UNITED STATES

DEPARTMENT O f THE INTERIOR

GEOLOGICAL SURVEY

Late Mesozoic and Cenozoic Tectonics

and the Age o f Porphyry Copper Prospects;

Chignik and Sutw i k Island quadrangles, Alaska Peninsula

by

Freder i c Hen 1 ey W i 1 son

OPEN-FILE REPORT 80-543

This report i s preliminary and has not been e d i t e d or reviewed for conformi ty w i th

Geological Survey standards and nomenclature. Menlo Park, C a l i f o r n i a

1980 -

Abst rac t

During Tertiary t i m e , two major episodes of i g n e o u s a c t i v i t y occurred in the Chignik and Sutwik Is land quadrangles o f A l a s k a . Leuco-basaltic t o d a c i t i c volcanism and granodioritic plutonism occurred during Eocene to Early Oligocene time, whereas leuco-basaltic to d a c i t i c volcanism and q u a r t z d i o r i t i c t o granodioritic plutonism occurred f r om L a t e Miocene t o Holocene t i m e .

The Eocene t o Ol igocene vo lcan ic arc , t h e To ls to i -Mesh ik a rc , was apparent ly or iented east-northeast t o west-southwest; this i s related to a more northerly d i r e c t e d convergence direction than present and t o subduction o f the Kula Plate. Fol lowing the model o f Delong and others, subduction of t he K u l a - P a c i f i c Ridge i n Late Oligocene t ime caused cessation o f volcanism. A f t e r ridge subduction, volcanism resumed t o f o r m the present Aleut ian arc, which i s oriented i n a nor theas t t o southwest direc t ion . This i s essent i a1 ly perpendicu 1 ar t o theU present convergence d i r ec t ion a t the A l e u t i a n trench near the Chignik region. Isotopic da t i ng results indicate a r a p i d mig ra t i on (<I0 my. ) o f the locus of volcanism across the Alaska Peninsula from southeast t o northwest since l a t e Miocene time. Less conc lus ive i s evidence suggesting m i g r a t i o n o f the locus o f volcanism in t h e Tolstoi-Meshik arc from nor th t o south i n Eocene to Early Oligocene t ime .

Hydrothermal a1 t e ra t i on and copper porphyry-type mineralization has been related to both per iods o f igneous a c t i v i t y . Potassium-argon da t ing o f hydrothermal m i n e r a l phases and geologic re la t ions i n d i c a t e a distinct t ime period between emplacement o f an igneous phase and m i n e r a l i z a t i o n of t h a t phase.

Three porphyry p r o s p e c t s were examined i n detail. Bee Creek i s a copper porphyry prospect assoc ia ted ~ i t h a dacite intrusion in t h e Upper Jurassic Naknek Format ion . No primary minerals from the dacite were da tab le ; therefore no emplacement age was determined. A hornblende date on post-mineralization d a c i t e was 2.15 my.. whereas potassium-argon ages on hydrothermal b i o t i t e , sericite, and ch lo r i t e suggest a 3.7 m y . age for minerali t a t i o n .

Warner Bay i s a p l u t o n i c porphyry deposit in the multiphase Late Miocene D e v i l s b a t h o l i t h . Mineralization occurs i n the Sweater Bay phase o f the bath01 i th; potassium-argon dating o f primary and hydrothermal ( ? ) mineral phases i n d i c a t e s a t ime span o f a t least 2.5 m.y. between emplacement and m i n e r a l i z a t i o n . I n t r u s i o n of the Northwest Arm phase o f the batholith may have thprmally overprinted these dates o r may be contemporaneous ~ i t h mineralization, Another interpretation can explain the i s o t o p i c results t h r o u g h slow c o o l i n g o f the Sweater Bay phase followed by rapid coo l i ng o f the l a t e r Northpiest A r m phase.

Mallard Duck Bay i s a v o l c a n i c porphyry prospect i n volcanic and volcaniclastic rocks of t h e Meshik Forrndtion. Potassium-argon dates on the volcanic rocks range f r o m 27 m.y. on a dike i n t r u d i n g the volcanic rocks t o 21 m.y. on fresh leuco-baszlt peripheral to the prospect. Hydrothermal b i o t i t e and ch lo r i t e mix tu res and serici t e a l s o yield a 21 m.y. ages, suggesting mineralization occurred very late i n the more t h a n 7 m.y. history o f the v o l c a n i c center .

Geolog ic mapping and potassium-argon da t ing has indicated a t least th ree and possibly f o u r episodes of volcano-pltuonic arc activity i n t h e study area. These episodes took p lace i n Early t o Middle Jurassic, e a r l y

L a t e Cretaceous(?), Eocene t o Early 01 igocene, and L a t e Miocene t o Holocene t imes. A geologic history o f the Chignik region has been derived f r o m this work; t h i s history has been used t o h e l p construct a speculative southern Alaska model. The model describes the motion o f t h e Baja Alaska terrane f rom Early Jurassic t o Holocene time and relates t h i s mot ion t o o t h e r ma jo r terranes i n Alaska and t o c r a t o n a l North America. Large scale northward translation and rotation o f portions o f southern Alaska, suggested by paleomagnetic data , are reconciled with the known geologic history. The proposed model involves accretion o f these te r ranes t o N o r t h America and str ike-slip trans1 a t i on along t h e western margin o f N o r t h America.

Acknowledgements This open- f i l e report was a t h e s i s submitted t o Dartmouth College,

Hanover, New Hampshire by this author, i n partial fulfillment o f requirements f o r a Ph.D. i n geology.

Any thesis depends in large part on support of a l l kinds extended t o the writer. I gratefully acknowledge t he f i n a n c i a l support I have recieved from Dartmouth College which has allowed me to attend. I am pal-ticularily indebted t o Professor J.B. Lyons f o r sincere encouragment, good humor, and help ing me to m a i n t a i n a sense o f what t h i s i s a l l a b o u t . Professor R . C . Reynolds p rov ided many respites from t h i s thesis and time to d i s c u s s soye "'real scienceu.

I am also grateful to the U.S. Geological Survey f o r field and laboratory support and in particular the Branch o f A l a s k a n Geology f o r ,

constant and steady encouragement. Particularily helpful were M.L. Silberman, R.L. Detterman, J.E. Case, H.C. Berg, and A.T. Ovenshine. Many aspects o f t h i s t h e s i s , particularily o f economic o r i e n t a t i o n benef i ted greatly from t h e suggest ions o f , discussions with, and encouragement from M.L. (W i l d B i l l ) Silberman.

F e l l o w students a t Dartmouth helped t o create an atmosphere conducive t o dork on this thes is , while f r i e n d s and former classmates at the U.S. Geological Survey, particularily John Decker, participated i n stimulating discussions o f Alaskan geology.

I wish t o t h a n k t h e Geological Society o f America and the U.S. Geological Survey for the opportunity t o attend the Penrose Conference, Mesozoic and Cenozoic Microplate Tectonics o f Western N o r t h America.

In any project r e q u i r i n g as much l a b work as t h i s , one f i n d s t h a t many have helped a long the way. William C. Gaum, Paige L. Herzon, and Leda Beth Gray helped w i t h mineral separations and argon extractions and Barbara Myers helped w i t h problem s o l v i n g and mak ing the l a b a n i c e r p l ace t o be. Finally, b u t by no means least , the deeply f e l t and l ong term support o f my parents and friends i s extremely g r a t e f u l l y acknowledged.

The Bristol Bay N a t i v e Corporation and Bear Creek Mining Company k ind ly permitted use o f maps and data collected by Bear Creek Mining Company on t h e porphyry prospects o f the Chignik region. The Economic Geology Pub l ish ing Company k i n d l y gave permission for use o f Figure 5. The Canadian I n s t i t u t e o f Mining kindly p rov i ded permission for use o f Figures 6, 7, and 8.

Chigni t Formation 84 Hoodoo Formation 86 Jolstoi Formation 87 Stepovak Formation 88 Meshik Formation 89 Bear Lake Format i on 89 Milky River Formation 90

Appendix 4 Chemical and Normative data, and Rock Descriptions for analyzed rocks 91 and P l a t e 2

Table 1

Table 2

Table 3

Table 4 Table 5

Table 6

Table 8

Table 9

L i s t of Tables Table showing percent o f silica and normative color index for analyzed volcanic and nypabyssal rocks 16

Potassi um-argon ages - Kodiak-Shumagin P lu ton ic Series 19

Potass i um-argon ages - Devi 1 s B a t h o l i t h and Warner Bay Prospect 21,

Potass i um-argon ages - Meshi k Formati on 23 Potassium-argon ages - Cape Kumlik and

Cape Kunmi k 25 Potassium-argon ages - Miscellaneaus

Intrusive and Volcanic Racks 26 Potassium-argon ages - Bee Creek Prospect 38

Potassi um-argon ages - Ma1 1 ard Duck Bay Prospect 43

Potassium-argon ages - Miscellaneous Prospects 45

v i i

L i s t o f Illustrations Figure 1 Location map showing the Alaska Peninsula

and the Chignik and Sutwik Island region Figure 2 Stratigraphic section, Chignik region Figure 3 Normative Q:Or:Pl ternary diagram for

analyzed rocks Figure 4 A:F:M ternary diagram f o r analyzed rocks Figure 5 Porphyry copper deposit model, after

S i 11 i toe (1973) F igu re 6 Type I or phallic porphyry model, after

Sutherland-Brown (1976) Figure 7 Type I1 or volcanic porphyry model, after

Sutherland-Brown (1976) Figure 8 Type I I I or plutonic porphyry model,

a f t e r Sutherland-Brown (1976) Figure 9 Temperature versus time plot f o r the

Devils batholjth F igu re 10 Map showing l o c a t i o n o f the Wrangellia

and Baja Alaska terranes in southern A1 aska

Figure 11 Map showing correlation between aero- maqneti c anomalies and geochronological ly dated igneous rocks, Chignik and Sutwik Island quadrangles

F iqu re 12 Histogram o f dates f rom the Chignik and Sutwik Island quadrangles

F i sure 13 Schernati c cross-sect ion - Tectonic development o f t h e Chignik region Part I

Figure 14 Schematic cross-sect ion - Tectonic development o f the Chignik region P a r t I I

Figure 15 Schematic cross-secti on - Tectonic development o f the Chignik region Part I11

Figure 16 Tectonostratigraphic terranes o f Alaska exc lus ive o f southeastern Alaska and Seward Peninsula, modi f ied after Jones and Siblerling (1979)

Figure 17 Schematic diagram - Generalized Southern Alaska model

Figure A 1 Schematic diagram - Argon e x t r a c t i o n l i n e

P l a t e 1 Generalized geologic map o f the Chignik and S u t w i k Is1 and quadrangles, Alaska (~etterman and others, 1979) in pocket

Plate 2 Map showing potassium-argon age sample locations and named features, Chignik and Sutwik Island quadrangles, Alaska in pocket

Plate 3 Geologic map o f the Bee Creek Prospect, after Fields (1977) i n pocket

Plate 4 A l t e r a t i o n map o f the Bee Creek Prospect, a f t e r Fields (1977) i n pocket

plate 5 Geologic map o f the Mallard Duck Bay Prospect, a f t e r Fields (1977) i n pocket

v i i i

Introduction and Purpose This dissertation i s a partial result o f studies carried out as part

o f the Chignik and Sutwik Is land quadrangles AMRAP (Alaska Mineral Resource Assessment Program) o f t h e U.S. Geological Survey, Branch of Alaskan Geology. Parts o f t h e study area were being considered f o r i nc lus ion i n t o Aniakchak National Monument and i n t o the Na t i ona l W i Id1 i f e Refuge System. I n addit ion, areas are claimed by the Bristol Bay Nat ive Corporation under the Alaska Nat ive Claims Settlement Act of 1971. As a result o f these considerations, an evaluat ion o f geologically related resources, coupled w i t h a general reconnaissance geologica l evaluation was begun. The purpose o f this dissertation i s to report on a study o f t h e t i m i n g o f plutonism and minera l i za t ion i n the Chignik and S u t w i k Island quadrangles o f Alaska. This d isser ta t ion attempts to place intrusive events w i t h i n a time framework es tab l i shed using potassium-argon dating and t o present a coherent tec ton ic synthesis of i n t r u s i v e a c t i v i t y w i t h i n t h i s framework. Within the constraints o f t h i s work and other geologic data, a regional t e c t o n i c synthesis i s attempted. I n addi t ion , the t iming o f mineralization i s s tud ied by potassium-argon dat ing o f the a1 teration assemblages formed concurrently with mineralization.

Location and Geography The study area i s composed of the Sutwik Island and Chignik

quadrangles spanning the Alaska Peninsula i n an east -west d i r e c t i o n just southwest of Kodiak Island (Figure 1). The extent o f the area i s approximately 18,000 sq km (7,000 sq m i ) and i t encompasses parts o f two physiographic r eg ions (Wahrhaftig, 1965), the A l e u t i a n Range Prov ince and the Nushagak-Bristol Bay Lowland Province.

The A l e u t i a n Range Province i s characterized by jagged glaciated peaks wi th e l e v a t i o n s averag ing 10UOm, w i t h sone peaks e x t e n d i n g t o over 2000m. The valleys are U-shaped and o f t e n fjord-like though many are filled w i t h recent ash f rom the active volcanos o f M t . Veniarninof, Aniakchak and Black Peak. The northwestern edge o f t h i s p rov ince in the study area i s bounded by these three volcanos.

The Nushagak-Bristol Bay Lowland i s characterized by extreme low re l ie f (4100m). I n general, the t e r r a i n has been h i l t by Quarternary processes, the material usually g l ac i a l d r i f t o r ash-flow and ash-fall debris. Near the Bristof Bay coast, there are mar ine terraces a t an e l e v a t i o n near 30m, ev idenc ing a higher sea level stand (oral communication, R.L. Detterman, 1978), o r relative post-glacial uplift o f t h e Alaska Peninsula.

Human habitation i s concentrated around Chignik Bay in the villages o f Chignik, Chignik Flats, Chignik Lake, and Chignik Lagoon and at P o r t Heiden i n the village o f Meshik with a t o t a l year-round population near 300. The area can be reached by a i r w i t h scheduled s e r v i c e by Reeve A l e u t i a n Airways t o Port Heiden and connecting s e r v i c e by Peninsula Airways t o o the r po in t s . There are no roads main ta ined i n t h e area except w i t h i n villages. Much o f the area l i e s within the territorial boundaries o f t h e Bristol Bay N a t i v e Corporation and Koniag Inc., the Kodiak Native corporation.

Methods and Techniques

F i e l d Work F i e l d ~ o r k i n a s s o c i a t i o n w i t h t h i s p r o j e c t was done i n the early

summers o f 1977 and 1978 by t h i s w r i t e r and assoc ia tes of t he Branch of Alaskan Geology of t h e U.S. Geological Survey. The f i e l d p a r t y i n c l u d e d R.L. Detterrnan (Team leader), D.H. Rich te r , T.P. Miller, M.E. Yount, myself and guest scientists. Each year, i n late May and during t h e month o f June, a mobile f i e l d camp was established on board t he Alaska Branch's s h i p , t he R.V. Don J . Miller I I . T h i s ship i s equipped w i t h f o u r s k i f f s f o r shore work, a he l i cop te r land ing deck f o r helicopter support, rocK sads a n d associated equipment f o r s l a b b i n g and s t a i n i n g o f rocks, and a work room with map cabinets and drafting tables for compilation and map work. The s h i p w i l l support up t o an 11 person geologic f i e l d party. Each year during the first two weeks o f July the f i e l d p a r t y moved t o a land-based camp at Columbia Ward Fisheries in Chignik Lagoon. Helicopter suppor t was contracted to ERA Hel icopters i n 1977 and a Bell 2068 was f lown by Rober t McDonald. I n 1978, A i r L o g i s t i c s o f Alaska received the contract an3 Gordon Hine was our pilot, also i n a Belt 2068. While ship-board, log i s t i c support was provided by the s h i p ' s crew; on land, food and lodging were c o n t r a c t e d t o Columbia Ward f isher ies.

Frequent storms o r ig ina t ing i n the A l e u t i a n Low passed through the area during the f i e l d season, often f o r c i n g us t o remain i n camp or c u r t a i l work i n progress. Winds of ten exceeded 50 knots and reached 35 knots (109 mph) i n one storm. Calmer weather was o f t e n accompanied by thick fog . The result was t h a t we were a b l e t o work about h a l f t h e time, generally under marginal cond i t i ons . When good days occurred they were truly spectacular, and made the rest o f the s t a y worthwhile.

During 1977, t r a v e r s e s were of t e n made alone b u t a number o f unfor tunate bear encounters in our and o t h e r f i e l d partizs caused us t o change our working procedures i n 1978, and as a result we usually worked i n p a i r s . Each p a i r c a r r i e d two-way rad ios , allowing communication w i t h t h e h e l i c o p t e r , o ther work ing p a i r s and the ship. Mapping was done on a reconnaissance b a s i s , using 1:63,360 scale topographic sheets as f i e l d maps except a t the Bee Creek prospect where a steel tape and compass survey was made by M.L. Silberman and myse l f . The f i n a l c o m p i l a t i o n map i s being made a t 1:250,000 scale .

Laboratory Work A f t e r t h e f i n i s h o f the f i r s t f ie ld season, hand sample and t h i n

sec t i on s tudy o f the ava i l ab l e r o c k s was begun. Samples collected for age determination were evaluated i n t h i n sec t ion , and those judged s u i t a b l e f o r fur ther work were selected. Fossils collected by the f i e l d pa r ty were submitted to the U.S. Geological Survey Branch o f Paleontology and Strat igraphy f o r study and identification.

Potassium-argon Age Determinat ions A l a r g e p a r t o f t h i s dissertation is based on potassium-argon dat ing

work by t h i s author; t he re fo re some discussion o f t h e procedures used i s warranted. Two strategies were used i n dating; 1. d a t i n g of primary mine ra l s or whole rocks t o determine crystallization ages and 2. d a t i n g of secondary o r hydrothennal minerals t o determine the t i m e of a l t e r a t i o n and minera l i z a t i on.

Whole-rock samples, o ther t h a n quar t z sericite samples, were crushed t o 60-100 o r 100-200 mesh, dependent on mineral g r a i n size, f o l l owed by a hydrofluoric a c i d leaching (Appendix 1). The leaching process removes. secondary minerals and g l a s s and etches the remainder. Experimental ly, t h i s procedure has been shown t o dramat ica l ly increase the relat ive percent radiogenic argon in the sample while apparent ly n o t affecting the I

determined age, t h u s measurably decreasing the a n a l y t i c a l e r r o r i n the age determination. U s i n g a microsplitter, aliquots of the sample were s e p a r a t e d

, and submitted t o the U.S. Geological Survey Branch of A n a l y t i c a l Laboratories f o r potassium analysis. Other a1 i q u o t s were r e t a i ned for argon analysis .

Rocks t h a t were t o y i e l d mineral separates were crushed, washed, and separated u s i n g rnethylene iod ide fo r density separat ion, a Franz Isodynamic Separator for magnet ic sepa ra t i on , a n d a v i b r a t i n g inclined table f o r separation o f p la ty minerals. Purities of the mineral separates in most cases exceeded 99 percent. Sp l i t s o f these separates were then made for potassium analysis and argon extraction.

Argon ex t rac t ion and mass spectrometry were carried ou t i n t he U.S. Geological Survey Branch o f I so tope Geology l abora to r i e s i n Menlo Park, C a l i f o r n i a , generally following the methods o f Dalrymple and Lanphere (1969). Procedures and equipment have been modi f i ed somewhat since 1969 and the argon extraction procedure i s descr ibed in an appendix (Appendix 2). Potassium analyses were by Paul Klock and were made by flame photometry using a lithium metaborate f l u x and lithium internal standard (Engels and Ingamells, 1970). A 1 1 potassium analyses were i n duplicate and most were i n quadruplicate.

The e x t r a c t e d argon sample was loaded on one o f two mass spectrometers, one of so-called N i e r design and the other of similiar, though multi-collector, design. Argon measurement i s based on i s o t o p e d i l u t i o n techniques, using t he argon-38 introduced i n t o the sample d u r i n g ex t rac t ion a s a tracer o f known amount. The Nier mass spectrometer works e s s e n t i a l l y as described in Dalrymple and Lanphere (1969) ; however, there have been numerous improvements i n the control electronics since 1969, which have improved t h e p r e c i s i o n o f the d a t a ,

Data reduction from the spectrometer charts was made using a Fortran-language computer program w r i t t e n by myse l f . L inear and exponent ia l l i n e f i t s were made of each se t of peak da ta (i .e. argon-40, argon-38, and argon-36) and the b e s t f i t was chosen between the linear and exponential fits on the'basis of the correlation coefficients calculated in t h e program. From t h e l i n e f i t chosen f o r each peak, the l i n e intercept or the peak he igh t a t "T-zero" (the time o f s a m p l e introduction i n t o the spectrometer) i s determined; t h i s y i e l d s the argon i s o t o p e r a t i o s o f the gas introduced t o the spectrometer. Corrections are t h e n made f o r

C- spectrometer mass d i s c r i m i n a t i o n , background, and t r a c e r composi t ion t o yield the " t ruef t argon i s o t o p e r a t i o s (Ar-40/Ar-38 and Ar-38/Ar-36). The standard potassium-argon age equation;

t (yrs) = 1.804 x 109 1n(9.541(~%,/~%) + 1.0)

was used to calculate the age and t h e n the program calculated an e s t i m a t e d analytical error based on the l i n e f i t chosen, measured tracer error, the percent radiogenic argon, and t h e e s t i m a t e d error i n t h e potassium a n a l y s i s . The procedure used t o calculate the analytical error is modified

from Cox and Dalrymple (1967). A number of d i f f e r e n t ind ices are calculated t o provide i n fo rmat ion in

order t o a s s i s t i n the in terpre ta t ion o f the qua l i t y o f the analysis . These include t h e percent rad iogenic argon yie ld , spectrometer mass discrimination and spectrometer s e n s i t i v i t y . Most samples d a t e d were extracted a t leas t i n duplicate; often the results were unacceptable ana ly t i ca l ly , so a number o f t e s t s and modifications were i n t roduced t o m in im ize the analytical difficulties. The procedure described i n Appendix 2 was found t o be the most successful means of argon e x t r a c t i o n .

As o f July, 1976, a l l potassium-argon age determinations from t h e U.S. Geological Survey are reported u s i n g new isotope abundance and decay constants . These constants are:

4 = 5,72 x 10 -I1 year 4.z 8.78 x 10 -13 ,,,-I A = 4.963 x 10 - ' G e a r -1 4 d ~ / ~ = 1.167 x rnol/mol

(Beck insa le and Gale, 1969; Garner and others, 1975).

For ages between 1 and 100 million years, t h i s change adds approximately two percent t o ages calculated using the previous constants. All ages reported i n t h i s thesis have been calculated using the i976 constants, i n c l u d i n g previously published ages, which. have been recalculated.

Prev ious Work Prior work i n the area has not been extensive and most o f i t i s

discussed and amplified i n Burk's (1965) study of the geology o f the Alaska Peninsula. This work laid the framework f o r most o f the present study, which i s the beginning of a major effort by the U.S. Geological Survey, Branch o f Alaskan Geology in Alaska Peninsula geology. Some of the earliest geologic ~ o r k here was done by Spurr (1900) on a hurried traverse across the northern part of the peninsula, near Naknek l a k e (Figure 1). Atwood (1911), Capps (1923) and Knappen (1929) a l l c o n t r i b u t e d ear ly reconnaisance geologic stud ies to the literature o f the region. With respect to dating work, Imlay and Detterman (1973; 1977; Imlay, 1953; 1975) and Wolfe (1972) have repor ted on p a l e o n t o l o g i c and paleobotanic s t u d i e s . P r i o r to the present study on ly two r a d i o n e t r i c dates have been published on rocks i n t he study area: one by Armstrong and others (1976) and one by Kienle and Turner (1976).

The geologic literature contains a b u n d a n t reports concerning porphyry-type mineralization, though f e w have at tempted major radiometric s tud ies in a s s o c i a t i o n with work on the m i n e r a l i z a t i o n . M o s t important have been those by Moore and Lanphere (1971), Page and McDougall (1972a, 1972b), Page (1975), Theodore and others (1973) and C h i v a s and McDougall (1978).

General Geologic Setting The A l a s k a Peninsula i s a te r rane primarily composed of clastjc

sedimentary rocks r a n g i n g from T r i a s s i c t o Quarternary i n age. The abundant vo lcan ic rocks are generally o f andesitic composition, ranging from leuco-basalt t o d a c i t e , and are o f Permian, Tert iary, and Quarternary age. I n t r u s i v e rocks o f Jurassic t o Quarternary age are a l s o known. Major geologic f e a t u r e s include t he Quarternary t o Recent volcanos: Iliamna. Avgust ine Island, Katmai, Chiginagak, Aniakchak, Veniaminof, Pavlof and other less spectacu lar volcanos (Figure 1). The Alaska-Aleutian Range b a t h o l i t h forms t h e geo log ic backbone o f the Alaska Peninsula i n i t s northern h a l f . Aeromagnetic (u .S . Geological Survey, 1978) and d r i l l ho le d a t a (Brockway and others, 1975) i n d i c a t e t h a t this batholith may extend at least as far south as Port Heiden in t h e subsurface. The batholith i s formed o f quar tz -bear ing i n t r u s i v e rocks o f g r a n i t i c t o d i o r i t i c composition (Reed and Lanphere, 1969). Potassium-argon work by Reed and Lanphere (1974) has shown that these b a t h o l i t h i c rocks f a l l i n t o three age clusters; 180 t o 158 my., 85 t o 59 m.y., and 39 t o 26 m.y..

Along a n o r t h e a s t t o southwest trend parallel t o t h e A l a s k a Peninsula in t he Kodiak, Semidi and Shumagin Islands (Figure 1) i s a belt o f plutons generally o f i n te rmed ia te composit ion, a l l o f approximately 60 may. age. K ien le and Turner (1976) called t h i s the Shumagin-Kodiak ba tho l i t h , a n d suggested i t represents a Paleocene magmatic arc, although no evidence i s presented demonstrat ing any continuity o f the rocks i n the subsurface.

The remainder of t h e Alaska Peninsula i s composed o f sedimentary and volcanic rocks, p r imar i l y o f M i d d l e Jurassic t o late Tertiary age, P le is tocene t o Recent volcanjc rocks, and the Kodiak and Shurnagin Formations, a s l a t e and rnetagraywacke sequence apparently o f Upper Cretaceous age. The older sediments tend t o be arkos ic , whereas the younger are more volcaniclastic and tend to conta in more shale. There does no t appear to be an overal l f i n i n g upward t rend, though i n d i v i d u a l sediment packages may show t h i s . The volcanic rocks are primarily o f T e r t i a r y age, are generally a n d e s i t i c in composition, and are represented i n pa r t by many geornorphic features t ha t a r e apparent ly volcanic necks. There are Permian t o Jurassic volcanic and volcaniclastic rocks on the w e s t s i d e o f Kodiak I s l a n d and a t Puale Bay on the Alaska Peninsula (Moore and Connelly, 1977; Burk, 1965).

The Alaska Peninsula, a t present, i s the subarea1 expression o f a volcanic arc assoc ia ted wi th the convergent margin between the Nor th American and P a c i f i c plates. The p resen t l y active volcanos o f the Alaska Peninsula and A leu t i an archipelago can a l l be related t o subduction a long the A l e u t i a n Trench.

Potassium-argon d a t i n g undertaken as part o f t h i s study has shown t h a t a northwestward d ipp ing thrust fault truncating t h e Bee Creek prospect (Plate 2) has been a c t i v e d u r i n g the past 3.6 my., t h i s movement may cont inue t o the present day.

The Chignik and S u t w i k Island r e g i o n straddles the Alaska Peninsula (F igure 1) just south o f i t s midpoint and probably conta ins the most complete stratigraphic s e c t i o n to be seen on the Pen insu la . Stru~turally the study area i s dominated by t h e Chign ik anticline, an overthrust ant i t 1 i ne trending subpara l l el to t h e A1 aska Peninsula. Hypabyssal, vo lcan ic and plutonic rocks of T e r t i a r y age outcrop i n the area, though the Alaska-Aleut ian Range b a t h o l i t h o f Jurassic, Cretaceous, and Tertiary age does not. Within the s tudy area, our work has i n d i c a t e d numerous changes

Flgure 1 Map o f the Alaska Peninsula showing the Chignik and Sutwik - Island region.

must be made on Burk's (1965) geologic map (see P l a t e 1).

Stratigraphic Review The sedimentary and volcanic formations of t h e Chignik and Sutwik

Is land area i n d i c a t e a number o f d i f f e r e n t geologic environments and t h e r e f o r e p lace important constraints on t h e geologic h i s t o r y o f t h e Chignik and Sutwik Island reg ion . Each l i t h o l o g i c package defines a particular environment and , t h rough t i m e , each different environment m u s t fo l low the other i n a log ica l sequence. Each formation i s more f u l l y described i n Appendix 3; what f o l l o w s i s a sumaary o f these descriptions.

B r i e f l y r e v i e d i n g the s t r a t i g r a p h i c column o f the area (Figure 2) , from oldest to youngest, t he re i s t h e f o l l o w i n g sequence of l i t ho log ic packages and presumed e n v i ronments :

I. The She l i ko f Format ion i s composed o f f i n e - t o medium-grained sandstone and siltstone of Middle Jurassic age tha t i s up t o 2000m t h i c k . There are th ree members, 1. a lower siltstone con ta in ing v o l c a n i c ash depos i ts , 2. a mass ive gray-green sandstone w i t h interbedded s i l t s tone and plutonic pebble conglomerate and, 3 . an upper mass ive black sCale with minor l imestone lenses and nodules. The Shelikof was deposited i n a rapidly subsid ing basin and f rom a terrane yie ld ing an abundant supply o f v o l c a n i c detritus. The source terrane was t o the present n o r t h and northwest o f the depositional basin. The lower si l ts tone was deposited below wavebase and t h i n interbedded ashes i n d i c a t e a c t i v e volcanism a t t h e t i m e . The m idd le sandstone was deposi ted near shore, and apparent ly minor volcanism continued. Imlay (1953, p. 57) suggested deposition i n a m a j o r ocean Sas i n.

2. The Naknek Format ion i s a sandstone and conglomerate u n i t unconformably overlying t h e Shelikof and equ iva len ts . The lower Naknek i s a pale green, fine-grained, arkosic sandstone w i t h some coarse plutonic cobble conglomerate. The upper Naknek is a dark o l i v e green siltstone w i t h abundan t f o s s i l s and t h i n calcareous layers o f large l a t e r a l extent . The environment of depos i t i on f o r the Naknek was a nearshore, moderate t o high-energy b r a i d e d stream or beach environment w i t h r a p i d sediment influx, shor t sediment t r a n s p o r t d is tances and gradual subsidence w i t h time t o a farther o f f sho re environment.

3 . The Staniukovich Formation i s l i t h o l o g i c a l l y continuous w i t h t h e Naknek and ind ica tes t ha t a very sirniliar depos i t iona l environment extended i n t o t h e Lower Cretaceous. The source terrane f o r both fo rmat ions was plutonic. Lithologically, t h e Staniukovich i s identical t o the Naknek and can o n l y be reliably separated f rom i t on paleontologic c r i t e r i a . T h i s i s further discussed i n Appendix 3.

4. In limited areas, the Herendeen Limestone (Burk, 1965, p.45) is exposed i n the study area. The Herendeen Limestone may be indicative of subsidence o f the Naknek and Stan iukov ich Formations source area. The Herendeen Limestone i s a f o s s i l i f e rous calcarenite. Inoceramws prisms are extremely common, i n some areas c o n s t i t u t i n g three fourths o f the rock. The noncalcareous port ions of the formation are sirniliar l i t ho log ica l ly to the S tan iukov i ch .

5. The Shurnagin Format ion i s a poorly da ted Late Cretaceous ( M a e s t r i c h t i a n ? ) f lysch derived f rom a volcanic terrane t o the northwest, except i n the Sanak I s l ands where the source terrane was apparently t o the east or southeast . No stratigraphic t o p to t h i s f o rmat ion i s known. T h i s formation i s f o u n d i n the Outer Shurnagin and Sanak I s lands a t the present

Figure 2 Stratigraphic section o f t h e Chlgnik regfon.

edge of the continental shelf. ~aestricht i an(?) f o s s i l s have been reported from t he Shumagin Formation (Jones and Clark, 1973, p . 134). The Shumagin may be as old as Albian; it i s probably no younger than Late Maestrichtian in age. It was probably deposited in a fore-arc basin by turbidity currents. It i s apparently correlative w i t h the similiar Kodiak Formation, from which Maestriclitian fossils nave been reported. The Sllu~nagin i s complexly f o l d e d and faulted and has undergone z e o l i t e fac ies metamorphism. Fold axes parallel the e x i s t i n g c o n t i n e n t a l s h e l f edge and deformation may have been caused by underthrusting a t a convsgen t p l a t e margin (Moore, 1973b). Moore thought d e p o s i t i o n occurred at abyssal or near-abyssal depths.

6. The Chignik Formation i s an Upper Cretaceous a]-gillaceous and arkosic sandstone and conglomerate u n i t . The Coal Valley Member ir~cludes much of the conglomerate, i n addition to s i l t s t o n e and coal horizons. The Coal Valley is a nonmarine depos i t , t h e conglomerates are alluvial fan deposits, and the s i l t s t o n e s and coals represent deltaic o r iagoonal fac ies . he remainder o f the Chignik Formation i s made up o f well-bedded argillaceous sandstones and siltstones. The volcanic component o f t h i s format ion is somexhat greater than that of t h e Naknek and Staniukovich Formations which the Chignik overlies unconformably. Bentonitic shales may i n d i c a t e active volcanism at t h e time o f deposition. The source area for the Chignik was to the nor th or northwest and yielded mixed plutonic and vo lcan ic rocks.

7. The Hoodoo Formation conformably overlies(?) the Chignik and is a black, well-bedded siltstone. It was depos i t ed in fairly deep water, possibly offshore of t h e nearly contemporaneous Chignik Formation and has characteristics o f turbidity c u r r e n t depos i t i on . Depth o f deposition is not thought t o be grea t , though the Hoodoo might represent a d i s t a l turbidite. A t present , the direction o f sediment inflow i s unknown. The Hoodoo has been sub jec ted t o a very low grade o f metamorphism.

8. The Tolstoi Formation i s an Eocene-Oligocene(?) vo lcan ic l a s t i c unit unconformably overlying the o lder formations. A number o f hypabyssal i n t rus ions i n t o the Tolstoi have yie lded l a t e Eocene to Oligocene ages. The ex t reme ly coarse grain size of some o f t h e conglomerates and the nonmarine f l o r a l assemblages found i n the Tolstoi indicate short transport distances; t h e interbedded flows and agglomerates (possibly including volcanic breccias and lahars) i n d i c a t e nearness to a volcanic source area. Occurrences o f p y r i t e i n the siltstones may indicate euxnic bas ins or lagoonal deposition o f the siltstones.

9. Conformably overlying the Tolstoi is t h e Meshik Formation, a leuco-basalt to d a c i t e volcanic u n i t . This u n i t is cornpose+d o f flows, agglomerates and breccias, w i t h minor interbedded volcaniclastic sediments. The Meshik is thought to represent a Late Eocene to Oligocene volcanic a r c d e p o s i t , related t o a convergent p l a t e margin.

10. Unconf ormabl y over ly i ng the ear 1 i er uni t s , the Bear Lake Format i on reflects a hiatus i n v o l c a n i c a c t i v i t y and possibly a change i n the subduction r e g i m e o f t h e proto-Aleutian Trench. The Bear Lake i s composed o f sandstones, conglomerates, and minor s i l t s t o n e c o n t a i n i n g abundant nonvolcanic clasts. The Formation i s considered to be Middle ta Late Miocene in age based on pelecypods, gastropods and echinoids (Detterman and others, 1980b). The Bear Cake i s essentially de r i ved f r o m reworking o f earlier sediments in a non-marine to inner neritic environment. It probzbly reflects a marine regression, followed by a minor transgression.

11. The M i l k y River Formation reflects the reinitiation o f vo lcan ism a long the A l a s k a Peninsula. The u n i t unconformably overlies the Bear Lake Format ion. The coarse volcanic las t ic rocks , porphyritic f l o w s and lahars i n d i c a t e short t r a n s p o r t d i s tances and nearness t o source. The sedimentary s t r u c t u r e s and t e x t u r e s indicate nonmarine depos i t i on . This u n i t probab ly represents a reg ime v e r y s i m i l i a r t o t h a t o f the Tolstoi-Meshik package.

The sediment source terrane f o r the study area has always (except possibly for the Hoodoo) apparently been t o t he north o r northwest. Volcanism has been an important contributor t o the Tertiary u n i t s , except f o r the Bear Lake Format ion. The L a t e Jurassic through Early Cretaceous Format ions and t h e Bear Lake Format ion r e f l ec t t h e erosion o f a p l u t o n i c source terrane, or reworking o f p r e - e x i s t i n g sediments. The M idd le Ju rass i c Shelikof and possibly the Late Cretaceous Chignik Format ion i n p a r t may refect erosion o f a qu iesen t v o l c a n i c arc . Except for t h e Shumagin and Hoodoo, sedimentation has always been relatively nearshore or nonmarine. The Shumagin was probably depos i ted i n a fore-arc b a s i n and l i t h o l o g i c , a l l y may be i n d i c a t i v e o f the erosion o f a volcanic arc.

Igneous Rocks

Introduction Intrusive rocks i n t h e Chignik and Sutw ik Is land area range i n age

from Paleocene t o Quarternary. In t h i s t h e s i s , the igneous terminology will f o l l o l t ~ t h a t of Streckeisen (1976, 1979). Many hypabyssal rocks are i n t r u s i v e i n form and texturally intermedi a t e between f i n e - and medium-grained; t o a v o i d confusion, hypabyssal rocks w i l l be named using v o l c a n i c rock names because they are of ten i n d i s t i n g u i s h a b l e from t h e i r extrusive equivalents. Compositionally they range from leuco-basalt t o d a c i t e , though the b u l k of the i n t r u s i o n s are near andesite in composition. Most in t rus ive rocks are hypabyssal; r e l a t i v e l y deep seated plutons are represented by three m a j o r occurrences, which are: 1. the large late Miocene quartz diori t e ( ? ) body extending along the coast t o t h e northeast f r o m Chiginagak Bay ( P l a t e 1 j in t h e Sutwik I s l and and Ugashik quadrangles; 2. the Devils batholith, a late Miocene granodiorite to t o n a l i t e multiphase i n t r u s i v e south o f Chignik a t Devils Bay (Plate 1) i n the Cnignik quadrangle; and 3. t h e Paleocene biotite granodiorite ( ~ i e n l e and Turner, 1976) i n the Semidi Islands in t h e Sutwik island quadrangle. Minor occurrences o f p lu ton i c rocks include a number o f small tonalite b o d i e s a t Mallard Duck Bay. A major f e a t u r e o f the A l a s k a Peninsula i s t he Alaska -A leu t i an Range bathol i th described by Reed and Lanphere (1969,1973). Though t h i s b a t h o l i t h i s n o t exposed i n the study area, aeromagnetic d a t a (U. S. Geological Survey, 1978) imp1 i e s i t s southwesterly continuation i n the subsurf ace of t h e Nushagak-Bri st01 Bay Low1 and near Port Hei den. Eroded debris f rom i t i s common i n the Naknek, Chignik and possibly Bear Lake Format ions.

Figure 3 and Table 1 conta in the d a t a used t o classify and name the volcanic and hypabyssal rocks i n t h i s thesis. Figure 3 i s a Q:Or:P1 t e rnary diagram upon which normative quar tz , orthoclase, and plagioclase have been p l o t t e d . Samples t ha t f a l l w i t h i n t h e andesi te , b a s a l t f i e l d are named af te r being p l o t t e d on the small normat ive color index versus % s i 1 i ca diagram i n Figure 3. Complete l i s t i n g s o f chemical analyses and normative minera ls are i n Appendix 4. An A:F:M diagram of the igneous rocks o f the study area is shown i n Figure 4. A l s o shown i n Figure 4 i s an ou t l ine of the calc-alkal ine trend (Ringwood, 1977) to provide a reference frame f o r comparison wi th t h e rocks o f t h i s thesis .

The exposed i n t r u s i v e rocks o f the s tudy area can be considered the roots o f now eroded volcanos and as such these rocks tend t o be s i l i c a oversaturated and alkal i -poor . The intrusive rocks are o f t e n closely assoc ia ted wi th s i l l s , dikes, and vo lcan ic rocks in the immediate area o f outcrop. The major m a f i c mineral i s hornblende, t h o u g h pyroxenes and biotite are a l s o present . No rocks, other than eroded plutonic debris o f t h e Alaska-Aleutian Range ba tho l i t h , conta in primary muscovite, though Burk (1965, p . 110) reported some muscovi te a long the margin of- the batholith i n the Shurnagin Islands ( F i g u r e 1). Primary b i o t i t e and potassium feldspar are rare i n a l l b u t the large b a t h o l i t h i c plutons o f the Kodiak-Shumagin P l u t o n i c Series, and the Devils b a t h o l i t h .

Alaska-Aleutian Range Batho l i th The A 1 aska-Aleuti an Range bathol i t h was descr ibed by Reed and Lanphere

(1969, 1973) and the name r e f l e c t s a phys iograph ic grouping o f g r a n i t i c rocks ra the r than a genetic unit. Th i s u n i t does not crop ou t i n t h e study

Table 1.--Percent o f Si02 and normative color index for analyzed samples from the Chignik and Sutwik Is land area

Refer t o Appendix 4 for complete chemical data

P o i n t number Sample S i02 Co 1 or

(on number index Rock type f i gures

3 and 4 )

Andesi t e Andesi t e Daci te Dac i te leuco- bas a1 t Andesi t e Leuco-basal t Andesi t e Andesi te Andesi t e Daci t e Leuco-basal t Leuco-basal t Andes1 t e Leuca-bas a1 t Andesi t e Pacite Dac i t e Dac i t e DacSte Granodi ori t e Tonal i t e Tonal i te Granite

- *Normalized; analysis recomputed t o 100 percent water and C02-free.

a r e a o f t h i s thesis, ye t i t s appare . i t presence i n the subsur face and the c o n t r i b u t i o n o f i t t o sediments o f the Naknek and Staniukovich Formations i n the C h i g n i k and Sutwik Is land region w a r r a n t some discussion o f i t here. I n t r u s i v e rocks mapped with in the u n i t range i n age from 180 m.y. t o 26 m.y. (Reed and Lanphere, 1973). A Jurassic intrusive event s t a r t i n g near 180 m y . and continuing f o r some 20 t o 25 m.y. contributed most of the r o c k s to the b a t h o l i t h in the area between Becharof Lake and Chakachamma Lake (see Figure 3, Reed and Lanphere, 1973, p . 2588-2589). These J u r a s s i c plutonic rocks range f r o m gabbro to rarely granite; tonal i te and granodiorite are most common (Reed and Lanphere, 1973, p.2595). The presence o f granite c las t s i n sed imentary debris f rom the batholith suggests t h a t f e l s i c rocks are more common in the now-buried plutons o f the batholith than would be inferred f rom present surface exposures. There i s some evidence t o suggest t h a t parts of the Jurassic batholith were rapidly cooled and unroofed, contributing to sediments o f the Middle Jurassic (Ba joc ian ) Tuxedini Group and Kialagv ik Formation within a f e w m i l l i o n years o f emplacement (Reed and Lanphere, 1973, p . 2596; Burk, 1965, p . 73; Detterman and Hartsock, 1966). Large i n t r u s i o n s t e n d t o be elongate and structurally concordant, and to induce the development of a foliation in the country rock parallel t o the contact. Reed and Lanphere (1973, p.2595) note that where intrusive r e l a t i o n s and potassium-argon age determinations have been made w i t h i n an intrusive sequence, the m a f i c intrusions are oldest, and younger intrusions become -succe;si v e l y more f e l s i c . Closely associated areally wi th the Jurassic p l u t o n i c rocks is a t h i c k Lower Jurassic v o l c a n i c and volcaniclastic sequence (Ta lkee tna Formation. see Detterman and Hartsock, 1966); v o l c a n i c rock types range at least from andes i te to d a c i t e . Reed and Lanphere (1973) suggested the Talkeetna Formation implies an Early Jurassic volcano-plutonic arc following the trend o f the Alaska-Aleutian Range b a t h o l i t h .

Reed and Lanphere (1973) a lso distinguish a Late Cretaceous t o Early Tertiary i n t r u s i v e e v e n t i n t h e batholith; potassium-argon age de te rm ina t ions on these rocks range from 85 t o 59 m.y.. Rocks from this episode have only been found on t h e northwest side o f the batholith north o f Iliamna Lake (see Figure 1). I t was not possible to distinguish a contact between these Cretaceous to Early T e r t i a r y and the Jurassic plutonic rocks. The Cretaceous to Early T e r t i a r y plutonic rocks r ange in composition from tonalite t o rarely granite; the b u l k are t o n a l i t e (Reed and Lanphere, 1973, p.2597). No associated volcanic rocks are k n o ~ n for the C r e t a c e o u s - t o Early Terti ary plutons.

The third group of p l u t o n i c rocks described by Reed and Lanphere (1973) i n the A l a s k a - A l e u t i a n Range b a t h o l i t h are middle ~ertiar; intrusive rocks. These do not demonstrate t h e relat ively homogeneous chemi;try o f the older suites; compositions r a n g e f rom tonalite t o alkali-feldspar g ran i t e . However, those near Nonvianuk Lake (Figure 1) cluster i n the tonal i t e and granodiorite fields and have similiar modal and chemical compositions. The ages of these rocks range f r o m 37.0 t o 25.7 my. , with a cluster a t 29.1 t o 25.7 m y . and one pluton a t 37.0 m.y. (hornblende) and 35.6 m.y. (biotite)(~eed and Lanphere, 1973).

Kodiak-Shurnagin Plutonic Series The Kodiak-Shumagin Plutonic Series i s t h e name used here f o r the

Shumagi n-Kodi ak batholith o f Ki enle and Turner (1976). They a hypothesized a batholith based on t he occurrence o f g r a n i t i c rocks o f similiar age and compos i t ion i n the Kodiak, Semidi, and Shumagin I s lands and on Sanak Island

i n southwestern A l a s k a (see Figure 1). Burk (1965, p.110) discussed a portion of the Kodiak-Shumagin Plutonic

Series, the Shumagin batholith. of t h e Shurnagin Is lands . Th is pluton i s a medium-grained, partly porphyritic b i o t i t e granodiorite (~rantz, 1963, p. B108) and intrudes meta-sediments of the Shumagin Formation. It i s o f hypidiomorphic granular texture, wi th late potassium feldspar crystals up to 4 mrn i n diameter (Grantz , 1963, p. 8108; Burk, 1965, p.110). Muscovite occurs loca l ly along the margins of t h e body; otherwise the character o f the p lu ton i s q u i t e uniform (Burk, 1965, p.110). Potassium-argon ages range between 65.6 and 57.4 m.y. (Paleocene, Table 2, Burk, 1965, p.111; Moore, 1974a). In t h e Semidi Islands and on Sanak I s land a biotite granodiorite o f simi 1 iar appearance and age was reported by Burk (1965, p . 110) and Moore (1974b)(see Table 2 ) . Kienle and Turner (1976) obtained samples of these rocks and reported new potassium-argon age determinat ions (Table 2) which are generally concordant (62.0 to 59.3 m.y.) w i th previous determinations and wi th ages on similiar rocks on Kodiak I s l a n d (Burk, 1965, p.111; Moore, 1974a; 1974b; and Kalstrom and Ball, 1969, p .28) . However, it i s doubtful that these plutons represent a continuous b a t h o l i t h ; more likely they are a ser ies o f a n a t e c t i c plutons intruding the Kodiak and Shurnagin Formations (Hudson and others, 1979). On t h e Kodi ak Is lands , t he Kodi ak batholith, a part o f t he Kodi ak-Shumagin Plutonic Series, intrudes the Kodiak Formation. Here t h e continuity o f the Shumagi n-Kodi ak bath01 i t h o f Kienle and Turner (1976) i s n o t demonstrated by outcrop patterns from i s l a n d to island. Therefore i n t h i s thesis the name Kodiak-Shumagin Pluton ic Ser ies i s used to refer to t h i s s e r i e s o f plutons.

Devils Batholith The Devils batholith i s a multiphase granodiorite to tonalite

batholith approximately centered on Devils Bay i n t h e southeastern part of t h e Chignik quadrangle (P la te 1) and cove r i ng some 500 square kilometers (200 square m i 1 es) . The rocks are medium-grai ned hypidiomorphic granular- textured. Hornblende and b i o t i t e are in approximately equal proportions in the rock and average about 5 t o 10 percent each. A number o f t h i n sections show a discontinuous reaction series f rom clinopyroxene t o hornblende to biotite. Plagioclase has a composi t ion o f An 40-45, and i s often i n phenocrysts . Pot ass i urn feldspar (orthocl ase) may constitute as much as 15 percent o f the rock and quartz as much as 30 percent. A particularily mafic-rich phase corresponds t o an aeromagnetic high (U.S. Geological Survey, 1978) a t Seal Bay. The maf i c phase conta ins cl ino- and

5 orthopyroxene, hornblende, and b i o t i t e . The plagioclase i s s ieve- textured and o f composition An 40-45. A sample o f t h i s m a f i c phase has yielded a potassium-argon age o f 7.84 + .04 m.y. on b i o t i t e and 10.08 + .04 m.y. on hornblende (sample 78AWs 9 5 , 7 a b l e 3 ) . In contact wi th this-mafic phase i s a s l i g h t l y f o l i a t e d f i n e - g r a i n e d granodiorite w i t h biotite veinlets or ien ted sub-paral l e l to the foliation,

In a number o f localities along the periphery o f the batholith are earl ier hornblende-bear ing f e l s i c intrusive rocks that have been c o n t a c t metarno~*pho~ed by i n t r u s i o n o f the ba tho l i t h . These i n t r u s i v e rocks were noted a t Fishhook Bay, two bodies located near northeastern Kuiukta Bay, one northeast o f VABM "Goat" (56 O l ' N , 158 26'W), and others a t Ship Mountain, Castle Bay, and Castle Cape. These rocks range from a hypabyssal hornblende d a c i t e t o coarser -gra ined rocks o f similiar composi t ion and to dikes o f uncertain, but poss ib ly original a n d e s i t i c composition. Feldspars

Table 2.--Potassium-argon ages - Kodiak-Shumagin plutonic series

Loca ti on Reference

Kodiak Islands

Kodiak Is 1 and-------- Granodiortte Bio 59.5 - + 2.6 KaJstrom and Baf 1, 1969. -- - . .- . . .

Semi d i Is 1 ands

Chowiet Island-------Granodiorite Bio 5 9 . 3 + l . B Kfenle and Turner, 1976.

Shumagfn Islands

Nagaf Is1 and--------- Granodf orf t e Bio 60.7 - + 1.8 Kienle and Turner, 1976. Granite Nus 57 - 4 Burk, 1965.

B i o 58.4 Burk, 1965. Granite Mus 65.6 Burk, 1965.

Sirneonof Is 1 and------ Granodiori te B io 59.9 - + 3.0 Moore, 1974a.

BSg Koniu j i i Island--6ranodiorite Bio . 60.2 - * 1.8 Kfenle andTurner, 1976.

Sanak Is1 and--------- Granodi or4 t e B i o 62.W - 1,8 Kfenle andTurner, 1976. Granodiorite B i o 61.4 2 1.8 Moore, 1974b.

*All ages recalculated us lng new constants (see text). **Reported estimated analytical error.

i n these older rocks t e n d to be less calcic (An 25-30) than i n the batholith, and ma f i cs are generally a l te red t o chlorite, epidote and calcite. One o f these bodies, located i n the v i c i n i t y o f nor theastern Ku iuk ta Bay, was originally rich i n b i o t i t e , a l l o f which, except for traces, i s now altered t o chlorite. Plagioclase feldspars i n these older bodies are strongly sericitized i n contrast t o those of t h e b a t h o l i t h which are charcteristically fresh-appearing. An age d e t e r m i n a t i o n on hornblende f rom the Castle Bay body yielded 22.4 + 1.86 m.y. which i s cons idered a minimum age o f emplacement because i t Fas been contact metamorphosed by the D e v i l s batholith. The batholith also intrudes and contact metamorphoses rocks o f the Chignik, Hoodoo and Tolstoi Format ions. Of ten t h e parent rock i s readily recognizable; however in some a reas (Ship Mounta in and Necessity Cove (Plate 2 ) ) the country rock resembles the Shumagin or Kodiak Formation, b o t h o f w h i c h a re n o t expected t o be p resen t i n t h i s a rea on s t ruc tu ra l grounds.

Potass i urn-argon age de te rm i na t i ons on the b a t h o l i t h y ie ld a 1 a t e Miocene age (10 my. ) ; however, these ages are discordant . A t Sweater Bay,

C an age determination on biotite (7.78 + -23 m y . ) i s t w o m i l l i o n years younger than one on hornblende (9.86 +-.36 m y . ) at the same locality (Table 3, sample 77AWs 100). This discordancy i s s i m i 1 iar t o t h a t o f t h e m a f i c phase a t Seal Bay. There i s evidence that the northeastern part o f the Devils batholith may rep resen t a younger phase ( 6 m.y., Table 6, sample 77AWs 125). The thermal event assoc ia ted with the i n t r u s i o n o f this younger phase may have partially reset the p r e - e x i s t i n g b i o t i t e o f the Sweater and Warner Bay p o r t i o n s of the b a t h o l i t h . These ages will be discussed more f u l l y in the s e c t i o n on t h e Warner Bay Prospect .

Chiginagak Bay b a t h o l i t h Extending along the P a c i f i c coast t o the northeast f r o m Chiginagak Bay

i n the northeastern p a r t o f t he S u t w i k I s l a n d quadrangle f o r some 25 t o 30 km (17 to 20 m i ) i s a large diorite to q u a r t z d i n r i t e ( ? ) b a t h o l i t h . Most o f t h i s body l ies o u t s i d e o f the study area and t h e r e f o r e our examinat ion o f this was q u i t e brief. It apparently i n t r udes v o l c a n i c and v o l c a n i c l a s t i c rocks that may be co-genet ic along i t s southern margin, the only p o i n t where the c o n t a c t has been examined by t h i s writer. Burk (1965, p.108). descr ibed t h i s batholith as being very similiar to the D e v i l s batholith (his Kuiukta Bay batholith) and considered i t to be o f mid-Tertiary age. Based on very l i m i t e d sampl ing, the Chiginagak Bay body is apparently somewhat more m a f i c t h a n t he Devils b a t h a i t h . A t Chiginagak Bay, a volcanic neck thought to be cont inuous with the larger batholith has been examined i n detai l and dated. The rock i s gabbroic o r b a s a l t i c i n appearance and thin-section examination reveals phenocrys ts o f plagioclase (An 55 t o An 6 0 ) , augite and hypersthene i n a glassy groundmass. A potassium-argon age of 9.25 + .25 m.y. has been determined on a whole-rock sample o f this neck (Table 6 , sample 77AWs 09). Chemically, t h i s sample i s v i r t u a l l y i d e n t i c a l t o the Northwest Arm phase o f the D e v i l s b a t h o l i t h , a medium-grained b i o t i t e h o r n b l e n d e t o n a l i t e (Appendix 3, samples 77AWs 09 and 125); andesi te i s possibly the b e s t name t h a t can be a p p l i e d t o t h i s rock. A dating sample f r o m t h e Chiginagak Bay b a t h o l i t h collected by Lee Smirnov o f Amoco i s a pyroxene d i o r i t e and may be more representative o f the batholith. Further examina t i on o f t h i s batholith i n the f u t u r e i s p 1 anned .

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Meshik Volcanic Rocks The volcanic rocks mapped as the Meshik Formation are dacitic to

leuco-basal t i c f 1 o r s , agglomerates, and brecci as. These are usual ly hydrothermally altered, though in a number o f areas, fresh, columnar-jointed andesite and d a c i t e p l u g s are included i n the formation. A number o f these plugs have been dated and t h e r e f o r e provide a probab le upper l i m i t i n g da te on the formation. The ages range from 35 t o 30 m.y. (Tab le 4, samples 78AWs 31, 32, 58, and 51).

The samples dated ( T a b l e 4 ) range f r o m hornblende dacites t o two-pyroxene andesi tes. The hornblende o f ten appears t o be resorbed and is commonly rimmed in opaque oxides. The andesi tes o f t e n have two populations o f plagioclase, An 35 and An 60, or a s i n g l e p l a g i o c l a s e a t about An 45-55. Textures range f r o m glomeroporphyritic to suboph i t i c an3 some racks have glassy groundmasscs. A few o f t h e t h i n sections examined contain o l i v i n e ; these rocks i n general are more m a f i c and may inc lude leuco-basalt o r basal ts.

In a number o f areas, hypabyssal hornb l ende leuco-basal t and andes i t e porphyry p lu tons intrude sediments o f the Tolstoi Formation, particularily on Cape Kumli k and Kuml i k Island ( P l a t e 2 ) . Potassium argon ages o f these intrusions all cluster about 35 my. o r Oligocene ( T a b l e s 5 and 6 , samples 77AWs 30, 40, 46, 74, 78AWs 17, and 24). Th i s i n d i c a t e s t ha t these i n t rus ions may be the roots o f volcanos t ha t were sources f o r some o f the Meshik vo lcan ic rocks. The previously mentioned dacite and andesite plugs may also be necks of volcanos t h a t were sources f o r some o f the Mes!i ik.

Eocene t o Oligocene I n t r u s i v e Rocks On Cape K u m l i k and Kum?ik I s l a n d , a number of hypabyssal hornblonde

andes i te and l e u c o - b a s a l t porphyry i n t rus ive rocks are found. Each of these rocks i s assgciated with areas of extens ive a l t e r a t i o n and p o s s i b l e m i n e r a l i z a t i o n , phenoc rys t i c amphibole i s the dominant m a f i c m ine ra l i n a l l of these rocks; however a few samples (samples 77AWs 30, 78AWs 17) c o n t a i n re l ic ts o r pseudomorphs o f pyroxene. The amphibole commonly shows resorbed edges and s ieve texture. The plagioclases, a lso phenocrystic i n part, are usually strongly zoned, Carlsbad- and jlbite- twinned and sometimes show g l o m e r o p o r p h y r i t i c tex tu re . A number o f these rocks have two d i s t i n c t populations o f plag ioc lase phenocrysts having compositional ranges of An 15-30 and An 50-60. P a r t i a l a l t e r a t i o n o f the feldspars t o sericite, and/or clays i s ubiqui tous. Calcite and epidote are q u i t e common and in some rocks the groundmass i s chloritized. Apatite i s an important and sometimes abundant accessory. The groundmasses o f these rocks are f i n e g r a i n e d and are composed of plagioclase feldspars and opaque minera ls . Qua r t z phenocrysts have been seen i n a few of the t h i n sections; these usually have resorbed edges. The disequilibrium minera l assemblages o f these rocks strongly sugg2st mixed magmas; particularily the t d o feldspar compositions and the resorbed edges on t h e quartz phenocrysts. Potassium-argon age de te rm ina t i ons on the amphiboles range f rom 39.1 + 2-54 t o 34.2 + 2.07 my . (Eocene-Oligocene)(Table 5, samples 77AWs 30, 40,-46, and 7 8 ~ ~ r 17) and are approximately co-eva l w i th t h e Meshik-Formation.

Similiar t o the above r o c k s are t h e intrusive rocks found on Cape Kunmik. At Cape Kunmik, autolithic aggregates of pyroxene and amphibole are common, and from a d is tance g i v e the rock a p o r p h y r i t i c appearance. Mineralization and a l t e r a t i o n o f the surrounding country rocks i s no t as extensive as a t Cape Kumlik. The eros ion l e v e l does not appear t o be as

Table 4.--Potassium-argon ages o f the Meshfk Formation

Locati on Sampl e number Rock type Phase K2° 40Ar(rad) Age h e y m )

(percent) (percent) and error*

ChSgni k quadrangle

Ma1 1 ard Duck Bay---70AWs 98 teuco-basal t WR 0.201 20.36 22.31 + 0.33 25.80 20.05 .38

Meanl--lll----llwIII--I-- 21.18 T 1.68 - Chigni k Is1 and-----78AWs 134 Dael t e WR 1.116 57.33 2 6 . 5 4 ~ .55

66.56 25.92 3 .S1 Meanmm---m-l------I-l-I-l 26.23 .87 -

-Gulf #I Port Core 1 42 + 4 Helden. ? 36 - 8

Sutwi k Is1 and quadrangle

Foggy Cape---------78AWs 31 Andesi t e** WR 1.260 90.48 30.35 + .47 84.85 30.10 T .14

Mean-I-CLH-I-II-mI-lII-l 30.33 7 .52 - Sutwik Island------78AWs 32 Andes1 teH* WR ,983 67 -05 31.05 + .48 - " E a ~ y " ~ ~ ~ ~ - ~ - ~ ~ - - ~ 7 8 A h 58 Dac l te*** WR 1,543 90.56 34.21 + .61

89.76 34.56 7 "36 Meanlllllrl-L-L---IIILIII 34.44 '3 .73 -

u J~ 1 i k"------------ 78AWs 61 Daci te*** WR 1,210 94.84 34.74 f: .I6

Ugashi k quadrangle

**Great Bas1 ns Cure 1 #1 UgashSk. Core 2

*See footnote ** on table 2. **Reference, Brockway and others, 1975.

**Assac1 ated w l t h To1 st01 Formati on.

grea t as a t Cape Kumlik and some samples were apparently emplaced a t very shallow dep ths . The rocks on Cape Kunmik e x h i b i t some abnormal alteration features; i n one sample the plagioclase appears t o be fresh, yet the arnphi bole i s completely a1 tered to a ye1 low mineral, possibly another amphibole. In other samples, amphiboles are fresh-appearing b u t plagioclase phenocrysts are thoroughly sericitized. A potassium-argon age o f 33.5 + 2.33 m.y. (Oligocene) has been determined on a green p l e o c h r o i c amphibole f rom a ho rnb l ende andesite i n which the plagioclase i s so altered as to be virtually unrecogn izab le (Table 5, sample 78AWs 24).

A very strongly a1 tered rock that was apparently a b i o t i te-hornblende d a c i t e dome was found on t h e northwest side of Sutwik Island (P la t e 2). Th is one o f the few rocks i n the s tudy area wi th primary biotite. A l t e r a t i o n o f the plagioclase is such t ha t the coaposition i s i nde termina te ; hornblende has been resorbed or a1 tered to c a l c i t e and opaques. A whole-rock chemical analys is of this rock i n d i c a t e d 3.5 percent CQ2 (Appendix 3). Quartz i s common and a p a t i t e i s extremely abundant. The biotite appears q u i t e fresh and has yielded a potassium-argon age o f 34.5 - + 1.66 m.y. (~ligocene)(Table 6 , sample 77AWs 74) .

P innacle Mountain, 7 miles southeast o f Aniakchak Crater (Plate 2), i s a hornblende d a c i t e porphyry pluton intruding rocks o f the Tolstoi and +leshik Formations. Th i s i n t r u s i v e has abundant green pleachroic hornblende phenocrysts, plagioclase o f composi t ion An 60, and quartz phenocrysts w i th resorbed edges. The texture i s porphyritic, w i t h the Sine-grained groundmass comprising about 50 percent o f the rock. A potassium-argon age o f 34.5 + .85 m.y. was determined on hornblende from this rock (Table 6, sample 7 8 A ~ s 35).

A two pyroxene andes i te f rom near Nakalilok Bay in the Sutwik Is land quadrangle has been col lected, and a plagioclase da ted a t 48.1 + .89 m.y. The pyroxenes, both cl i no and orthopyroxene are s 1 i ght 1 y a1 terd a1 ong cleavage planes and grain boundaries. The feldspar is f resh, Carlsbad- and albite-twinned, zoned, and approximately An 45 composit ion. The t e x t u r e is porphyritic, wi th plagioclase comprising the bulk of the phenocrysts and pyroxene primarily i n the groundmass.

Late T e r t i a r y Igneous Rocks Throughout the study area are many intrusive and volcanic rocks o f

Late Tertiary age, particularily in t h e v i c i n i t y of the p resen t l y active volcanos. Thexe i n c l u d e numerous volcanic rocks around Mt. Veniaminof, most o f the small intrusive rocks that are associated wi th a l t e r a t i o n zones, and other scattered intrusive rocks i n the northeastern p o r t i o n o f the Sutwik Island quadrangle.

Most o f the small i n t r u s i v e - r o c k s are pet rographica l ly similiar and a general description is as f o l l o w s . E i t h e r phenocrystic hornblende or hypersthene and/or a u g i t e is the m a f i c mineral; i t has resorbed edges and i s probably p a r t i a l l y chloritized. The plagioclase, where i t can be determined is of An 45 to An 55 conposition. The plagioclase i s usually al te red , o f t e n to the point that i n d i v i d u a l phenocrysts are d i f f i c u l t to d i s t i n g u i s h . Quartz may or may not be present i n the groundmass or as phenocrysts. The tex ture i s porphy r i t i c , with a fine-grained to aphan i t i c groundmass and medium-grained phenocrysts of plagioclase and the m a f i c mineral.

Two o f these rocks have been da ted , b u t unfortunately no chemical analyses are available. A hornblende separa te from a sample collected at

m Q I N ah- * * . - 4

m . .

t-c N

Tab1 e 6 .--Pot ass i um-argon ages of mfscel l aneous S n t rus i ve and volcanlc rocks

Locati on Sample number Rock type Phase K2° 40*r ( r ad) Age (m-y.)

(percent) (percent) and error*

Chignik quadrangle

Kametol ook River--- 77bWs 112b Leuco- bas a1 t. WR 0.645 9.56 0.60 0.013 7.57 -48 + ,015 3.87 "35 1 ,016

Mean--------------------- .4& + 012 -

Chjgnik Lagoon----- 77Ws 186 GranQ te cobble. 8 i o 6.98 70.5 91.8 2 .56 Chignik Format1 on. 78-2 87.7 2 1.38

Mean------------------- $9.8 2 3.3

Anchorage Bay------ 7 7 N s 176 Andesi t e s f 11. Hbd .277 12.37 15-6 f: -25 Int rudes Tolstoj 14.87 17.4 + -71 Fornation. Mean-------------------- 16.5 11 .48

Sutwf k Is land quadrangle

Sutwik Island------ 77AWs 74 Dacite. Bio 8.48 43.65 35.5 4 '64 Associated with 73.95 33,5 -59 To1 5 t 0 i Formation. Mean--------------------- 34.5 5 1-66

North quadrangle 78AWs 11 Daci te, Hbd .491 30.53 19.4 2 -16 edge. Intrudes TolstoS 30.96 19.08+ -16

Foxmati on, Mean--------------------- 19.25 E -33

Naka l i lok Bay------ 78AWs 5 Andes i t e ( ?) . Plag .483 45.45 48.55 + -55 Int rudes Chignjk 40.38 47.64 7 .29 and Hood00 Mean--------------------- 48.10 - -89 F o m a t i ons.

Pinnacle Mountain-- 78AMs 35 Dacf te. Hbd .365 36-02 33.93 + -16 Associated with 18 -86 35.01 T .33 T o l s t o i Formation, Mean--------------------- 34.47 -85

Ugas h i k quadrangle

Chiginagak Bay----- 77AWs 9 Andesite. WR 1.78 60.97 9.39 2 -14 Intrudes Meshfk(7) 52.24 9.11 f; .W Formation. Mean---------------------- 9.25 - + -25

*See footnote " on table 2.

-

Anchorage Bay (P la t e 2) near Chignik from an andesite sill has yielded a 16.5 + 1.48 m.y. age (Table 6, sample 77AWs 176). This sill is q u i t e similTar to other s i 11s intruding sediments of the Chign ik Formation i n the area o f Anchorage Bay. Such s i l l s o f t e n i n t r u d e a long coal horizons, thermally upgrading t h e coal f r o m lignite t o bituminous. The d a t e d sample i s p a r t i a l l y hydrothermal ly a1 tered, the feldspars, both i n phenocrysts and groundtnass, are strongly sericitized and kaolinized, and there is c l i nozoisi t e i n the groundmass. The amphi b o l e i s p l eochroic in deep broko~ns and has resorbed edges, whereas the clinopyroxene i s very p a l e green and has sharp subhedral g r a i n boundar ies. T h i s i s one o f t h e very f ew r o c k s t h a t has been d a t e d t h a t y ie lds an age between 10 and 20 m.y. The age i s considered a minimum age. A sirnil jar sill, from across Anchorage Bay has proved t o be d i f f i c u l t t o da te , t h o u g h t h e a p p a r e n t age may be near 10 t o 6 my.

A d a t i n g sarnple col lec ted from the north edge of the Sutw ik Island quadrangle (Plate 2) i s a hornblende d a c i t e t h a t i s virtually a p e r f e c t match for the general description o f the L a t e Tertiary rocks g i ven ea r l i e r i n t h i s sec t ion . The amphibole i s i n good condition and has yielded an age of 19.4 + -16 m.y. (Table 6, sample 78AWs 11). The feldspar i s nearly comp~eteTy a1 tered to serici te; what remains i s strongly zoned, and Carlsbad-twinned, w i t h minor albite-twinning. The rock texture i s c a t a c l a s t i c - p o r p h y r i t i c , w i t h phenocrysts sha t te red and dislocated. The phenocrysts r a n g e up t o 7 mm i n diameter.

Economic Geology and Age o f Mineralization

Porphyry Models I t i s generally accepted t h a t porphyry copper deposits f o rm i n

converging p l a t e m a r g i n environments ( S i l l i t o e , i972, 1976), b u t agreement on a genet ic model or models of how they form has n o t been reached. Several models are currently in use. S i l l i toe (1973) has described a volcanic-plutonic model for copper porphyries as an extens ion of h i s plate tectonic model ( S i l l i t o e , 1972). Th i s volcanic-plutonic model proposes that the metals o f porphyry copper systems a r e derived from the mantle a t divergent p l a t e marg ins and t ransported in the oceanic crust to a convergent p l a t e margin. Through partial melting o f metal-enriched oceanic crust in subduct ion zones, calc-alkaline magmatism i s thought to occur, and porphyry copper systems are believed t o c o n s t i t u t e a no r~na l f a c e t of t h i s magmat ism. There f ore t h e t i m e and space distribution o f porphyry copper depos i ts i s dependent on the subduction o f metal-enriched oceanic crust and on calc-a1 kal i ne magma generat ion. Oyarzun and Frutos (1974) have shovin t h a t the requirement f o r enr iched oceanic crust i s unnecessary and p o i n t out that according to Krauskopf (1967), the average abundance o f metals i n sediments and magmas i s s u f f i c i e n t to supply the meta ls i n a porphyry system; therefore t h e important problem i s the d e f i n i t i o n o f t he segregat ion and t r a n s p o r t mechanisms. As an example, o f Oyarzun and Frutos ' (1974) model, a calculation using data from Krauskopf (1967) on crustal abundances o f copper was made. In order t o produce t h e amount o f metallic copper i n the E l S a l v a d o r porphyry copper deposit o f Ch i le (Gustafson and Hunt, 1975), i t would on ly require the subduct ion of a slab o f oceanic crust 10 krn wide and 2 t o 10 krn long, assuming a 25% s e g r e g a t i o n e f f i c i e n c y . Assuming a subduction rate o f 5 crn/yr, this is on the order o f 40,000 t o 200,000 years o f subduction.

S i l l i t o e (1973) has r e f i n e d his 1972 model (F i gu re 5), p ropos ing t ha t porphyry copper depos i t s typically occur sub-volcanical ly in t h e "cupolam reg ion o f p h a n e r i t i c i n t r u s i v e rocks comagmatic w i t h a calc-alkaline vo lcan ic p i l e . Grading downward from the typ ica l porphyry copper deposit i s a transitional zone o f stockwork mineralization and potassium-silicate a l t e r a t i o n succeeded further down by an essentially unaltered pluton o f large dimensions relative t o t h e porphyry stock. S i l l i t o e (1973) feels t h a t the evidence f a v o r s the interpretation t h a t t h e tops o f porphyry systems a t e emplaced a t depths of 1.5 t o 3.0 km beneath the summits o f the related stratovolcano, and t h a t the' porphyry system ( e s s e n t i a l l y the cupo la) may have a v e r t i c a l e x t e n t o f 8 km.

In d e t a i l , a porphyry copper deposit consists of c o n c e n t r i c a l l y arranged shells o f hydrothermal a1 t e r a t i o n and mineralization approx imate ly centered about a high level calc-alkaline stock. Lowell and Guilbert (1970) have descr ibed t h e arrangement o f t h e alteration ha los in porphyry d e p o s i t s and S i l l i t o e (1973) a t temp ts t o characterize the na tu re of these halos vertically throughout t h e porphyry system. F igure 5 i s a drawing showing the t y p i c a l s i m p l e porphyry copper model a s envisioned by Sillitoe (1973). Impor tan t po in ts t o notice are: 1, the limited ver t i ca l ex ten t o f s e r i c i t i c alteration; 2. t h a t epithermal copper, lead, z inc and precious metal ve ins are an integral p a r t o f t he porphyry system, accompanying p r o p y l i t i c alteration; and 3. t he advanced argillic a l t e r a t i o n , silicification and/or p r o p y l i t i c a l t e r a t i o n which occurs i n t h e o v e r l y i n g comagmatic v o l c a n i c p i l e . The ore body typical ly occurs i n t h e

High - tamprra~urm ROCK TYPES

- b KM {IN If t A T ION TO SEA LEVEL) PORPHYRY STOCK PHANERITIC GRANODlORl f E

- 3

PIIE -VOLCANO

ADV.4NC ED ARGILLEC

-a2

- -a badim r 1

POTASSIUM 0 2

HOllZOMTAL SCAZE t 6 m n m om vmr t f t a l ) : I KiIanmrmr8, SILICATE

Figure 5 Porphyry copper deposit model, after S i l l i toe (1973).

pre-existing older and g e n e t i c a l l y unrelated country rock below the volcanic p i l e . Regarding t he v o l c a n i c pile, this "..... need not be a simple cone, but may be multiple in character and inc lude the dev~lopment of domes and collapse c a l d e r a s , perhaps resurgent" (Sillitoe, 1973, p. 802).

Rather than a t t e m p t to model porphyry systems w i t h i n the restrictions of one model, Sutherland-Brown (1976) uses three d i s t i n c t models based on h i s o b s e r v a t i o n s i n the Canadian Cordillera. Sutherland-Brown's (1976) three models a r e b a s e d i n p a r t on morphology, magma pressure and v o l a t i l e content, degree and isotropy o f the ex te rna l stress f i e l d and the type and condition o f the host rocks . He does n o t r e a l l y d iscuss most o f these factors d i r ec t ly and h i s c l a s s i f i c a t i o n i s heavily based on the first and last, i . e . morpilology and t h e c o n d i t i o n o f the host rocks. His models are p a r t i c u l a r i l y a p p l i c a b l e to Canadian porphyries though he f e e l s many porphyries worldwide a r e "...recognizible within the proposed framework" (Sutherland-Brown, 1976).

Type 1 or phallic porphyry deposits are ideally centered on a small s i n g l e or multiphase cylindrical pluton (Figure 6). Brecc ias are a distinctive fea ture , as p i p e s , dikes or shells and are an indication t h a t the body was formed h i g h i n the crust. The pluton is usually l a t e or post-erogenic, comaonly has vented t o the surface, as evidenced by the breccias, yet had not built up an ex tens ive v o l c a n i c pile and intruded i t . "Rad ia t ing o r concentric dikes are normal and may form a swarmu (Sutherland-Brown, 1976, p.46). The alteration shells are essentially as descr ibed by Lowell and Guilbert (1970) and as i n S i l l i t o e ' s (1973) model. An important f a c t o r Sutherland-Brown (1976) ment ions is t h e thermal metamorphic aureole in t h e country rock around the pluton and the possible confusion o f the potassium-silicate alteration zone w i th this aureole. A charac te r i s t i c mineral o f the thermal aureole i s a very f i n e f e l t e d b i o t i t e , easy t o confuse w i t h the secondary or hydrothermal biotite o f t he alteration system. The ore zone i s . generally synonymous w i th the sericitic zone, though i t can also lie partly within the potassium-silicate zone. Examples o f a phallic type porphyry copper depos i t are the Casino deposit (Godwin, 1976) in the Yukon Territory o r the deposit at El Salvador , Chile (Gustafson and Hunt, 1975).

Type 2, volcanic porphyry deposits "...ideally are formed adjacent t o variably shaped porphyry intrusions. . . to intrude a coeval and partly consanguineous volcanic pile i n an orogenic s e t t i n g t h a t i s commonly marine" (Sutherland-Brown, 1976, p.46). These deposits typically have large ore zones in relation to the s i z e o f t h e porphyry body, which i s i n essence a d i k e swarm. Breccias are o f several types within a particular deposit and are an important aspect o f t h e system. Breccias are pre- and post-ore, and the post-ore breccias often conta in fragments o f the porphyries t h a t decrease i n s i ze and abundance may from the porphyry con tac ts (Sutherland-Brown, 1976)(Figure 7). Alteration i s not as regular as i n t h e phallic porphyries though the types o f a l t e r a t i o n , including the thermal aureole are generally similiar. Often the con tac t metamorphic b i o t i t e i s destroyed by later alteration; therefore it i s not a l w a y s readily apparent. Potassium-silicate a l t e r a t i o n i s slight, s e r i c i t i c a l t e r a t i o n i s r e l a t i v e l y widespread but weak, and propylitic alteration i s distributed over a broad peripheral zone and i s pervasive. Alteration in quar t z -poor alkalic systems is minerallogically q u i t e different and can lead to alteration o f the volcanic host rocks t o "pseudo-syeni tes or f e n i t e s u (Sutherland-Brown,

1976). The pattern of mineralization i n a volcanic porphyry i s analogous t o

the alteration p a t t e r n , and of ten forms the core of the altered zone. However pre-ore faults, dikes and/or b r e c c i a bodies i n the system can l o c a l i z e the mineral izat ion. Along w i t h a d i s t i n c t type o f alteration, a1 k a l i c v o l c a n i c porphyries t e n d to be mol ybdenum-poor and are solely copper deposits. There are no molybdenum volcanic porphyries, regardless o f i n t r u s i o n chemistry (Sutherland-Brown, 1976). An example of a volcanic type copper porphyry deposit i s t h e Island Copper deposit (Cargill and others , 1970) on n o r t h e r n Vancouver Is1 and.

The t h i r d model proposed by Sutherland-Brown (1976) i s the plutonic porphyry model ( F i g u r e 8 ) . These form i n petrological l y zoned medium-sized plutons which may i n t r u d e a coeval volcanic pi le o r l i e within unrelated hos t rock. These p l u t o n s are i n t r u d e d a t the end o f t h e ea r l y s tage or a t t he middle stage of orogenesis and dynamothermally metamorphose the enclosing host rocks to amphibolite facies. A s c h i s t o s i t y i n the country rock is formed pa r a l l e l t o t he contact , and p a r t s o f the pluton itself may show foliation. Typical ly the pluton does not display a porphyritic t e x t u r e . Taken as a whole, t h e evidence seems to indicate that this t ype "porphyry" i s emplaced a t a greater depth t h a n t he p r e v i o u s two models. In contrast to the other two types, breccias are rare i n t hese systems and therefore un impor tan t . However, fracturing of h igh dens i ty is always seen, and veining i s common. The v e i n i n g i s commonly o f f o u r t o f i v e ages though, Sutherland-Brown (1976, p.50) s t a t e s t h a t isotopic d a t i n g work has shown these to br a n a l y t i c a l l y indistinguishable.

Alterat ion in p l u t o n i c porphyry copper deposits i s typically weak and quite unlike the model o f Lowell and Guilbert (1970). Contact metamorphism i s descr ibed by Sutherland-Broivn as not b e i n g p a r t of the alteration array; however from t h e p rev i ous d i scuss ion the metamorphism a s s o c i a t e d w i th the i n t r u s i o n i s q u i t e obvious. Sutherland-Brown (1976) re la tes the ore deposit to a late felsic core (sse Figure 8); contact metamorphism i s not d i r e c t l y assoc ia ted w i t h t h i s core. Apparently, t h e core of the f e l s i c zone i s usual ly barren o f mineralization but i t may show a form of sericitic a l t e r a t i o n , and the alteration undergoes a t r a n s i t i o n outward to a propylitic-kaolinitic a l t e r a t i o n or a quartz-chlorite + sericite a l t e r a t i o n . As ore deposits, the p l u t o n i c porphyries are usually copper-molybdenum or molydenurn deposits. Veining can be q u i t e important economica l l y w i th ve ins up t o a mete r wide and hundreds o f meters long . Copper mineralizat ion i s t y p i c a l l y related t o e a r l y veins, whereas molybdenum mineralization seems to come l a t e r i n the v e i n sequence. Plutonic porphyry copper deposits i n the Guichon Creek batholith of British Columbia include Valley Copper (Ustenko and Jones, 1976) and J.A. (I~IcMillan, 1976).

Potassium-argon Ages i n Porphyry Copper Deposits One o f the major cons t ra in ts on the use of potassium- argon d a t i n g i s

t h e sensitivity o f many datable minerals to argon loss in response to thermal events. T h i s sensitivity may be ref lected by r e c ry s t a l l i z a t i on in response to changing temperature and pressure conditions, as for example the e x s o l u t i o n o f potassium feldspars or t h e replacement o f b i o t i t e by chlorite. Another corninon e f f e c t i s the outward d i f f u s i o n o f argon caused during a weakening of lattice bonds during h e a t i n g . In the dating o f mineralization even ts as d i s t i n g u i s h e d from emplacement or crystallization

events, both o f these types o f thermal responses become asse ts . The b a s i c assumption in applying potassium-argon d a t i n g t o porphyry ores is t h a t dur ing r e c r y s t a l l i z a t i o n minerals wi l l degas argon and r e s e t the radiometric clock. There fore i f separates or concent ra tes can be made of the replacement or hydrothermal mine ra l s which form d u r i n g the mineralization process, t h e resulting age determinations will r e f l ec t the mineralization e v l n t s . Regard ing simple thermal d i f f u s i o n o f argon, complete r e s e t t i n g of the radionetric c l o c k is arguable; however discordant dates will general ly i n d i c a t e a thermal event s i n c e d i f f e r e n t minerals, f o r exarnple b i o t i t e and hornblende, have d i f f e r e n t argon d i f f u s i o n characteristics, resulting i n d i f f e r i n g degrees o f argon r e t e n t i o n a t a g i ven temperature art, 1964).

In porphyry systems, a number o f minerals are characteristic uf hydrothermal a1 t e r a t ion and i n i n e r a l i z a t i on (Lowe 11 and G i l be)-t, 1970). In particular, b i o t i t e , sericite, and potassium feldspar have proven useful i n dating porphyry mineralization (S i lberman and others, 1977, Oainon and Mauger, 1966). Chlor i te , another c h a r a c t e r i s t i c a 1 teration m i n e r a l o f some zones of a l t e r a t i o n in porphyry coppers has also been successfully used t o d a t e a l t e r a t i o n and m i n e r a l i z a t i o n i n a group o f porphyry copper deposits i n south-central Alaska (S i lberman and others, 1977). In the present study, hydrothermal c h l o r i t e and b i o t i t e separates have been d a t e d f r o m r o c k s e x h i b i t i n g p r o p y l i t i c and potassic alteration. The ve ry f i n e g r a i n s i z e o f s e r i c i t e i n the porphyry systems s t u d i e d has precluded separation o f pure s e r i c i t e f rom t h e rock; however, i t was possible t o concentrate the sericite in many samples. In rocks t h a t t h i n s e c t i o n study showed to have been completely altered to quar tz , ser ici te , and sulfides, t h e s u l f i d e s and some q u a r t z were rernoved u s i n g magnetic and g r a v i t y techniques. In some cases, t h i s resulted in concentrates exceeding 2% K20. The quartz i s assumed to have no potassium and no argon ; therefore t h e potassium-argon date should r e f l e c t s i m p l y the se r i c i t e age. P rev ious age s tud ies using whole rock or modified whole rock potassium silicate and propylitic a l t e r a t i o n assemblages i n d j c a t e t h a t these quartz-sericite concentrates y i e l d r e l i a b l e ages y ort ton and others, 1977, Ashley and Silberman, 1976).

To date, very little information has been pub l i shed on Alaska P e n i n s h a prophyry copper occurrences. Armstrong and o t h e r s (1976) r e p o r t e d potassium-argon age de te rm ina t ions on two prospects, Dry Creek and Pyramid. T h e i r Dry Creek prospect i s t h e Bee Creek prospect discussed in d e t a i l i n a l a t e r s e c t i o n o f this thesis . Berg and Cobb (1957, p. 5-7) gave general i n f o rma t i on on lode deposits in the Alaska Peninsula. Bear Creek Min ing Co. (Fields, 1 9 7 0 on contract v i t h Bris tol Bay N a t i v e Col-p. examined a number of prospects i n the Ch ign i k area. T h e i r work included e x t e n s i v e geochemical sampling, geologic mapping, and core drilling a t t h e Bee Creek prospect . T h e i r report t o the Bristol Bay N a t i v e Corp. served as a d a t a base fo r some o f the age studies reported here.

Bee Creek Prospec t The Bee Creek prospect i s located about 22 krn north of Chignik,

approximately 5 km inland of Chignik Bay on Dry Creek (Plate 1). Elevation ranges from 150 t o 600 rn. Vege ta t i on i s sparse, t h o u g h t a l u s and alluvium cover much of the prospect (Plate 3). T h e sulfide system has an areal extent of 2.5 by 3 km and i s t r u n c a t e d on the nor th by a northward dipping low ang le thrust (Detterman and o thers , 1980). The country rock i s t h e Upper J u r a s s i c Naknek Formation. The main d a c i t e in t rus ion i s of

porphyritic texture, wi th abundant phenocrysts of plagioclase rang ing in composition from approx imate ly An 35 t o An 50. Interstitial t o t he phenocrysts i s a f i n e t o very f i n e grained groundmass o f plagioclase, quartz, and hornblende. The hornblende i s partially or completely altered t o b i o t i t e ; therefore t h i s phase probably predates mineralization. The b i o t i t e in all samples i s a p p a r e n t l y hydrothermal and i s often found i n clots r e p l a c i n g hornblende or along grain boundaries. Unfo r tuna te l y , no datable primary mineral phases were f o u n d for t h i s intrusion. Ciilorite is a1 so a common i~ydrothermal mineral . A number o f sa~nples hav2 very mi nor potass ium f e l d s p a r ; t h e bulk o f this is hydrothermal in o r i g i n . A second dac i te i n t r ~ s i o n i s very s i r n i l i a r except t h a t the hornblende i s una l te red. The hornblende has been dated a t 2.15 + .I5 m.y. able 7, sample 77AWs 215) . This d a t e indicates t h a t t h i s hypabyssal intrusion may p o s t d a t e mineralization. Studies a r e underway t o attempt t o confirm t h i s by o t h e r i s o t o p i c d a t e s . Regionally, this intrusion and t h e associated a l t e r a t i o n halo are p a r t o f a linear east -west intrusion t rend extend ing 65 krn f rom Weasel Mountain t o Black Peak, a la te Tertiary t o Recent volcanic contet (Plate 1).

The i n t r u s i o n o f the main pluton a t Bee Creek resulted in the development of c o n t a c t metamorphic b i o t i t e i n the arkoses o f the Naknek Formation and poss ibly more intense hornfelsing nearer the intrusion. In portions o f the "inner" zones of the prospect, a composite rock i s mapped. T h i s composite r o c k has both sedimentary and igneous f e a t u r e s and may be strongly contact-metamorphosed Naknek Formation obscured by later a l t e r a t i o n and mineralization.

Subsequent to the time of intrusion o f the p l u t o n , a l t e r a t i o n and mineralization occurred. An apparent alteration age of 3.66 + -18 my. h a s been determined on hydroti~ernal b i o t i t e from a number o f samcles (Table 7, sample 77AWs 152, 251). A potassium-argon age determinat ion on sericitic a l t e r a t i o n yields an age of 3.85 + -22 m.y. (Table 7, sample 77AWs 243) whereas dates on c h l o r i t e have yielded an age o f 3.58 + 1.01 my. (Table 7, samples 77AWs 235, 243). Armstrong and others (1976) Zetermined a 3.2 '+ .4 m.y. age on a serici te-chlori te separate (Table 7, sample Dry Ck.). ~ h F s e numbers are somewhat o lder than the age determined on hornblende f rom one dac i te i n t rus ion , and s tud ies are underway t o examine the hypothesis o f excess argon i n the hydrothermal minerals. Isochron plots do n o t i n d i c a t e excess argon in t h e hydrothermal minerals.

Th is i s one o f the b e t t e r known prospects i n the r eg ion as f i v e d r i l l holes were bored here b~ Bear Creek Mining Co. i n 1976. A l t e r a t i o n i s roughly centered on the intrusions (Plate 4) , w i t h a potassium-si licate core, sericitic outer halo and a propylitic periphery. There i s also the possible existence o f advanced a r g i l l i c alteration (Lowell and Guilbert. 1970) i n por t ions o f the inner zones of the prospect. A1 t e r a t i on assemblages are essentially as descr ibed by Lowell and Guilbert (1970), except f o r a quartz-magnetite assemblage seen i n one small area near the center o f t h e mapped potassium-silicate zone, and t h e presence o f a carbonate-actinolite asseinblage i n some arkoses a l s o found w i t h i n the potassi urn-si l i c a t e tone.

Mineralization decreases toward t h e core o f what was mapped as the d i o r i t e intrusion by F i e l d s ; this i s t rue b o t h on the surface and a t dep th [ ( F i e l d s , 1977). The bu lk of the mineralization i s in the composite rocks external to the intrusions, though s u l f i d e s replace some m a f i c minerals w i t h i n t h e d i o r i t e . Copper in chalcopyrite i s the m a j o r economic metal.

However, there i s a l s o molybdenum and assoc ia ted silver and go ld . Lead and z i n c are found peripheral t o the prospect a t sub-economic grades based on rock sample geochemistry (Fields, 1977; Yount and others, 1978).

Warner Bay Prospect The Warner Bay prospect is located at tidewater 15 km south o f

Chignik, w i t h i n the n o r t h e a s t e r n p o r t i o n o f t he D e v i l s B a t h o l i t h , a large g r a n o d i o r i t e t o tonalite p l u t o n i n t r u d i n g sediments of the Chignik, Hoodoo, Shurnagin(?), and Tolstoi Format ions ( P l a t e 1). The prospect was apparently d iscovered i n the e a r l y p a r t o f t h i s century and has two a d i t s d r i v e n i n t o i t (Atwood, 1911, p .124) . R e l i e f i s a p p r o x i m a t e l y 300m and the exposure i s almost entirely w i t h i n a v e r t i c a l cliff ex tend ing f rom sea l e v e l on Warner Bay. The mine ra l i za t i on occurs on j o i n t su r faces i n t h e closely j o i n t e d (N 45 W ) plutonic rock, i n ve ins p a r a l l e l t o t he j o i n t i n g , and also disseminated i n the small breccia,zone on t h e n o r t h edge o f t he exposure. Molybdenite i s q u i t e common, though it i s subordinate to chalcopyri te (Robinson, 1975; Youn t and others, 1978). A small breccia zone or diatreme on the nor th edge of the exposure c o n t a i n s rounded pebbles o f t h e granodior i te w i t h p e r v a s i v e p r o p y l i t i c a l t e r a t i o n . I n the interstices are found l a rge crysta ls o f ga lena , s p h a l e r i t e , and abundant pyrite. Excep t for the breccia zone, a l t e r a t i o n i n t h e rock seems t o be res t r ic ted t o incomplete a l t e r a t i o n o f amphiboles t o b i o t i t e , and minor c h l o r i t i z a t i o n o f t h e b i o t i t e . I n pa r t i cu l a r , there i s no s e r i c i t i c a l t e r a t i o n and f e l d s p a r s are f r e s h in t h i n section.

A number o f age d e t e r m i n a t i o n s have been made or are i n progress on rocks assoc ia ted w i th the Warner Bay prospect and t h e su r round ing Devils b a t h o l i t h . The p r e s e n t l y a v a i l a b l e results are i n accord w i t h the following suggested his tory; sometime i n the period between t he end o f the Eocene and the start o f t h e late Miocene a number o f small hypabyssal hornblende andes i t e and b i o t i t e d a c i t e p l u tons and d ikes in t ruded a sedimentary sequence c o n s i s t i n g o f Chignik, Hoodoo and Tolstoi f o r m a t i o n rocks. These bodies were described i n the previous section o f t h i s thesis t ha t discusses the Devils b a t h o l i t h . An age determination on hornblende f r o m one o f the i n t r u s i o n s a t Castle Bay (22.4 + 1.86 m y . , T a b l e 3 , sample 77AWs 122) suggests t h a t t h i s group of small i n h s i o n s may be related t o igneous a c t i v i t y a t Mallard Duck Bay (see next section). Following the emplacement o f the andesites and d a c i t e s , the medium-grained b io t i t e -ho rnb lende g r a n o d i o r i t e b a t h o l i t h a t Seal , Sweater, and Warner Bays, was ernplaced d u r i n g the La te Miocene a t about 10 my. based on the h

hornblende ages (Table 3 , samples 77AWs 100, 78A'rls 95) . T h i s pluton may account for the b u l k o f the i n t r u s i o n s o f the Devil's batholith. J o i n t i n g occurred a t Warner Bay as t h i s large pluton cooled. A t approximately the same time as the j o i n t i n g , t h e diatreme or breccia p i p e now seen a t the north end of the Warner Bay exposure was formed. In jec t ion o f t h e small pegmat i tes a t Warner Bay occurred a f t e r the j o i n t i n g . A t a still l a t e r time, Cu-140 mineralization at Warner Bay developed, p o s s i b l y c o n c u r r e n t l y with, b u t ,probably prior to emplacement o f a more feldspar-rich b io t i te -hornb lende t o n a l i t e represented by the 6 m.y. o ld sample from Northwest Arm, Castle Bay ( T a b l e 3, sample 77AWs 125). T h i s las t i n t r u s i v e phase has very minor mineralization assoc ia ted w i t h i t . I n some areas t h e in t rus ion i s so r ich in autoliths(?) that i t appears cong lomera t i c f r o m a d i s t a n c e . Age d e t e r m i n a t i o n s on biotite (7.38 + - 5 5 m.y.) and orthoclase (6.53 - + .17 my.) f rom the pegmat i tes and mixedprimary and secondary

Table 7.--Potasslum-argon ages of Bee Creek Prospect, Chlgnik quadrangle

System component Sampf e number

K2° 40

Rock type Phase q r a d ) Age (m.~. 1 (percent) (percent) and error*

Pot ass1 call y at terid 77AWs 152 Intrusive rack.

Intrusfve rock-------- 77AWs 215-2

Propy1ltica1ly 77dWs 235 altered Naknek Fm.

Quartz-ser k i t e 77AWs 243 altered Naknek Fm.

Potassicaf ly sf tered composite rack .

!

**Quartz-ser k i t e Dry Creek. a? t ered rock .

Altered Daclte. Bia 8.50 11.88 3.67 + 0.09 34.84 3.62 5: .07

Mean------------------- 3.65 T .12 -

Dacl t e . Hbd ,430 1.75 2.12 .t .I2 2.92 2.17 7 -09

Mean------------------- 2.15 .15 d

Arkose. Cht

WR .917 7.81 3.72 + .09 Chl ,626 9 .35 3.92 - .05

Ser 5.45 31.62 4.00 2 .03

Composite rock. Bio 8.16 24.11 3.73 + .07 18.25 3.60 .On

Mean------------------ 3-67 T -

*See footnote ** an table 2. *+Reference, Armstrong and ot+hers, 1976.

b i o t i t e ( 7 .65 + .41 m y . ) f r om granodiorite a t Warner Bay (Table 3, samples 77Ahls 96b, 179, 180) and b i o t i t e (7.78 + . 2 3 my.) a t Sweater Bay (Tab le 2, sample 77AWs 100) y ie ld ages between t h a t o f the hornblende a t Sweater Bay (9.86 - + .36 m.y., Tab le 3 , sample 77AWs 100) and a t Seal B a y (10.08 + .04 mmY- , T a b l e 3, sample 78AWs 95) and the a p p a r e n t 6 my. age a t ~ o r t h i k s t Arm.

Though we riere n o t a b l e t o reach thc. outcrops, e x t e n s i v e a l t e r a t i o n o f the coun t r y rock around t h e margins of' t h e Nor thwest A r m p l u t o n was v i s i b l e . The Castle Bay pluton i s closely assoc ia ted w i t h a mineralized area; t h i s m i n e r a l i z a t i o n probably predates enplacement of the Devils batholith.

Harrison and others (1973) s tud ied thermal h i s t o r i e s of a number o f plutons i n t h e Coas t Plutonic Complex of Bri t i sh Columbia. Based on Rb-Sr, K-Ar, and fission-track d a t i n g and e s t i m a t e d isotopic closure temperatures f o r v a r i o u s mineral phases, they were ab le t o cons t ruc t temperature versus time plots f o r each pluton t h a t they s tud ied. Though Rb-Sr and fission-track d a t a are n o t available, a temperature versus time p l o t for the d a t a from t h e D e v i l s batholith yields a plot (F i gu re 9) similiar i n fo rm to their p l o t f o r the Ecstall pluton. Temperatures used were t h e b lock ing temperatures they derived for individual m i n e r a l phases; the potassium-argon age o f each phase i s p l o t t e d on the t i m e a x i s . Three possible cooling c u r v e s are shown on the diagram; each assumes t ha t cooling occurs a t an exponent ia l rate. The f i r s t , a dashed l i n e projected t h r o u g h t he hornblende, b i o t i t e , and or thoc la se d a t a p o i n t s f rom t h e Sweater Bay phase o f the batholith assumes a slow cooling history. The second i s t h e apparent cooling t rend o f t h e Northwest Arm phase o f the batholith shown by the combination dot ted and dashed l ine, The t h i r d curve is s dotted line very s i m i l i a r to that drawn for the Northwest Arm phase, drawn from the hornblende p o i n t f o r the Sweater Bay phase.

Th is d i ag ram (Figure 9) can be interpreted in a number o f ways and t h e a v a i l a b l e data do not allow s e l e c t i o n of a b e s t case based on the i n f o r m a t i o n i n the diagram. One possible interpretation would e x p l a i n the thermal history by slow c o o l i n g o f t h e i n i t i a l Sweater Bay phase of the bath01 i t h , w i t h the hydrothermal a1 t e r a t i o n and m i n e r a l i z a t i o n tak ing place dur ing th i s period. This slow coo l ing c o u l d be due t o near proximity t o the still-active magma chamber. The intrusion and rapid cooling o f the Northvest Arm portion o f t h e b a t h o l i t h fo l lowed hydrothermal a l t e r a t i o n . Slow c o o l i n g o f t h e Sweater Bay phase would expla in the discordance o f the olde; a g e s , whereas la ter r a p i d coo l i ng and u p l i f t would account f o r concordant ages i n the la te r phase. Hut~ever, w i t h t h i s sort o f thermal history, one would expect a grea ter thermal aureo le around t h e batholith than e x i s t s ; i .e. 1 i k e t h a t described by Sutherland-Brown (1976) for plutonic pol-phyry deposits. The shales of the Hoodoo Formation rnaintai n o r i g i n a l structure and t e x t u r e w i t h the development o f con tac t metamorphic b i o t i t e near the p l u t o n ; no dynamothermal e f f e c t s have been described.

The d a t a shown in Figure 3 do not rule o u t the possibility t h a t coo l ing o f the i n i t i a l Sweater Bay phase o f the Devils batholith may have occurred along a p a t h similiar t o that suggested for the Northwest A r m phase. This rapid cooling may have then been folloxed by a thermal event o f s u f f i c e n t i n t e n s i t y and duration t o partially reset the b i o t i t e ages o f the plu ton. The presence o f hydrothermal b i o t i t e in t he mineralized phases a t Xarner Bay, a long w i th primary b j o t i t e i n the same rocks suggests t h a t t h i s p o s t - c r y s t a l l a t i o n thermal even t may have been associated wi th

Figure 9 fernperaturn versus Elm plot for data from the Devfls bathol i th , using blocking temperatures o f Harrl son and others (1979 and ages determined I n th l s thesl s 1 (see Table 3 ) . Dashed 1 ine Indicates poss b l e slow coaling h i s t o w o f Sweater Bay phase; d o t t e d and dashed l f ne I nd ica tes rapid cooling of Northwest Am phase; dotted 1 i ne shows para1 1 e 1 rapi d cool 1 ng ext rpol ated far Sweater Bay phase.

mineralization b u t i t may also reflect t he emplacement o f the Northwest A r m pluton. The hydrothermal m i n e r a l and pegmat i t e potassium-argon ages would also ref lect this therma l event; formation of the hydrothermal minerals may be related to the thermal event. In this postulated history, the i n t r u s i o n and rapid cooling of the Northwest A r m phase was the l a s t event ; t h i s may ;lave occurred simultaneously w i t h t h e thermal event. If the therrnal event occurred at t he t i m e of intrusion o f the Northwest Arm phase , then almost complete resetting of the or thoc lase a t Warner Bay a n d less complete r e s e t t i n g o f the b i o t i t e occurred.

The approach used by Harrison and others (1979) yields equivocal results i n the Devils batholith based on the l imited d a t a available. Hot~ever, i n con junc t ion w i t h o t h e r geological data, a post-crystalliztion thermal event a s s o c i a t e d w i th the intrusion o f the Nor thwest Arm pluton i s t he e x p l a n a t i o n preferred by the kriter to explain the potassium-argon age pattern i n the D e v i l s batholith. This thermal event must have been o f sufficient intensity and duration to parially reset m i c a and potassium feldspar ages. The e f f e c t o f t h i s event on older hornblende ages is uu known.

Mallard Duck Bay Prospec t The Mallard Duck Bay prospect i s a 4 by 10 k.1 altered zone exposed i n

the headwaters o f Mallard Duck Bay, approximately 10 km southwest o f Chignik. E l e v a t i o n ranges f r o m near sea level to approximately 600 meters. Outcrop exposure i s poor on the well-vegetated slopes and most outcrops are on steep cirque walls or along fresh meander c u t s in t h e braided st reams d ra in ing the prospec t .

The rocks o f the prospect area a r e a thick Tertiary (Ol igocene?) andes i t i c volcanic sequence (Meshik?) i n t r u d e d by swarms o f andes i te dikes and small tonalite stocks ( P l a t e 5). (No te : Fields (1977) c a l l e d the vo lcan ic rocks andesites, but they probab ly include leuco-basalts and d a c i t e s also. He also cal led t h e tonalites g r a n o d i o r i t e , undoubtedly b o t h are present.) Bear Creek Mining Company (F i e l ds , 1977) found t h e v o l c a n i c rocks to be composed of subhor izonta l andesite f l ows , volcanic las t ic sediments and tuffaceous u n i t s wi th a t o t a l t h i c k n e s s o f a t f e a s t 3000 f e e t (900m). I n t r u d i n g the volcanic rocks i s a pre-mineralization t o n a l i t e complex of s m a l l plutons and large d ikes . A sample c o l l e c t e d f rom one o f t h e s tocks i s a porphyritic hornblende t o n a l i t e w i t h large phenocrysts o f q u a r t z , p lagioclase o f approx ima te l y An 45 composition,,and ho rnb l ende . The hornblende phenocrysts are entirely replaced by f i n e b i o t i t e and chlorite. The groundmass i s composed of f i n e r - g r a i n e d plagioclase, quartz, and hydrothermal ( ? ) b i o t i t e . The p l agiocl ase i s generally fresh; sericitization i s not comnon. The d ikes t r e n d nor thwest and are up to a - kilometer i n l e n g t h and up t o 100 mete rs wide. Apparently, hydrothermal alteration i s centered on one small tonalite exposure on the southwest edge o f the a l l u v i a l valley. F i e l d s (1977) reported t h a t all the tonalites are of pre-mi neral i z a t i o n age, a1 though he notes that m i n e r a l i z a t i o n decreases towards the core o f the plutons. A 21.3 + '79 m y . age has been determined on c h l o r i t i z e d biotite f r o m on one o f the-tonalite stocks and a 21.2 + 1.31 m.y. age has been determined on a quartz -serici t e hole rock sample Tram this prospect (Tab le 8, samples 77AWs 137 and 183). The andesitic d i k e swarm was considered post-mi neral i z a t i o n by Fields (1977), yet t h i n s e c t i o n s r e v e a l pervasive propylitic alteration. A d i k e sample collected i n 1977 by M.L. Silberman i s a f i n e - g r a i n e d hornblende andes i te porphyry

w i t h large h o r n b l e n d e phenocrysts and abundant plagioclase phenocrysts up to 1 cm i n s i z e . The rock con ta ins a b u n d a n t epidete and c h l o r i t e . A potassium-argon age of 27 .1 + 1.58 m.y. h a s been determined on hornblende (Table 8, sample 77AMs 1) from t h i s sample. Other samples o f these d i k e s were a lso hornblende andes i te porphyries; however, p e r v a s i v e s c r i c i t i c and propylitic alteration has a f fec ted these, and t h e amphibole and plagioclase are largely destroyed. Whole-rock dates on una l t e red volcanic rocks assoc ia ted w i t h the Mallard Duck Bay vo l can i c pile range from 22 to 26 m.y. (Tab le 4, szrnples 78AWs 98, 134) and probab ly represent l a t e s tage flows f r o m the v o l c a n i c center.

The geology o f t h e Mallard Duck Bay prospect has a close correspondence t o the volcanic model o f Sutherland-Brown ( i 9 7 6 ) o r possibly t he upper reaches o f t h e volcanic-plutonic model o f Silli toe (1973). Geochemical sampl ing shows the Mallard Duck Bay prospect t o be low i n t h e m e t a l s that were o f economic interest (Cu, Mo, Zn, Pb, Ag, Au); however, r e l a t i v e to the o t h e r prospects i n t h e area (except p o s s i b l y Warner Bay), Mallard Duck Bay i s proportionately higher in lead and z i n c . Molybdenum concentrat ions are particularily low. Mineralization tends t o be f racture-control l ed, wi th i n t e n s e shattering throughout t h e area (F i e l ds , 1977 ) .

A1 t e r a t i on i s characterized by widespread p r o p y l i t i c a1 teration, irregular p e r v a s i v e s e r i c i t i c a l t e r a t i o n and potassium-sil icate alteration concent ra ted i n a small area of t h e g r a n o d i o r i t e pluton (Fields, 1977).

Cathedral Creek Prospect T h i s prospect i s a large color anomaly in t h e d ra inages o f Cathedral,

Milk, and Braided Creeks, and lies nor th-nor thwest o f Chignik. Pan American (now Amoco) drilled 9 t o 10 core holes on the prospect i n 1965 t o 1967 and Bear Creek Mining Co. evaluated t h e prospect i n 1975 ( F i e l d s , 1977). Pan Averican's drilling was to e v a l u a t e the po ten t i a l of a number o f sphalerite, galena, chalcopyrite, silver, and gold v e i n s cropping out in t he headwaters o f Braided Creek; Fields (1977) reported t h a t their records were los t , and Bear Creek Mining Co. was only a b l e t o locate 4 o f the drill s i t e s . Bear Creek Mining Co. spent 3 days examining and sampling the prospect i n 1975 and concluded t h a t i f any s i g n i f i c a n t copper mineralization e x i s t s , i t i s a t depth.

A t this prospect , a large andes i t e pluton i n t r u d e s sediments of t h e Chignik, Hoodoo, Tolstoi, and Meshik Formations. T h i s pluton lies near t h e nor thwest end o f t h e b e l t o f i n t r u s i v e rocks ex tend ing from Weasel Mountain t o Black Peak, p r e v i o u s l y ~nant ioned i n r e l a t i o n t o t h e Bee Creek prospect . Samples collected from the C a t h e d r a l Creek area i n d i c a t e t h a t the p l u t o n i s

. a hypabyssal body rang ing i n cornpositon f r o m a two-pyroxene andesite t o a hornblende andesite porphyry. Plagioclase composi t ions are q u i t e calc ic and rang? from An 50 t o An 70. B i o t i t e i s rare i n the t h i n sections examined, though chlorite has replaced the m a f i c phase i n one sample. Fields (i977) reports chlori t izat ion i s common in the i n t r u s i v e rocks w i t h i n the a l t e r e d zone. The t e x t u r e s in these rocks and the geologic s e t t i n g i n d i c a t e t h a t they were i n t r u d e d a t very shallov d e p t h s and may i n p a r t be e x t r u s i v e . The sediments intruded by these igneous rocks l i e on the n o r t h flank o f t h e Chignik a n t i c l i n e and accord ing t o Fields (1977), d i p some 55 t o 70 degrees t o the north-northeast . Dacitic and andesitic f l o w s , pyroclastics, and agglomerates from Slack Peak, a Quarternary t o Recent volcano overl ie the northwest portion of t h e area. Sericitic

Table 8.--Potassium-argon ages o f Mallard Duck Bay Prospect, Chignik quadrangle

K2° 40

System component ~ o c k t y p e Phase Ar(,,d) Age (mmym) number (percent) (percent) error*

A1 tered i n t r u s i v e 77AWs 137 Tonal i t e rock .

Quartz s e r i c i t e 77AWs 183 altered volcanic.

Andesi t e Wbd Pre-mineralization 77AMs 1 dike.

0.343 29.56 26.2 + .57 15.31 27.9 7 .86

Mean111-33e----L-133----- 27.1 T 1.58 -

*See footnote ** on table 2.

a l t e r a t i o n i s extensive in the sedimentary rocks surrounding the a n d e s i t e and chloritization i s comaon i n the p l u t o n . Potass ic alteration i s not apparent. Dennis Cox collected samples o f sericitically a l t e r e d sediments; a potassium-argon d a t e o f 1.38 + .19 m.y. has been determined on one o f these ( T a b l e 9 , sample 77ACx 4); A potassium-argon age of 3.08 + .14 m.y. has been determined on hornblende from a n d e s i t e t h o u g h t to be paFt o f the main i n t r us i on (Table 9, sample 7 8 W s 125) .

Nakcharni k I s1 and Nakchamik I s l a n d i s h i g h l y mineralized offshore i s l a n d lying south of

Cape Kumlium i n Chignik Bay ( P l a t e 1). Ye spent one day there i n the summer o f I977 and a s i n g l e d a t i n g sample was collected. T h i s sample was porphyr i t i c hornblende dacite (S t reck i sen , 1979) w i t h phenocrysts o f hornblende, plagioclase , and possibly potass ium feldspar. The hornblende phenocrysts a r e large subhedra l crystals with p a r t i a l l y resorbed edges an3 inclusions o f plagioclase. There i s minor secondary chlorite assoc ia ted w i t h the hornblende. The zoned plagioclase phenocrysts a r e generally shattered, are apparently p a r t i a l l y s e r i c i t i c a l l y al tered and are o f An 45 t o An 55 composi t ion. The more s e r i c i t i c a l l y a l te red plagioclase phenocrysts may be o f a d i f f e r e n t conposition. The groundmass i s f i n e - g r a i n e d and composed o f plagioclase, quartz and hornblende. An age of 10.1 + -92 m.y. (Table 9, sample 77ANs 134) was determined on hornblende f rom This sample. Tile sample was unmineralized and l i t t l e altered; therefore the sample and i t s age probably represent a p o s t - m i n e r a l i z a t i o n event . The rock da ted was a cataclastic hypabyssal porphyritic tonalite and i t i n t r u d e s propylitically and s e r i c i t i c a l l y altered i n t r u s i v e rocks and volcanic rocks making up the island.

Unavikshak I s l and Unavikshak Island i s a l s o a small i s l a n d in Chign ik Bay lying e a s t o f

Cape Kumlium (Plate I). Hornblende f rom a hornblende andesite sill that has been d a t e d a t 36.4 + .I5 m.y. (Table 9, sample 78AWs 42) intrudes volcanic rocks and coal bearTng sediments o f unknown age. T h i s hornblende andes i te i s similiar petrographically to the hornblende d a c i t e on Nakchamik I s l a n d , t h o u g h not as f resh. The fe ldspar phenocrysts are o f An 50 composition and are strongly sericitized. Subhedral quar tz phenocrysts are present in small amounts (+ 10%). The groundmass i s composed o f plagioclase, q u a r t z , hornblende and ~ccessory a p a t i t e . A small zone of serici t i c a1 t e r a t i on l ies near t h e nor thern end of the i s l a n d and this alteration h a s Seen dated a t 29.9 + -65 m y . ( T a b l e 9, sample 78AWs 43) . The two d a t e s r e p o r t e d here are singTe determinat ions , unlike most others i n t h i s t h e s i s , therefore the reliability of the age determination i s unknown. I t can be assumed t h a t the analytical error i s somewhat larger than that indicated. The m i n e r a l i z a t i o n system a t Unavikshak I s l a n d i s best classified as a v o l c a n i c porphyry deposit, similiar t o M a l l a r d Duck Bay on a much smaller scale. The 6.5 million year gap between emplacement o f the sill and mineralization i s s i g n i f i c a n t and o f the same order o f magnitude as t h a t at Mallard Duck Bay.

Conclusions Regarding the Genesis o f Porphyry Systems

In a number o f s tud ies , Page and McDougall (1972a, 1972b, Page, 1975), have distinguished a d i f f e rence of 1 o r more mi l l i on years between the age

o f emplacement o f an i n t r u s i v e rock and the age o f hydrothermal minera ls re la ted t o the mineral izat ion of t h e i n t rus ion . As would be expected and hoped, the hydrothermal m i n e r a l ages are cons is ten t ly younger than emplaceaent ages o f t h e hos t rocks . Roore a n d Lanphere (1971) reported a s i m i l i a r resu l t a t Bingharn. One o f the i n i t i a l goals o f this dissertation was t o d e t e r m i n e i f a s i m i l i a r t i m e s p a n cou ld be measured by dating o f concentrates and separates of hydt-othermal and primary mineral phases f rom m i n e r a l i z e d zones i n t h e Chignik and Sutw ik Island r eg i on .

T l ~ e r e s u l t s ob ta i ned on t h e Warner Bay, Unavikshak I s l a n d , and 14allard Duck Bay prospects provide a d d i t i o n a l evidence towards the e x i s t e n c e o f th i s t i m e span between einplacem2nt and a l te ra t ion-minera l iza t ion . These r e s u l t s a l s o ind ica te the c o m p l e x i t y o f t h e intrusion/alteration systems. At the Warner Bay a n d Mallard Duck Bay prospects, m i n e r a l i z a t i o n developed a f t e r t he i n i t i a l in t rus ive events and just before or du r i ng the l a t e s t magmat i sm.

The Bee Creek prospect s h o u l d have p r o v i d e d a clear cut case f o r study. However, problems arose when i t became c lear t h a t i n o n l y one sample would i t be p o s s i b l e t o separate primary hornblende for da t ing . An i n i t i a l separa te was made, and a f te r repeated argon extractions i t was apparent t h a t no da te was going t o be ob ta ined . Each s p l i t yielded an age older than any hydrothermal phase, yet no two splits were concordant. A t about t h e same time, difficulty wi th a number o f hornblende separates o f s imi l ia r age was encountered and no cause or solution fo r th i s problem was ever determined. A second hornblende separa te was made o f the sample from Bee Creek (77AWs 215-2) and a f t e r t h r e e attempts a duplicate analysis was obtained. However, t h e 2 m.y. old r e s u l t suggests t h a t the i n t r u s i o n is younger t h a n m i n e r a l i z a t i o n , which was unexpected, and the low yie ld of rad iogen ic argon makes t h e r e s u l t s ana ly t i ca l l y suspic ious. The good agreement between the two extractions could be mere coincidence. Further s t u d i e s w i l l have t o be undertaken t o resolve t h e problem and t o a r r i v e a t a solution.

The r e s u l t s o f t he study o f porphyry systems i n the Alaska Peninsula, coupled wi th the e a r l i e r s t u d i e s c i t e d , i n d i c a t e t h a t porphyry m i n e r a l i z a t i o n and the assoc ia ted hydrothermal a1 t e ra t ion systems a re probab ly no t related t o the emplacement and c o o l i n g o f a par t icular igneous body. Most d e s c r i p t i o n s of porphyry deposits i n d i c a t e t h a t m i n e r a l i z a t i o n occurred after igneous emplacement, and of ten af ter j o i n t i n g or b r e c c i a t i o n o f t h e p l u t o n . T h i s may i n d i c a t e t h a t the hydrothermal a l t e r a t i o n system i s more re l a t ed t o the overa l l magmatic center ra ther t h a n any p a r t i c u l a r igneous phase. T h i s i s particularily apparent w i t h respect t o v o l c a n i c p o r p h y r y systems. In these, the t i m e span between the emplacement and a l t e r a t i o n events i s p a r t i c u l a r i l y large. The p l u t o n i c system a t Warner Bay may have been thermally a l t e red , yet i t is clear t h a t the t i m e span between emplacement o f the igneous phase t h a t i s now mineralized and t h e a c t i v a t i o n o f the hydrothermal system \$as much shorter t h a n t h a t i n t he vo l can i c systems. The d a t a are not conc lus i ve b u t may suggest an inverse relationship between depth o f mi neral i z a t i o n and the t i m e span between emplacement and mineral izat ion.

Ch ign ik Region T e c t o n i c Synthes is Paleomegnetic S t u d i e s

Stone and P a c k e r (1977) reported on a paleomagnetic i n v e s t i g a t i o n of r o c k s f r o m t h e A l a s k a Pminsula . They co l l ec ted samples f rom a number of l o c a l i t i e s , including rocks o f the Naknek, Chignik, Hoodoo, and T o l s t o i Formations. Some o f t h e i r sawple l o c a l i t i e s were w i t h i n the study area o f this t h e s i s . Though their paleopole error limits were large, t he i r best f i t r e c o n s t r u c t i o n requires a large n o r t h w a r d migra t ion and c l o c k w i s e r o t a t i o n o f a block they named " B a j a A l a s k a n , relative t o the North American P l a t e s i n c e Jurassic and prior t o Eocene time. On the b a s i s o f ea r ly d a t a , Packer and Stone (1974) had proposed the S a j a A1 aska model f o r the Alaska P ~ n i n s u l a ; l a t e r work (Stone, 1979) supports and extends the proposed model. T h i s model describes t he motion o f the B a j a A l a s k a block i n t h e time range from Jurassic t o Eocene. The Jurassic paleopoles f o r the B a j a A?aska block i n d i c a t e t h a t i t l a y i n the southern hemisphere. Cretaceous paleopoles i n d i c a t e a r a p i d migration northward u n t i l , by Eocene t i m e , t h s b l o c k was i n a p p r o x i m a t e l y i t s present p o s i t i o n relative t o cratonal Nor th h e r i c a . Stone ' s (1379; Stone and Packer, 1977) d e f i n i t i o n o f Baja Alaska was based e n t i r e l y on t h e a r e a from which t h e i r samples were col lected, and not on any geologic or t e c t o n i c boundaries. In essense, t he i r Baja A la ska i s t h e Alaska P e n i n s u l a ; geologically t h e rock u n i t s on t h e A laska P a i n s u l a a re n o t l imited t o i t s geographic boundaries, b u t extend i n t o southern Alaska in the Cook Inlet region.

Jones and others ( 1 9 7 7 ) have defined a large allochthonous terrane in southern Alaska, western Canada, and Idaho as Wrangellia, which i s d i s t i ngu i shed by a characteristic stratigraphic sequence, structure, f o s s i l fauna, and overall geo log i c his tory ( F i g u r e 10). The pr incipal p a r t s o f the stratigraphic sequence include a t h i c k Middle and Upper Triassic u n i t of t h o l e i i t i c b a s a l t disconforaably o v e r l a i n by ca lcareous sediments deposited dur ing late Karnian or earliest Norian (Late T r i a s s i c ) t i m e (Jones and others , 1977, p.2565). Older p a r t s o f t h e terrane where exposed, "are dominantly o f sedimentary and arc-re la ted ( ? ) vo l can i c rocks t h a t nowhere may be older than Pennsylvanian and, from i n d i r e c t evidence, rnay have Seen i n p a r t , deposited on oceanic crust4' (Jones and others, 1977, p. 2656).

Th i s terrane, according t o Jones and others (1977, p.2571-2572) i s d i s t i n c t f r o m the Triassic and older rocks exposed a t Pua le Bay (Cape Kekurnoi , Figure 1) and other places t o the northwest on t h e Alaska Peninsula. D.L. Jones (o ra l communication, i979) believes t h a t these two Triassic terranes were separa te during T r i a s s i c and Early J u r a s s i c t imes, as i s i n d i c a t e d by t he d i s t i n c t l y d i f f e r e n t Triassic f aunas , and t h e i r apparently d i f f e r en t Early Jurass ic histories. I n t h e Talkeetna Mountains t h e A l a s k a - A l e u t i a n Range batholith i s continuous across b o t h terranes, i n d i c a t i n g they were joined by Middle Jurassic t i m e . The Puale Bay Triassic rocks are p a r t o f the Baja A l a s k a block o f Stone and Packer (1977) . Jones and others (1977), c i t i n g earlier work by Jones (1963), p o i n t o u t t h a t the two T r i a s s i c t e r r anes were cont iguous by Cretaceous t i m e because the faunas and stratigraphy ac ross t h e two terranes s i n c e t h a t time are simi 1 i a r . Hi l lhouso (1977) presented paleomagnetic d a t a for t h e Nikolai greenstone f rom t h e Wrangellia t e r r a n e i n sou th -cen t ra l Alaska. H i s interpretation was t h a t th i s terrane had undergone 27 degrees of nor thward m i g r a t i o n and 90 degrees of counter-clockwise r o t a t i o n , o r 57 degrees o f northward m i g r a t i o n and 90 degrees o f clockwise r o t a t i o n r e l a t i v e t o t h e

North American Plate s i n c e T r i a s s i c t i m e . To this writer, the fairly close agreement between Stone and Packer's (1977) interpretation and Hillhouseis (1977) interpretation lends credence to the Baja A l a s k a model. Stone and Packer (1977) d a t a restr icts t h e t i m i n g o f t he nor thward m i g r a t i o n and r o t a t i o n of the combined te r ranes t o the late Mesozoic or ear ly Tertiary.

Other Geophysical Studies As part o f t h e g e o l o g i c i n v e s t i g a t i o n of t h e Chignik and S u i w i k Island

quadrangles , the U . S . Geological survey c o n t r a c t e d t o - h a v e an aeromagnetic survey (U. S. Geological Survey, 1978) f lown o f the area by CKB Resources, Inc. Interpretation o f the data i s underway by J.E. Case (oral communication, 1973); some o f t h e preliminary results can be discussed here. As mentioned earlier, the data show the continuation o f a trend thought to be t h e A l a s k z - A l e u t i a n Range batholith i n the subsurface as f a r south as t h e v i c i n i t y o f Port Heiden. Pratt and o t h e r ' s (1972) shipborne magnet ic d a t a indicate a possible continuation o f this trend o f f s h o r e i n the Bering Sea (Bristol Bay).

Case interprets a number o f the other magnetic highs to be the roots of older volcanic centers. Case ( o r a l communication, 1979) has interpreted t h e l i n e o f aeromagnetic highs f o l l o w i n g t h e t r e n d o f the Ch ign ik a n t i f o r m j u s t west of Chignik Bay as b u r i e d plutons. Figure 11 i s a map showing t h e out l ine o f aeromagnetic anomaly h i g h s based on the aeromagnetic map o f the Chignik and Sutwi k Island quadrangles (u. S. Geological Survey, 1978). A1 so shown i s an interpretation o f t h e age o f each anomaly based on ages determined on rocks f r o m each anomalous area. From t h i s map, the f o l l o w i n g t r e n d i s apparent . She Eocene t o Ol igocene volcanic centers (Tolstoi-Meshik arc) cluster in t h e center of the Ch ign ik and Sutwik Island area; however t h i s may be an artifact o f the dat ing, which i s quite sparse on t h e perimeter of the area. Assuming the appa ren t distribution i s meaningful, there i s a nor theas t -southwest e longa t ion i n the cluster o f magnetic h i g h s w i t h a bend t o the e a s t t o encompass S u t w i k I s l and . I n general t h i s trend i s markedly more eas t -wes t than other magnet ic anomaly trends in the Chignik and S u t w i k I s l a n d area (Figure 11). Another trend of magnetic highs i s aligned parallel t o the trend o f t h e Alaska Peninsula and corresponds to t h e be1 t o f late Miocene (5-10 m.y. ) intrusive rocks, i n c l ud ing the D e v i l s ba tho l i t h , the Chiginagak Bay pluton, and the pluton dated on Nakchamik I s l and . These p lu tons form the P a c i f i c c o a s t o f the Alaska Peninsula. A third trend of 0-5 m.y. anomal ies a lso p a r a l l e l s the Alaska Peninsula and corresponds to t h e Holocene volcanos on the B e r i n g Sea side of the peninsula. h

No compilation gravity anomaly map i s yet available f o r the study area; Case 's (oral communication, 1973) suggest the following interpretation: simple Bouguer anomaly g r a v i t y d a t a i n d i c a t e a +50 mgal anomaly a l o n g the axis o f the A l a s k a Peninsula i n the Chig'nik area. T h i s anomaly occurs throughout the southern Alaska P e n i n s u l a and Aleutian arc, and suggests crust with a gravity signature t rans i t ional between oceanic and c o n t i n e n t a l crust beneath t he arc. There i s a major change to a negative anomaly in the region of Becharof Lake, (Figure 1) approximately 100 krn northeast o f t h e Chignik and Sutwik I s l a n d r e g i o n , suggest ing continental crust there. In the s tudy area, t h e positive anomaly has two m in ima , one paralleling the Meshik River and the other just south o f Mt. Veniaminof (P l a t e 1). J.E. Case believes these may be " sp layM basins off the Bristol Bay b a s i n . They a l s o may be m a j o r crustal breaks. In

particular, the gravi ty anomaly trend of the Peninsula i s offset in a r i g h t lateral sense across t h e Meshik River. The surface geology i s permissive f o r either case; however, the major structure of t h e Chignik and Sutwi k Island region, the antiform west o f Chignik Bay (Plate l), i s also offset at the Meshik River. To the n o r t h o f the river a similiar feature i s mapped as the Wide Bay Anticline (see Burk, 1965, P a r t 3). The presently active volcanos, Mt. Veniaminof and Aniakchak do not lie on t h e gravity anomaly trend.

Geochronologic Studies The geochronologic work undertaken here has ind ica ted two impor tant

Terti ary magmatic episodes in the Chignik and Sutwi k Island region area. These magmatic episodes may correspond to s i m i l i a r events i n t h e Aleut ians reported by Delong and others (1978); however, the ages reported in the Aleutians are metamorphic ages. The magmatic episodes i n t h e Chignik and Sutwik Island region occur 5 to 10 million years later than in the Aleutians, possibly i n d i c a t i n g the propagation r a t e o f magmatic a c t i v i t y related to subduction along the Aleutian Trench. Figure 12 is a histogram o f potassium-argon dates from t h e Chignik and Sutwik Island region. Each date p l o t t e d represents a single discrete event, i .e. only a s i n g l e date was used fo r the alteration age at Bee Creek even though a number o f different age deteminations were run. Also shown on t h i s graph by a long and short dashed line i s a schematic estimate of the "true'hiistribution of ages in the region, based both on t he age determinations and on the known and in fer red geologic distribution o f the igneous rocks. The large peak, centered about 34 may., represents the Tolstoi-Meshik Arc. The large volume and wide distribution o f Tolstoi Formation volcaniclastic rocks and Meshik Formation volcanic rocks would tend to confirm the importance o f t h i s event. A smaller peak on the flank of the 34 m.y. old peak, centered about 22 m.y. i s based primarily on the distribution o f age determinations. Most of the dates i n this range come from the vicinity o f the Mallard Duck Bay prospect; it is unclear that t h i s event i s discrete from the Tolstei-Meshik Arc and t h e r e f o r e the dotted line may be a more accurate representation. The second peak is incompletely shown, because it may be centered about presently a c t i v e vo lcan i sm related t o plate convergence at the Aleutian Trench. Radiometric work has shown t h a t there i s little evidence for Paleocene igneous a c t i v i t y except for the plutons o f the Kodiak-Shumagin Plutonic Series. In particular, my work to date suggests that the previous Paleocene (Burk, 1965) age assignment for the Tolstoi Formati on i s incorrect. Some s t r a t i g r a p h i c evidence (discussed later) i s suggestive o f a late Cretaceous volcanic arc, there i s still no evidence of a source terrane, nor have any samples from the study area yielded Late Cretaceous ages. A single mid-Cretaceous age (Tab le 6, sample 77AWs 186) on a plutonic cobble from the Chignik Formation is thought to represent a minimum age on a weathered sample o f t h e Jurassic portion o f the Alaska-Aleuti an Range batholith by this writer.

The potassi urn-argon stud ies r e p o r t e d here have been used primari ly to interpret the Tertiary history of t h e Chignik and Sutwik Island area. They have not been of use to interpret o f older events, except to indicate t h a t no older igneous rocks outcrop in the Chignik region. The only pre-Tertiary igneous rocks known are found as sedimentary debris i n Mesozoic formations.

The studies on mineralized areas have been o f some use i n defining the

t i m i n g and sequence o f events in these areas. A t Warner Bay, d a t i n g o f hydrothermal minerals has i n d i c a t e d t h a t mineralization occurred between the time o f enplacement of t w o phases of the Devils b a t h o l i t h . Secause of t h e p a u c i t y o f hydrothermal phases a t Warner Bay, f u r the r ref inement o f t he t i m i n g o f the a l t e r a t i o n event i s n o t possible. Dat ing at Mallard Duck Bay i n d i c a t e s m i n e r a l i z a t i o n o c c u r r e d d u r i n g the l a t e stages o f the l i f e o f the volcanic center, was a c t i v e for a t leas t 7 m.y. A sirniliar result ~ 3 5

obta ined a t Unavikshak I s l a n d . A t Bee Creek, t h e prospect I studied most intensively, the d a t i n g h a s n o t been as successfu l , nor the results as c l e a r cu t . The hydrothermal or secondary m i n e r a l s a l l yield similiar ages and isochron p l o t s i n d i c a t e no i m p o r t a n t loss or g a i n o f r a d i o g e n i c argon. Unfortunately, p r ima ry minera ls have been difficult t o collect and the one hornblende separate y ie lds an age sonewhat younger t h a n the other minerals. I view hornblende age w i t h some suspicion; however no good case can be inade for rejecting the date, except for t h e low yield o f radiogenic argon. The geologica l relations a t t h e Bee C r e ~ k prospect suggest that t h e hornblende s h o u l d be a t l eas t as old a s the hydrothermal minera ls ; however, i t may represent a late intrusive phase.

A problem w i t h dating i n t h i s so r t o f terrane i s the almost ubiquitous a l t e r a t i o n of igneous rocks, and therefore t h e v e r y l i m i t e d popu la t ion o f f m s h igneous rocks from r ~ l ~ i c h samples can be collected. In addition, t he generally l o w potass ium content of t h e r o c k s limited d a t i n g t o whole rocks, p l a g i o c l a s e , and hornblende. Since it was rarely possible to date m i n e r a l p a i r s , a good knowledge o f the geological relations between rocks units i s r e q u i r e d i n order t o determine the consistency o f the ana ly t i ca l work. Unfo r tuna te ly , our knowledge o f the geology of t h e Alaska Peninsula i s often inadequate i n t h i s respect.

Another problem i s due t o t he very young age o f many of the units. Low potassium content and young ages require large samples t o generate s u f f i c i e n t argon f o r accura te and p r e c i s e measurement. There i s ho~ever, a practical l i m i t t o the s i z e sample t h a t can be loaded i n the argon e x t r a c t i o n lines and o f t e n the desired sample s ize exceeded t h i s l i m i t ,

In dealing with the geological and analytical problems encountered as pa r t o f t h i s study, a n~rnber o f avenues of research a r e be ing pursued o r considered. These include:

1. Continued d a t i n g o f igneous r o c k s from t h e area i n an e f f o r t t o determine i f t h e early Fliocene h i a t u s i n igneous activity i s real.

2. F u r t h e r d a t i n g o f phases from the Bee Creek prospect t o a t tempt t o resolve the apparent inconsistencjes; t h i s will also i n v o l v e t h e d a t i n g of hydrothermal quar tz to examine the likelihood o f excess argon.

3. Fission-track dat ing o f s u i t a b l e phases f rom mineralized or a l tered zones, to p r o v i d e more information for decipher ing t h e thermal h i s t o r i e s o f these areas.

4. Laboratory work t o determine i f t h e HF a c i d leaching technique can be applied t o amphiboles t o improve the analytical results. Amphiboles, particularily the younger ones, exhibit poor r e p r o d u c i b i l i t y , i n p a r t because o f the low yield o f r a d i o g e n i c argon. The small amount of r a j i o g e n i c argon evolved during extraction i s easily ~bscured by the large a tnosphe r i c component. Any m a n s t h a t can reduce the amount o f atmospheric argon w i thou t significantly a f f e c t i n g t he radiogenic argon component will improve the precision of t h e age determination. Other authors (Dalrymple and Lanphere, 1969, p.189) have stated t h a t the HF leaching technique s h ~ l d n o t be used on m a f i c minerals; yet apparently no tests o f the

technique have actually been carried out. The use of the technique on basalts and andesites has not been theoretically j u s t i f i e d , yet empi r i ca l d a t a (M.L. Silberman and E.H. McKee, oral communication, 1979; Morton and others, 1977) clearly i n d i c a t e t h a t i t works.

Conclusions A series o f schematic cross sections are shown in F i g u r e s 13-15. The

d iscuss ion t h a t f o l l ows i s generally re la ted t o these cross sections, which g i v e a v i s u a l representation o f the geologic h is to ry o f the study area th rough t ime.

Jurassic s e d i m e n t a t i o n in the Chignik and Sutwik Island region commenced a f t e r volcanism in a p r o t o - A l e u t i a n arc ( T a l k e e t n a A r c ? ) waned. The ea r l i e s t rocks exposed in the Chign ik and Sutwik Island area are p a r t s o f the S h e l i k o f Formation, depos i ted as the l a s t phases o f v o l c a n i s m ended ( ~ i g u r e 13a). The volcaniclastic rocks assoc ia ted w i t h t h i s arc are exposed much further north as the Talkeetna Formation (Detterman and Hartsock, 1966) which belongs stratigraphically to the Lower Jurassic Blueschist inc luded i n melange on Kodiak Island (Carden and others, 1977), -

the Jurassic parts of the Alaska-Aleut ian Range Batholith, and t h e Ta lkee tna Formation are a l l evidence o f t h i s arc. The spatial arrangement o f these components svggests t h a t t he arc formed over what in the present would be a northwest-dipping subduction zone. The Shelikof Format ion was deposi ted i n an open ocean basin, and indicates an i m p o r t a n t change in the tectonic regime o f the B a j a A l a s k a Block. Its deposition records the ending o f a phase o f convergent plate margin a c t i v i t y , and the beginning of a p a s s i v e p l a t e marg in phase, lasting through m i d d l e Cretaceous time. A f t e r depos i t i on of the She l i ko f , the S h e l i k o f underwent p a r t i a l erosion and its volcanic-plutonic source terrane was eroded further.

Approximately 150 m y . ago ( e a r l i e s t Oxfordi an, La te Jurassic t i m e ) deposi t ion of the Naknek began (F igure 13b). I believe i t had the same northerly source terrane as the Shelikof, except t ha t the terrane was being eroded a t a deeper level and t h e r e f o r e the percentage o f plutonic rocks i s significantly greater. Deposition continued uninteruptedly, along with gradual subsidence, u n t i l Valang in ian (Early Cretaceous, 125 m.y.) t ime . The source of sediment supply decreased progressively, ultimately a l l o w i n g deposition o f the Herendeen Limestone. The Herendeen Limestone i s a calcarenite and may indicate s t a r v e d basin condi t ions. A similiar unit of t h i s age i s common i n many parts o f southern A l a s k a . kb

During the Late Cretaceous the ex is tence o f another volcanic arc (Shumagin Arc) , i n approximately the same position as t h e Jurassic arc, may be i n f e r r e d (F igure 14a). Th is i nferrence i s based on ev idence showing t h a t the source terrane for the Shumagin Formation was a volcanically a c t i v e region to the no r th and northwest o f the d e p o s i t i o n a l basin (~oore, 1973a). In a d d i t i o n , the 85-59 my. plutonic event i n t h e ~laska-Aleutian Range batholith may i n d i c a t e the sub-volcanic plutonic phase o f t h i s arc. The deposits o f the Shumagin Arc were apparently carried across the shelf and dumped into a forearc b a s i n as f l y s c h o f the Kodiak, Shumagin, and, possibly , the Hoodoo Formations. Correlative sequences also include p a r t s o f the Valdez and Yaku ta t groups in southern Alaska, and t he S i t k a Graywacke i n southeastern Alaska (Plafker and others, 1977). There i s some disagreement among g e o l o g i s t s working in southern Alaska on the existence of t h i s arc; Arthur Grantz (oral communication, 1979) b e l i e v e s t h e source f o r the extensive flysch terrane was erosion of the Lower Jurassic

(a. Late ~retaceous ( pre-campan1 an)

Late Cretaceous (Campani an = Maestri chtf an)

L 1 3 - - - S , L a

Kc' = Chignl k Formation Kcv Coal Valley Member Kh Hoodoo Fomat lon Ks a Shumagin Formation K-Jgd, AARb = Alaska-Aleutian Range batholith

Figure 14 Schematic Cross-section. - Tectonlc development o f the Chigni k reg ion - Part 2

Talkeetna Format ion. Ho!qever, this does n o t account fo r the evidence for a c t i v e v o l c a n i s m a t the time o f deposition o f t h e f l y s c h , no r does i t seem geologically poss ib le t h a t t h e Talkee tna Formation cou ld act as a source f o r t h e f l y s c h in the Shumagin or Sanak Islands. No reasonable case can be made fo r e r o s i o n of the T a l k e e t n a Formation on the southern A l a s k a Peninsula at any t i m e during the Cretaceous kecause, among other things, tlrere js no gv idence t h a t the formation extends t h a t f a r southwest. There remains the possibility t h a t the t u r b i d i t y currents responsible f o r deposit ion of the f lysch f lowed l ong distances z long the a x i s o f t he ancestral A l e u t i a n trench. T h e v o l c a n i c and volcaniclastic rocks t h a t formed the ac tua l arc a re now missing, and the t i m e - e q i i i v a l e n t shallow-v~ater sediments o f t h e Alaska Pen insu la do n o t r e f l ec t an a c t i v e volcanic arc. There i s , ~ O W O V Z ~ , ev idence of m i n o r volcanic a c t i v i t y based on the b e n t o n i t i c sha les i n the Chignik Formation. In t h e Chignik and Sutwik I s l a n d region, m ino r f o l d i n g p r i o r t o the depcsition o f the Chignik Format ion may be r~lated t a a renewal of subduction related t o the p o s t u l a t e d Shumagin Arc (Figure 14b). The geologic record o f t h e Alaska Peninsula c o n t a i n s a depositional h ia tus t h a t may range from t h e Valanginian to t h e Campanian (Cretaceous, see Figure 2 ) . The source terrane for t he Ch ign ik Formation probably was, i n p a r t , t h e Upper Jurassic Naknek Formation, and i n part v o l c a n i c de t r i tu s incorporated f r o m what m igh t havo k e n the Sbumagin Arc. According t o Stone and Packer (?977) , most o f the migration of B a j a Alaska took place a f t e r the d e p o s i t i o n of these Cretaceous rocks and before o r du r i ng t h e i n i t i a t i o n o f the ea r ly Tertiary volcanic arc. A t near ly the sjine t i m e as the depos i t i on of the Chignik an3 Shumagin Format ions , the Hoodoo Formation was a l s o being depos i ted. The Hoodoo Formation, apparen t ly a d i s t a l turbidite, represents a major transgression. I t may be a shelf f a c i e s lying inboard of the Shumagin and generally outboard of the Chignik. The location and type o f source te r rane fo r t h e Hoodoo i s uncerta in, as i s t h e d i r e c t i o n o f f l o w of the t u r b i d i t y currents. A provenance s tudy, sirnil i a r t o Mooref s (1973a) on the Shumagin, would be o f great v a l u e for the Hoodoo. Intrusion of the Kodiak-Shumagin P l u t o n i c Ser ies g r a n o d i o r i t e i n the Semidi I s lands du r i ng Early Paleocene t i m e may be re la ted t o z n a t e x i s i n the thick Shumagin f lysch (Hudson and others, 1977, p.170).

Dur ing Eocene time, a new vo l can i c arc was initiated, the Tolstoi-Moshik Arc (Figure 15a) . Paleornagnet i c d a t a o f Stone and Packer (1977) indicate t h a t the T o l s t o i was depos i ted essentially a t i t s present latitude. I n i t i a t i o n o f t h i s a rc p robab ly >/as coeval w i t h the end o f subduct ion t o t h e n o r t h i n t h e Bering Sea b a s i n (Hopkins and Silberman, 1978) and t he complete t r a n s f e r o f subduct ion t o t h e s o u t h t o a p r o t o - A l e u t i an trench, located in probably the same relative position as the present A l o u t i an Trench. B a j a Alaska-Wrangel l i a was no longer be ing ra f ted w i t h the Kula or Pacific p la tes , and other than read jus tments along v a r i o u s f a u l t s , t h i s block was in position on t h e North American Pla te . Volcanism ;,?as probably i n i t i a t e d in the f a r western Aleutian Arc ( E a r l y Series, Marlow and others, 1973); t h i s i s discussed more fully later. Volcanism i n the fa r m s t e r n A l e u t i a n Arc d u r i n g Eocene t i m e may indicate t h a t t h e arc was straighter or t h e convergence direction between the North American P l a t e and the Kula p l a t e was more northerly or northeasterly than t h e present convergence between the North American and the P a c i f i c pla tes . T h i s p a r t of the Aleutian Arc i s n o t presently active, poss ib l y due to a large transform c o ~ p o n e n t t o t h e subduction o f the P a c f i c Plate in the

(I, m

western p a r t o f t h e Aleutian Arc. The apparently more e a s t - w e s t e l onga t i on o f the To ls to i -Mesh ik Arc (F igure 11) compared t o l a t e r structural features agrees w i t h this in te rp re ta t ion o f a Inore norther ly convergence d i r e c t i o n d u r i n g Eocene time. The presence o f plutons of t h i s age (45-25 m.y.) in many places i n southern Alaska m a y i n d i c a t e o t h e r ex tens ions o f this arc fron the Chignik and Sutw ik I s l and r e g i o n (Turner and o the rs , 1975; Wilson and others, 1979; Dadisman, 1980). As yet, there are no Paleocetle age de termina t ions on r ocks a s s o c i a t e d w i t h the To ls to i - r fesh i k a rc in t h e Chignik and Sutwik Island area. The 85-59 m.y. e v e n t distinguished by Reed and Lanphere (1973) i n t h e A l a s k a - A l e u t i a n Range Batholith i s n o t assoc ia ted w i t h the Tolstoi-bleshik arc; the younger 37 t o 26 m.y. :diddle T e r t i a r y even t i s . The Tolstoi Formation was deposited on a n u p l i f t e d terrane, relative t o the Hoodoo, and i t i s channeled i n t o t h e Hoodoo i n some areas (R.L. Detterman, o ra l communication, 1978). There i s no s t r a t i g r a p h i c cont inui ty between t h e L a t e Cretaceous Chign ik and Hoodoo Format ions and the younger Tolstoi Formation. No plutons or volcanic rocks i n the Chignik and Sutw ik Is land reg ion or on t h e e n t i r e A l a s k a Peninsula a r e known t o have ages i n the range between 59 and 48 m.y.

Radiometric ages i n d i c a t e t h a t t h e Tolsto i - l4esh- i k arc essenti a1 ly ceased a c t i v i t y i n the Oligocene; t h i s may correspond t o t h e beginning o f subduc t ion o f t h e K u l a - P a c i f i c Ridge i n t h i s area (Delong and others , 1578) as i t mig ra t ed northward. Delong and o the rs (1978) proposed a model f o r subduct ion o f t h e K u l a - P a c i f i c Ridge t h a t would require a temporary cessation o f vo lcan ism a t the time o f a c t u a l ridge subduction. Assuming the model i s correct, t h e p r e d i c t e d c e s s a t i o n of volcanism should occur sometime near t h e e n d o f t h e Ol igocene i n the Chignik and S u t w i k I s land region. The t i m i n g i s somewhat dependen t on the o r i e n t a t i o n o f the ridge and its rate o f m i g r a t i o n along t h e trench. Also, segmentation o f the ridge w i l l exert another control on the t i m i n g o f ridge subduction.

I n M idd le t o Late Miocene t i m e , deposition of t he Bear Lake Format ion began, probably by reworking o f pre-existing sediments ( R . L . Detterman, oral communication, 1979). I t s 1 i t h o l o g y i n d i c a t e s a quiescence o f volcanism and a m a r i n e trangression i n the region. The present ou tc rop d i s t r i b u t i o n o f t h e Bear Lake Formation i s primarily i n t h e southwest por t ions o f the Chign ik quadrangle. The fo rmat ion i s encountered i n d r i l l ho les as f a r n o r t h as Becharof Lake (Figure 1); therefore t h i s transgression must have covered a lar,ge p o r t i o n o f the A laska Peninsula. A t t h e end o f the Miocene, major volcanism and plutonism began a g a i n in the

- study area , a long t h e t r e n d o f t h e D e v i l s Batholith, Nakchamik I s l a n d and Chiginagak Bay. This vo lcan ism has apparen t l y m i g r a t e d q u i c k l y inland t o i t s present position along a l i n e from M t . Ven iaminof t o Aniakchak, as subduction has cont inued a long t h e A l e u t i a n TI-ench. Migration i n l a n d o f t h e l ocus o f vo l can i sm cou ld be due t o acceleration i n the ra te o f subduct ion and an associated s h a l l o w i n g i n the angle o f subduction. However, accord ing t o G.B. Dalrynple ( o r a l conmunication, 1979), work i n t h e Hawai ian I s l a n d s i n d i c a t e s a relatively c o n s t a n t r a t e o f movement ( o r o f v o l c a n i c propaga t ion o f the Hawaiian Islands) o f t h e P a c i f i c P l a t e ovsr the l a s t 60 m.y., w i t h p o s s i b i l i t y o f a slight change i n the 28 t o 20 m.y. age range. Therefore i t i s unlikely t h a t t h e Fliocene t o Recent migration of volcanism i n the Chign ik and Sutwik 1s:and r e g i o n could be d u e t o a change i n the r a t e o f subduct ion. Another pos s ib i l i t y i s an i nc rease i n the d e p t h o f p a r t i a l melting w i t h time. Mapping and geochemical d a t a are as yet i n s u f f i c i e n t t o apply petrogenet ic models t o rocks o f t h e area t o

t e s t t h i s hypothesis. I n Pliocene time, u p l i f t , thrusting t o the s o u t h ~ e s t , and minor

i n t r u s i o n occurred along the eastern s i d e o f t h e Peninsula, while d e p o s i t i o n of the Milky R i v e r Formation occurred on t h e wes te rn s i d e (F igu re 15b). Thrusting f rom the northwest occurred along t h e core o f t he Chignik a n t i c l i n e and continued u n t i l la ter than 3.6 m.y., as d a t e d by the northwest d i p p i n g thrust a t the Bee Creek prospect.

Speculations - Southern A l ~ s k a Tectonics

I n t roduc t i on Specu la t ing on a model f o r the t e c t o n i c development o f southern

Alaska, by extrapolat ion o f t he syn thes is f r o m t h e Chignik and Sutwik Is land reg ion, one is struck by the sheer number and d i v e r s i t y o f small t e c t o n i c blocks or terranes involved. Jones and Silberling (1979) on s t r a t i g r a p h i c g rounds have d i v i d e d southern Alaska , exclusive o f sou theas tern Alaska, i n t o a t l ea s t nineteen d i f f e r en t t e c t o n o s t r a t i g r a p h i c terranes. F i g u r e 16 i s m o d i f i e d f r o m Jones and Silberling (1979) and i n a general sense shoui~s some o f t h e terranes they have de f i ned for A l a s k a , exclusive of southeastern Alaska and the Seward Peninsula. Many o f these terranes are not y e t described i n t h e l i t e r a tu re ; however, the Pen insu la r t e r r a ~ e as defined by Jones and Silberling i s ess~ntially the same as Baja Alaska. Scutheastern Alaska has been d i v i d e d i n t o s i x d i s t i n c t terranes a lone (Berg and o t h e r s , 1978) and as mapping proceeds i n other p a r t s o f southern Alaska, other terranes are sure t o be recognized. The f i r s t terrane in southern Alaska t o be d e s c r i b e d i n detail was Wrangellia (Jones and others, 1977). (Ba ja Alaska predates t h i s in the l i t e r a tu re (Packer and Stone, 1974) b u t h a s not been geological ly described or de l im i t ed . ) Some models f o r t h e t e c t o n i c development o f southern Alaska assume t h a t these torranes a re "rnicroplates" ra f t ed i n and welded t o t h e A l a s k a margin (i .e. Jonss and others, 1977). Other models suggest t h a t these are 'rslivers" o f continental material ca r r i ed along t h e western margin o f nor th (and s o u t h ? ) America (Stone, 1977) . None o f the p u b l i s h e d models i s f u l l y satisfactory for one or more s i g n i f i c a n t reasons.

Speculative Model The above geologic history o f t h e Ch ign i k r e g i o n places a number o f

constraints on the t e c t o n i c history o f southern Alaska . Additionally, the apparent island a r c n a t u r e o f some o f the T r i a s s i c rocks involved i n Mrangellia and Baja Alaska; the southern hemisphere paleopoles suggested by Stone (1979) and Hillhouse (1977) fo r Ba ja Alaska and the T r i a s s i c N i k o l a i Greenstone r e s p e c t i v e l y ; and t h e lack o f any obvious con t i nen ta l c o l l i s i o n zones, except possibly the v o l c a n i c and sedimentary unit (KJvs) of Hoare and Coonrad (1978a), suggest t o t h i s au thor t h a t a model i n v o l v i n g both accreted tectonic blocks and a shifting p l a t e edge (Stone, 1977, p.30) i s supported by available d a t a and best accomodates t h e s e c o n s t r a i n t s .

Reflected i n t h i s model i s t h a t no c o l l i s i o n boundary w i t h main land Alaska h a s been mapped n o r t h of the Baja Alaska-Wrangellia b lock , yet p a l e o m a g w t i c ev idence c l ea r l y i n d i c a t e s la rge s c a l e t r a n s p o r t . However, t he dismembered Jurassic ophi 01 i t e o f southwestern A1 aska (Goodnews t e r rane ) may be a small block o f oceanic crust c a u g h t i n a f a u l t s l i ce or sliver as other blocks ro ta ted and translated nort! iward. The l a c k o f a major collision boundary would seem t o constrain m o t i o n t o translation along the North American cont inental margin unless t h e KJvs unit (Hoare and Coonrad, 1978a) o f southwestern Alaska i s the remnant of a collision te r rane. A modern example o f such t r a n s l a t i o n i s B a j a C a l i f o r n i a (source o f the name, Baja A l a s k a ) which has at times been on the Pacific and the North American p la te s and i s translating northward along the San Andreas and other f a u l t s . F i g u r e 17 i s a schematic ske tch of a crude, very generalized model a long these lines, as envisaged by t h i s writer. I t i n d i c a t e s the general positions o f the Yukon Crys ta l l ine Terrane,

- -. .- - - - - - .- .

Figure 1 7 S c h e m a t i c d i a g r a m - Generalized S o u t h e r n A l a s k a r n 0 . 1 ~ 1

' A 8 Early Cretaceous La te Cr~taceous

C

Arrow. ; o.lic#w I r ~ i i l t l v ~ ~

moilon I r r ~ l w p r n Or:e,~rt~r. .ind Nor1 t i A i h v r ~ r ~ ~ ~ ~ t ' l a t t ~ f :

C Focene

Kula Pl+r 1 1,

4 . Kuta-Paci f ic R l ;dq~

1 -- - >LC/ - L I1,ici F i c P l a t e - L

w

I-EGEIdD

Terranes ' s a j a Alaska

63 Urangel l ia

' Yolcanlc and sedimentary uni t ( ~ 4 1 ~ 5 )

l=l Yukon Crystal 1 ine T e r r a n ~

I[IIIl thuq.ch

Orca and Ghost Rocks

K U I L O X W ~ ~ Cmup

/ Subduction zone

- Spreading center

Wrangellia, Baja Alaska and other terranes f rom Early Cretaceous to Holocene t ime . I n the f o l l o w i n g paragraphs my speculat ive model f o r t h e t ec ton ic development o f southern Alaska, consistent wi th t h e history o f the Chignik region, i s b r i e f l y descr ibed. A key assumption o f my model, based on d i scuss ions with Beld Csejtey, and on the continuity o f the Alaska-Aleut! an Range b a t h o l i t h across the two terranes, i s t h a t B a j a Alaska and Wrangellia were ad jacen t i n Middle Ju rass i c t i m e , th is larger terrane i s hzrein called the Southern Alaska b l o c k . Numerous other terranes o f southern Alaska (Jones and S i lberling, 1979) Idere accreted t o t h i s block a t v a r i o u s times. Not a l l w i l l be discussed here, as they a re beyond t h e scope o f this thes is , and in any case t h e i n f o rma t i on i s often n o t a v a i l a b l e .

(1) Midd le Jurassic The Baja Alaska and Wrangel l i a terranes were joined a t a latitude near

t h e paleoequator. This j o i n i n g was d u e t o subduction, possibly f r o m the west or south, under the Southern Alaska block and the formation o f t h e Jurassic port ion o f the A l a s k a - A l e u t i a n Range batholith. The block may have undergone some northward m i g r a t i o n during Middle Jurassic t ime.

(2) Late Jurassic-Middle Cretaceous The Southern Alaska block underaent m a j o r nor thward m i g r a t i o n a long

the North American p l a t e edge (Figure 17a) i n a manner similiar t o Baja California. Apparent ly , no subduction was i nvo l ved ; like Baja C a l i f o r n i a , the Southern A l a s k a block was p a r t o f an oceanic ( ~ u l a ? ) p l a t e .

(3) Middle Cretaceous The depositon o f t h e Chugach t e r r ane flysch and the 85-59 m.y.

p l u t o n i c event i n the A l a s k a - A l e u t i a n Range batholith may indicate t h a t some convergence occurred between the oceanic plate and the Southern Alaska block. The melange f a c i e s of the Chugach terrane was accreted t o the Southern Alaska b l o c k during this time, as p a r t of t h i s subduct ion event.

(4) Late Cretaceous The Southern Alaska block cont inued northward w i t h rotation ( F i g u r e

17b). Some underthrusting of the Sou the rn A1 aska block ( i n c l u d i n g other accreted terranes) under the North American pla te probably occurred on the northeastern (now northern and northwestern) or leading edge. Apparently, l i t t l e or no subduct ion took place under t h e northern part ( B a j a A l a s k a ) o f t h e Southern A l a s k a block. Subduction occurred to t h e far north along the eastern ancestral Bering Sea continental margin and nor th-south(?) transform mot ion occurred a long the southeastern ancestral Bering Sea margin.

( 5 ) Early Ter t ia ry (Paleocene-Eocene) The Southern Alaska block overrode t he Ber ing Sea traasform f a u l t

( F i g u r e 17c) caus ing the step-out o f subduct ion f r o m t h e northern Ber ing Sea margin to an "ancestfalM Aleutian trench. Th is s tep -ou t o f subduction caused entrapment of a p m t i o n o f the Kula p l a t e beh ind t h e newly forming . A leu t ian Arc.

The Paleogene subduct ion complex (Orca Group and G q c s t Rocks) o f southern Alaska began t o f o rm du r i ng Paleocene t i m e . I n add i t ion , t he volcanism of the Tolstoi-Meshik Arc was i n i t i a t e d d u r i n g Eocene t ime.

( 6 ) Midd le T e r t i a r y t o Recent (Oligocene-Holocene) The Paleogene subduction complex was accreted t o t h e Southern A laska

block dur ing subduction of the Kula-Pacific Ridge a t the proto-Aleutian trench. A f t e r subduct ion of the K u l a - P a c i f i c Ridge, the convergence d i r e c t i o n at the Aleutian Trench had a more westerly component (F igure

1 7 d ) . Volcanism a long t h e A l e u t i a n Arc began again, s t a r t i n g t h e p r e s e n t c y c l e of a c t i v i t y .

E x p l a n a t i o n A b r i e f e x p l a n a t i o n o f some aspects of the speculative model f o l l o ~ ~ s .

Each e x p l a n a t i o n i s numbered t o correlate wi th t h e same t i m e interval descr ibed i n the f o r e g o i n g sec t i on .

(1) Pre-Jurassic rocks i n the B a j a Alaska t e r r a n e i n c l ude T r i a s s i c carbonates and v o l c a n i c rocks and Permian c a r b o n a t e s a t Puale bay (Burk, 1965, p.19) and s i r n i l iar r o c k s f u r t h e r n o r t h a long Cook I n l e t (Detterman and Reed, 1980). In the Naknek quadrangle n o r t h o f Becharof Lake (F i gu re 1) l imes tone of probable S i l u r i a n o r Devonian age have been r e p o r t e d by Detterman and o t h e r s (1980a).

Stone's (1979) and Hillhouse's (1977) Triassic southern hemisphere paleopoles for B a j a Alaska and Wrangell i a may require a m i g r a t i o n o f these terranes from t h e South American plate across t o the North American p la t e . Since the two American plates may not have been connected u n t i l a f t e r the Tr i a s s i c , t h i s would r e q u i r e 8 a j a Alaska and Wrangellia t o be r a f t ed on t h e K u l a ( o r P a c i f i c or Faral lon) p l a t e d u r i n g t h i s per iod , u n t i l they sncountered t he North American p l a t e . Recent mapping i n A l a s k a and wes te rn Canada has indicated other s l i v e r s o r blocks t h a t possibly have been i nc luded i n northward translation a l o n g the western N o r t h American c o n t i n e n t a l margin. These other terranes (not shown on F i g u r e 16 or 17) include the Alexander Ten-ane o f southeastern Alaska, the Taku, Tracy A r m (or Coast P l u t o n i c Complex), Stikine, and Cache Creek terranes (Berg and o thers , 1978). Further i n l z n d i s the Yukon Crys ta l l ine Terrane (Templeman-Kl u i t , 1976) which includes the Yukon-Tanana terrane o f Jones and S i l berl i ng (1979). In southyestern A l a s k a , the Kanektok terrane (Hoare and Coonrad, 1978a; Kilbuck terrane of Jones and Silberling, 1979) and t h e dismembered o p h i o l i t e o f the Cape Nevenharn a rea (Hoare and Coonrad, 1978b; Goodnews t e r r a n e o f Jones and Silberling, 1979) a re two o f possibly many originally(?) d i s t i n c t terranes.

( 2 ) The paleomsgnetic d a t a for t h e A l a s k a Pen insu la suggests large displacement o f the B a j a A l a s k a block from the south an3 r o t a t i o n , yet the Mesozoic s t ra t igraphy requires a c o n t i n e n t a l source t e r r a n e d u r i n g much o f t h i s migra t ion , in t he opinion o f t h i s writer. Possibly t h e Baja Alaska-Wrangellia block was large enough t o a c t as i t s own source since the Jurassic, but one can a l t e rna t ive ly assume t h a t t h i s block maintained contact with the North American p l a t e t h r o u g h much i t s m i g r a t i o n nor thward.

Two u n i t s t h a t he lp t o d e f i n e t h e m o t i o n of t h e Southern Alaska block are the v o l c a n i c and sedimentary u n i t (KJvs) and the Kuskokwim Group o f Hoare and Coonrad (1978a) sho;qn on Figure 16. These are briefly described be1 ow.

. The KJvs u n i t o f southwestern Alaska desc r i bed by Hoare and Coonrad (1978a) is a thick and widespread marine volcanic and sedimentary rock u n i t r a n g i n g i n age f r o m Middle J u r a s s i c t o Early Cretaceous. Accord ing t o them, i t i n c l u d e s v o l c a n i c r o c k s ranging " i n compos i t ion from m a f i c p i l l o w basa l t s t o more abundant andesitic and t r a c h y t i c flows, t u f f s , and b r e c c i a s . In te rbedded w i t h the volcanic r o c k s a r e t h i c k sect ions o f t u f faceous s i l t s tone , tuffaceous cher ts , and massive o r th in-bedded argillite."

The Kuskokwirn Group (Hoare and Coonrad, 1978a, Cady and o thers , 1955, p. 35-47) i s a t h i c k sedimentary u n i t o f i n f e r r e d Early Cretaceous ( A l b i a n )

and ea r ly La te Cretaceous age. The lithology o f the u n i t includes a basal conglomerate up t o 1500m th ick overlain by micaceous shales and siltstones. Overlying this i s a thick s e c t i o n o f interbedded graywacke, siltstone, and shale. In one area, Hoare and Coonrad (1978a) report abundant plant material , t h i n coal beds, and impure limestones which they suggested may be nonmarine; otherwise, the Kuskokwim i s considered marine. The Kuskokwim Group i s extensive in southwestern Alaska; unfortunately, other t h a n the reconnaissance work o f Cady and others (1955) and Hoare and Coonrad (1978a), l i t t l e has been published.

( 3 ) Very p r o m i n e n t on any would-be t e c t o n i c map o f Alaska would have t o be the e x t e n s i v e accretionary f lysch terrane o f southern Alaska. Extending con t inuous ly f r o m Baranof I s l a n d in southeastern Alaska t o the Sanak Islands o f f the southwestern A l a s k a Peninsula, i t i s possibly the largest such terrane i n the wor ld . This te r rane , L a t e Cretaceous i n age, and including the Shurnagin and K o d i a k Formations, t h e Valdez Group, p a r t of the Yakuta t Group and t h e Sitka Graywacke (Plafker and others, 1977) corresponds to the younger Chugach terrane o f Berg and others (1972, 1978) (see Figure 16). The Chugach terrane i s separated by the Border Ranges F a u l t (MacKevet t and Plafker, 1974) on i t s inboard m a r g i n from an assoc ia ted melange. The Border Ranges f a u l t i s mapped i n the Cook Inlet region and on K o d i a k I s l a n d ; i t i s approximately t h e boundary between the Chugach and Peninsular t e r r a n e s shown on Figure 16. Jones and Silberling (1979) inc lude the melange in the Peninsular terrane, though Berg and others (1972) considered i t part o f the Chugach. As o f yet, no source terrain f o r the largely volcaniclastic accretionary flysch o f the Chugach terrane i s d e f i n i t e l y k n o ~ n . In discussing the Late Cretaceous history o f the Chignik region ( p . 86), I s ~ g g e s t e d t h a t the volcanic debris was derived f r o m an i n fe r red Shurnagi n Arc. Pal eocurrent a n a l y s i s (Moore, 1973a) agrees with this hypothesis; however, as mentioned above, a source terrane i s still d i f f i c u l t to pinpoint .

The melange associated with t h e Chugach Terrane includes blocks o f la te Paleozoic t o Early Cretaceous age i n a matrix o f middle t o upper Cretaceous pe l i t e o r t u f f aceous pel i t e (PI afker and others , 1977). This writer suggests t h a t a possible equ iva len t o f the melange i s found i n southwestern A 1 aska as t h e vo lcan ic and sedimentary u n i t (KJvs) o f Hoare and Coonrad (1978a).

( 4 ) Recent work by M e a n (1979) shows t h a t the extens ion of t h e Chugach terrane along the Bering Sea shelf margin as proposed by other au thors (Patton and others, 1976; Hopkins and Si lberman, 1978) d i d n o t occur; he suggested t h a t the s h e l f margin was probably a Cretaceous* transform f a u l t .

(5) Rather weak evidence for a l a t e Cretaceous age o f parts o f t h e A leu t i an Ridge i s reported i n Scholl and others (1970, p.3589). Marlow and others (1973) postu la ted a middle to late Eocene age for the origin o f the A leu t ian Ridge. I n light o f the translational and r o t a t i o n a l movement of the numerous blocks o f southern Alaska, I believe there was a gradual ro ta t ion of the present eastern Bering Sea continental margin until late Cretaceous or early Tertiary time. This may have led to an instability in the e x i s t i n g subduction regime and the postu la ted Bering Sea transform boundary (~igure 17b), which caused the p l a t e margin t o jump t o t he then-forming A l e u t i a n trench and i s l a n d arc f rom t h e northern Ber ing Sea edge. This step-out of t h e p l a t e margin would result in entrapment o f a p o r t i o n of oceanic crust (F igure 17c). This entrapment h a s occurred and

has been i n t e r p r e t e d as a portion of the Kula Plate (Cooper and others , 1976, Scholl and others, 1975, Patton and others, 1976).

Subduction or "leakytt t r ans fo rm mot ion may have cont inued t o occur a l o n g the eastern Ber ing Sea marg in u n t i l e a r l y Ter t ia ry t i m e b u t by Eocene t i m e , mo t i on was taken up a t the A l e u t i a n t rench. Eocene i n i t i a t i o n o f volcanism i n t h e A l e u t i a n Is1 ands therefore corresponds L E ~ t h Eocene i n i t i a t i o n o f volcanism i n the Tolstoi-Meshik arc i n t h e Ch ign i k and Sutwik I s l a n d r e g i o n . A Paleugene sgbduction complex has been descr ibed (Plafker and others, 1977, p.B42) a l o n g t he northernmost southern A l a s k a coas t ( P r i n c e W i l l i a m Sound) and along the eastern shore o f Kodiak Island. The complex i s represented by the Orca Group i n Prince William Sound and by the Ghost Rocks Format ion on Kodiak Is land (f4oore, 1969); these have been accreted ou tboa rd of t h e Cretaceous rnelangeAand f l ysch . Tysda l a n d Case (1979) repor ted t h a t portions of the Orca Group are sirniliar t o those t o be expected for a "fossil" spreading cen te r o f Paleocene t o ear ly Eocene(?) age. They mapped a pillow b a s a l t and sheeted d i k e complex i n t h e Seward and B l y i n g Sound quadrangles o f P r i n c e W i l l i a m Sound, and interpreted i t as fo rming near t h e junction of the con t inen ta l margin w i t h a "leaky" transform f a u l t (Tysdal and others, 1977). They a l s o suggested t h e p o s s i b i l i t y t h a t these rocks may represent remnants o f the K u l a - P a c i f i c Ridge; hoiqever, they p o i n t o u t a number o f d i f f i c u l t i e s w i t h such an interpretation. T h e i r i n t e r p r e t a t i o n sugges ts t h a t t h e Kula-Pacific Ridge was probably still a c t i v e and n o t near t h e continental margin during Paleocene t i m e , as was suggested by Byrne (1979).

Byrne (1979) reconstructed the h i s t o r y o f t h e K u l a - P a c i f i c Ridge u s i n g sea-floor magnet ic anomaly pa t te rns and speculated t h a t the K u l a - P a c i f i c Ridge had ceased a c t i v i t y d u r i n g Paleocene t ime. He a l s o suggested t h a t the near proximity o f t he Kula-Farallon Ridge t o the Aleutian Trench at that time i s responsible f o r the anomalous near-trench plutonism o f t h e . Kodiak-Shurnagin P lu ton ic Series. Byrne's reconstruction would allow l i t t l e or no subduct ion of oceanic crust a t t h e A leu t i an Trench s ince Paleocene t i m e . Th i s i s because h i s reconstruction produces virtually the presen t sea- f l oo r magnet ic anomaly p a t t e r n in Paleocene time. This i s i n serious disagreement wi th t h e presently known geo log ic history. Assumi ng the present subduction rate i s 6 crn./y., over the l a s t 10 my., the amount o f c r u s t subducted i s greater than Byrne's reconstruction would allow. A d d i t i o n o f the amount o f crust subducted dur ing t h e l i f e o f the To ls to i -Mesh ik Arc on l y makes the fit o f the presently k n o ~ n facts to Byrne's model worse.

Byrneish(1979) suggest ion t h a t the t u rb id i t e s o f the A l e u t i a n Abyssal P l a i n were derived f r o m A l a s k a across the defunct K u l a - P a c i f i c Ridge i s apparently based on a misinterpretation o f a paper by S t e w a r t (1976) who suggested sou theas te rn A laska and western British Columbia as a source for the sediments o f the Aleutian Abyssal P l a i n ; this would require t r anspor t across the Pacific-Farallon Ridge; not t h e Ku la -Pac i f i c Ridge.

( 6 ) Subduction o f the Kula-Pacific Ridge i n Ol igocene t o Miocene time (Delong and others, 1978) was the f i n a l major t e c t o n i c episode, leading t o the present t e c t o n i c s e t t i n g . The r e i n i t i a t i o n of volcanism i n l a t e Miocene time t h a t cont inues t o the present suggests t h a t t he current c o n f i g u r a t i o n o f under th rus t ing o f t he P a c i f i c P l a t e a t t h e Aleut ian Trench and the general distribution and placement o f tectonic b locks i n sou the rn Alaska was established by the end of Miocene time.

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Appendix 1 - H y d r o f l u o r i c Acid Leaching Procedure

\* /hole rock sarnples were prepared f o r E r g o n e x t r a c t i o n by f i r s t t rea t ing them w i t h a hydroflvoric a c i d l e a c i ~ i n g procedure t o rernovz secondary phases and glass. The procedure is as follows:

1. Place 50-i03 gins o f crushed 50-i00 or 100-200 mesh sainple in a p l a s t i c beaker znd det 4 , v i t h distilled wzter. Excess ivaier s!lould be &canted o f f ,

2. Add 200-300 rnl o f 5 t o 6% hgdrofluoric a c i d t o the sample and a l l 0 2 1 i t t o react for 1 t o 1.5 minutes ~ 5 t h cvns tan t stirring. A t the end o f t h e allotted time the a c i d i s decanted o f f .

3 . W a s h t l re samplz about 3 times, deca17ting o f f water and l i g h t suspfinded mater ia l with each cashing.

4. T r a n s f e r the sample t o a gli:ss bbxker by washing i t i n i d i t h distilled ~{iater and d e c ~ n t excess watzr ,

5, Add 200-300 m l o f 14% nitric ac id and al low t o r eac t 30 minutes wi th occas iona l stirring.

6 . Decant a c i d and i ~ a s h wi th s z t e r until fines are r e m ~ v e d . 7. Suspend Lezker w i t h the sample i n xa to r in the b a t h o f an

ul t rasonic vibrator. Vibrate ssraple no more than 2 m i n u t e s and wash t o rmove f i nes,

8. Dry sample and split f o r potassium and argon analysis.

Apgendix 2 - Argon E x t r a c t i o n Procedure In preparat ion fo r argon extraction, a s p l i t o f the sample is weighed

and loaded i n t o a 0.03 mm molybdenum inner crucjble. This i s then loaded into a degzssed (used) outer crucible o f 0.05 mm molybd2num and placed i n the sample holder on the a rgon ex t rac t ion l i n e (Figure A-1 ) . T h i s l i n e i s essent i a 1 l y t h a t descri bod by Dal rymple and Lanphere (1963), thougll v i i t h some morlificztions. A sample t a k e - o f f and 3 8 ~ r t racer are blown on t o t i le l i n e , the simple b o t t l e sealed (copper gaske t lnetal t o ae ta l seal ) and the l i n e evacuated w i t h t i l e r n i l g i ~ i n g pump. A f t e r 12ak t c s t i n g , t h e mercury diffusion pump i s s t a r t e d , t h e s s s o c i a t r d l i q u i d nitrogen (LNg) cold t r a p f i l l e d and t h e bake -ou t oven s t a r t e d . The bake-ou t t a k e s overnight, a t 30U0 C, and i s d o n 2 i n order t o d r i v e o f f atmuspi~eric argon and o the r gzses a d s o r b ta on t h e sainple a n d t h e e x t r a c t i o n line and t h e r e f o r e r n i ~ ~ i m i z e atmospkric c o a t a n i n a t i o n in the gas t o be collected. i4itl1 older samples and good c o n d i t i o n s , y i e l d s o f 90% r s d i o o e n i c argon a r e corninon. Tile saiaples i n t h i s s t u d y generally yieldd I t o 43% radiogenic argon; less than 5% i s normally cons ide red analytical I y a n a c c q t a b l e .

The n e x t morning the l i n e pressure i s c !~ecked w i th the i o n gauge. I f the i n i t i a l pressure r e a d i n g i s 5.0 x c-5 torr o r less the extraction i s continued. A t t h i s p o i n t t h e i o n gauge is degassed u s i n g the internal filament a:ld t h e n a f i n a l or outgas pr2ssure i s obtained. T h i s must be lzss than 5.0 x e-7 torr and frequzntly i s less t h a n 1.0 x e-7 t o r r . I f an acceptable pressure i s obtained, the Cu-CuO fu r l lace i s outgassed by pumping on i t while i t hea ts t o 525 t o 60Q°C. T h i s furnace con ta ins pellets of metallic copper 2nd copper o x i d e and i t s purpose i s t o o x i d i z e t he gases t h a t a r e evolved d u r i n g the e x t r a c t i o n , p a r t i c u l a r i l y hydrogen. A t the end o f th i s per iod , the bottle v a l v e i s closed ( C u - c ~ O furnace s t i l l on) and a LNg t r a p i s placed an tlre charcoal finger, a glass t u b e c o n t a i n i n g a c t i v a t e d charcoa l . The r a d i o frequency ( R F ) generator and c o i 1 are energ ized and ~ h i l e t he b o t t l e i s f a n cooled on the outs ide , the molybdenua crucibles and sample are heated by i nduc t i on . Heat ing i s carr ied o u t i n s tages and gzr~era l l y takes between one h a l f h o u r and one and one half hours t o come t o f u s i o n tetnperature, which is as h i g h as an estimated 1600°C based on optical pyrometry. Q~artz-serisite whole rock and volcanic whole rock samples are prone t o "puffing" i f hea ted t o o f a s t and therefore these take the longest t i m e t o b r i n g t o temperature. "Pu f f ing" i s t he result o f the sarnpls exploding o u t o f t i l e crucible rluz t o I -ap id release o f adsorbed or absorbed ?later. Once o u t o f the crucible the earnpi3 can" be fused and t l ~ e r e f o i - e tl12 extraction i a failare. While the sainple is being b r o u g h t

3 8 t o f u s i o n t e n p e r a t a r e , the Ar t r a c e r i s introduced t o the l ine t h r o u g h t h e brcak-seal. This break-seal i s a t h i n l o ~ p o f glass t h a t can he broken w i t h a steel w i g h t manipulated w i t h an e x t e r n a l magnet. Once f u s i o n temperature i s reached, the sample i s equilibrated 10 t o 15 minutes and then over a p e r i o d o f about f i v e m inu tes the RF i s t u r n e d down and o f f . During t h e e n t i r e f u s i o n process, t h e LNg t r a p i s maintained f u l l t o m in im ize pressure i n t h e line, which i nc reases the e f f i c i e n c y o f the induction hsz t ing and prevents readsorption o f the rehased gases on t h e cooling snmple, A f t e r t h e RF i s o f f and t h e sa;nple h a s cooled , t h e LNg t r a p i s reinoved, t h 2 charcoal warms and tile gases are released. These gases react :r,ith Cu-CuO furnace , and tile i eo l i t e t r a p absorbs and some carbon d i o x i d z . To t e s t .For canpl~tenass o f t h i s "cleanup" a t e s l a c o i l i s used. I n i t i a l l y a b l u e g l o w due t o t;ater vapor and carbon d i o x i d e i s apparent ; upon completion o f the react ions a red t o pir.': glow (primarily

from nitrogen) i s obtained. In many o f the samples used i n t h i s study, so much carbon d iox ide was generated i n the fusion t h a t the z e o l i t e trap was unable t o remove it a l l . If a sample doesn't show evidence o f cleaning up after about one h a l f hour, LNg i s placed on the carbon d iox ide t r a p t o freeze o u t t h i s gas and water vapor. This trap is an empty tube o f glass, unl ike the charcoal finger. Therefore the argon doesn't freeze out along w i t h the other gases since the LNg i s n o t o f low enough temperature t o freeze argon. While the cleanup takes place, the Ti0 furnace i s brought t o te rn~era tu re and degassed a t 825 t o 9006C. 2

When the sample cleanup i s complete, the pump va lve i s closed, LNg i s placed on the sample t a k e o f f and the b o t t l e v a l v e i s opened to cryogenically t ransfer the sample t o the "pump s ideH o f t h e e x t r a c t i o n l i n e . Once t h e transfer begins, the mercury diffusion pump can be turned o f f . If the C02 t r a p had t o be used, the LNg i s maintained on i t during the t ransfer t o prevent the simultaneous t ransfer o f the frozen-out carbon dioxide and water. The transfer procedure takes one h a l f hour; upon completion the b o t t l e valve i s closed and t he LNg trap can be removed from the sample t akeo f f . The Cu-CuO furnace can also be turned o f f and the LNg removed from the carbon dioxide trap. The remaining gases on the pump s ide of the l i n e are allowed t o react w i t h the T i 0 2 furnace and after about 20 minutes the furnace i s turned o f f and allowed t o cool slowly. Most o f t he "getter ing" ac t ion of the furnace takes place as i t cools; when t h e cooling furnace reaches 100DC a fan can be used t o complete the cooling t o room temperature. F i f t een minutes be fore sample freeze-out LNg may be placed on the hydrocarbon t r a p , a g l a s s tube identical t o the CO2 trap; otherwise LNg i s immediately placed on the sample t a k e o f f and the sample i s f r o z e n out . After a h a l f hour, the capillary tube j o i n i n g the sample takeof f t o t h e -traction l i n e i s collapsed with a t o r c h and t he sample takeof f removed. LNg is maintained on the sample takeoff and the hydrocarbon t r a p , i f used, during the c u t o f f procedure. The e n t i r e argon sample i s now i n the sample takeof f and i s ready t o load on the mass spectrometer. The extraction l i n e i s opened and the cruc ib le checked t o be certain the sample has been fused; usually only a glass remains, though micas o f ten vaporize leav ing only a coa t ing on the c r u c i b l e .

Appendix 3 - Stratigraphy Shelikof Formation

The Shel ikof Formation i s only exposed in one small f a u l t s l i c e i n t h e Chignik and Sutwik Is land area; it i s best seen along Shelikof Strait west o f Kodiak Island (particularily Puale Bay) where it was originally described by Capps (1923, p . 97-101). I t i s of Cal lov ian (Middle Jurassic) ase and was considered by Capps (1923, p. 101) t o be general ly c o r r e l a t i v e w i t h the Chinitna Formation o f lower Cook Inlet. The formation i s usually d i v i d e d i n t o three lithologic members; a lower siltstone ranging from 250 t o 500 m (800 t o 1800 f t) thick; a middle sandstone ranging f r om near 300 t o over lOOOrn (1000 t o 3500 ft) th ick and an upper siltstone ranging from 275 to 450m (900 to 1500 f t ) th ick (Imlay, 1953, p. 49). The lowermost contact of the Shelikof i s probably conformable wi th the K i a l a g v i k Formati on, a Midd le (Bajoc i an) Jurassic unit ( Iml ay and Detterman, 1977, p .609) . The upper contact i s an unconformi ty with t he Naknek Formation ( ~ m l a y , 1975, p . 2 ) .

The lower siltstone member i s a gray sandy siltstone t ha t becomes harder and darker upsection. Sandy interbeds from a few centimeters t o 70m thick are common, and t h i n beds o f white to yellowish brown material, interpreted as volcanic ash are f a i r l y abundant and distinguish t h i s u n i t f rom the s i l t s t o n e s in the underlying Kia lagvik Formation (Irnlay, 1953, p. 49). Limestone concretions are abundant i n many places as are ammonite fossils (Imlay, 1953, p . 49).

The m i d d l e member o f the Shelikof i s dominantly a massive gray-green sandstone wi th interbeds of siltstone and lenses o f p lutonic pebble conglomerate (Imlay, 1953, p. 49). I t i s concre t ionary i n many places (Capps, 1923, p. 98). Imlay (1953, p. 57) mentions the occurrence o f some volcan ic cobbles i n some conglomerates.

The upper Sheli kof i s a massive b lack shale conta in ing some l imestone lenses and nodules. It i s poorly fossiliferous and may be sandy or calcareous in places (Capps, 1923, p. 9 7 ) .

Foss i l s qeneral ly occur throughout the Formation i n calcareous concretions and are generally ammonites though belemnites, pelecypods, brachipods, crustaceans and p l a n t fragments have been reported (Imlay, 1953, p. 58-59).

The fault s l i ce exposing the Shelikof Formation i n the Chignik area occurs north o f Chignik Bay at the head of Through Creek (Figure 2). The lower and upper members are exposed here, and cons fs t chiefly o f black shales with calcareous concretions and layers, and abundant ammonites.

The 800 to 2000m thickness of the Cal lovian (Shel ikof and equiva lents) rocks impl ies r a p i d subsidence o f the sea f l o o r and an abundant supply of sediment (~mlay, 1953, p.57). Th i s sediment supply Imlay (1953, p. 57) believes came f rom a mountainous source to the north and northwest of the present outcrop area, based on the southward th inning o f massive sandstones and a general decrease in grain s i z e southward. The nearshore environment of the sediments he believes i s shown by the pebbly character o f some beds and the local occurrence of granitic boulders 0.7m ( 2 ft) i n diameter. In the lower siltstone member, t h i n seams o f apparent volcanic ash imply deposition below wave base and indicate active volcanism a t t h e time; Imlay (1953, p . 57) also implied t h a t ash may be the source of the greenish color in some of the sandstones. He believes t h a t the deposi t ion o f the Shel ikof took place along a steep slope i n a major ocean basin, rather than i n a shallow arm of the sea such as Cook Inlet.

The faunal assemblages described by Iml ay (1953) led him t o i n t e r p r e t the depositional environment of the She1 i k o f Formati on as ra ther deep water, yet ouite nearshore based on the l i thologic assemblage. The sediments also show intraformational unconformit ies, as evidenced by scouring seen i n the lower siltstone and some unconformable boundaries between the lower siltstone and the middle sandstone members (Imlay, 1953, p . 5 7 ) .

Naknek Formation The Naknek Formation i s well exposed i n the study area, and i n general

along the Alaska Peninsula. The Naknek Formation (Series) was named in 1898 by Spurr (1900) after type occurrences on Naknek Lake and i n the Katmai area. The formation i s Upper Jurassic (earliest Oxfordian to early Tithonian) in age and i n general consists o f arkos ic sandstone, siltstone, condomerate and claystone (Imlay and Detteman, 1973, p. 10). The format ion i s known t o extend from t he Talkeetns Mountains i n south-central Alaska to the Port Moller area i n the southwestern Alaska Peninsula. According t o Imlay and Detteman (1973, p . 15), the Naknek Formation always unconformably overlies older u n i t s with rocks o f l a t e Callovian to ear ly Oxfordian age (150-165 m.y. ) missing below it.

Within the study area, the Naknek Formation has a number o f d i f f e r e n t lithologies. The lower Naknek is usually a pale green f ine-gra ined sandstone with t h i n laminae o f magnetite, some arkosic sandstone, and some coarse cobb 1 e conglomerate. The conglomerate is composed p r i m a r i ly o f . ran i t ic clasts w i t h minor metamorphics in an arkosic mat r i x , and i s lithologically similiar to many o f the conglomerates i n the younger Chignik Format i on. The arkosi c sandstones are composed primari l y of quar tz and feldspar; hornblende may make up 5 percent o f t h e rock with b i o t i t e sl ight ly less common (Burk, 1965, p . 36). Burk (1965, p. 36) reported that fe ldspar i s dominant i n the coarser-grained rocks, whereas quartz i s dominant in the f iner-grained (siltstone) rocks.

The upper Naknek formation tends t o be a dark olive green s i l t s t o n e , mudstone and sandstone wi th abundant Buchia pelecypods, belemnites and other m o l l u s k s (see Atwood, 1911, p . 33-38; Martin, 1926, p. 203-218; Irnlay and Detterman, 1973, p. 25). Plant fragments have also been repor ted (Spurr , 1900; Martin, 1926, p . 211; Burk, 1965, p . 30) though no major s t u d i e s o r collections have been made o f these. Thin calcareous layers are often found i n the upper Naknek; these layers often have large la tera l extent relative t o t h e i r thickness. I n add i t ion , horizons wi th abundant calcareous concretions are also present in t he Naknek as well as in the Chignik Formation.

The lithology o f the Naknek Formation i n general i nd i ca tes a nearshore environment o f moderate t o high energy. The conglomerates probably are i n d i c a t i v e of braided stream or high-energy beach environments, based on the large size and well-rounded nature of the cobbles. This, plus the presence o f f r e s h angular hornblende and b i o t i t e in the arkosic sands i s probably i n d i c a t i ve o f shor t transport and rapid deposition (see Detterman and Hartsock, 1966, p. 54). R.L. Detteman (wr i t ten communication, Jan., 979) i n t e r p r e t s the lower sand-rich p a r t of the Naknek as representing a

nearshore high-energy environment, whereas the upper silt-rich part probably represents s 1 i ght ly deeper water deposits . Burk (1965, p. 39) reported t h a t i n general the Naknek i s finer grained t o the southwest; poss ib ly due t o greater distance from the apparent source i n the present

day Alaska-Aleut ian Range b a t h o l i t h . Knappen (1929, p . 184) and Burk (1965, p . 39) note t ha t the sedimentary structures seem t o indicate a south-to-southeast t ranspor t d i r e c t i o n f o r the sediments o f t he Naknek .

Staniukovich Formation The Stan iukov ich Forrnati on i s found general ly i n the northern p o r t ions

o f the Chignik and Sutwik Island map area, and t o t he southwest of the map area at the type l o c a l i t y , S tan iukov ich Mountain. Atwood (1911, p. 38) f i r s t descr ibed the Stan iukov ich as a s e r i e s o f shales t h a t were l i t h o l o g i c a l l y d i s t i n c t f rom t h e unde r l y i ng Naknek Formation. Atwood considered the formation t o be o f Early Cretaceous age and possibly conformable w i t h the Naknek. Atwood's (1911) aqe de te rm ina t ion was based on f o s s i l collections of Buchia - crassicoliis and 8. plochii. - B . c r a s s i c o l l i s and B. ~ i o c h i i have n o t been found i n the ~ a k n x f o r m a t i o n , Burk (1965. p. 3922recognized a much wider extent o f the Staniukovich based, -in part on i t s lithologic character and i n part on the contained f o s s i l s . Burk (1965, p . 39-40) i n re-examining t h e type s e c t i o n described it as V i n e t o medium grained f e ldspath ic sandstones and arkoses which weather t o a characteristic tan o r yellowish brown and contain very abundant large Buchia." Burk (1965, p . 43) discusses t h e f o s s i l da ta f rom the m k o v i c h Formation and concludes t h a t the age range o f the f o rma t i on i s f r o m Kimner idg ian ( 155 my., Late Ju rass i c ) t o Valanginian ( 125 m y . , Early Cretaceous). This age range, a t i t s oldest, overlaps t h a t o f t h e Naknek Formation and implies t h a t the basal S tan iukov ich i s equ i va len t t o the younger p a r t s o f the Naknek. F o s s i l s o the r than Buchias found i n the Stan iukov ich inc lude l i m i t e d belemnites. rare Inoceramus and carbonaceous - a

p l a n t deb r i s (Burk, 1965, p. 42). Lithologically, the Stan iukov ich cons i s t s o f about 600m (2000 f t) o f

"sandstones and siltstones ( t h a t ) are composed almost exclusively o f subangular grains o f feldspar and quartz. Hornblende and b i o t i t e are by-far the most dominant accessory minera ls , each comnon l y c o n s t i t u t i n g about 5 per c e n t o f the- rock and together as much as 15 per cent. (Burk, 1965, p . 42)."

Rock fragments are ra re , although Burk (1965, p. 42 ) reported rare felted volcanic fragments and p o s s i b l e c h e r t i n coarser-grai ned rocks. I n a d d i t i o n he reported t h a t t he biot i te i s commonly chloritized and t h a t t r aces o f epidote and garnet ( ? ) have been recognized (Burk, 1965, p . 42).

Environmentally, the Stan iukov ich and the Naknek appear t o have been deposited under s imj 1 i ar condi t ions f rom a p robab ly i d e n t i c a l source t e r rane . Because o f t h i s and because parts o f t h e Naknek weather the same c h a r a c t e r i s t i c bu f f t o y e l l o w i s h c o l o r as t h e Staniukovich, and because p a r t s o f the Staniukovich weather gray-green as does the Naknek, it o f t e n proved impossible t o distinguish t h e two format ions i n the f i e l d . The d i f f e r e n t lithologies reported by Atwood ( 9 1 , p 38) and by Burk (1965, p . 39-42) i n t h e type l o c a l i t y o f the Stan iukov ich only f u r t h e r confuse t h e i d e n t i f i c a t i o n o f t h e fo rmat ion . I n t h e au thor ' s personal experience, i t was found t h a t only w i th expertise i n the i d e n t i f i c a t i o n o f the var ious Buchia species cou ld the Stan iukov ich Format ion be r e l i a b l y i d e n t i f i e d ; t he re fo re it should n o t be considered a separate formation,-but p o s s i b l y an upper member o f the Naknek.

Herendeen Limestone The Herendeen Limestone has been recognized i n limited areas o f the

C h i g n i k region. The formation was or ig ina l ly described by Atwood (1911) i n the Port Moller-Herendeen Bay area south o f the Chignik and Sutwik Is land quadrangles (Figure 1). Burk (1965, p.45) re-exam1 ned the type exposures and expanded Atwood's descr ipt ion. The descript ion t h a t follows i s from Burk (1965, p.45-48); this wri ter was no t able t o examine exposures of the Herendeen Limestone i n the Field.

Lithologically, the Herendeen Limestone i s a l i g h t gray t o gray, dense arenaceous limestone and calcareous sandstone. Maximum thickness i s 150111 (500 feet); i n the C h i g n i k region i t i s only 3011 th ick (Detterman and others , 1980b). Cl a s t s are we1 7-sorted, angular t o subrounded quartz and fe ldspar ; quartz i s about twice as abundant as fe ldspar . Hornblende and biotite are the most common accessory minerals . The noncalcareous materi a1 cons t i tu te s from one quar te r t o three quar te rs of the rock a t any par t i cu la r loca l i ty . Except f o r the large component o f calcareous material , the Herendeen Limestone i s very similiar to t h e Staniukovich Format i on.

The calcareous portion o f the formation i s composed o f fragments and prisms o f Inoceramus shells. B u r k (1965, p . 4 7 ) mentions t ha t i n the past B. c r a s s i c o l l i s was reported f rom the Herendeen Limestone; he believed t h a t - these were actually from the Staniukovich Formation. Therefore, Inoceramus i s the only f o s s i l known f rom the Herendeen Limestone. Burk assigned a latest Valanginian age t o the Herendeen Limestone based on i t s conformi ty wi th the Staniukovich and i t s d i s t i n c t l i t h o l o g i c difference f rom the over ly ing Ch i gn i k and Hoodoo Format i ons .

Strati graphically, the Herendeen Limestone conformably over1 i es the Staniukovich Formation. The con tac t i s gradational over a short ve r t i ca l distance. Burk suggested t ha t the Herendeen Limestone might be considered a member o f the Staniukovich d u e t o the i r very grea t l i t ho log ic s im i l i a r i t y ; apparently, the only important di f ference i s the presence of abundant lnoceramus I n the Herendeen Limestone. In view of t h i s au thor ' s suggestion t h a t the Staniukovich m i g h t be considered a member o f the Naknek, might the Herendeen Limestone be a f a c i e s of the Naknek?

The Herendeen Limestone i s unconf ormably over 1 a1 n by the Upper Cretaceous Chignik Formation. The C h i g n i k a l s o unconformably overlies the Naknek and Staniukovich Formations, therefore the areal d i s t r ibu t ion i s presently limited due t o cu tou t and no accurate estimate o f i t s original thickness and d i s t r ibu t ion can be made.

Shumag 1 n Format i on The Shumagin Formation i s an Upper Cretaceous (Maestrichtian?) f lysch

sequence found on the Outer Shumagin Islands and on the Sanak Islands. Equivalent u n i t s are found on Kodiak Island (Kodiak Formation) and throughout southern Alaska i n t h e "slate and graywacke beltM (Plafker and others , 1977). I t was f i r s t described and named by Burk (1965, p.63-72) from exposures i n the Shumagin and Sanak Islands. Inoceramus kusiroensis i s probably o f Maestrichtian age and i s known from t h e Kodiak Formation; an apparently s im i l i a r related spec ies (? ) has been collected from the Shumagin Formation i n the Shumagin Islands (Jones and Clark, 1973, p. 134) and t h i s formation has also been assigned a Maestrichtian age. An upper age l imi t o f earl iest Ter t i a ry i s provided by the radiometric ages on the plutonic rocks o f the Kodiak-Shumagin p l u t o n i c ser ies t h a t intrude the

Shumagi n Format ion (Table 2). S t r a t i g r a p h i c control on the Shumagin Formation i s lacking; the base i s exposed in a small area on Long Island i n the Sanak Islands (Moore, 1973a, p. 597). Here an unnamed, undated p i 1 low 1 ava, bedded cher t and graded-bed sequence conformably under1 i es t h e Shumagin. The t op o f t he Shumagin has not been recognized and may not be exposed. The Shumagin Formation i s not known t o crop out i n the study area, yet a description o f i t i s important in establishing the t e c t o n i c framework o f the Alaska Peninsula, and i n particular the study area.

Interbedded dark-gray sandstones and mudstones t h a t have undergone z e o l i t e f acies metamorphism characterize the 1 i thology o f the Shumagin Formation. Moore (1973a, p. 598-603) describes t h e formation i n some detail, and a b r i e f summary o f h i s work follows.

Two end member lithologies and a continuous spectrum o f intermediate rock types are f o u n d . Massive, evenly bedded, medium-grained sandstone beds, ranging from less than im t o g rea te r than 20m i n thickness, but usually I to 5m t h i c k comprise one end member. Moore (1973a, p.598) reported these have high sandstone/shale ratios, very poor to nonexistent grading and no i n t e r n a l structure. Basal contacts a r e sharp whereas the contact w i t h overlying mudstones i s genera l ly " d i s t i n c t . " Mudstone breccia i s often seen a t the base of or within the massive beds and i s composed of clasts r a n g i n g f rom about 3 t o 30cm in diameter. The massive sandstones are well indurated, are medium-light to medium-dark gray i n color, and weather to l i g h t e r gray, green or brown hues.

The second end member lithology i s characterized by t h i n graded beds, usually t o 20cm t h i c k and evenly bedded. The grading is from fine sand or si 1 t upwards to clay-s ize particles. Moore (1973a, p . 599) reported sharp ,pper and lower contacts. SandstoneJshaIe r a t i o s are always less than one, u s u a l l y 1:3 o r 1:4. These u n i t s are medium t o dark gray or grayish-black on f r e s h surfaces; weather ing t o lighter grays and buf fs .

These two end member lithologies account for 70 percent of t h e variation o f the Shumagin Formation. An intermediate type accounts f o r almost all t h e rest of the formation. Th is in termedia te type, by de f i n i t i on , includes all mixtures o f the two end members.

' I A l l variations e x i s t , making the description of an average bed d i f f i c u l t . However, a typical intermediate bed may be 20 to 60cm th ick with basal sandstone o f medium-fine grain s i z e , and a sandstone/shale r a t i o o f 111 t o 69. Generally the beds show we 11 -developed internal and e x t e r n a l structures (Moore, 1973a, p . 599) .'I

Rare conglomerates, w i t h pebbles o f calcareous s i 1 tstone and sandstone, chert, and volcanic and p l u t o n i c rocks i n variable proportions, and greenish-gray volcanic ash beds make up the remainder o f the Shurnagin Formation; these constitute less than 0.5 percent o f t h e rocks. Moore (1973a, p. 603) considers the massive sandstones to represent Bouma IIAII

i n t e r v a l s and t h e t h i n graded beds represent Bouma "C-D-Eu o r "D-E" intervals (See Bouma, 1962).

Sandstone o f t h e massive and intermedi ate l i thologies i s primari ly composed of volcanic l i t h i c fragments, plagioclase and quartz. Volcanic lithic fragments comprise 51 percent o f t h e framework grains and Moore (1973a, p . 601) estimates 44 percent of t h e total rock volume. These volcan ic fragments are pilotaxitic-textured andesites. Considering the cont r ibu t ions o f feldspar, heavy minerals and clays o f volcanic origin, and

the volcanic l i t h i c fragments, Moore (1973a, p.602) estimates, that probably 60 percent of t h e rock i s o f volcanic origin. The remainder i s made up o f half plutonic l i t h i c fragments and h a l f sedimentary lithic fragments w i th very minor metamorphic l i t h i c fragments. The t h i n graded beds are cornpositionally q u i t e s i m i l i a r t o the massive sandstone.

Moore (1973a, p. 610) suggests t h a t the Shumagin Formation was derived from an a c t i ve, predomi nantly andesi t i c , volcanic terrane from which some p lu ton ic and sedimentary rocks were also being eroded. There i s no agglomerate or volcanic breccia in t h e Shumagin, therefore a volcan ic source wi th in the depositional basin i s unlikely. Because the formation apparently contains no calcareous micro- or nannof o s s i Is, Moore (1973a, p. 608) suggested t h a t t h i s ind ica tes deposition below t he calcium carbonate compensation depth. The lack of shallow-water features and the evidence for t u r b i d i t y current deposi t ion led him to propose t h a t the depth o f deposi t ion was greater t h a n 1000m, or a t abyssal depths. Paleocurrent analysis i n the Shumagin indicates t h a t the predominant direction o f zed iment transport in the Shumagi n Is1 ands was southwestward, whereas in the Sanak Is1 ands the direction of t ranspor t was west-northward (Moore, 1973a, p.605-607).

Moore (1973a, p . 610) suggested t ha t i f the age assignment o f the Shumagin i s correct, then it i s an age equivalent o f the Chignik and Hoodoo Formations. This implies t h a t a l l are fac ies o f one another. D i f f i c u l t i e s w i th t h i s interpretation include: 1. the less abundant volcanic 1 ithic fragments in the Chignik, which should have been deposited close to the presumed volcanic source area, though the amount o f volcanic debris does increase upward in the Chignik Formation; 2. the apparently non-volcaniclastic nature o f the Hoodoo which i s also close t o the supposed volcanic source region; and 3 . the l ack o f a known suitable source terrane for the overwhelmingly volcanic composition of the Shumagin. As yet, no volcanic rocks o f s u f f i c i e n t l y old age t o be a source for the Chignik, Hoodoo or Shumagin Formations have been found on the Alaska Peninsula. Burk (1965, p. 59) and Moore (1973a, p. 610) suggested t ha t the missing volcanic source terrane may be unrecognized among the later Cenozoic volcanic rocks o r , t h a t the source terrane i s buried under the Nushagak-Bristol Bay Lowland northwest o f the present volcanic arc. Reed and Lanphere's (1974) 85-59 m.y. plutonic event represents igneous a c t i v i t y w i t h i n the expected t ime interval . Burk (1965, p.69-71) recognized the d i f f i c u l t y of corre 1 a t i ng the Shurnag i n and the other Upper Cretaceous Formations o f the Peninsula and suggested t h a t the Shumagin may be somewhat older than the other formations. This still leaves the question o f i t s volcanic source region an unanswered enigma.

Ch i gn i k Format i on The Chignik Formation i s a major Upper Cretaceous u n i t seen throughout

the Chignik and Sutwik Island area. Recently, Mancini and others (1978) have attempted t o demonstrate t h a t i t may be equ i va len t in age to t h e Hoodoo Formation, which i s also widespread i n the study area. However, R.L. Detterman and D.L. Jones (written communication, Jan. 1979) state t h a t megafossil data indicate t h a t the Hoodoo i n general i s younger than the Chignik.

The Chignik Formation was first described by Atwood (1911, p . 41) from a trpe sect ion exposed in Whalers Creek near Chignik Lagoon, and from other exposures on the shores of Chignik Lagoon and Chignik Bay. The Chignik

Formation consists of approximately 250m (800 ft) o f sandstones, shales, conglomerate and coal. Fossil assemblages i n c l u d e Inoceramus and other pelecypods, ammonites and an excellent f l o r a l assemblage.

Burk (1965, p. 50) studied the Chignik Formation and determined t ha t the basal portion of the formation was usually non-marine. He separated this u n i t as the Coal Valley Member based on type exposures at Herendeen Bay. The Coal Valley Member consists o f carbonaceous t o lignitic shales, s i 1 tstones, sandstones, and conglomerate, local ly bentoni tic and weathering to typical orange and reddish-brown colors. The conglomerate contains largely volcanic, gran i t i c , and che r t clasts (Burk, 1965, p. 52). Mancini and others (1978, p . 438) describe the conglomerates as alluvial f a n deposits; they interpret the Coal Valley Member as a fac ies of the Chignik, not necessarily basa l . Detterman (1978, p. 8-62) shows i n a measured sect ion on the shore o f Chignik Lagoon that the Chignik i s depositionally cycl ic and he documents four cycles i n the measured sect ion. Fossils found in the Coal Valley member include pollen and spores (Mancini and others, 1978, p. 438) and abundant carbonaceous plant fossils (Hollick, 1930; J.A. Wolfe, oral communication, 1978).

The main po r t i on o f t he Chignik Formation consists of upwards o f 500m of well-bedded, fairly homogeneous argillaceous sandstones and siltstones. The sandstones are composed of subangular to subrounded grains consisting o f feldspar, quar tz and l i t h i c fragments. L i t h i c fragments consis t predominantly of fine-grained sediments and a lesser amount o f yolcanic rocks. Unl ike the older sedimentary u n i t s i n the region, hornblende i s a scarce accessory, but chloritized biotite may represent 5 percent o f the rock. Calcite i s common as cement or replacing feldspar grains; t h i s i s most abundant i n rocks rich i n p l a n t remains (Burk, 1965, p. 52). "Trough crossbedding, ripple 1 ami nation and b i o t u r b a t i o n are common1' and these sedimentary structures, along wi th the pollen spores, occasional phytoplankton and open mari ne ammonites and bivalves i n d i c a t e t h e Chigni k was deposited i n an inner-neritic, continental shelf environment (Mancini and others, 1978, p.439)."

Difficulty i n d is t ingu ish ing the Chignik Formation from the underlying formations has been mentioned in t h e literature (Knappen, 1929; Burk, 1965) and occurred o f t e n i n the f i e l d in the Chignik and Sutwik Island area. The conglomerates are very similiar t o those in the Naknek; the abundant volcanic clasts i n the conglomerates, and t h e volcanic lithic fragments i n the sandstones can also be quite similiar to those in overlying Tertiary formations. In many ways, the main body o f the Chignik i s lithologically transitional between the earlier and later units. In the field, the Chignik usually can be distinguished f rom the Naknek by the greater volcanic component in the Chignik. Granitic and some metamorphic clasts are still common i n Chignik conglmerates though, and th i s criterion can be used to distinguish the Chignik f rom the Tertiary formations.

The Upper Cretaceous Ch igni k Formati on unconf ormably over1 i es the Upper Jurassic to Lower Cretaceous Naknek, Staniukovich, and Herendeen Limestone Format ions, o f t e n c u t t i n g out the Staniukovich, Herendeen Limestone, and parts o f the Naknek. Where the Chignik Formation, including the Coal Valley Member, i s best developed, t he underlying sect ion i s most complete; this may indicate

" t h a t the Chignik Formation was deposited on an erosion surface cut into broad folds rather than on a regionally tilted crust,

and t h a t the i n i t i a l nonmarine Chignik deposi ts f i l l e d the sync l ina l depressions whi le the adjacent a n t i c l i n a l arches were being eroded (Burk, 1965, p . 57) ."

Burk (1965, p. 56) assigned a Campanian (La te Cretaceous) age t o the Chignik based on the Inocerarnus f o s s i l s present. Recently, Mancini and others (1978, p. 438) have shown the Ch ign ik t o range upward i n t o the early Maestr icht ian, based on pollen recovered from both the Coal V a l l e y Member and the main body o f the Chignik. A g r a n i t i c cobble from a Chignik exposure across Chi gni k Lagoon from Dettennan s measured sec t i on has yielded a 89.8 + 1.60 m.y. potassium-argon date on c h l o r i t i z e d b i o t i t e (Table 4, sample 77AWs '186). This i s taken as a min imum age.

Hoodoo Format i on The Hoodoo Formation i s an Upper Cretaceous Formation d i s t r i b u t e d i n

general, throughout t h e study area. The u n i t was o r i g i n a l l y named and described by Burk (1965, p. 59-63) f r om exposures southeast o f Mt. Hoodoo i n the Port Moller quadrangle (F i gu re 1, southwest o f Chignik. The formation cons is ts o f more than 600m (2000 f t ) o f @%lack t o dark-gray, well-bedded s i l t s t o n e and silty shale, w i t h some claystone, c l a y shale and small amounts o f very f ine-gra ined sandstone" i n the type sect ion (Burk, -965, p.60) . The type section i s incomplete and disturbed, and no complete sect ion has been descr ibed i n the l i t e r a t u r e . Mancini and others (1978) consider the Hoodoo t o be a d i s t a l f a c i e s o f the Chignik and there fo re age-equivalent whereas Burk (1965, p. 62) considers i t t o be the upper, younger p a r t of an Upper Cretaceous t r ansg ress i ve sequence i n c l u d i n g the Chignik and Hoodoo Formations. According t o R.L. Detterman and D.L. Jones ( w r i t t e n communication, Jan. 1979) fossil data from ammonites and pelecypods support Burkts i n t e r p r e t a t i o n .

I n general the Hoodoo Formation i s composed o f a black t o dark-gray poor ly f o s s i l i f e r o u s well-bedded s i l t s t o n e and shale. Outcrop exposures a re genera l ly poor, as these rocks tend t o weather eas i l y . However, where exposures are good, f o r example along the southwestern shore o f Chignik Bay and along the margins o f the Devils batho l i th , graded bedding i s v i s i b l e i n t h i n layers o f large l a t e r a l ex tent . On the west shore o f Chignik Bay, between Lumber and Lake Bays, the Hoodoo exposures are o f gray-green sandstone layers genera l ly about 0.5m t h i c k but up t o 1.5m th ick , interbedded w i t h and grading i n t o green-gray s i l t s t o n e layers 20 cm th ick , bu t ranging u p t o 1 m i n thickness. Th i s outcrop also has some i n t e r v a l s of the more t yp i ca l , rhythmical ly - layered shale and s i ltstone c h a r a c t e r i s t i c o f the Hoodoo. Just south o f Jack Bay, the Hoodoo exposures are thin-bedded shales and s t 1 tstones rhy thmica l l y i nterlayered and having 1 arge 1 a te ra l ex ten t . Normal f a u l t i n g 1s common along t h i s exposure.

In many places, dark-gray t o b lack shales and s i l t s tones are seen and mapped as Hoodoo; o f ten these are i nterbedded w i t h Chigni k-appeari ng rocks ( P l a t e 2, Black Creek and B l u f f Creek areas) and some confusion results. Often the rhythmic layering and graded bedding which character ize the Hoodoo are no t seen, because o f poor exposure. In addition, the overlying T e r t i a r y u n i t s contain dark shales t h a t can be confused w i t h the Hoodoo; therefore f i e l d i d e n t i f i c a t i o n often has a degree o f uncertainty.

Foss i l occurrences i n the Hoodoo are a u i t e r a r e and most co l l ec t i ons - .

have ~nocera& schmid t i as the prime speci'es. Mancini and others (1978, p. 439) r e p o r t a Campani an outer -ner i t i c t o bathyal fo ramin i fera1 assemblage co l l ec ted from the Hoodoo and Jones and M i 1 ler (1976) r e p o r t two species of

Amninoids. Burk (1965, p.62) and Mancini and others (1978) consider the Hoodoo to be of Campanian to early Maestrichtian aqe,

In general the' Hoodoo has the appearance o f a-di stal t u r b i dite (Bouma and Hollister, 1973, p. 89). Both "A" and I4BM d i v i s i o n s of the Bouma turbidite f a c i e s model are missing and the "CM divison i s of ten poorly developed. (see Bla t t and others, 1972, p.122-124) If it can be assumed the Upper Cretaceous coastal margin was at all similiar to that o f the oresent southeast s ide o f the Alaska Peninsula, turbidite deposition could DOSS~ bly occur very near-shore and the present pattern o f close juxtaposition o f the Chignik and Hoodoo Formations represents minor, if any, telescoping o f the section by faulting. X-ray diffraction analysis o f the clay minerals from silts o f the Hoodoo i n d i c a t e a low grade metamorphism as t h e r e are no mixed-layer clays (P.D. Nadeau and R.C. Reynolds, oral comnunication, 1979). This may be due to burial in an a c t i v e volcanic terrane.

Tolstoi Formation The Tolstoi Formation i s an Eocene(?) volcaniclastic u n i t widespread

on the Alaska Peninsula. Burk (1965, p. 83-84) originally defined i t f rom type exposures "along the east shore o f Pavlof Bay, north of Cape Tolstoi and Tolstoi Peak" (Figure 1). He considered it to be Paleocene to Eocene i n part based on floral assemblages collected from it.

The formation consists of 1000 t o 150011 (3000 to 5000 ft) o f black siltstones with interbedded volcaniclastic sandstones and conglomerates, flows, sills and agglomerate. A r i c h fossil floral assemblage has been collected from these rocks and Eocene marine invertebrates have also been recovered (Burk, 1965, p. 83). Burk (1965, p.83-101) mentioned t h a t often the T o l s t o i i s d i f f i c u l t to d i s t i n g u i s h from the overlying Stepovak Formation and tha t the black to dark-gray s i ltstones from the underlying Hoodoo have at times been erroneously included in the Tolstoi. R.L. Detterman (oral communication, Nov., 1978) has recowended abolishing the Stepovak Formation and mapping all the Eocene (Ol igocene(?)) sediments on the A 1 aska Peninsula as T o l s t o i Formation,

The black siltstones in the Tolstoi are generally nonmarine rocks as distinguished from those o f the Hoodoo. Burk (1965, p.84) describes them as commonly very sandy and carbonaceous, sometimes contain ing pyrite crystals. The sandstones and conglomerates o f the Tolstoi consist essentially o f mafic volcanic fragments, though chert pebbles are seen in sane conglomerates. Rounding varies f rom angular to subrounded, and sorting is almost universally poor. The rocks o f the Tolstoi tend to have a strong green to black cast, though Burk (1965, p. 84) reported the presence of "a characteristic, pale pebble to granule conglomerate.. . . o f pastel (usually geenish) volcanic clasts, w i t h a large part o f the f i ne r constituents eroded away." This pale conglomerate crops out on the ridge west o f Sleepy Creek (Plate 2), interbedded wi th black siltstones o f the Hoodoo ( ? ) and Tolstoi Formations. Apparently, at this locality it forms the base o f the Tolstoi.

The Tolstoi e x h i b i t s a general reverse grading, and angular volcanic ~reccia i s much more common in the upper half of the formation (Burk, 1965, p. 98).

During the summers o f 1977 and 1978 extensive collections o f floral assemblages of the Upper Cretaceous and Tertiary u n i t s in the study area were made by J.A. Wolfe and R.L. Spicer. Work i s underway on these fossil

determinations. Prior work by Wolfe (1972, p.205-206) ind icates an ear ly Paleocene f 1 ora i n the To1 stoi o f probable subtropic provenance. Eocene f loras of southern Alaska are i n d i c a t i v e o f a tropical o r near-tropical climate r e f l e c t i n g a warmer climate than in the late Paleocene. However, according t o R.W. A l l i s o n (oral communication, 1979) no evidence exists among known mega- or microfossil assemblages for a Paleocene or even early Eocene age assignment t o t h e Tolstoi. This assertion and the r a d i o m e t r i c dates on apparently interbedded volcanic rocks (presently considered Meshik Formation, see Tables 4,5, and 6) br ing into question Wolfe's assignment o f the fossil floras to t he Paleocene. I f the f loras o f the Tolstoi are Eocene, rather than Paleocene as Wolfe (1972) suggests, a much simpler tectonic interpretation i s p o s s i b l e . Paleomagnetic (Stone and Packer, -977) and other geologic interpretations i nd i ca te that there probably had been a northward movement o f the rocks o f the Alaska Peninsula between Paleocene and Eocene time, rather than southward as Wolfe (1972, p.206) suggested.

Channeling i n t o the Hoodoo by the Tolstoi was seen in the area o f Cape Kunmi k (oral comunication, R.L. Detterman, 1978) and the T o l s t o i unconformably overlies t he Naknek i n the area south of Chignik Lake (Plate 2). The upper boundary o f the T o l s t o i i s d i f f i c u l t t o place; where present, the Stepovak Formation i s conformable and Burk (1965, p. 85) mapped these two formations together as the Beaver Bay Group. The Meshik Formation overlies the Tolstoi in the Chignik Bay area and northward. The Meshik has been mapped by our f i e l d party as a predominantly volcanic un i t . In any case, the top o f the Tolstoi i s difficult t o d i s t i n g u i s h in the field. Isotopic work reported herein indicates a number o f late Eocene t o Oligocene volcanic centers w i t h i n the Tolstoi.

Stepovak Format ion The Stepovak Formation i s a Late Eocene to Oligocene, andesit ic ( ? )

volcanic and voicaniclastic un i t conformably overlying the T o l s t o i Formation. Lithologically it i s a continuation o f the T o l s t o i , though Burk (1965, p.88) reported t h a t the amount o f coarse volcanic debris (conglomerates, agglomerates and breccia) i s less than t h a t i n the T o l s t o i and bedding i s more even. Stratigraphical ly, the Stepovak i s thought to be equivalent to the Meshik Formation and in f i e l d mapping it i s o f t e n easy t o confuse the Tertiary units. Palache (1904), while on the 1899 Harriman hunting expedit ion was f i r s t t o examine and name rocks of the Stepovak i n the area o f Chichagof Bay ( P o r t Moller quadrangle and Figure 1). R.L. Detterman (ora 1 communication, Nov., 1978) has recommended abol i shi ng t h i s format i on name.

The type Stepovak section c o n s i s t s of more than 2 1 0 h (7000 f t ) o f i nterbedded volcanic sandstones and conglomerates with nearly 300111 (1000 f t) o f interbedded black s i l tstone containing abundant calcareous concretions (Burk, 1965, p.86). Burk (1965, p.86) reported t ha t " t h i c k conglomerates are rare and angular, poorly bedded volcanic deb r i s is uncommon except in the lower 2000 ftam (600111). The exposure examined by the author with J.A. Wolfe and R.L. Spicer on Boulder and Fox Bays i n the Stepovak Bay quadrangle consisted almost entirely o f volcanic-flows, rubble f l o w s and volcanic breccia with rare s i l t s t o n e and rarer sandstone; t h i s lithology seems at some variance w i th t he measured section described by Burk (1,965, p.205-208) a t the same locality. Experience during the summers o f 1977 and 1978 indicates t h a t a t least i n the study area, the Stepovak i s

nenerally a vo lcanic u n i t , consisting o f andesit ic rubble flows, water la id t u f f s , volcanic breccias, and 1 ahars. It i s virtually indistinguishable from the Meshik Formation and R.L. Detterman (oral communication, Nov., 1978) has proposed t h a t the volcanic portions o f the Stepovak formation be included i n the Meshik as one u n i t and t h a t t he sedimentary portions o f the Stepovak be mapped as the Tolstoi Formation.

Meshik Formation The Meshik Formation was named by Knappen (1929, p.198) from exposures

along the Meshik River and around Meshik Lake (Figure 2 ) . He described a reverse-graded andes i t i c vol can4 c sect i on i ncl udi ng i nterbedded cl ay, f i ne sand, volcanic ash, and coarse agglomerate (Knappen, 1929, p.198-199). The clays a r e b e n t o n i t i c and the lower part o f the formation contains many pal eoso 1s. Knappen (1929, p. 200) noted abundant s i 1 icif i ed and carbonized p l a n t fragments in the lower sediments, including s i l i c i f i e d stumps and tree trunks up to 15 inches (0.4m) in diameter. The upper part o f the formation was composed e n t i r e l y o f coarse volcanic m a t e r i a l s including numerous f 1 ows (Knappen , 1929, p .198) . Based on 1 i tho1 ogi c correl a t i on wi th the T o l s t o i and Stepovak Formations, Knappen (1929, p.201) believed the Meshik was o f Oligocene to early Miocene age. According to Burk (1965, p.99), the Meshik rests conformably on the Paleocene t o Eocene Tolstoi formation and i s overlain unconformably by Pliocene and younger volcanic rocks or a l l u v i a l and glacial debris. Burk (1965, p.99) estimated the formation t o be 5000 f t (1500m) thick. I t was distinguished by Burk (1965, p.99) from the underlying Tolstoi by the brighter colors, brown and yellow "wi th local pastel t i n t s of green, red, and purple." The Meshik is thought to be equivalent i n age to the Stepovak Formation though no f o s s i l s have been collected from the Meshik. As a result o f our work in the Chignik area, R. L. Detterman (oral comunicat ion, Nov., 1978) has proposed mapping the Meshik Formation as an entirely volcanic u n i t and inc luding the sediments i n the T o l s t o i Formation. Potassium-argon work to date has indicated t ha t some o f the volcanic rocks Sncluded in the Meshik are o f 1 ate Eocene to Oligocene age.

Bear Lake Format i on Burk (1965, p. 89) named the Bear Lake Format i on a f t e r a sequence o f

Middle t o Late Miocene sandstones, conglomerates, and minor siltstone exposed along the eastern shore o f Port Moller and i n the mountains around Bear Lake. Burk (1965, p.89) considered the u n i t t o be a t least 5000 f t (1500111) thick. This un i t i s d i s t i n c t i v e w i t h respect t o the other Tertiary units i n t h e study area owing to the "abundance of nonvolcanic clasts, i n the greater rounding and better sorting of the grains, and in the poor consolidation of much o f the rock" (Burk, 1965, p . 9 1 ) . R.L. Detterman ( o r a l communication, 1979) suggests these sedimentologic features are probably due to the reworking of older u n i t s , as does Burk (1965, p.91). The author has seen few exposures of the Bear Lake Formation; the bu lk of t h i s description i s based on Burk (1965) and Detteman and others (1980b).

The type section, designated by Detterman and others (1980b), consists of 1525m of inner n e r i t i c and nonmarine sandstone, conglomerate, siltstone, shale, and t h i n coal beds. A thickness of 2400111 has been encountered i n d r i l l i n g (Brockway and others, 1975). The Formation i s local ly abundantly fossiliferous, containing pelecypods, gastropods, and echinoids of late Miocene age (Detterman and others, 1980b). Burk (1965, p.91) reported the

fossiliferous beds were m o s t comnon i n the upper parts o f the formation. The type sect ion i s rhythmically bedded i n units 1 to 1h th ick w i t h i n d i v i d u a l beds ranging from 6cm t o l m th ick (Detterman and others, 1980b). The i nterbeds are composed o f conglomerate and sandstones i nterbedded w i t h dark-gray si l tstones and carbonaceous shale (Burk, 1965, p .91). In the conglomerates, the clasts are about one t h i r d quartz and chert, one t h i r d volcanic fragments and one t h i r d sedimentary l i t h i c cl asts (Detterman and others, 1980b). The rock i s generally dark-brown to pale ye1 lowi sh-brown i n color.

The Bear Lake Formation unconformably overlies the Tolstoi and Meshik Formations and i s disconformably overlain by the Milky River Formation (Detterman and others, 19806).

Mflky River Formation The type sect ion for the Mi 1 ky R i v e r Formation was designated by

Detterman and others (1980b) as 1525111 o f vol canigenic nun-mar i ne sedimentary rock w i t h interlayered andesite f l o w s and s i l l s . Though t h i s Formation i s not of ten encountered on the surface, i t i s seen i n dri 11 holes bored on the Nushagak-Bristol Bay Lowland. The type section i s located j u s t east o f Bear Lake i n t he Chignik quadrangle. The descr ip t ion t ha t follows i s taken from Detterman and others (1980b).

T h i s formation unconformably overlies the Bear Lake Formation and i s the youngest Tertiary u n i t mapped i n the study area. I t i s unconformably over l a i n and preserved by volcan ic flows o f apparent Quarternary age. The lower lOOOm i s almost entirely o f coarse, f l u v i a l , highly cross-bedded and channeled volcanic sandstones and cobble to boulder conglomerates. The upper parts o f the u n i t contain numerous porphyritic andesi te flows, iahars, and tuffaceous units interlayed with volcaniclastic rocks. The proportion o f volcanic rocks increases upward i n the unit. The rocks are ooorly indurated and are dark brown t o gray i n color.

Appendix 4 Rock Descriptions

77AWs 9 Andesite Chiginagak Bay L a t . 57 00.3'N Long. 156 40.4'W Very dark gray andesi t e wi th cl i no- and orthopyroxene phenocrysts and plagioclase phenocrysts (An 55). Much o f groundmass i s composed of f i n e grained opaque oxides and glass(?). Plagioclase phenocrysts are glmeroporphyritic.

77AWs 30 Andesite Cape Kumlik Lat. 56 38.2 'N Long. 156 35.3IW L i g h t green i s h gray porphyritic andesi t e w i t h hornblende phenocrysts and strongly zoned glmeroporphyritic plagioclase phenocrysts (about An 40 though some phenocrysts are An 15-20). C a l c i t e w i t h chlorite or ta lc , possibly pseudomorph i ng pyroxene. Abundant accessory Mn-beari ng purple apa t i t e . Groundmass w i t h abundant opaque oxides and possibly devitrified glass. Thoroughly fractured, probably proclastic texture.

77AWs 40 Leuco-basalt Cape Kuml ik Lat . 56 38.2" Long. 157 28.9'W Dark green porphyr i t ic leuco-basalt wi th hornblende phenocrysts up t o 10 mm i n size. Altered plagioclase phenocrysts (Anorthite greater t h a n 50). Abundant c a l c i t e , chlorite and epidote. Hornblende phenocrysts p a r t i a1 ly a1 tered t o chlorite. Phenocrysts and groundmass fractured, may be c a t a c l a s t i c rather t h a n p r o c l a s t i c . Dike i n mineralized area.

77AWs 46 Andesite(?) Cape Kurnlik Lat. 56 38.2IN Long. 157 27.7'W Dark green porphyritic andesite(?) w i th chloritized amphibole phenocrysts and altered zoned plagioclase phenocrysts (An 35(?) and An 55). Minor quartz. Rock i s deuterically altered, calcite i s common. In m i neral i zed area.

77AWs 74 Dac i t e S u t w i k Island L a t . 56 32 .5 'N Long. 157 20NW Tan dac i te . Fresh b i o t i t e phenocrysts(?) i n a thoroughly altered rock. Hornblende phenocrysts altered to opaque oxides and calc i te , s e r i c i t i z e d p lag ioc lase o f indeterminate composition, quartz and abundant accessory a p a t i t e and sphene.

77AWs 96b Pegmatite Warner Bay Lat . 56 09.7 IN Long. 158 24IW Small ~ e q m a t i t e o f feldspar and b i o t i t e formed i n granodior i te o f D e v i l s batholith.

77AWs 100 Granodiorite Sweater Bay Lat. 56 05.0tN Long. 158 30.01W L i g h t gray medium-grained granodiorite. Major minerals are plagioclase (An ~5-30), potassi urn feldspar, b i o t i t e , hornblende and quartz. Minors include pyroxene(?), c h l o r i t e , and opaque oxides. Accessory apat i te. Very fresh rock, plagioc lase zoned An 30 core, An 25 rim. Hypidiomorphic-granular texture.

77AWs 112b Leuco-basalt Kametolook River L a t . 56 01.7'N Long. 159 11.2'W Dark gray-black porphyritic leuco-basal t . Plagioclase i n phenocrysts and groundmass (An 60-70), clinopyroxene and o l i v i n e ( ? ) . Flow structure, nearly opaque groundmass of d e v i t r i f i e d g las s (? ) .

77AWs 122 Dacite Castle Bay L a t . 56 17.7 IN Long. 158 19.04W L i g h t gray iron-stai ned daci te wi th hornblende and plagiocl ase phenocrysts (An 30 (? ) ) i n a f i ne-grai ned groundmass. Some quartz, hydrothermal ( ? ) biotite, and sericite. Hornblende i s sparsely distributed and i s i n long l a t h s . Hyalo-ophitic texture.

77AWs 125 Tonal i te Northwest Arm, Castle Bay Lat . 56 13.2'N Long. 158 23m3'W Light gray medium-grained tonalite wi th b i o t i t e and hornblende. Minor pyroxene, some c h l o r i t e and epidote from hornblende. Minor s e r i c i t i z a t i o n of plagioclase (An 45). Accessory zi rcon. Hypidimorphic-granular t ex tu re .

77AWs 134 Dacite Nakchamik Is land Lat. 56 19.8'N Long. 157 19.6'W L i g h t gray dac i te w i th hornblende and plagioclase phenocrysts. Large phenocrysts o f hornblende f l o a t i n g i n a groundmass of f ine-grai ned plagioclase (An 40-60, possibly two populations). Rock i s s l i g h t l y a1 tered and strongly brecci ated.

77AWs 137 Tonalite Mallard Duck Bay Lat. 56 13.6'N Long. 158 28.9'W Gray tonalite wi th h ighly altered phenocrysts of hornblende in a f i ne-grained groundmass of plagioclase (An 46) and quartz. C h l o r i t e and b i o t i t e a1 teration products from hornblende abundant, some sericitization o f plagiocl ase.

77AWs 152 Dac i te Bee Creek Drill Hole 8-1 Lat . 56 31.01N Long 158 23.5'W Light gray dacite w i th amphibole phenocrysts altered t o b i o t i t e and chlorite. Plagioclase phenocrysts (An 45) show minor sericitization. Abundant quartz and very minor potassium feldspar, Hypidiomorphic-granular texture.

77AWs 176 A n d s i t e ( ? ) Lat. 56 18.0tN Long. 158 22.8'W Dark gray-green andesi te(? ) wi th coarse-grai ned brown pleochroic par t i a1 ly resorbed amphibole and some clinopyroxene. Amphi bo le f a i r l y fresh, f ine-grai ned groundmass strongly a1 tered to epidote, kaolinite, and serici te. Hyal e-oph i t i c porphyritic texture .

77AWs 179 Granodiorite Warner 8ay Lat . 56 09.7'N Long. 158 24.04W L i g h t ,ray hornblende b i o t i t e granodiorite. Hornblende p a r t i a l l y altered to b i o t i t e , primary and hydrothermal(?) b i o t i t e , f resh zoned plagioclase (An 48), quartz and potassium feldspar. Trace chlorite. Epidote i n fractures. Hypidiomorphic-granular texture.

77AWs 180 Pegmatite i n Devils b a t h o l i t h Warner Bay Lat. 56 09.7'N Long. 158 24.01W Biotite, feldspar pegmatite i n D e v i l s batholith.

77AWs 183 Quartz-sericite altered rock Bee Creek Lat . 56 13.8'N Long. 158 29.O'W Strongly i roned a1 tered sediment(?) o f Naknek Format i on. Rock i s completely altered t o quartz, s e r i c i t e , and pyrite. Apparent re1 i c t bedding(?) . Possibly some argillic alteration.

77AWs 186 Granite Chignik Lagoon Lat. 56 20.O'N Long. 158 30.01W Coarse- t o medium-grained b i o t i t e granite cobble f rom Chignik Formation. Biotite partially altered to chlorite, some s e r i c i t i z a t i o n o f plagioclase. Abundant quartz and potassium feldspar . Hypidiomorphic-granul ar texture.

77AWs 190b Andesite Anchorage Bay Lat. 56 18.5W Long. 158 24.5'W Dark gray-green porphyr i t i c hornblende andesite. Large phenocrysts of hornblende i n a very f i ne groundmass o f altered feldspar and d e v i t r i f i e d g lass (? ) . Hornblende phenocrysts are p o i k i l i t i c textured w i t h resorbed edges.

77AWs 215 Daci te Bee Creek Lat. 56 30.6'N Long. 158 2 4 . 0 ' ~ L igh t gray porphyritic dacite wi th phenocrysts of hornblende and plagioclase. Hornblende shows minor alteration to chlorite, plagioclase (An 43) slightly sericitized. Fine-grained groundmass o f plagioclase, quartz, and b i o t i t e . Plagioclase phenocrysts are glomeroporphyritic. Small 1 ate quartz- and ch lo r i t e - f i 1 led fractures.

77AWs 235 Propylitically altered sandstone Bee Creek Lat, 56 30.4'N Long. ~ 5 8 23.5'W Green propyl i ti cal ly a1 tered sandstone of ~ a k n e k Formati on. Arkose w i t h sedimentary plagioclase, quartz, q u a r t z i t e , and potassium feldspar ( ? ) . Hydrothermal c a l c i t e , chlorite, s e r i c i t e and kaolinite(?) are found interstitial t o grains and disseminated through grains.

77AWs 243 Sericitically and propylitically altered sandstone Bee creek Lat .

56 30.5'N Long. 158 24.11W Green s e r i c i t i c a l l y and propylit ically altered sandstone o f Naknek Formation. Slightly iron-stained, w i t h re1 i ct bedding. Primary quartz, hydrothermal chlorite, serici te , and pyrite. Chlorite i s fracture filling, sericite i s disseminated.

77AWs 251 Potassically altered dac i te (? ) Bee Creek Lat. 56 31 .O'N Long. 158 24.O'W L i g h t gray, slightly iron-stained medium-grained composite or hybrid rock. Appears to have been hornblende daci te or arkose. B i o t i t e and chlorite pseudomorph amphibole. Plagioclase and quartz abundant, some h i n t o f hypidiamorhic-granular texture.

78AWs 5 Andesite Nakalilok Bay Lat . 56 56.5 'N Long. 156 57'W Gray f i ne - t o medium-grained andesite. Ortho- and clinopyroxene with unaltered pl agi ocl~ ase phenocrysts (An 45). Mi nor groundmass of opaque oxi des, f ine-grained plagioclase, and i d d i n g s i t e ( ? ) . Some c a l c i t e . Intersertal porphyr i t ic texture.

78AWs 11 Dac i te North edge, Sutwik Island quadrangle Lat . 56 59.01N Long. ~ 5 7 07.5'W Gray porphyr i t i c dac i te w i t h phenocrysts o f green hornblende and pl agiocl ase. Strongly zoned and car1 sbad twinned plagioclase phenocrysts are o f indeterminate composition due to sericitic alteration. Cataclastic porphyritic texture, phenocrysts are shattered and dislocated. Miarolitic(?) c a v i t i e s f i 1 led wi th rusty quartz crystals.

78AWs 17 Leuco-basalt Kumllk I s land Lat. 56 38.01N Long. 157 25.01W Green oorphyritic leuco-basalt. Large phenocrysts of hornblende (-1 cm) and augite. Unevenly distributed autolithic aggregates or zenoliths o f amphibolite, r i c h in hornblende and plagioclase. Minor plagioclase phenocrysts (An 60) are s e r i c i t i c a l l y altered. Abundant c a l c i t e , c h l o r i t e and sericfte i n fine-grained groundmass.

78AWs 24 Andesite Cape Kunmik Lat. 56 47.3'N Long. 157 11.8'W Light gray ~ o r p h ~ r i t i c hornblende andesite. Green pleochroic hornblende phenocrysts in a groundmass o f hornblende and s e r i c i t i c a l l y altered pl agioclase. Sphene and actinol i te a1 teration products f rom pyroxene. Weakly developed flow structure. Sample was c o l l e c t e d from rubble mixed with hornfels, apparent ly upper con tac t of in t rus ion .

78AWs 31 Andesite Foggy Cape Lat. 56 33.OUN Long. 156 59.6IW Dark gray porphyritic andesi t e with pl agiocl ase phenocrysts (An 55) and phenocrysts o f ortho- and cl i nopyroxene. Groundmass of very f ine-grained plagioclase and pyroxene. Weak development o f f low structure. Very minor alteration of plagioc lase .

78AWs 32 Andesite Sutwi k Is land Lat. 56 31.6'N Long. 157 11.7'W Dark gray andesite w i t h phenocrysts of plagioclase (An 55-60) and minor cl i nopyroxene. Complete gradation from medium to very f i ne-grai ned crysta l sizes. Apparently, orthopyroxene phenocrysts are altered to b a s t i te. Weak development o f f l o w structure. Feldspar i s unaltered.

78AWs 35 Daci te Pinnacle Mountain Lat . 56 49.1'N Long. 157 56.3'W Light gray dacite w i t h phenocrysts of green pleochroic hornblende and plagioclase (An 60). Rare quartz phenocrysts(?), part ia l ly resorbed. Very f i ne-grai ned groundmass, primari ly o f feldspar . Most p l agi ocl ase phenocrysts are broken and dislocated, hornblende-phenocrysts are general 1 y a1 i gned indicating a weak development of flow structure.

78AWs 42 Andesite Unavikshak Is land Lat. 56 30.4'N Long. 157 43.1'W Gray porphyr i t ic andesi te with phenocrysts o f pl agiocl ase (An 55) hornblende and opaque oxides. Phenocrysts are variable i n size from 4 mm t o groundmass and a1 1 show resorption. Pl agiocl ase zoned,

a1 t e r a t i o n var ies w i t h zoning. Hyalo-ophi t ic porphyr i t ic tex tu re . 78AWs 43 Quar t z -se r i c i t e a l t e r e d andesite Unavikshak Is land Lat. 56 30.4'N

Long. 157 43.Z1W i ron-sta ined quar t z -se r i c i t e a l t e r e d andesite. Rock i s completely a1 te red t o quartz, s e r i c i te , su l f ides , oxides, and clays. Pseudomorphs o f p lagioclase and hornblende phenocrysts. Accessory apat i te .

78AWs 58 Dacite VABM llEasyM Lat . 56 30.4IN Long. 157 49.4'W Gray-brown porphyritic d a c i t e wi th hornblende and p lag ioc lase (An 55) phenocrysts . Groundmass o f f ine-grai ned pl agioc l ase, glass, and opaque oxides. P i l o t a x i t i c tex ture.

78AWs 61 Daci te VABM "Juli k w Lat . 56 36.11N Long. 157 58.9'W Gray-green p o r p h y r i t i c dac i te . Phenocrysts o f hornblende and p l a g i o c l ase (An 55?) i n an extremely f ine-grained groundmass o f p lag ioc lase(?) and glass. Hornblende phenocrysts ou t l i ned i n opaque oxides. I n t e r i o r s o f some olas ioc lase phenocrysts altered; m o s t a re f resh. Hand sample appears coarse-grained due t o abundance o f phenocrysts and the nature o f the groundmass.

78AWs 95 Quartz d i o r i t e Seal Bay Lat. 56 Ol.lBN Long. 158 30.3'W Gray med i urn-grained quartz d i o r i t e phase of Devi 1 s bathol i th. Major minerals are plag ioc lase (An 40-45), b i o t i t e and c h l o r i t e , hornblende, augite, and quartz. Minor potassium fe ldspar. Accessory sphene, apat i te, and i lmenite. Augite shows reac t i on r e 1 a t ionsh ip t o hornblende, which goes t o b i o t i t e , and f i n a l l y t o c h l o r i t e p l u s epidote. Plagioclase i s sieve-textured w i t h inc lus ions of pyroxene; i t also shows minor s e r i c i t i c a l t e ra t i on . Hypidiomorphic-granular texture .

78AWs 98 Leuco-basalt Ma l l a rd Duck Bay Lat. 56 12.7'N Long. 158 19.9'W Dark gray f ine-gra ined leuco-basalt . Glomeroporphyrit ic phenocrysts o f p l agiocl ase (An 65?) and orthopyroxene i n a groundmass o f p l agioclase, clinopyroxene, orthopyroxene, d e v i t r i f i e d glass, and opaque oxides. Plagioclase f r e s h and unaltered. P i l o t a x i t i c tex ture.

78AWs 125 A n d s i t e ( ? ) M i l k Creek Lat. 56 29.5'N Long. 158 48.4'W L ight gray p o r p h y r i t i c andesi t e ( ? ) . Phenocrysts of hornblende and p l agocl ase (An 55) i n a groundmass o f f ine-grained p lag ioc lase and opaque oxides (magnetite?) . P lag ioc l ase phenocrysts and g lmeroporphyr i t i c .

78AWs 134 Dacite Chignik Is land Lat. 56 17.11N Long. 158 35.2'W Dark gray medium-to f ine-grained p o r p h y r i t i c daci te. Medium-grained phenocrysts o f p l ag ioc lase (An 40-45) i n a f ine-gra ined groundmass o f plagioclase, ortho-, and clinopyroxene. Late quartz. Some p lag ioc lase crystals show f r a c t u r e s and d is locat ions. F rac tu res are f i 1 led w i t h d e v i t r i f i e d glass, i n d i c a t i n g a p r o c l a s t i c subophi t ic tex ture.