, K.J., Sims, P.K., and Peterman, Z.E., 1988, A post-tectonic rare-metal-rich granite in the southern complex, Upper Peninsula, Michigan: 34th Annual Institute on Lake Superior Geology, Marquette, Michigan, Proceedings and Abstracts, p. 95-96.

Simpson, Carol, 1986, Determination of movement sense in mylonites: Journal of Geological Education, v. 34, p. 246-261.

Sims, P.K., 1976, Early Precambrian tectonic-igneous evolution in the Vermilion district, northeastern Minnesota: Geological Society of America Bulletin, v. 87, p. 379-389.

1980, Boundary between Archean greenstone and gneiss terranes in northern Wisconsin and Michigan, rn Morey, G.B., and Hanson, G.N., eds., Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper 182, p. 113- 124.

1990, Geologic map of Precambrian rocks, Marenisco, Thayer, and Watersmeet 15- minute quadrangles, Gogebic and Ontonagon Counties, Michigan, and Vilas County, Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series Map 1-2093, scale 1:62,500.

1991, Great Lakes tectonic zone in Marquette area, Michigan-Implications for Archean tectonics in north-central United States, rn Sims, P.K., and Carter, L.M.H., eds., Contributions to Precambrian geology of Lake Superior region: U.S. Geological Survey Bulletin 1904-E, 17 p.

1992, Geologic map of Precambrian rocks, southern Lake Superior region, Wisconsin and northern Michigan: U.S. Geological Survey Miscellaneous Investigations Series Map 1-2185, scale 1:500,000.

Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, The Great Lakes tectonic zone- A major crustal structure in central North America: Geological Society of America Bulletin, pt. 1, p. 690-698.

Sims, P.K., and Morey, G.B., 1973, A geologic model for the development of early Precambriancrust in Minnesota: Geological Society of America Abstracts with Programs, v. 5, p. 812.

Sims, P.K., and Peterman, Z.E., 1981, Archean rocks in the southern part of the CanadianShield--A review: Geological Society of Australia Special Publication 7, p. 85-98.

Sims, P.K., Peterman, Z.E., Prinz, W.C., and Benedict, F.C., 1984, Geology, geochemistry, andage of Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area, northernMichigan: U.S. Geological Survey Professional Paper 1292-A, p. A1-A41.

Sims, P.K., Peterman, Z.E., Zartman, R.E., and Benedict, F.C., 1985, Geology andgeochronology of granitoid and metamorphic rocks of Late Archean age in northwesternWisconsin: U.S. Geological Survey Professional Paper 1292-C, 17 p.

Sims, P.K., and Southwick, D.L., 1985, Geologic map of Archean rocks, western Vermiliondistrict, northern Minnesota: U.S. Geological Survey Miscellaneous Investigations SeriesMap 1-1527, scale 1:48,000.

Southwick, D.L., 1972, Vermilion granite-migmatite massif, in Sims, P.K., and Morey, G.B., eds.,Geology of Minnesota--A centennial volume: Minnesota Geological Survey, p. 108-119.

Southwick, D.L., and Chandler, V.W., 1983, Subsurface investigations of the Great Lakestectonic zone, west-central Minnesota: Geological Society of America Abstracts withPrograms, v. 15, p. 692.

Southwick, D.L, and Morey, G.B., 1991, Tectonic imbrication and foredeep development in thePenokean orogen, east-central Minnesota--An interpretation based on regionalgeophysics and the results of test drilling, j Sims, P.K., and Carter, L.M.H., eds.,Contributions to Precambrian geology of Lake Superior region: U.S. Geological SurveyBulletin 1904-C, 17 p.

Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map of the Penokeanorogen, central and eastern Minnesota, and accompanying text: Minnesota GeologicalSurvey Report of Investigations 37, 25 p., scale 1:250,000.

Taylor, W.E.G., 1967, The geology of the lower Precambrian rocks of the Champion-Republicarea of Upper Michigan: Evanston, Illinois, Northwestern University Report 13, 33 p.

Thomas, WA., 1977, Evolution of Appalachian-Ouachita salients and recesses from reentrantsand promontories in the continental margin: American Journal of Science, v. 277,p. 1233-1278.

1983, Continental margins, orogenic belts, and intracratonic structures: Geology, v. 11,p. 270-272.

Van Hise, C.R., and Bayley, W.S., 1897, The Marquette iron-bearing district of Michigan: U.S.Geological Survey Monograph 28, 608 p.

129

Sims, P.K., and Morey, G.B., 1973, A geologic model for the development of early Precambrian crust in Minnesota: Geological Society of America Abstracts with Programs, v. 5, p. 812.

Sims, P.K., and Peterman, Z.E., 1981, Archean rocks in the southern part of the Canadian Shield-A review: Geological Society of Australia Special Publication 7, p. 85-98.

Sims, P.K., Peterman, Z.E., Prim, W.C., and Benedict, F.C., 1984, Geology, geochemistry, and age of Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area, northern Michigan: U.S. Geological Survey Professional Paper 1292-4 p. A1-A41.

Sims, P.K., Peterman, Z.E., Zartman, R.E., and Benedict, F.C., 1985, Geology and geochronology of granitoid and metamorphic rocks of Late Archean age in northwestern Wisconsin: U.S. Geological Survey Professional Paper 1292-C, 17 p.

Sims, P.K., and Southwick, D.L., 1985, Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1527, scale 1:48,000.

Southwick, D.L., 1972, Vermilion granite-migrnatite massif, Sims, P.K., and Morey, G.B., eds., Geology of Minnesota--A centennial volume: Minnesota Geological Survey, p. 108-119.

Southwick, D.L., and Chandler, V.W., 1983, Subsurface investigations of the Great Lakes tectonic zone, west-central Minnesota: Geological Society of America Abstracts with Programs, v. 15, p. 692.

Southwick, D.L., and Morey, G.B., 1991, Tectonic imbrication and foredeep development in the Penokean orogen, east-central Minnesota-An interpretation based on regional geophysics and the results of test drilling, Sims, P.K., and Carter, L.M.H., eds., Contributions to Precambrian geology of Lake Superior region: U.S. Geological Survey Bulletin 1904-C, 17 p.

Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p., scale 1:250,000.

Taylor, W.E.G., 1967, The geology of the lower Precambrian rocks of the Champion-Republic area of Upper Michigan: Evanston, Illinois, Northwestern University Report 13, 33 p.

Thomas, WA., 1977, Evolution of Appalachian-Ouachita salients and recesses from reentrants and promontories in the continental margin: American Journal of Science, v. 277, p. 1233-1278.

1983, Continental margins, orogenic belts, and intracratonic structures: Geology, v. 11, p. 270-272.

Van Hise, C.R., and Bayley, W.S., 1897, The Marquette iron-bearing district of Michigan: U.S. Geological Survey Monograph 28,608 p.

White, S.H., 1976, The effects of strain on the microstructures, fabrics, and deformationmechanisms in quartzites: Royal Society of London Philosophical Transactions, ser.A283, p. 69-86.

Wise, D.U., and seven others, 1984, Fault-related rocks--Suggestions for terminology: Geology,v. 12, p. 39 1-394.

Wood, John, 1980, Geology of the Hewitt Lake area, District of Kenora, Patricia portion:Ontario Geological Survey, Report 186, 122 p.

130

White, S.H., 1976, The effects of strain on the microstructures, fabrics, and deformation mechanisms in quartzites: Royal Society of London Philosophical Transactions, ser. A283, p. 69-86.

Wise, D.U., and seven others, 1984, Fault-related rocks-Suggestions for terminology: Geology, v. 12, p. 391-394.

Wood, John, 1980, Geology of the Hewitt Lake area, District of Kenora, Patricia portion: Ontario Geological Survey, Report 186, 122 p.

Field stops and road logField stops shown on figure 6

Miles0 Depart from Ramada Inn in downtown Marquette on Michigan highway 28 (M28)

and proceed west.

3.8-6.3 Numerous outcrops of pillow basalt along M28. Basalt comprises a major part ofArchean greenstone-granite terrane (Wawa subprovince) in the northern complexof Marquette district.

Early Proterozoic rocks of Marquette syncline lie to the south of highway.

7.2 Turn left (south) from M28 onto M35.

10.8 M35 intersects Marquette County road 480 (Co. 480). Continue south on M35.

11.2 Outcrops of Early Proterozoic Negaunee Iron-formation.

14.1 High dumps at 12 o'clock, Empire iron mine waste rock. Pass through Palmer.

16.9 Turn right (west) on County highway 565.

18.1 At road fork turn right onto County highway 476.

19.5 Stop 1. Roadcut in "Tilden granite" (Hammond, 1978).

The "Tilden granite" is a pinkish-gray to pink, medium-grained, locally porphyritic,massive to weakly foliated, homogeneous granite that locally contains orientedxenoliths of mafic gneiss. It intrudes gneisses and in turn is cut by pink pegmatite.The granite is highly fractured; the fractures have slickensided surfaces and a thincoating of chlorite and other propylitic alteration minerals. The granite has a U-Pbconcordia intercept age of 2,585±15 Ma (R.E. Zartman, oral commun., 1991).Chemically, the granite has characteristics of post-orogenic, within plate granite ondiscrimination diagrams (T. Bornhorst, written commun., 1990), which is consistentwith its post-tectonic field relationships.

Continue southwest on Co. 476.

19.7 Turn around and return to M35.

22.4 Junction of Co. 476 and M35. Proceed southeast on M35.

23 Stop 2. High roadcut on M35 in Archean gneiss (of Archean gneiss terrane).Road shoulder is narrow. Be careful.

Excellent outcrop of layered gneiss (of Archean gneiss terrane) on the northeastside of road. Amphibolite and quartzofeldspathic gneiss are interlayered, and arecut by red pegmatite. Pegmatite and aplite are boudined and brecciated; boudins

131

Field stops and road log Field stops shown on figure 6

Miles 0 Depart from Ramada Inn in downtown Marquette on Michigan highway 28 (M28)

and proceed west.

Numerous outcrops of pillow basalt along M28. Basalt comprises a major part of Archean greenstone-granite terrane (Wawa subprovince) in the northern complex of Marquette district.

Early Proterozoic rocks of Marquette syncline lie to the south of highway.

Turn left (south) from M28 onto M35.

M35 intersects Marquette County road 480 (Co. 480). Continue south on M35.

Outcrops of Early Proterozoic Negaunee Iron-formation.

High dumps at 12 o'clock, Empire iron mine waste rock. Pass through Palmer.

Turn right (west) on County highway 565.

At road fork turn right onto County highway 476.

Stop 1. Roadcut in Tilden granite" (Hammond, 1978).

The "Tilden granite" is a pinkish-gray to pink, medium-grained, locally porphyritic, massive to weakly foliated, homogeneous granite that locally contains oriented xenoliths of mafic gneiss. It intrudes gneisses and in turn is cut by pink pegmatite. The granite is highly fractured; the fractures have slickensided surfaces and a thin coating of chlorite and other propylitic alteration minerals. The granite has a U-Pb concordia intercept age of 2,585Â 15 Ma (R.E. Zartman, oral commun., 1991). Chemically, the granite has characteristics of post-orogenic, within plate granite on discrimination diagrams (T. Bornhorst, written commun., 1990), which is consistent with its post-tectonic field relationships.

Continue southwest on Co. 476.

Turn around and return to M35.

Junction of Co. 476 and M35. Proceed southeast on M35.

Stop 2. High roadcut on M35 in Archean gneiss (of Archean gneiss terrane). Road shoulder is narrow. Be careful.

Excellent outcrop of layered gneiss (of Archean gneiss terrane) on the northeast side of road. Amphibolite and quartzofeldspathic gneiss are interlayered, and are cut by red pegmatite. Pegmatite and aplite are boudined and brecciated; boudins

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plunge parallel to recumbent fold axes. Tight folds with axial planar foliationplunge about 25° with a strike of N. 50 W., as does a mineral lineation.

This outcrop is typical of the gneiss of the Archean gneiss terrane, but amphiboliteis uncommonly abundant here. Tight recumbent folds that plunge gently northwardare the dominant structures in this part of the gneiss terrane.Return to vehicle and continue southeast on M35.

23.7 M35 intersects private road (Cleveland Cliffs Iron Company).

24.2 Stop 3. Low outcrop on northeast side of road in grassy area. Outcrop has beencovered by loose sand and is lichen free.

This outcrop contains northwest-trending folds with an axial planar foliation visibleon polished surfaces that strikes N. 65° W., and dips 600 NE. This structure issuperposed on recumbently folded gneiss, and therefore is younger. Thick layers offelsic gneiss retain a dominant older foliation that strikes N. 250 W.

The steep northwest-trending foliation and associated folds are parallel to the trendof the antiformal southern complex (of Marquette district). Although minor in thisarea, upright northwest-trending folds are the dominant structures in the Republicarea, 30 km to the west.

Continue southeast on M35.

24.3 Turn around at Community Club and proceed northwest on M35 toward Palmer.

24.9 M35 intersects private road (CCI). Continue north on M35 to intersection withCo. 480.

32.3 Intersection of M35 and Co. 480. Turn right (east) on Co. 480.

35.6 Turn right (south) on secondary road. Roads for remainder of field excursion areunmarked; accordingly, we have arbitrarily assigned letter designations A through L(see fig. 3) to specific segments as an aid in location. The main road into area(road A) is a graded sandy dirt road; other roads followed on the trip are notgraded and may be rutted or soft, requiring some care in driving.

39.6 Road A crosses Chicago and Northwestern railway near Gentian.

39.7 Take right road fork (B) and continue southwest on B.

40.6 Junction with east-west road (C). Proceed west on C for 0.3 mi.

plunge parallel to recumbent fold axes. Tight folds with axial planar foliation plunge about 25' with a strike of N. 5' W., as does a mineral lineation.

This outcrop is typical of the gneiss of the Archean gneiss terrane, but amphibolite is uncommonly abundant here. Tight recumbent folds that plunge gently northward are the dominant structures in this part of the gneiss terrane. Return to vehicle and continue southeast on M35.

M35 intersects private road (Cleveland Cliffs Iron Company).

Stop 3. Low outcrop on northeast side of road in grassy area. Outcrop has been covered by loose sand and is lichen free.

This outcrop contains northwest-trending folds with an axial planar foliation visible on polished surfaces that strikes N. 65' W., and dips 60' NE. This structure is superposed on recumbently folded gneiss, and therefore is younger. Thick layers of felsic gneiss retain a dominant older foliation that strikes N. 25' W.

The steep northwest-trending foliation and associated folds are parallel to the trend of the antiformal southern complex (of Marquette district). Although minor in this area, upright northwest-trending folds are the dominant structures in the Republic area, 30 km to the west.

Continue southeast on M35.

Turn around at Community Club and proceed northwest on M35 toward Palmer.

M35 intersects private road (CCI). Continue north on M35 to intersection with Co. 480.

Intersection of M35 and Co. 480. Turn right (east) on Co. 480.

Turn right (south) on secondary road. Roads for remainder of field excursion are unmarked; accordingly, we have arbitrarily assigned letter designations A through L (see fig. 3) to specific segments as an aid in location. The main road into area (road A) is a graded sandy dirt road; other roads followed on the trip are not graded and may be rutted or soft, requiring some care in driving.

Road A crosses Chicago and Northwestern railway near Gentian.

Take right road fork (B) and continue southwest on B.

Junction with east-west road (C). Proceed west on C for 0.3 mi.

40.9 Stop 4. Rock knob north side of road C.

This outcrop is typical of the Archean greenstone-granite terrane. It consists ofgranite gneiss (or foliated granite) that contains lenses of biotitic and chioriticamphibolite. These rocks are cut by granite pegmatite and aplite, which crosscutthe foliation in the granitoid rocks. Note that crosscutting aplite and pegmatite arenot sheared out. Foliation strikes N. 70° W. and dips 65° Sw.

Although this outcrop lies near the north margin of the Great Lakes tectonic zoneand although it has a foliation subparallel to that in the GL1'Z, the outcrop andthose to the west-northwest are excluded from the GLTZ because the crosscuttingpegmatite and aplite are not sheared out and the rocks are protomylonite. Thenorthern boundary is placed at the abrupt transition from "straight-banded"mylonite in the GL1'Z to rocks that retain recognizable igneous structures, such ashere. The rocks to the north of the GLTZ are protomylonite.

Return to vehicle and continue west on C for 0.2 mile.

41.1 Intersection of roads C and D. Turn left on D and continue for 0.4 mi.

41.5 Stop 5. Outcrops of milky quartz and altered Archean rocks about 50 yards west ofroad D.

Milky quartz and silicified country rock, such as those exposed here, characterizethe hifis to the west. These outcrops comprise the most eastern exposed part of aquartz lense, oriented about east-west, that is about 1.4 km long. The quartz bodymay occupy a major tension fissure zone related to dextral wrench shear within theGLTZ.

Turn around and proceed north on D to intersection with C.

Then follow C and D to road fork at Gentian crossing.

42.8 Turn sharply right from road B onto road E; continue south for 1.2 ml.

44 Road fork; continue south on road F.

44.35 Road fork; continue south on right fork (G), then turn left at road fork onto H andcontinue east on H to locked gate.

45.0 Park and walk on private road east and northeast to west shore of Powell Lake(distance 0.3 mi). walk is easy.

Stop 6. This is an exceptionally good series of outcrops of mylonite within theGLTZ because they are periodically washed clean. The rock is dominantly granitegneiss containing pods of migmatized amphibolite and hornblende schist. Theprotolith is granite and amphibolite of the Archean greenstone-granite terrane.

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40.9 Stop 4. Rock knob north side of road C.

This outcrop is typical of the Archean greenstone-granite terrane. It consists of granite gneiss (or foliated granite) that contains lenses of biotitic and chloritic amphibolite. These rocks are cut by granite pegmatite and aplite, which crosscut the foliation in the granitoid rocks. Note that crosscutting aplite and pegmatite are not sheared out. Foliation strikes N. 70Â W. and dips 65' SW.

Although this outcrop lies near the north margin of the Great Lakes tectonic zone and although it has a foliation subparallel to that in the GLTZ, the outcrop and those to the west-northwest are excluded from the GLTZ because the crosscutting pegmatite and aplite are not sheared out and the rocks are protomylonite. The northern boundary is placed at the abrupt transition from "straight-banded" mylonite in the GLTZ to rocks that retain recognizable igneous structures, such as here. The rocks to the north of the GLTZ are protomylonite.

Return to vehicle and continue west on C for 0.2 mile.

41.1 Intersection of roads C and D. Turn left on D and continue for 0.4 mi.

41.5 Stop 5. Outcrops of milky quartz and altered Archean rocks about 50 yards west of road D.

Milky quartz and silicified country rock, such as those exposed here, characterize the hills to the west. These outcrops comprise the most eastern exposed part of a quartz lense, oriented about east-west, that is about 1.4 lun long. The quartz body may occupy a major tension fissure zone related to dextral wrench shear within the GLTZ.

Turn around and proceed north on D to intersection with C.

Then follow C and D to road fork at Gentian crossing.

42.8 Turn sharply right from road B onto road E; continue south for 1.2 mi.

44 Road fork; continue south on road F.

44.35 Road fork; continue south on right fork (G), then turn left at road fork onto H and continue east on H to locked gate.

45.0 Park and walk on private road east and northeast to west shore of Powell Lake (distance 0.3 mi). Walk is easy.

Stop 6. This is an exceptionally good series of outcrops of mylonite within the GLTZ because they are periodically washed clean. The rock is dominantly granite gneiss containing pods of migmatized amphibolite and hornblende schist. The protolith is granite and amphibolite of the Archean greenstone-granite terrane.

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Foliation in the gneiss strikes about N. 75° W. and dips 75° SW., but is variable. Aconspicuous stretching lineation plunges uniformly about 550 and bears S. 45° E.Both dextral and sinistral asymmetric folds plunge subparallel to the lineation;dextral folds predominate. Porphyroclasts are common, but they do not clearlyhave internal monodlinic symmetly and elongate tails, necessary to determinemovement sense (Simpson, 1986).

Return to parked vehicle at locked gate. Proceed west on road H to road fork(distance 0.25 mi), then turn right on H to junction of H with J (distance 0.15 mi).

45.4 Junction of roads H and J. Turn sharply left on J and continue for 0.2 mi.

45.6 Road fork. Continue on right fork (K) for 0.1 ml.

45.7 Park and walk on trail to northwest, a distance of about 750 ft, to outcrops.

Stop 7. Low outcrops of pink mylonitic gneiss with lenses of biotite schist.Protolith is considered to be felsic gneiss of Archean gneiss terrane. Foliationstrikes N. 750 and dips 65° SW. A quartz rodding lineation plunges 50° andbears S. 45° E. Note abundant ribbon quartz. This outcrop is in the southern partof the GLTZ.

Return to road. Turn around and follow roads K and J to road fork with H.

Proceed south on H to junction (1.5 mi).

46.2 Continue south on L.

47.3 Road L intersects power line. Park and walk 750 ft southwest to low outcropsalong power line.

Stop 8. Outcrops comprise pink granite gneiss with amphibolite layers and lenses,cut by pink granite pegmatite. Foliation strikes N. 40° W., and dips 80° NE. Rocksare not sheared. Two parallel diabase dikes cut the Archean gneiss.

These outcrops are within the Archean gneiss terrane, a distance of about 0.65 kmsouth of the south boundary of the (3L1'Z. Thus, we have crossed from theArchean greenstone-granite terrane, through the GLTZ, into the Archean gneissterrane to the south of the GLTZ.

End of field trip. Suggest returning to Co. 480 via dirt roads L, G, F, E, and A.

Foliation in the gneiss strikes about N. 75' W. and dips 75' SW., but is variable. A conspicuous stretching lineation plunges uniformly about 55' and bears S. 45' E. Both dextral and sinistral asymmetric folds plunge subparallel to the lineation; dextral folds predominate. Porphyroclasts are common, but they do not clearly have internal monoclinic symmetry and elongate tails, necessary to determine movement sense (Simpson, 1986).

Return to parked vehicle at locked gate. Proceed west on road H to road fork (distance 0.25 mi), then turn right on H to junction of H with J (distance 0.15 mi).

45.4 Junction of roads H and J. Turn sharply left on J and continue for 0.2 mi.

45.6 Road fork. Continue on right fork (K) for 0.1 mi.

45.7 Park and walk on trail to northwest, a distance of about 750 ft, to outcrops.

Stop 7. Low outcrops of pink mylonitic gneiss with lenses of biotite schist. Protolith is considered to be felsic gneiss of Archean gneiss terrane. Foliation strikes N. 75' W. and dips 65' SW. . A quartz rodding lineation plunges 50' and bears S. 45' E. Note abundant ribbon quartz. This outcrop is in the southern part of the GLTZ.

Return to road. Turn around and follow roads K and J to road fork with H.

Proceed south on H to junction (1.5 mi).

46.2 Continue south on L.

47.3 Road L intersects power line. Park and walk 750 ft southwest to low outcrops along power line.

Stop 8. Outcrops comprise pink granite gneiss with amphibolite layers and lenses, cut by pink granite pegmatite. Foliation strikes N. 40' W., and dips 80' NE. Rocks are not sheared. Two parallel diabase dikes cut the Archean gneiss.

These outcrops are within the Archean gneiss terrane, a distance of about 0.65 km south of the south boundary of the GLTZ. Thus, we have crossed from the Archean greenstone-granite terrane, through the GLTZ, into the Archean gneiss terrane to the south of the GLTZ.

End of field trip. Suggest returning to Co. 480 via dirt roads L, G, F, E, and A.

U)w-J

In —

U)

w

I

0— -0

Figure 6. Field trip stops. Marquette and most of Michigan highway 28 are just north of themap area.

135

Figure 6. Field trip stops. Marquette and most of Michigan highway 28 are just north of the map area.

Issues of the Proceedings of this meeting may be ordered from

M.G. Mudrey Jr., Secretary-Treasurerdo Wisconsin Geological and Natural History Survey

3817 Mineral Point RoadMadison, Wisconsin 53705-5 100

Part 1: Program and Abstracts: $7.00 U.S.Part 2: Field Trip Guidebook: $7.00 U.S.

Orders will be filled while supplies last.

All volumes back to 1955 are available for photocopying at the prevailing rate, from the MichiganTechnological University Library, through Mr. M.S. Spence, Archivist.

Telephone (906) 487-2505

Issues of the Proceedings of this meeting may be ordered from

M.G. Mudrey, Jr., Secretary-Treasurer c/o Wisconsin Geological and Natural History Survey

38 17 Mineral Point Road Madison, Wisconsin 53705-5 100

Part 1: Program and Abstracts: $7.00 U.S. Part 2: Field Trip Guidebook: $7.00 U.S.

Orders will be filled while supplies last.

All volumes back to 1955 are available for photocopying at the prevailing rate, from the Michigan Technological University Library, through Mr. M.S. Spence, Archivist.

Telephone (906) 487-2505