Reconnaissance Geol(l§f of 

Southern Atka Island, 

Aleutian Islands, Alaska 

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Reconnaissance Geology of 

Southern Atka Island, 

Aleutian Islands, Alaska 

By James R. Hein, Hugh Mclean, 

and Tracy Vallier 

U.S. GEOLOGICAL SURVEY BULLETIN 1609 
DEPARTMENT OF THE INTERIOR 

WilliAM P. ClARK, Secretary 

U.S. GEOlOGICAl SURVEY 

Dallas l. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE 1984 

For sale by the 

Distribution Branch, Text Products Section 

U.S. Geological Survey 

604 South Pickett St. 

Alexandria, VA 22304 

Library of Congress Cataloging in Publication Data 

Hein, James R. 

Reconnaissance geology of southern Atka Island, Aleutian 

Islands, Alaska. 

(U.S. Geological Survey bulletin 1609) 

Bibliography: p. 18-19 

Supt. of Docs. no.: I 19.3:1609 

1. Geology-Alaska-Atka Island. I. Mclean, Hugh, 1939-

. II. Vallier, Tracy lowell, 1936- . Ill. Title. IV. 

Series: United States. Geological Survey. Bulletin 1609. 

QE75.B9 no. 1609 557.3s 84-600043 

[QE84.A84] [557.98'4] 
CONTENTS 

Abstract 1 

Introduction 1 

Stratigraphy and paleontology 2 

Paleogene sedimentary rocks 2 

Middle and upper Miocene (synplutonic) sedimentary rocks 5 

Petrography of sedimentary rocks 6 

Igneous rocks 7 

Plutonic rocks 8 

Volcanic rocks 11 

Geochemistry 11 

Regional patterns of alteration 14 

Structure 15 

Folds 15 

Faults 15 

Resource potential 15 

Summary and conclusions 16 

References cited 18 

FIGURES 

1. Geologic map of Atka Island 3 

2. Structural map of Atka Island 4 

3. Schematic and interpretative cross section of Atka Island 5 

4. Photographs showing Paleogene sedimentary rocks of Atka 

Island 6 

5. Photographs showing synplutonic and postplutonic rocks of 

Atka Island 7 

6. Plot of Si02 content versus FeOT/MgO ratio for Tertiary rocks 

of Atka Island 13 

7. Ternary diagram of Sr-Ti-Zr contents of plutonic and volcanic 

rocks 13 

8. Rare-earth-element plot for selected rocks of Atka Island 13 

9. Histograms showing alteration products of Atka Island rocks 14 

TABLES 

1. Locations of samples of Atka Island rocks 2 

2. Carbon content of Atka Island sedimentary rocks 8 

3. Fossils from Atka Island 9 

4. K-Ar analysis of Atka Island igneous rocks 10 

5. U-Pb analyses of zircon from tonalite of the Beaver Bay 

pluton, Atka Island 11 

6. Classification, ages, textures, and mineralogy of chemically 

analyzed Tertiary igneous rocks of Atka Island 11 

7. Chemical analyses of representative Tertiary igneous rocks of 

Atka Island 12 

8. Percentages of extensively altered rocks of Atka Island 15 

9. Metallic-element contents in bulk and heavy-mineral separates 

of sediment fractions from drainages in the Beaver Bay pluton, 

Atka Island 16 

10. Chemical analysis of silica and hematite hydrothermal deposit 

in Nazun Bay, Uyak Island 17 

III 

Reconnaissance Geology of Southern Atka Island, 

Aleutian Islands, Alaska 

By James R. Hein, Hugh Mclean, and Tracy Vallier 

Abstract 

Atka Island, one of the central Aleutian Islands 

is composed of two distinct parts: This report treat~ ~he southern part, consisting mostly of Tertiary 

Igneous and volcaniclastic rocks, and ignores the 

northern part, consisting of the late Cenozoic Korovin 

volcanic complex. The oldest sedimentary rocks 

(preplutonic), middle Eocene to middle Oligocene in 

age, compose a sequence of bedded marine sandstone 

m~dstone,. and minor conglomerate that interfinge; 

~Ith massive volcanic flows, volcanic breccia, and rare 

pillow flows. Younger sedimentary rocks, of Miocene 

(synplutonic) age, occur only on the north shore and 

consist of sandstone, volcanic conglomerate, black 

carbonaceous mudstone, and volcanic debris flows 

bearing fossil logs. 

Sandstone consists of volcanic plagioclase, 

volcanic-rock fragments, clinopyroxene, opaque grains, 

and rare fragments of microfossils and quartz. 

Secondary minerals are pore-filling cements of 

~mectite, chlorite, and zeolites. Metamorphic grade 

mcreases from east to west (toward the plutons) in the 

sedimentary and volcanic rocks, from zeolite through 

prehnite-pumpellyite to greenschist-hornfels facies, 

characterized by chlorite, epidote, amphibole, and 

quartz. Overall, metamorphic grade on Atka is higher 

than on adjacent Amlia Island. 

Volcanic rocks, mostly basalt, basaltic andesite, 

and andesite, were extruded during three eruptive 

episodes. The earliest episode was submarine and of 

middle(?) and late Eocene and early Oligocene age; the 

second episode extruded basalt and basaltic andesite in 

middle and late Miocene time; and, finally, 

stratovolcanoes of mostly Quaternary age arose on the 

northern part of the island. Plutonic rocks were 

intruded during the early, middle, and late Miocene. 

The three plutons that were dated apparently 

young toward the west. Plutonic rocks are gabbro, 

diorite, norite, tonalite, and quartz diorite; quartz 

diorite is the dominant plutonic-rock type. Most of 

these rocks have hypidiomorphic-granular textures, 

and euhedral to subhedral crystals of plagioclase, 

mafic minerals, and Fe-Ti oxides occur with anhedral 

quartz. 

Plutonic and volcanic rocks show both calcalkaline and tholeiitic trends. Differences in the state 

of differentiation can explain the two series. 

Folds are generally gentle and open; however, 

dips of rocks locally exceed 70° where drag folds 

result from faulting. Faults of small displacement are 

common. Faulting produced a pronounced east-westtrending topographic lineament that extends from the 

head of Egg Bay to Atka Village. Most faulting 

appears to be extensional, related to the emplacement 

of plutons and unrelated to compression associated 

with plate convergence along the Aleutian Trench. 

Although no metallic ore deposits of economic 

importance were found on Atka Island, geochemical 

indicators suggest that further exploration is 

warranted for Cu, Pb, Ag, Sn, and possibly other 

metals. Reservoir and source-rock properties of rocks 

that crop out on Atka show little promise for 

petroleum primarily because of alteration resulting 

from plutonism. Offshore basins, however, offer a 

greater potential for petroleum development. 

INTRODUCTION 

Atka Island is situated in the central part of the 

Aleutian Islands and is part of the Andreanof Island 

gro~p (f.ig. 1). Atka sits atop the Aleutian Ridge, an 

ensimatiC volcanic archipelago that separates Pacific 

oceanic crust south of the ridge from older oceanic 

crust north of the ridge that forms the floor of the 

deep Bering Sea. The southern part of Atka Island is 

about 85 km long and averages about 10 km in width. 

Atka is separated from adjacent Amlia Island to the 

east by the 2-km-wide Amlia Pass. Adak Island, 175 

km west of Atka, is the nearest large community and 

was our logistic support center. Atka Village, located 

at the head of Nazan Bay, is inhabited by a small 

population of Aleuts. Besides the naval station on 

Adak Island, Atka Village is the only community in the 

Andreanof group of islands. 

Atka Island is geographically and geologically 

composed of two parts, which are joined by a 3-kmwide isthmus separating Korovin and Nazan Bays (figs. 

1, 2). The northern part of the island consists of an 

uppermost Tertiary and Quaternary volcanic terrane 

the informally named Korovin volcanic complex, that 

includes historically active Korovin Volcano and an 

older stratovolcano, Mount Kliuchef. The elongate 

southern part of the island consists of Tertiary rocks 

underlying an east-west-trending mountainous ridge. 

Numerous elongate bays, rocky headlands, and small 

islands mark the shoreline of the southern part of the 

island. 

Atka and Amlia Islands were among a small 

number of Aleutian Islands that had not previously 

been mapped geologically, even on a reconnaissance 

scale. Former work on southern Atka consisted of a 

brief description of fossil mollusks from shore 

exposures near Martin Harbor by Grewingk (1850), who 

visited the area during the period of Russian 

occupation. The Quaternary volcanic terrane of 

northern Atka Island was mapped by B. D. Marsh 

(Johns Hopkins University) in 1977 but not published. 

We did reconnaissance mapping, mostly 

shoreline geology, and sample collecting of Tertiary 

rocks on Atka Island in 1977 and 1979 (table 1). This 

report presents data from our work and some broad 

interpretations based on them. 

Introduction 1 
Table 1. Locations of samples of Atka Island 

rocks 

Field Latitude Longitude 

No. N. w. 

877-6-3a2,a4 52°15 1 174°15 1 

877-7-3A,B,F 52°10 1 174°(5 1 

877-7-4C 52°1 o• 174°25 1 

877-9-1 52°15 1 174°15 1 

877-9-2A 52°15 1 174°15 1 

877-10- LA ,b 52°05 1 174°05 1 

877-10-2 52°05 1 174°10 1 

877-H1-5A,R 52°05 1 174°15 1 

877-11-1A 52°05 1 174°15 1 

877-1?-1 52°05 1 174°25 1 

877-12-4 52°0 5• 174°35 1 

877-12-H 52°05 1 174°40 1 

877-12-9,A 52°00 1 174°45 1 

877-13-1A 52ono• 174°25 1 

877-14-2d 52°00 1 174°45 1 

877-14-3R 5 2°00 I 174°55 1 

877-14-5 I I 52°00• 175°00 1 

877-15-4 52°05• 175°10 1 

877-15-5 52°00 1 175°05 1 

877-16-5 52°05• 174°45 1 

779-1fi-2 52°1 o• 174°10 1 

779-18-1A,C 52°10 1 174°10 1 

779-1g-1A 52°15 1 174°20 1 

779-24-1A 5 2°05 1 174°10 1 

779-24-2 52°05 1 174°40 1 

779-24-3 52°05• 174°40 1 

77g-24-4 52° o s• 174°40 1 

779-?4-5 52°05 1 174°40 1 

779-24-n 52°05 1 174°401 

779-24-7 52°05 1 174°40 1 

779-24-8 52°05 1 174°40 1 

Acknowledgments.-Many people assisted us in 

the various phases of our field and laboratory studies. 

We thank Comdr. J. C. McCrork, Adak Naval Station; 

R. E. Portlock, U.S. Bureau of Indian Affairs, B. 

Mcintyre, U.S. Geological Survey; and the captain and 

crew of the U.S. Coast Guard cutter Midgett for 

logistic support. The Atka Village Council kindly 

permitted us to use their community as a base of 

operations. We thank the people of Atka Village for 

providing lodging and supplies, especially George 

Kudrin and Mike Snigaroff, and our small-boat crew 

Nicolas Dirks, Ronald Snigaroff, and Edward 

Nevzaroff. We also thank D. W. Scholl of the U.S. 

Geological Survey for encouragement and logistical 

support. Scholl and M. S. Marlow, U.S. Geological 

Survey, critically reviewed the paper. B. D. Marsh 

shared results of his fieldwork on northern Atka 

Island. M. A. Fisher was our able field assistant. 

Rosemary Sliney, Sara Monteith, Marla Wilson, Mary 

Ann Allen, Eva Vanek, and Lisa Morgenson, all of the 

U.S. Geological Survey, provided technical assistance 

in the laboratory. Radiometric ages were provided by 

J. M. Mattinson, University of California at Santa 

Barbara, and M. L. Silberman and L. B. Gray, U.S. 

Geological Survey. Paleontologic ages were 

determined by D. Bukry, J. A. Barron, K. McDougall, 

J. A. Wolfe, R. A. Scott, and L. Marincovich of the 

U.S. Geological Survey, and F. H. Wingate of Cities 

Service Co. Chemical data were provided by H. T. 

Millard, D. McKown, R. Knight, J. Budahn, M. Taylor, 

M. Cremer, L. Espos, J. S. Wahlberg, J. Taggart, J. 

Baker, J. Kent, R. Lerner, and S. Neil, all of the U.S. 

Geological Survey. 

STRATIGRAPHY AND PALEONTOLOGY 

Southern Atka Island consists of a sequence of 

middle and upper Eocene and lower Oligocene 

(preplutonic) sedimentary rocks, interbedded with 

volcanic flows and breccias, that are intruded by early, 

middle, and late Miocene plutons. A sequence of 

middle and lower upper Miocene (synplutonic and 

postplutonic) volcanic flows, breccia, and sedimentary 

rocks unconformably overlies the older rocks; and in 

the vicinity of the isthmus separating the Korovin 

volcanic complex and the southern part of the island, 

the postplutonic rocks are, in turn, overlain by mostly 

Quaternary volcanic rocks of the Korovin volcanic 

complex (figs. 1-3). 

Paleogene sedimentary rocks 

The oldest rocks on Atka Island are a sequence 

of bedded marine sandstone, mudstone, and minor 

conglomerate that interfinger with massive volcanic 

flows, volcanic breccia, and rare pillow flows. 

Sedimentary rocks exposed along the west shore of 

Amlia Pass are mainly massive coarse-grained 

volcaniclastic sandstone with planar bedding and 

gentle dips (figs. 1, 2). Framework grains consist of 

moderately sorted subrounded fragments of basalt and 

andesite in a matrix of authigenic clay and zeolite. 

Marine fossils include fragments of oyster shells and 

rare fragments of thick-walled gastropod shells. Shell 

fragments occur most commonly in massive pebbly 

sandstone. The section northwest of Cape Utalug 

(figs. 1, 4~) is thinner bedded and includes graded 

turbidite beds from 0.5 to 2.0 m thick; rare flute casts 

indicate westerly flow. 

Thin-bedded to laminated fine-grained 

sandstone and siliceous mudstone sections are well 

exposed along the northeast shore of Egg Bay (fig. 4!2) 

on the north side of Atka and at Tillamook Cove on the 

south side. The rocks in both areas are highly 

indurated and altered owing to their proximity to 

intrusive rocks. Calcium carbonate, mostly 

contributed by foraminifers, is abundant in some 

samples (for example, 877-7-3F, tables 2, 3). Beds of 

massive breccia that contain angular clasts of basalt 

and andesite interfinger with the sedimentary rocks. 

Volcanic breccia is most abundant along the north 

shore, especially between Bechevin and Wall Bays. 

2 Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska 
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EXPLANATION 

Sandstone and volcanic conglomerate and breccia 

(upper and middle Miocene) 

Plutonic rocks (upper, middle, and lower Miocene) 

Sandstone, siltstone, and mudstone interbedded with minor volcanic 

breccia and flows(lower Oligocene through middle Eocene) 

Altered basaltic and andesite flows and breccia 

(Oligocene and Eocene?) 

5 10 15 20 KILOMETERS 

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Salt 

Island 

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Island 

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ALASKA 

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Island Atka Island .. 't .lumnak Island o 10 0 

<>~- • ""' • .,.,,: ~~- au? ~ Andrea~of Island Gl 

Figure 1. Geologic map of Atka Island, showing location of cross section A-A' in figure 3. 
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-tEXPLANATION 

Inferred fault 

Strike and dip of beds . Dashed 

where estimated 

Syncline 

Anticline 

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KOROVIN BAY 

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Sadatanak 

o Island 

Sagchuda k 

Is land 

Irregular outlines are 1000-ft contour. 

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Massive flows of dark-gray andesite and basalt 

also interfinger with strata of the sedimentary. 

section. Pillowed flows, however, were observed at 

only three places along the south shore-3. 75 km 

- northwest of Cape Tadluk and on the north and south 

sides of the entrance to Vasilief Bay. The thickness of 

the preplutonic sedimentary and volcanic section, 

which is not accurately known, is probably between 

1,000 and 2,000 m. 

Near-vertical faults of unknown displacement 

interrupt most sections. Correlations are further 

hampered by rapid facies changes and by truncation of 

strata by conglomerate, breccia, and volcanic flows. 

A conspicuous characteristic of the sedimentary 

sequences is rapid changes in bed thick-nesses and 

grain sizes, both laterally and vertically. Total 

organic-carbon content of these beds is typically less 

than 0.5 weight percent (table 2). 

Age-diagnostic fossils from the preplutonic 

sequence were recovered from only three places (table 

3). Poorly preserved fragments of a bivalve, identified 

as Ostrea cf. Q. tigiliana Slodkewitsch (1938), occur in 

pebbly sandstone along the shore northwest of Cape 

Utalug. The species is reported in Eocene and 

Oligocene rocks of the Alaska Peninsula and 

Kamchatka (L. N. Marincovich, written commun., 

1979). Calcareous nannofossils from the same vicinity 

suggest a middle Eocene through early Oligocene age 

on the basis of lower-latitude correlations (table 3). 

Foraminifers were observed in thin sections of highly 

indurated fine-grained sandstone from the northeast 

shore of Egg Bay; one rock from Egg Bay is considered 

to be of Eocene age on the basis of the occurrence of 

Globigerina linaperta Finlay (table 3; Gerta Keller, 

written commun., 1982). Lithologically correlative 

strata on adjacent Amlia Island have an age of late 

Eocene and early Oligocene on the basis of several 

nannofossil assemblages (McLean and others, 1983). 

Thus, fossils from Atka Island and correlations with 

dated rocks from other Aleutian Islands suggest that 

the preplutonic sedimentary rocks were most likely 

deposited during part of or all of the middle and late 

Eocene and the early Oligocene, approximately 50-30 

m.y. B.P. 

A 

(j) 1000 SOUTHWEST a: 

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LEVEL 

Middle and Upper Miocene (Synplutonic) Sedimentary 

Rocks 

Synplutonic and postplutonic (on the basis of 

paleontologic and radiometric ages) sedimentary rocks 

crop out mainly around Martin Harbor on the south 

shore of Korovin Bay, and along· the north shore of 

Korovin Bay east of the abandoned Russian settlement 

of Korovinski. Sedimentary rocks on the west shore of 

Martin Harbor include massive sandstone, volcanic 

conglomerate, and a bed of black carbonaceous 

mudstone, about 50 em thick. This mudstone actually 

consists of coaly beds, 2 to 3 em thick, interbedded 

with light-gray volcaniclastic sandstone. The beds 

crop out in the intertidal zone and probably constitute 

the section from which Cretaceous(?) leaf fragments 

'had been collected (R. R. Coats, oral commun., 

1977). Samples we collected contain woody and 

resinous debris but are barren of palynomorphs. 

Because the strata on both sides of Martin Harbor have 

similar structural attitudes, are similar in lithology, 

and contain oxyhornblende, we believe that the strata 

on the west shore may also be middle to late Miocene 

in age. 

Previous work by Grewingk (1850) and R. R. 

Coats (oral commun., 1978) focused on sedimentary 

rocks bearing fossil logs, from the south shore of 

Korovin Bay at the mouth of Martin Harbor. Thes logs 

consist of carbonized and silicified stumps and 

·assorted fragments of an unidentifiable species of 

conifer (fig. 5A). The plant debris occurs within a 

massive poorly sorted volcanic debris flow that 

contains boulder-size fragments of unaltered 

hornblende andesite (fig. 5~). Sandstone interbedded 

with the debris flow contains fresh oxyhornblende and 

slightly altered tuff. 

Palynomorphs from the matrix of the logbearing unit indicate a probable Homerian age (late 

middle and early late Miocene), or about 14-8 m.y. 

B.P. (Jack A. Wolfe, written commun., 1979). 

Additional age control on the Martin Harbor section is 

provided by a K-Ar age of 6.6 m.y. from an andesite 

plug that appears to intrude the sedimentary rocks (B. 

A' 

NORTHEAST 

SARAN A KOROVIN 

COVE BAY 

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0 5 10 15 KILOMETERS 

VERTICAL EXAGGERATION X10 

Figure 3. Schematic and interpretive cross-section of Atka Island. 

Stratigraphy and Paleontology 5 
D. Marsh, written commun., 1979) and thus places a 

minimum age on the sedimentary section. A basalt 

flow interbedded with the sedimentary rocks at the 

head of Martin Harbor yielded a K-Ar age of about 11 

m.y. (table 4). Basaltic andesite associated with 

similar rocks on the south shore of Bechevin Bay 

yielded a whole-rock K-Ar age of about 12 m.y. (table 

4). 

A diatom assemblage from a 50-em-thick 

siltstone bed interbedded with massive volcanic 

pebble-cobble conglomerate along the northeast shore 

of Korovin Bay indicates a Miocene age (J. A. Barron, 

written commun., 1980). Thus, synplutonic and 

postplutonic sedimentary rocks were probably 

deposited during the middle and late Miocene, about 

14-7 m.y. B.P. 

PETROGRAPHY OF SEDIMENTARY ROCKS 

The sedimentary rocks in the area of Amlia 

Pass between Cape Utes and Cape Utalug are the least 

altered of the preplutonic strata. Westward from 

Cape Utalug toward Vasilief Bay, increasing alteration 

of the rocks probably reflects increased temperatures 

associated with the intrusion of Miocene plutons. 

Primary framework grains in clastic rocks in 

the Amlia Pass area consist of fresh volcanic 

plagioclase, various volcanic-rock fragments (mostly 

porphyritic rocks with altered vitreous groundmasses), 

clinopyroxene, opaque grains, and rare fragments of 

microfossils and quartz. Secondary minerals are 

mostly pore-filling cements of smectite, chlorite, and 

suites of zeolites, including heulandite, analcime, 

stilbite, and minor laumontite and phillipsite. Clay 

minerals form rim cement in some rocks; the 

remaining pore space is filled with zeolites. Calcite 

and quartz cement some rocks. Chlorite replaces 

volcanic-rock fragments, and chlorite and smectite fill 

vesicles in the altered-rock fragments. 

The sandstone and siltstone between Vasilief 

and Beaver Bays become increasingly altered toward 

Beaver Bay. Framework grains of plagioclase are 

albitized or replaced by laumontite. Volcanic-rock 

fragments are likewise replaced by suites of secondary 

minerals, including smectite, laumontite, chlorite, and, 

more rarely, epidote. Rare microfossils occur as 

ghosts or are strongly recrystallized. Clinopyroxene 

remains fresh; in places, ferromagnesian minerals are 

replaced by chlorite and calcite. Secondary quartz and 

chlorite commonly fill pores and replace glassy 

matrix. Laumontite is the dominant zeolite. 

Figure 4. Paleogene sedimentary rocks of Atka Island. /1, Section just northwest of Cape Utalug, showing a thick basal bed of pebbly sandstone overlain by an upward-thickening sequence of sandstone beds. ~' 

Section on the east side of Egg Bay, consisting of thin-bedded silicified sandstone and shale, dipping about 

70°. Some beds contain abundant foraminifers. 

6 Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska 
On the south side of Explorer Bay, prehnite 

becomes an abundant secondary mineral in altered 

feldspathic subwacke. Other secondary minerals that 

replace feldspar and glassy volcanic groundmass are 

quartz, chlorite, and laumontite. Minor pumpellyite, 

natrolite, and mordenite also occur. 

Prehnite and quartz are major secondary 

minerals in altered tuffaceous siltstone interbedded 

Figure 5. Synplutonic and postplutonic rocks of Atka 

Island. &, Carbonized and silicified logs near Martin 

Harbor, enclosed in volcanic debris flows. ,!2, Boulderto sand-size volcanic debris flows of Neogene age near 

:Vlartin Harbor. 

with pillow basalt about 3.5 km northwest of Cape 

Tadluk along the central coast. Interpillow chert is 

composed of quartz and minor amounts of prehnite, 

epidote, zeolites, and celadonite. Much interpillow 

sediment is replaced volcanic glass that was spalled 

from pillows when the flows entered the sea. 

Plagioclase is completely replaced by prehnite, and 

zeolites and silica replace volcanic glass in the 

·matrix. The pillow lava also contains abundant 

prehnite and pumpellyite and accessory epidote, 

chlorite, and stilpnomelane. Pumpellyite replaces 

plagioclase and fills some vesicles. Other vesicles are 

filled with quartz, chlorite, minor epidote, feldspar, 

pyroxene, and prehnite. Pyrite veins also occur in the 

section. 

Preplutonic sedimentary rocks in the western 

part of the island, from Tillamook Cove to Atka Pass, 

are strongly altered, owing to the intrusion of plutonic 

rocks that crop out discontinuously between Sergief 

and Kigun Bays. Some rocks close to the contact with 

the pluton at Sergief Bay are hornblende-hornfels 

facies and have a metamorphic granular texture. They 

'are composed of quartz, epidote, piedmontite, altered 

green hornblende with hematite rims, and large 

patches of pyrite. Pyrite also is common along the 

margins and within the Sergief Bay pluton. 

Sequences of altered sedimentary rocks on the 

north shore of Atka Island crop out in Podsopochni 

Bay, along Island Point, and along the northeast shore 

of Egg Bay. Primary framework grains of porphyritic 

pyroxene andesite contain albitized plagioclase 

microlites and groundmass glass replaced by chlorite, 

epidote, calcite, and quartz, with traces of prehnite 

and pumpellyite and suites of zeolites including 

natrolite, heulandite, and analcime. 

The Egg Bay section consists of highly indurated 

thin-bedded fine-grained sandstone and laminated 

siltstone. Sandstone is a poorly sorted wacke 

. composed of altered glassy volcanic-rock fragments, 

clinopyroxene, recrystallized foraminiferal tests, 

albitized plagioclase, and the opaque minerals 

hematite and magnetite. Secondary minerals include 

analcime, quartz, calcite, chlorite, and traces of 

several other zeolites that may be clinoptilolite, 

heulandite, and thompsonite. Rare ghosts of 

radiolarians appear in laminated siltstone. 

Miocene sandstone surrounding Martin Harbor 

consists of angular, fresh grains of plagioclase, 

oxyhornblende, quartz, and volcanic-rock fragments 

cemented by quartz and clay minerals, mostly 

smectite. Fossilized logs contained in interbedded 

volcanic debris flows are silicified. Black 

carbonaceous mudstone and sandstone consist of 

plagioclase, quartz, pyrite, rare zeolites and Kfeldspar, and a great variety of clay minerals, 

including chlorite, smectite, vermiculite, and mixedlayer varieties of these three clay minerals. 

IGNEOUS ROCKS 

The igneous rocks of the Aleutian Islands have 

been discussed mainly by Byers (1961), Coats (1952, 

1962), DeLong (1974), Marsh (1976, 1982), Kay (1977, 

Igneous Rocks 7 
1978, 1984), Kay and others (1978, 1982, 1983), Citron 

and others (1980), and Perfit and others (1980). In this 

section, we describe some selected igneous rocks from 

the Tertiary sequence on Atka Island and comment on 

their relations to other rocks in the arc. Details on 

the chemistry of all the igneous rocks of Atka Island, 

including the accuracy and precision of the analyses, 

are available for L. A. Morgenson and others (unpub. 

data, 1983). 

The igneous rocks from Atka Island include both 

volcanic and plutonic rocks. The volcanic rocks 

apparently were extruded during three eruptive 

episodes, whereas available radiometric data suggest 

that only one plutonic episode affected the geologic 

evolution of the island. The earliest volcanic episode 

(preplutonic) was submarine and of middle(?) and late 

Eocene and early Oligocene age similar to those of 

nearby Amlia Island (McLean and others 1983). A 

second volcanic episode (synplutonic and postplutonic) 

may be represented by the basalt and basaltic andesite 

flows that crop out near Martin Harbor, which have KAr ages of 11-12 m.y. (Table 4). The youngest volcanic 

episode is associated with the growth of the large 

volcanic cones of the Korovin volcanic complex of 

latest Miocene to Holocene age. According to Marsh 

(1982), the extrusive rocks of the complex range in 

composition from basalt to dacite (46-67 weight 

percent Si02); most are basaltic. Koniuji Island, a 

small island north of Atka, is composed of Quaternary 

basalt and andesite (50 and 58 weight percent Si02, respectively). 

The plutonic rocks on Atka were apparently 

intruded during the early, middle, and late Miocene 

(tables 4, 5), but there may have been an earlier phase, 

as described below. 

We classify the igneous rocks by texture and 

Si02 content (corrected for H20 and C02

) rather than 

by modal mineralogy, as favored by Streckeisen (1976, 

1979). The meaningfulness of a modal classification of 

these rocks would be tenuous because of extensive 

replacement of the primary minerals in some rocks and 

the difficulty in distinguishing the alkali feldspars 

petrographically. Basalt, gabbro, and norite contain 

less than 52 we~ght percent S~o • Diabase c?ntains 

less than 54 weight percent SI02 and has a diabasic 

texture. Basaltic andesite and meladiorite contain 52 

to 54 weight percent ~io , at:t_desJte ~!)d diorite frof!! 

54 to 59 weight percent Sio2• Dacite and quartz 

diorite contain than 59 weight percent Si02• 

The petrographic and chemical data for 31 

igneous samples from Atka Island are available from L. 

A. Morgenson and others (unp.ub. data, 1983). For this 

report, we have selected 10 samples to show the 

diversity and range in compositions. Mineralogic and 

chemical data of the selected samples (tables 6, 7) are 

supported by plots of chemical data from all the rocks 

analyzed (fig. 6), from only the mafic rocks (fig. 7), 

and of rare-earth-element (REE) data from five rocks 

(fig. 8). 

Plutonic rocks 

Plutons of gabbro, norite, meladiorite, diorite, 

quartz diorite, and quartz-rich tonalite, as well as 

hypabyssal dikes and sills of diabase, crop out 

primarily in the central and western parts of Atka 

Island. The largest pluton, informally named the 

Beaver Bay pluton, crosses the island from Beaver Bay 

on the south to Bluefox and Banner Bays on the north, 

and probably includes Salt Island (fig. 1). A smaller, 

second pluton, informally named the Sergief Bay 

pluton, crops out in the isthmus area of Sergief Bay. A 

third pluton, informally named the Crescent Bay 

pluton, forms much of the west end of the island 

between Bechevin and Kigun Bays. Each pluton 

intrudes a sequence of highly indurated sedimentary 

and volcanic strata that were described in preceding 

sections. Some plutons may have more than one 

intrusive phase and thus are more composite than our 

reconnaissance mapping shows. Near contacts, the 

preplutonic and older(?) plutonic rocks are riddled by 

.dikes and discolored by iron staining caused by the 

oxidation of pyrite. 

The plutonic rocks are mainly quartz diorite, 

Table 2. Carbon content of Atka Island sedimentary rocks 

[All values in weight percent. Samples 779-19-1A and 877-9-2A are from the 

synplutonic sequence; all others are from the postplutonic sequence] 

Sample Litho 1 ogy Inorganic Organic Total Caco carbon carbon carbon 3 

877-7-38 Silicified siltstone---- 0.04 0.07 0.11 0.33 

877-ln-5 Silty sandstone--------- .02 .10 .12 .16 

877-11-lA Volcanic lithic wacke--- .Hi .11 .27 1.32 

877-10-lA Feldspathic wacke------- .01 .14 .15 .07 

877-14-3!3 Siltstone--------------- .01 .14 .15 .09 

779-Hi-2 Silty sandstone--------- .02 .15 .16 .12 

779-19-lA Sandy siltstone--------- .02 .17 .19 .16 

877-7-3A Siltstone--------------- .28 .25 .53 2.30 

8 77 -10-5A do-------------------- .05 .36 .41 .41 

877-7-3F Black shale------------- 1.10 .69 1.79 9.15 

877-9-2A Coaly siltstone--------- .06 10.06 10.12 .51 

8 Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska 
but locally gabbro, norite, diorite, and tonalite are 

abundant. Most rocks have hypidiomorphic granular 

textures, and euhedral to subhedral crystals of 

plagioclase, mafic minerals, and Fe-Ti oxides occur 

with anhedral quartz and alkali feldspar (table 6). 

Some of the plutonic rocks (for example, samples 1 and 

3) are metamorphosed, probably by heat from later 

intrusions, and have actinolite, chlorite, epidote, 

Table 3. Fossils from Atka Island 

[Dating of fossils by J. A. Wolfe and R. A. Scott (palynomorphs), John Barron (diatoms), L. Marinkovich and F. H. Wingate 

(bivalves), Gerta Keller and K. McDougall (foraminifers), and David Rukry (nannofossils)l 

Sample 

R77-6-3a2 

877-6-3a4 

Fossils 

Lycopodium, Osmunda polyEodl aceae, ~ taxolaceae, Ab1es, Lar~ 

PSeU'd'Otsuga, Pi cea, Pi nus, 

Tsuga, AI nus ,""""'BetUTa-,--

Myri ca ,centrospermae, 

Erl"'C'aTe s • 

877-10-1b Pinnularia sp., 

Cosc1nodiscus sp. 

877-14-2d Freshwater pennates, 

779-19-1A 

including Pinnularia sp.; 

benthic mar1ne: Cocconeis 

spp.; other marine: 

Grammatophora angulosa 

(Ehrenberg), Cosc1nod1scus 

fragment, Rhizosolenia 

fragment. 

Biddulphia aurita (Lyngbye) 

Brebisson-ana-Godey, B. 

rhombus (Ehrenberg) Smith, 

Cci'C'C'Oiiei s costata Gregory, 

C. dirupta Gregory, C. 

scutelTi:iiilEh renberg ,-C. spp. 

Cosc1nod1scus cf. C.-

kutZ1ng1i Schmidt,-

Ceratulus sp., Endictya sp., 

Hyalodiscus sp., Melosira 

clav1gera Grunow, M. sulcata 

(Ehrenberg) Kutzi ng, fifaVTCUTa 

directa (Smith) Ralfs, 

PlnnUTaria sp., Rhabdonema 

japon1cum var. rectum? 

Tempere and Rru~os ira 

maxima (Kutzing) Brunow, ~sionema sp. 

799-24-1A Ostrea cf. 0. tigiliana ~kewitsch (1938). 

Age (r1.y .) Locality and remarks 

Palynomorphs 

Late middle and 

early 1 ate 

Miocene (R-14). 

f)iatoms 

Eocene to Holocene 

(most likely 

Miocene through 

Holocene). 

Most likely Miocene 

through Holocene. 

Locality: Confluence of Korovin Bay with Martin Harbor. 

The two pollen assemblages are similar to one another 

and are dominated by Taxodiaceae, Alnus, and Betula. 

In the Kenai Group, Taxodiaceae persist into~ 

lower part (latest M1ocene) of the Clamgulchian Stage, 

but Abies last occurs in the Homerian Stage (late 

midrlre-IO early late Miocene). The absence of broadleaved exotic genera (particularly Ulmaceae and 

,Juql andaceae) favors a Homeri an rather than a preHomen an age. 

Locality: Cape Utalug. Nannofossils from correlative 

strata on Aml i a Island indicate a 1 ate Eocene to early 

Oligocene age (Mclean and others, 1983). 

Locality: Coastline northwest of Cape Tadluk. The 

freshwater and benthic marine specimens suggest a nearshore environment. The freshwater pennate diatoms 

indicate an age no older than Eocene. 

Miocene (5.5-24)------ Locality: Northeast shore of Korovin Bay. Environment 

is shallow marine with a minor freshwater component. 

Planktic species are very rare, and a more refined age 

estimate is not possible. 

Bivalves 

Eocene or Oligocene 

( 24-58). 

Locality: Coast nortf-Jwest of Cape lltalug. This species, 

questionably identified here because of poor preservation, occurs in Eocene and Oligocene rocks of the 

Alaska Peninsula and Kamchatka. It is the only 

Tertiary oyster that could be described as common on the Alaska Peninsula, and occurs in large numbers in 

sor1e fossil beds. Oysters always live cemented to 

hard substrates, such as rocks or other shells, and in 

modern seas live in depths ranging from the intertidal zone to 35 m, in regions extending from the Equator 

almost to the polar seas. 

Igneous Rocks 9 
prehnite, calcite, quartz, sphene, and zeolites as 

secondary minerals. Other rocks are relatively fresh 

(for example, samples 2, 4, and 5). The 

metamorphosed plutonic rocks may represent an 

earlier intrusive phase, but the age of that suspected 

event is not known. 

Radiometric ages of plutonic rock samples 

range from 8.5 to 18 m.y. (tables 4, 5). Two zircon 

Table 3. Fossils from Atka Island--Continued 

Sample Fossils Age (m.y.) 

analyses from tonal~\fs of 2:fRe Beaver Bay pluton 

yielded a concordant Pb/ U age of 15.1±0.1 m.y., 

which is considered to be an accurate cooling age 

(table 4; J. M. Mattinson, written commun., 1980). KAr dates from quartz diorite and meladiorite of the 

same pluton yielded ages of 17.5 and 18.1 m.y., 

respectively (table 4). K-Ar ages of 13.0 m.y. (biotite) 

and 11.8 m.y. (plagiocla~e) were obtained on quartz 

Locality and remarks 

Foraminifers 

R77-7-4C 

R77-7-3A 

R77-7-3F 

R77-10-2 

Globigerina linaperta Finlay--- Eocene (37-58)-------- Locality: Central north shore of Egg Ray. Thin 

section contains fragments of unidentifiable planktic 

foraminifers and rare Globigerina linaperta Finlay, 

which ranges in age from late Paleocene through 

Eocene and is most common in the Eocene. 

Coccolithus, Dictyococcites, 

Ret1culofenestra?. 

Coccolithus eopelagicus, 

Cycl1cargol1thus floridanus, 

Dictyococcites scrippsae, 

Reticulofenestra sp., among 

sparse placol1th fragments. 

Reticulofenestra, Dictyococcites scripps~ 

bisectus?, Coccol1thus 

eopelagicus?. 

Nannofossils 

Eocene to Pliocene 

(5.5-58). 

Middle Eocene to 

middle Miocene 

( 16-52) 0 

Mi dd 1 e Eocene 

through 1 ower Oligocene (30-52), on the basis of 

lower-latitude 

correlations. 

Locality: Samples 877-7-3A and 877-7-3F are from the 

north side of Egg Bay, and sample 877-10-2 is from the 

northwest shore of Cape Utalug. The best evidence 

from coccoliths is that the samples are middle 

Tertiary (not Paleocene or Pliocene). Fragmentation 

and low diversity indicate that environmental and 

diayenetic effects are indistinguishable. 

Table 4. K-Ar analyses of Atka Island igneous rocks 

[l•lhole-rock samples were ground and sieved through 35- and 60-mesh screen. The powder was washed and treated with dilute hydrofluoric 

acid, then with dilute nitric acid, and briefly placed in an ultrasonic bath. These techniques enhance the radiogenic-argon content and 

increase precision but do not adversely affect the accuracy of the age determination. Error limits represent estimated ana lyt~ cal 

precision based on uncertainties in the analyses of K and Ar. Constants used in the calculation of age are: Ae+e'=0.581x1o-1 yr-1, As= 

4.91i3x1o-10 yr-1, and 40K/K=l.167xln-4 atom percent] 

Average 

40Arrad 401\rrad 

Sample Location Rock type K 0 Apparent Remarks content (1 o-1 ° mol/g) 40

Artotal age 

(wt pet) 

877-15-4 Middle of headland Quartz diorite------ n.'iOfi o.onsn 0 .1fi 8.9±0.4 Hornblende separate. 

between Crescent .506 • 059? .10 8.1±0.8 Do • 

and Rechevin Rays. 8.5±0.4 Average. 

877-9-1 Southwest shore of Rasa l t-------------- .878 .1408 .28 11.1±0.4 Whole rock. 

Martin Harbor. 

877-15-5 South shore of Rasaltic andesite--- .6?4 .1076 .17 11.9±0.6 Do. 

Rechevin Ray. 

877-14-5-I I West-central shore Tonalite------------ 8.53 1.575 • 63 13.0±0.4 Biotite separate • 

Sergief Ray. 1.639 .32 -------------------------------- 1. 772 .3032 .35 11.8±0.05 Plagioclase separate. 

877-12-9 West-central shore Quartz diorite------ 1.131i .2869 .35 17.5±0.07 Whole rock. 

of Reaver Bay. 

877-1?-8 North shore of Mel ad i orite--------- 4.1i70 1. ~50 .35 18.1±0.8 Biotite separate. 

Reaver Ray. 

10 Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska 
diorite of the Sergief Bay pluton; quartz diorite from 

the Crescent Bay pluton (hornblende) is 8.5 m.y. old. 

Thus, the plutons appear to young toward the west. 

The radiometric ages, however, were defermined on 

relatively fresh and unaltered rocks, and we 

recognized more highly altered plutonic rocks both in 

the field and in thin section. Therefore, other, older 

plutons probably occur on Atka Island. Confirmation 

of the apparent younging of plutons to the west and 

the possibility that older plutons occur will require 

more detailed sampling and dating of a statistically 

significant number of rocks. 

Volcanic rocks 

Most of the Tertiary (pre-Korovin volcanic 

complex) volcanic rocks are basalt, basaltic andesite, 

and andesite. They are typically porphyritic and 

contain euhedral phenocrysts of plagioclase, 

clinopyroxene, and Fe-Ti oxides in a groundmass of 

secondary minerals. Plagioclase is commonly mottled 

and partially albitized; in some rocks, it is completely 

replaced by calcite or prehnite. Pyroxene phenocrysts 

are generally unaltered, but in some rocks they are 

completely replaced. 

Secondary minerals replace glassy 

groundmasses, rim or fill vesicles, and fill fractures. 

Secondary minerals include several types of zeolites, 

calcite, chlorite, epidote, prehnite, pumpellyite, and 

smectite. Radiometrically dated volcanic rocks (!!!ble 

4) are extensively altered. Sample 877-9-1 (table 6), 

for example, contains calcite, smectite, zeolites, and 

chlorite; other samples are similarly altered. 

Massive flows of pale-gray holocrystalline finegrained andesite crop out along the south shore 

between Vasilief Bay and Sadatanak Island. The rocks 

are essentially unaltered and form much of the 

highland area between Kobakof and Vasilief Bays. On 

the point of land directly north of Sadatanak Island, 

massive flows of holocrystalline andesite that 

concordantly overlie relatively unaltered sedimentary 

rocks grade laterally into discordant intrusive diorite. 

Field relations and petrographic similarities suggest 

that the extrusive rocks are comagmatic with the 

plutonic rocks. Although time did not permit closer 

examination, we believe that much of the island above 

300-m elevation may be composed of these 

comagmatic middle Miocene andesitic flows and 

breccia. 

Geochemistry 

A thorough discussion of the igneous-rock 

petrogenesis, including alteration, is beyond the scope 

and purpose of this report and, in fact, should be 

attempted only after more field relations are 

Table 5. IJ-Pb analyses of zircon from tonalite (sample 779-25-1) of the Beaver Ray pluton, 

Atka Island 

[All isotopic compositions normalized to absolute values by means of replicate analyses 

~n~ng ~a~· ~Jational ~~rea~ ~f Standards P~ st~~%ards qp,1 to gp,3, Cornman Pb corrections: 

Pb/ Pb, 19.0; 2 Pb/ 0 Pb, 15.6. 20 Pb/ Pb ages for the sample are insignificant 

because of the ~h}ght enrichment of radiogenic Pb and the uncertainty in correc2~gg f8r the common isotope - Pb, although these ages are concordant. Thus, the composite - U/ 6pb 

age of 15.1±0.1 m.y. should represent an accurate cooling age for the pluton] 

Age (rn.y.) 

Fraction size 23811 206pb ?08pb 207pb 204pb 

(ppm} (ppm) 20fiph 20fipb 206pb 206pb 207pb 

238u 206pb 

Coarse------ 330.8 0.674 0.4188 0.1122 0.004475 15.15±0.15 15±30 

Fine-------- 426.8 .861 .2827 .08724 .002754 15.00±0.15 34±?.0 

Table fi, Classification, ages, textures, and mineralogy of chemically analyzed Tertiary igneous rocks of Atka Island 

[Sample numbers correspond to those in figure 8 and table 7. Ages: P, preplutonic; S, synplutonic; --, not determined. Textures: n, 

diabasic; F, felty; H, hypidiofT!orphic granular; I, intersertal; P, porphyritic; X, pilotaxitic. Minerals: Act, actinolite; An, analcime; 

Bio, biotite; Ca, calcite; Chl, chlorite; Cpx, clinopyroxene; Ct, clinoptilolite; Ep, epidote; Gl, glass; H, hornblende; He, heulandite; Hm, 

hematite; Ol, olivine; Opx, orthopyroxene; Or, orthoclase; Ox, Fe-Ti oxides; Pl, plagioclase; Pr, prehnite; Otz, quartz; Sm, smectite; Sp, 

sphene; Ze, zeolites. (x), determined by X-ray diffractionl 

Sample Field No. Rock type Age (fTI.y.) Texture Primary minerals Secondary minerals Remarks 

1 877-13-1A Gabbro------------ H Pl ,Cpx,Ox,Op, Act ,Chl ,Pr ,Otz ,Or(x) Metamorphosed. 

2 ,''77-12-9A Hornblende norite- 17 ,'i H Pl ,H,Opx,Bio,Cpx,Ox Ct( x) Relative-ly unaltered. 

3 877-12-1 [)i abase----------- [) Pl ,Cpx ,Ox Ch l , P r, Ca , Hm , nt z ( x) , Ze Metamorphosed. 

4 877-12-8 Mel ad i ori te------- 18.1 H Pl ,H,Bio,Ox,rJpx Act ,Chl ,Sp Rel ati ve1y una 1 tered. 

5 877-14-5II [)11artz diorite---- 12.4 H Pl ,Cpx,Opx,Rio,Otz,H,Ox Act Do. 

6 779-::>5-1 Quartz diorite---- 15.1 H Pl ,Bio,H,Qtz,Cpx,Ox,Opx Act no. 

7 877-10-58 Basalt------------ p p ,X Pl,Cpx,Ol,Ox Sm,Ca,An(x) ,He(x) Abundant zeo1 i tes. 

8 877-9-1 Rasa 1 t------------ 11.1 p, X Pl ,Cpx,Ox,Gl ,H,Opx Ca , Sm, Ze, ffn, Ch l Altered. g 779-18-1A Basaltic andesite- p F ,P Pl , Ox Ep,He(x) ,Chl ,Hm,Otz,Sm Highly altered. 

10 877-1::>-4 Andesite---------- s I, p Pl , Ox ,Gl Ch1 ,Sm,Ca,Ep,Pr,Sp Metamorphosed. 

Igneous Rooks 11 
determined and additional laboratory data are 

available. We can, however, discuss the range of rock 

compositions, comment on the differentiation trends, 

and compare the rocks with other rocks from the 

·Aleutian Islands. 

Tables 6 and 7 list the ranges in composition of 

the modal mineralogies and chemical analyses, 

respectively. Si02 contents, corrected for H2o and 

C02, range from ~7 .5 to 66.3 weight percent (L. A. 

Morgenson and others, unpub. data, 1983). Al20 3 content is relatively high, similar to that of rocks in 

other island arcs (Gill, 1981). Ti02 content ranges 

widely, from 0.46 to 1.43 weight percent plutonic 

rocks and from 0.57 to 1.32 weight percent volcanic 

rocks. The average Ti02 content for 16 mafic rocks 

(less than 52 weight percent Si02) is 0.82 weight 

percent. Na2o content ranges widely; some high 

values are apparently related to zeolitization and (or) 

albitization. K2o content does not exceed 2.73 weight 

percent in the plutonic rocks and 1.26 weight percent 

in the volcanic rocks. In general, the range in Si02 

contents, the high A120 3 and low Ti02 contents, and 

Table 7. Chemical analyses of representative Tertiary igneous rocks of Atka Island 

[Major-element analyses in weight percent; minor-element analyses in parts per million. Major ele~ents and some minor 

and trace elements determined by X-ray fluorescence, and FeO by wet chemistry; *, determined by neutron activation. 

Sample numbers correspond to those in table 6. not determined or helow detection limitJ 

Sample-----

Si02------- 49.66 

TiOz------- 1.17 

Al 2o3------ 15.61 

Fe2o3------ 4.05 

FeO-------- 7.62 

MgO-------- 6.00 

CaO-------- 9.41 

Na 2o------- 3.49 

K20-------- .54 

P2o5------- .zo 

H

2

o+_______ 1.76 

H2o-------- .88 

C02-------- .12 

Total--- 100.51 

Mn--------- 1,208 

Rb---------

Sr---------

Zr---------

Cr---------

sc*-------- * 

Ba ~~*=~====~= * -------- * 

~=*======== * 

~~:======== 

~r~~~~~~ * 

~~*======== * 

u i~*======== * ---------

c66 

59 

89 

42 

38 

31 

134 

5.02 

12.00 

10.40 

2.72 

.9n 

3.84 

.57 

2.12 

.35 

1.46 

.78 

.43 

2 

50.12 

.63 

14.84 

4.fiR 

5.71 

7.20 

10.99 

3.fi6 

.90 

.18 

.fi7 

.14 

.30 

100.02 

1,805 

23 

388 

37 

294 

38 

27 

55 

110 

9.78 

22.30 

14.10 

3.31 

.87 

3.21 

.47 

1.35 

.23 

1.50 

.46 

1.7n 

.82 

3 

50.82 

.94 

16.48 

4.30 

5.30 

5.50 

8.10 

4.28 

1.11 

.18 

2.39 

• 74 

.35 

100.49 

1,053 

17 

295 

104 

Clfj 

32 

31 

39 

225 

6.45 

14.50 

10.70 

2.78 

.89 

3.34 

.55 

2.04 

.34 

1. 70 

.11 

1.1fi 

.fi3 

4 5 6 

Major-element oxides 

52.fi0 

.86 

19.17 

2.91 

3.94 

5.20 

8.6(1 

4.~3 

.74 

.25 

1.25 

.16 

.05 

100.08 

60.02 

.58 

16.02 

2.31 

2.87 

4.40 

fi.28 

4.11 

?.OR 

.15 

.5fi 

.50 

.09 

99.97 

fi1.10 

.63 

1fi .10 

1. 55 

3.0(1 

4.04 

5.44 

3.61 

2.52 

.14 

.85 

.10 

.46 

99.56 

Minor and trace elements 

651 

fi97 

74 

75 

If; 

18 

47 

194 

7. 71 

17.50 

11. no 

2.58 

1. 02 

2.18 

.42 

1.3fi 

.20 

1.07 

.fl7 

.25 

.14 

759 

403 

148 

164 

18 

32 

71 

128 

5.29 

13.30 

9 • .17 

3.12 

.98 

.56 

2.12 

.34 

1.36 

.14 

1.09 

.69 

629 

45 

501* 

181* 

119 

14 

16 

388 

13.00 

27.90 

16.60 

3.60 

.92 

3.08 

.45 

1.50 

.22 

5.33 

.33 

3. 72 

2.12 

48.51 

.6fi 

14.40 

3.89 

4.35 

7.5() 

11.57 

4.31 

.70 

.14 

2.11 

2.02 

.8? 

100.98 

1,324 

11 

137 

44 

397 

47 

42 

71 

319 

2.12 

4.9fi 

5.9? 

1.53 

.58 

2.24 

.33 

1. J 7 

.?6 

.16 

12 Reconnaissance Geology of Southem Atka Island, Aleutian Islands, Alaska 

50.08 

.87 

16.94 

3.19 

4.63 

5.60 

11.13 

3.52 

.75 

.17 

.03 

.83 

1.90 

1,100 

11 

40L 

67 

157 

35 

26 

31 

22(1 

5.75 

13.50 

11.30 

2.77 

1.01 

2.4(1 

.41 

1.47 

.24 

1.74 

.07 

.89 

.58 

g 

53.20 

.84 

21.20 

3.53 

3.83 

1.12 

8.95 

3.26 

.25 

.13 

2.75 

.41 

.04 

99.53 

2,330* 

o34 

10 

?1 

20 

fi .13 

13.110 

9.55 

2.45 

.83 

2.75 

.49 

1.77 

.27 

1.8Ll 

.09 

.98 

.5n 

10 

58.45 

.96 

15.47 

4.44 

4.20 

3.70 

3.?2 

5.46 

.29 

.17 

3.10 

.63 

.37 

100.46 

1,123 

137 

74 

75 

?I) 

23 

47 

7.45 

18.50 

13.20 

3.89 

1.05 

4.65 

.7P. 

3.39 

.51) 

3.37 

.21) 

1.29 

l.Ofi 
the range in alkali contents are similar to those of the 

late Cenozoic volcanic rocks in the Aleutians (S. Kay 

and others, 1982). 

Many of the rocks have high FeOT/MgO ratios 

(Fig. 6) at low Si02 content, indicative of ironenrichment (tholeiitic) differentiation trends 

(Miyashiro, 1974; Kay, 1977; Kay and others, 1982). 

Other rocks are calc-alkaline and have relatively 

constant FeOT/MgO ratios at all Si02 values. For 

comparison, the young volcanic rocks from the Korovin 

volcanic complex on Atka are tholeiitic (Marsh, 1982). 

The trace elements also have a wide range of 

values. Particularly noteworthy are the relative 

abundances of Sr, Ti, and Zr in the mafic volcanic and 

plutonic rocks (fig. 7), and the REE abundances (fig. 

8). An Sr-Ti-Zr ternary diagram (fig. 7) confirms the 

tholeiitic and calc-alkaline composition of the mafic 

rocks. None of the Tertiary rocks, however, is really a 

70 

Calc- alkaline Tholeiitic 

65 • 

I-' 

z 

w 

u 

a: • • w 

a.. 60 I- • 

I • 

Q • w 

~ 

~ 

I--

z 

w 

Iz 55 

0 

u 

• 0 • X 

en • • X 

• , X 

50 

• Plutonic rocks 

X Volcanic rocks • 

2 3 4 5 6 7 

Figure 6. Si02 content versus FeOT/MgO ratio for 

Tertiary rocks of Atka Island after (Miyashiro, 197 4, 

and R. Kay, 1977). Analytical data from L. A. 

Morgenson and others (unpub. data, 1983). Values not 

corrected for H20 and co2 contents. 

low-potassium island-arc tholeiite (LKT), even though 

the trace-element distributions plot in that field (Fig. 

7). 

The geographic positions of young volcanic 

centers in the Aleutian Island arc were correlated with 

their magmatic differentiation trends by Kay and 

others (1982), who concluded that the tholeiitic 

centers form either between or at the end of arc 

segments, where magmas can more easily reach the 

surface (for example, by rapid ascent). Calc-alkaline 

volcanoes, in contrast, occur in the middle of these 

segments, where transit through the upper plate above 

50 

Ti/1 00 

• Plutonic rocks 

x Volcanic rocks 50 

Figure 7. Triangular diagram of Sr-Ti-Zr content of 

plutonic and volcanic rocks (after Pearce and Cann, 

1973). All mafic rocks from Atka Island plot in or near 

the A (low potassium tholeiite) and B (calcalkaline 

basalt) fields. 

50 

L.LJ 

t: 

a: 

0 10 

z 

0 

I 

u 

-...__ 5 L.LJ 

L.LJ 

a: 

· .•.........•. 

---.....-

....., 

La Ce 

·.·-\~ 

__.......-- ·'.'. /· ....... ·-···-·-·-·-·-·-·-·-·-·__...·-· --- ...... .......... . - ' 

. ·. :--..-------------. _,..... ______ •··-·· .......... 

I 

2 

6 

7 

- 10 

'•·-·. ··-....... 

Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 

RARE EARTH ELEMENT (REEl 

Figure 8. Rare-earth element (REE) plot for selected 

rocks from Atka Island (tables 6, 7). Sample 7 is 

extremely depleted in REE, and sample 10 has a small 

Eu anomaly. 

Igneous Rocks 13 
a subduction zone is more difficult. The correlation of 

tectonic position with differentiation trend is more 

difficult with the older Tertiary rocks because of 

problems with age determination and because the 

tectonic regimes change over time in island arcs. 

Kay and others {i982) concluded that the 

similarity between the most primitive basalts from 

both differentiation series suggests initial 

crystallization from the same primitive parent 

magma. Apparently, initial crystallization was along 

an Fe-enrichment trend that was continuous for the 

tholeiitic series but was interrupted for the calcalkaline series. Therefore, differences in the style of 

differentiation can explain these two distinct series. 

REGIONAL PATTERNS OF ALTERATION 

In all the rock types, the most common 

secondary minerals are quartz and chlorite. Smectite, 

80 

60 

40 : 

20 : 

~ 0 

«: 

a: 

w 

~ 

::i" 

>-

a: 

«: 

0 

z 

0 

~ U) 

~ 

«: 

1-

z 0 0 

0 

1-

«: 

I 

1- 60 U) 

::.:: 

0 

0 

a: 40 

0 

U) 20 w 

(') 

«: 

1- 0 z 

w 

0 

a: 

w 80 0.. 

60 

A 

D 

zeolites (mainly heulandite, analcime, and laumontite), 

calcite, pumpellyite, prehnite, epidote, and amphibole 

also commonly occur (fig. 9). In some sedimentary 

rocks, secondary calcite occurs as recrystallized 

foraminifers and nannofossils. Other secondary 

minerals are alteration products of volcanic debris, 

vesicle infillings, or cement in sedimentary rocks. 

Secondary minerals not plotted in figure 9 include 

kaolinite, albite, hematite, pyrite, goethite, apatite, 

and sphene. 

Secondary minerals suggest that most of the 

rocks were subjected only to zeolite-facies 

metamorphism. Albite-epidote-hornfels- and 

hornblende-hornfels-facies rocks also occur but 

primarily on the west half of the island (west of 

Explorer and Banner Bays). Secondary minerals from 

the west half of Atka are dominated, in decreasing 

order of abundance, by chlorite, epidote, quartz, 

prehnite, amphibole, and pumpellyite, whereas those 

from the east half of Atka are dominated by quartz, 

chlorite, smectite, heulandite, analcime, laumontite, 

8 c 

F 

EXPLANATION 

1 QUARTZ 

2 SMECTITE 

3 CHLORITE 

4 ILLITEICELADONITE 

5 STILPNOMELANE 

6 LAUMONTITE 

7 ANALCIME 

8 HEULANDITE 

9 THOMSONITE 

10 NA TROLITE 

11 CHABAZITE 

12 MORDENITE 

13 ERIONITE 

14 STILBITE 

15 PHILLIPSITE 

16 UNIDENTIFIED ZEOLITE 

17 PUMPELLYITE 

18 PREHNITE 

19 EPIDOTE 

20 AMPHIBOLE 

21 K-FELDSPAR 

22 CALCITE 

Figure 9. Alteration products of Atka Island rocks. f:!, All rocks, based on 138 samples. ~' Sedimentary rocks, 

based on 65 samples. Q, Volcanic and pyroclastic rocks, based on 41 samples. J2, Plutonic rocks, based on 32 

samples. §, All rocks from east half of island, based on 81 samples. f, All rocks from west half of island, based 

on 57 samples. Upper histograms are for only extensively altered rocks; lower histograms are for all altered 

rocks within each numbered category. 

14 Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska 
and calcite. Higher grade metamorphic-mineral 

assemblages on western Atka reflect the influence of 

the intrusion of plutons there. Even though the higher 

grade minerals are more abundant to the west, more 

rocks are extensively altered to the east, 39 versus 55 

percent (table 8), assuming that the rocks collected 

are representative of the bulk of rocks that make up 

the island. Of all the rocks collected on Atka Island, 

48 percent are extensively altered. 

Table 8. Percentages of extensively altered 

rocks of Atka Island 

Rock type F.:ast 

half 

Sedimentary-------- 64 

Volcanic----------- 49 

Plutonic----------- 26 

All---------------- 55 

West 

half 

42 

39 

35 

39 

Whole 

island 

56 

45 

32 

48 

The rocks were divided into sedimentary, 

volcanic and pyroclastic, and plutonic categories. The 

sedimentary rocks contain the most abundant suite of 

secondary minerals (56 percent extensively altered) 

because they are composed of partly altered volcanicrock fragments and are cemented by a variety of 

authigenic minerals. In contrast, the plutonic rocks 

include the fewest samples that are extensively 

altered (32 percent); the volcanic rocks are 

intermediate (45 percent extensively altered; table 

8). Extensive alteration of some of the plutonic rocks, 

primarily to chlorite, amphibole, and epidote minerals, 

suggests albite-epidote-hornfels- and, possibly, lower 

hornblende-hornfels-facies metamorphism at 

temperatures probably near 350° to 500°C. 

In comparison with adjacent Amlia Island 

(McLean and others, 1983), Atka Island rocks are much 

more extensively altered-30 versus 48 percent, 

respectively. The dominant clay mineral on Atka is 

chlorite, and on Amlia smectite. Amlia rocks contain 

more zeolites, whereas Atka rocks contain more of the 

low-grade metamorphic minerals pumpellyite, 

prehnite, epidote, and amphibole. Again, these 

differences probably reflect the positions of plutons on 

Atka Island; none crop out on Amlia. 

STRUCTURE 

Folds 

The overall east-westward trend of Atka Island 

probably reflects the major structural features of the 

southern part of the island. As seen in shoreline 

outcrops, stratified rocks in the southeastern J>art of 

the island generally strike east-west and dip 10 -20° S. 

(figs. 2, 3). Dips locally exceed 70° where drag folding 

has occurred adjacent to faults. A large open anticline 

that trends northeast-southwest crosses the midpoint 

of Explorer Bay. Small folds of presumed local extent 

are common along the southeastern part of the island. 

Attitudes of the stratified rocks west of Beaver 

Bay are more irregular, probably complicated by 

intrusion of plutonic rocks; however, dips rarely 

exceed 2 5°. On the north side of the island, beds of 

massive sandstone and volcanic breccia in Wall Bay 

strike northwest and dip 35°-55° NE. A thin-bedded 

sequence of fine-grained sandstone and siltstone 

exposed on the northeast shore of Egg Bay likewise 

strikes northwest and dips 65°-80° NE. (fig. 4J:!). 

Faults 

Faults of presumably small displacement are 

abundant within the stratified sequences. The dip of 

most faults is near-vertical, and adjacent strata are 

little deformed except where drag folding has 

occurred, for example, along the northeast shore of 

Egg Bay and northwest of Cape Utalug. 

Faulting has created a pronounced east-westtrending topographic lineament that extends from the 

head of Egg Bay to Atka Village. The fault zone is 

well exposed in the seacliff just south of the village. 

Most faulting appears to be extensional in 

origin, probably related to the emplacement of plutons 

in the central and western parts of_ the island, and 

unrelated to compression associated with plate 

convergence along the Aleutian Trench. 

RESOURCE POTENTIAL 

Resources in many island arcs, especially the 

Aleutian arc, have not been adequately evaluated 

because most of the arc is under water, because those 

parts above water generally are associated with active 

volcanism, and because of logistic problems. Massive 

sulfides, Kuroko-type deposits, tin, gold, and other 

mineral deposits, however, are known from the 

Tertiary rocks of volcanic arcs (for example, Sato, 

1977). Atka Island rocks are worthy of extensive 

studies for economic minerals. 

Bottom sediment from seven streams that drain 

the Beaver Bay pluton were sampled to analyze for 

concentrations of metals. Sample 779-24-8 (bulk) 

contains relatively high concentrations of Cu (700 

ppm), Pb (300 ppm), and Ag (1.0 ppm) (table 9); sample 

779-24-5 (bulk) contains similar enrichments. Sn is 

high (100 ppm) in one sample (table 9). The average 

contents of 6 minor elements in 32 igneous rocks of 

Atka Island are also listed in table 9; these average 

values are comparable to those in rocks of the Beaver 

Bay pluton alone. Comparison of the river silt with 

the igneous rocks clearly shows enrichments of Mn, 

Cu, Pb, Sn, Ag, and Zn in the silt. Ag content of 1 

ppm is an enrichment of 25 times over the average for 

granite, 10 times over that for basalt, and 14 times 

over that of average continental crust (Taylor, 1964). 

Sn at 100 ppm is enriched 33 times over the average 

for granite, 100 times over that for basalt, and 50 

times over that for average continental crust. From 

these few data, the possibility of ore deposits 

associated with the Beaver Bay pluton cannot be 

evaluated, but future explorations should concentrate 

on the elements listed in table 9; other elements are 

'not enriched. 

Resource Potential 15 
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Small amounts of pyrite are concentrated in the 

contact zone around the Sergief Bay pluton, in black 

shale on the west side of Martin Harbor, and occur 

with goethite, in the fault zone near Atka Village. 

None of these concentrations of iron is large enough or 

of high enough grade to warrant development. 

Small bodies of hydrothermally deposited silica 

and iron (now hematite) occur between volcanic flows 

on tiny Uyak Island in Nazan Bay (table 10). At about 

20 weight percent Fe2o3 these deposits are too small 

and too low in grade for development. Sb at 394 ppm 

is significantly enriched in these hydrothermal bodiesabout 2,000 times over that of average continental 

crust, average basalt, and average granite (Taylor, 

1964). On Atka Island, Sb averages 0.100 and 0.343 

ppm for 8 extrusive and 24 intrusive igneous rocks, 

respectively; a single anomalously high value of 55 

ppm Sb was measured for a basaltic andesite but was 

not included in the average, which would be 6.99 ppm 

Sb for the extrusive rocks with it included • 

The reservoir and source-rock properties of 

rocks that crop out on Atka Island show little promise 

for the generation and accumulation of petroleum. 

The proximity of the sedimentary rocks to young 

plutons is responsible in large part for the reduction in 

porosity and permeability. Offshore basins, however, 

may show greater promise (Scholl and others, 1975; 

McCarthy and others, 1984) because. the rocks are less 

altered and have reservoir potential. 

SUMMARY AND CONCLUSIONS 

Southern Atka Island consists primarily of 

middle and late Eocene and early Oligocene (approx 

30-50 m.y. old) volcaniclastic rocks deposited in 

nearshore (probably intertidal) upper slope 

environments. Deposition occurred on fan-delta, shelf, 

and Aleutian Ridge summit-basin depositional sites. 

Sediment transport was by rivers, debris flows, and 

turbidity currents. Lava flows, pyroclastic deposition, 

and the intrusion of hypabyssal dikes and sills were 

contemporaneous. This Eocene and Oligocene 

sequence is recognizable on many of the Aleutian 

Islands, and at each site the oldest sequence is datable 

by fossils (for example, Scholl and others, 1975; Hein 

and McLean, 1980; McLean and others, 1983; McLean 

and Hein, 1984). On Adak and some other Aleutian 

Islands, an older group of gabbroic and basaltic rocks 

containing minor amounts of intercalated sedimentary 

rocks also occurs (Citron and others, 1980; Hein and 

McLean, 1980; Kay and others, 1983). These older 

rocks, which are part of the initial or early series of 

Marlow and others, (1973), probably are about 55 m.y • 

old (Hein and McLean, 1980); the bulk of the Aleutian 

Ridge formed rapidly during the early Tertiary. 

The Paleogene sequence on Atka was intruded 

by plutons during early, middle, and late Miocene time 

(18-8 .5 m .y. B.P .). Intrusion of plutons caused albiteepidote- and hornblende-hornfels-facies metamorphism 

(350°-500°C) of the surrounding rocks on western Atka 

Island, and zeolite- to prehnite-pumpellyite-facies 

metamorphism (200°-300°C) of the more distant rocks 

on eastern Atka Island. Plutons may have been 

emplaced during three separate periods at about 15 

16 Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska 
m.y., 12 m.y., and 9 m.y. B.P., but our data are 

insufficient to confirm this possibility. We also 

believe that an older group of plutons may occur on 

the island. 

Subaerial to shallow-marine (shelf) sediment 

and volcanic debris flows were deposited during the 

Table 10. Chemical analysis of a silica 

and hematite hydrothermal deposit (sample 

779-18-1C) in Nazan Bay, Uyak Island 

[Major-element analyses in weight percent; 

minor- and trace-element analyses in parts 

per million. Analytical methods: 1, X-ray 

fluorescence; 2, neutron activation; 3, 

emission spectroscopy] 

Value 

Major-element oxides 

Si02--------------- 77.6 

Al203-------------- .43 

Ti02--------------- .003 

Fe 2o3-------------- 19.5 

MgO---------------- .015 

CaO---------------- .30 

Na 20--------------- .073 

K20---------------- <.02 

P20~--------------- .21 

MnO---------------- .004 

Loss on ignition--- 1.12 

Total--------- 99.26 

Analytical 

method 

3 

1 

3 

1 

? 

Minor and trace elements 

8------------------ Ra-----------------

Ce-----------------

Co-----------------

Cr-----------------

Cs-----------------

Cu-----------------

Dy----------------- Eu----------------- r,d-----------------

La-----------------

Lu-----------------

Nb-----------------

Nd-----------------

Ni-----------------

Sb-----------------

Sc-----------------

Sm-----------------

Sr-----------------

Tb-----------------

II------------------

V------------------ Y------------------ Yb-----------------

Zn-----------------

Zr-----------------

50 

50 

6.2 

1. 30 

1.6 

1.16 

20 

4.5 

.599 

4.2 

6.87 

.631 

15 

13.0 

15 

394 

3.84 

2.97 

20 

• 7 2 

.47 

300 

70 

3. 70 

70 

78 

3 

3 

2 

2 

2 

2 

3 

2 

~ 

2 

2 

2 

3 

2 

3 

? 

2 

2 

3 

2 

2 

3 

3 

~ 

3 

2 

Neogene on Atka Island, about 14-7 m.y. B.P., 

contemporaneously with some of the intrusive 

activity. Conifer forests, which no longer exist 

anywhere along the Aleutians, were burned and buried 

by volcanic avalanches and flows. Black carbonaceous 

shale, containing mostly terrestrial plant debris, 

accumulated in small subaerial basins containing 

stagnant water. Neogene igneous plugs and dikes 

subsequently intruded the sections about 6 m.y. B.P. 

Synplutonic sedimentary rocks were also altered, 

mostly to zeolite facies but, in places, to higher 

metamorphic grades. Secondary quartz and chlorite 

are ubiquitous in Atka Island rocks. 

The Miocene, especially the middle Miocene, 

was a time of extensive and voluminous igneous 

activity along the entire Aleutian Island arc (Coats, 

1952, 1962; Marlow and others, 1973; Scholl and 

others, 1976; Hein and others, 1978). Middle Miocene 

plutons and contemporaneous volcanic rocks, like those 

on Atka Island, are known from many of the Aleutian 

islands (Citron and others, 1980). For example, middle 

Miocene plutons intrude the volcaniclastic rocks on 

Adak, Unalaska, and Kagalaska Islands (Carr and 

others, 1970; Marlow and others, 1973) in the Aleutian 

chain. Middle Miocene volcanic rocks exclusive of 

plutonic rocks occur on Shemya Island (Cameron and 

Stone, 1970). An older plutonic event of Oligocene age 

(approx 32-30 m.y. B.P.) recorded in rocks only on 

Adak (Citron and others, 1980) and Umnak Islands 

(McLean and Hein, 1984), and, possibly, on Amlia 

Island, has not been found on Atka Island, although 

volcanism occurred at that time on Atka Island; we 

believe that with detailed mapping it may also be 

found on Atka Island. Two much older episodes of 

plutonism are recorded in the rocks at the east end of 

the Alaska-Aleutian Range batholith (early and middle 

Jurassic and late Cretaceous and early Tertiary; Reed 

and Lanphere, 1973), but not on any of the Aleutian 

islands. 

· Thus, igneous events recorded on Atka Island 

include major Eocene and Oligocene volcanism (approx 

50-30 m.y. B.P.), major early, middle, and late 

Miocene plutonism and volcanism (18-8 m.y. B.P.), and 

latest Miocene through Holocene volcanism which 

includes the Korovin volcanic complex of the northern 

part of Atka Island. 

A higher metamorphic grade is observed in 

rocks from Atka Island relative to adjacent Amlia 

Island to the east, where plutons do not crop out. 

Whereas zeolites and smectite dominate on most of 

Amlia Island, prehnite, pumpellyite, epidote, 

amphibole, and chlorite dominate on Atka. Some 

plutons of Atka were extensively altered, primarily to 

chlorite, amphibole, and epidote, indicative of albiteepidote- and hornblende-hornfels-facies metamorphism. Similar degrees of metamorphism were noted · 

for Adak Island (Paleogene) plutons and adjacent rocks 

175 km west of Atka (Hein and McLean, 1980; Kay and 

others, 1983). As with the Eocene and Oligocene 

sedimentary rocks on Adak and Amlia Islands, the 

rocks on Atka show styles of deformation and · 

metamorphism, and textures of sandstones, that 

indicate only shallow levels of burial without regional 

metamorphism. Alteration and metamorphism result 

from hydrothermal activity that is driven by shallowseated plutons and, locally, by dikes and sills. 

Summary and Conclusions 17 
Gentle open folds and extensional faulting are 

related to the emplacement of plutons and not to 

compression associated with plate convergence along 

the Aleutian Trench. The dominant tectonic regime 

since the Eocene has been mild extension and highangle normal faulting. As on Amlia and Umnak Islands 

(McLean and others, 1983; McLean and Hein, 1984), 

little vertical tectonic movement during the Holocene 

is indicated by the undeformed wave-cut terraces. 

RHFHRHNCF.S CITED 

Byers, F. M., Jr., 1961, Petrology of three volcanic 

suites, Umnak and Bogoslof Islands, Aleutian 

Islands, Alaska: Geological Society of America 

Bulletin, 72, no. 1, p. 93-128. 

Cameron, C. P., and Stone, D. B., 1970, Outline 

geology of the Aleutian islands with 

paleomagnetic data from Shemya and Adak 

Islands: Anchorage, University of Alaska, 

Geophysics Institute Research Report U AG R213, 153 p. 

Carr, W. J., Quinlivan, W. D., and Gard, L. M., Jr., 

1970, Age and stratigraphic relations of 

Amchitka, Banjo Point, and Chitka Point 

Formations, Amchitka Island, Aleutian Islands, 

Alaska, in Cohee, G. V., Bates, R. G., and Wright, 

W. B., Changes in stratigraphic nomenclature by 

the U.S. Geological Survey, 1969: U.S. 

Geological Survey Bulletin 1324-A, p. A16-A22. 

Citron, G. P., Kay, R. W., Kay, S. M., Snee, L. w., and 

Sutter, J. F., 1980, Tectonic significance of early 

Oligocene plutonism on Adak Island, central 

Aleutian Islands, Alaska: Geology, v. 8, no. 8, p. 375-379. 

Coats, R. R., 1952, Magmatic differentiation in 

Tertiary and Quaternary volcanic rocks from 

Adak and Kanaga Islands, Aleutian Islands, 

Alaska: Geological Society of America Bulletin, 

v. 63, no. 5, p. 485-514. 

--1962, Magma type and crustal structure in the 

Aleutian arc, in Macdonald, G. A., and Kuno, 

Hisashi, eds., The crust of the Pacific Basin: 

American Geophysical Union Geophysical 

Monograph 6 (National Academy of SciencesNational Research Council Publication 1035), p. 

92-109. 

DeLong, S. E., 1974, Distribution of Rb, Sr, and Ni in 

igneous rocks, central and western Aleutian 

Islands, Alaska. Geochimica et Cosmochimica 

Acta, v. 38, no. 2, p. 245-266. 

Gill, James, 1981, Orogenic andesites and plate 

tectonics: Berlin, Springer-Verlag, 390 p. 

Grewingk, Constantin, 1850, Beitrag zur Kenntniss der 

orographischen und geognostischen 

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