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Geology of the Delarof and Westernmost Andreanof Islands Aleutian Islands, Alaska By GEORGE D. F'RMER end H. FRANK BARNETT

INVESTIGATIONS OF A L A S K A N VOLCANOES

Prepared in coofirration with the Departments of the Army, Nawy, and Air Force

UN1TE.D STATES G O V E R N M E N T PRINTENG OFFICE, WASHINGTON 1 1959

UNITED STATES DEPARTMENT OF TffE INTERIOR

FRED A, SEATON, Smrettny

GEOLOGICAL SURVEY

Thomen B. Nolan, Mrectw

Rawer, George M i t t , 1926 GBolom of the Delnrof and Westernmost Andreanof Is-

lands, Aleutian Islands, Alaska, by Gear D. Fraser and H. Frank Barnett. JF7ashington, U.S. &vt. Print. Off., 1959.

v, 211-248 p. Ww, maw (4 fold. cal. in pocket) profile, tablea, 25 cm. (U.8. QmIoglcal Survey. Bulletin 1028-1. Investigations of Alaekan volcanws)

"Prepared in mperatlon with the D e m e n t s d the Army, Navy, and Air Force?'

Blbliograpag : p. 2444H. 1. (feology-Aleutian Island& 2. Rocks, Igneous. I. Barnett,

Henry Frttnk, 1921- joint author. u TltIe: I3ela.d and western- mmt Andreanof Idmds, Aleutian Man&, Alaska. (SerIea: C.8. Qeologlcal Bumey. Bulletin 10-I. Scrim : V.& ~eologlcal Sur- vey. Investigattona of Alnskan volcanoes)

557.984

The U. S. Geological S m y , in response to the October 1945. request of the War Department (now Department of the Army), made rr reconnsriassxrce during 1946-64 of volcanic activity in the Aleutian Islands-Alaska Penimula area. Rwdi% of the first year's research, fieId, and laboratory work were hastily wembled as mo administrative reports to the War Department. Some of the early findings, as recorded by Robert R, Coats, were published in Bulletin 974-B (1950), "Volcanic Activity in the Aleutian Arc," and in Bulletin 989-A (1961), "Geology of Euldir Island, Ahutian IsIands, Alaska!'

Unpublished results of the early work and all data gathered in later studies are being published u %parate cpterrt of Bulletin 1028.

The investigations of 1946 were financed principally by the Military Intelligence Division of the Office, Chief of Engineers, U. S. Army. From 1947 until 1955 the Departments of the Army, Navy, and Air Force jointly furnished financial and logistie wistrtnce.

m

CONTENTS

Lo~ation,,,------------------------,-.--,----~-------------~- Climate and ~ W t E o n ,,,----------- --, - ,- -- ,- - --------------*,

k 1 0 ~ y ,.,,,---- * ----- * ---------..,----- ****-* ..------------------. hydrncks------- , , , , , .***-------- - - - - -*------- - - - -*-*---*

Amatfgnak laland,, , - - - - - - - - + - - - - - -I - - -I I - - - - - - - -- - - - - - - Ulak Idand,,,----, , , ,--------------------~--------------- Central Delamf Islands and southern parts of Tanaga and Ramp

Xwlands-- ---, ,-- , - --- - - - - - - ------ - - - - - - - - - - - --- -- - ---- Early eMeld wlcanw -,-- -, , , , , , - - - - - - - ,- - -I - - - - - - - - - - - -- - - - Recent volcanic cones,,- ---, . ,,, ,------ --,-,-- --- - --,,----- Surficial deposits -----------,,,,,---,-----,---------,,,----

megrained intmive rocks-. .-,-,,-,,.,,,-,----,---,--- -- -- ----- Coamgmined intrusive rocks,,- - -, , , , , . , - - - _,- -- - - - - * - - ,,- - - - - - - - - -

References ~ i k d - - - - - - - - ~ - - - - - . - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - , - - - - , -

ILLUSTRATIONS

PLATE 27. Gmneralhd geoldc map of Delamf and weeternmost Andreanof Islands.

28. Geolouie map of westeft) peninmls of Kanaga Ialand. 29. Geologic map of Kavdga Island. 80. Geologic map of Ogliuga and Skagul Idands. 31. Garetoi Volcano from Skagul Island - - , , - --- ---- - - --- , Facing 212 a2. Kavalga Idaad, looking west-- ---,--.,----,,,,,,,,-- Facing 213

R a m 41. Index map of Aleutian Islands -----,--.,,,--------------- 213 46. Detsiledaubmarine topography of central Delamf Islands atea 236 46. Surfaces of planation and drainage map, central Ranaga

I ~ l a n d - - - - - - - - - , . . - - - - ~ - - - - - - - - - - - - ~ , ~ ~ - 239 47. Seleckd bpographic proAIea showing orientations of planar

aurfam of five islends of Delarof groap,, . -. ,,, - ,,, ----, 242

TABLES

T m 1. Fosrdls from Kansga and Tanaga Ialands- - - - - - - - - - , , , . . -, - 218 2. Chemical composition of roeka from D e l d and areshrnmoirt

Andreanof I a l a n d e - - - - - - - - - - , . . - . - - - + - - - - - - - - - , . . , , , , - 222 w

fn~~&~-.-~---~---*-----------~,-------------*--------.--~--. ~praphy-,--,---rr--------------c--------------It------.--.-c---

hcation,,,----------------------------------------------.--- Climate and v ~ h t i o n ,,--. ------,,,-.---- ---------- ---------------

Layared rocks ------------------.------------ *------- ---------- Amatdgnak Idand,, -------- --,, - ------------------- ------- Ulak Idand ---------------------------------~------------- Cenh.aI D e b f Ialanh and muthem parts of Tamp and Xanaga

Islands -,,- -, , - - - , - - - -, , , , , - - - - - - - - - - - - - - I - - - - - - - - - - - - Early shield v o l c a m -,--------- --, - -.-- - ------ -,---------,- Recent volcanic cones--- - - - - - - , , - - - - - - - - r-- - - - - - - - - - - - - - - - - Surficid deposits- - - -- -- - - - - - - , , - - -, , , - , . - . , - - - + - - --a - - - - a

nne-grained intrusive rucks-. . -- - - - , - -, , - - . , , - , ,- - - - - - - - - -- - +- - - Coam+graid intrusive rocks -----, - -----. ---- ,- - --+-+-------

Submarine topography,- ---- ---- ------------- -------------- ph@ogr~phy--.-~.-.~------------,---------,---,-------------~--- -logic history,. , -, , ,- , , .- - -- - - - - - - -- - - - - - - . - -- - Referencescited---,----------+------------------------------------

-

ILLUSTRATIONS

mrn 27. Generalized geologic map of Delarof and weaternmat Andreanot Islands.

28. Geologic map of western peninmula of Hmga Ialand. 29. Geologic map of Kavalga Inland. SO. Geologic map of Ogliuga and Skagul Idanda 31. Gareloi VoIcano from Sbgul Idand - - ,,----,----- .,-, Facing 212 32. Kavalga bland, looking west-, - ,------,-------,,-.,. Facing 213 44, Index map of Aleutian Islands ,-,,-,- ------- ---- -- - ,, - -,, 213 45. DebiIedaubmarine topography oZ central Delamt hLands area- 236 45. Surfacea of planation and drainage map, central Kanaga

Island--------------,--..,.--.---,-2----------------.- 239 47. Selected topographic profiles ehowing orientations of planar

surfaces of five islanch of Delarof group- ----------,,--,, 242

TABLES

1- 1. Fodm from lhnaga and Tanaga Islands,, , -, , , - - - - - - - - - - - 218 R Chemical composition of m k a from Delmf and w&enrrnost

Andreanof Islands -,--,,---,.---,.-,,,--,,,----------- 222 v

INVESnGATEONS OF A M U N VOLCANOES

GEOLOGY OF TWE DELAROF AND WESTERNMOST ANDREANOF ISLANDS, ALEUTIAN ISLANDS, ALASKA

By GEORGE D. FRAsER and H. FRANK BmmTT

Th8 layered m k s of the Delarof and westernmoat Andreanof I h d n are divisible Into dve unlta, generaIlp younger from south to north. Unit 1, the rockn of Amatignak IsIand, ia composed of sltered, well-bedded Macmua m k a including thick, finely banded, p e n 6 ~ ~ 1 3 I b e m p a ~ a ~ ~ t r ~ l ~ d e formed argillib. These rocks apparently grade northward into unit 2, the altered pillow lava3 and pyroclastic depwita of Ulak Island which are interpreted as a near-~oam facies of unit 1. The age of these twa d t a is unknown but preamablp 31 Tertiary. Unit 3 is a composite sequence of almost unaltered ahallow marine and mbaerial wleanic r& cmpoeed of tuf-breccia, lava, pillow lava, and mrbrdinate sedimentary r& Thin units extends in a belt acrasa the central Delarof Idands and the sonthem part^ of Tanage and Kanaga Islands. Upper Tertiary fwfla are found in the roc3ts on Tanaga and Kanaga; the rocks of the central D e h o f s are comlsted with these because of lithologic ~imilarities. The rocks in unit 3 may be partly qaivalent, extreme faciea of the Amstignak OF they may be younger. T h e mtact is not exposed. Unit 4 is composed of Pleistmene precaldere lam that fonna fragments of old sbidd volcamw near the presently active cones. This lava, younger than most of unit 3, Etpp~terI* intertongum in places with the upper part of the older sequence. Unit 6 comprises the composite cones of presently active Gareloi, Tanaga, and &ma@ Volcanoes. All five bedrock units are andesitic to basaltic in composition.

Unita 1, 2, and 3 are cut by many dlkea, dike mrrns, and sills of andeb ikic of basaltic cornpodtion. One limburgite Intrusive mass and one Eight rhpdacite dike represent divergent compodtiona.

The older nnib are alao cut by coaree-grained tlills, dikes, and plnhna, mostly of granodioritic cornpo~ition, 31ak Island is part of a granodiorlte pluton cut by aplite and andedte diben; none of the invaded rock b mposed and the age is unknown.

None of the r& have been m'onally mhmorphoeed: mem l o w e e foliated meks, auch as slate and phyllite, are absent. Submarine contour8 reveal a major trench and ridge abuctare for tho

Alsutian arc Islands are actnally peah of a great arcuata mountain r- In this area the range rises 25,000 feet from a foxdeep on the Pacific ~ I d e and 13,000 feet from a deep on the north. The main ridge in apparently

an arch botlnded on the uouth bp; a norkhaipping thrust soae and cnr the nwth by a high-angle fault mne, downthrown to the north. Volcanoes are located dong the inferred fault an the north. A grmp of transverse mb- marine and rmbaerial linear elements nuggesh further that the area bas been segmented by high-angle faulting. An A l d a n island arc wan probably formed in this general area as early as Miocene time, but the pment voleanoss end the ieland loeationa and Plhapas were formed much labr, probably In P l e i w n e time.

Several stage8 of Iate Tertiary or Quaternary wave planation nnd mb- aequmE differential tilting e b a ~ the late hiatory of the era; sporadic central-type volcanism at ita north edge began in early Pleiistocene and has continued to the present time. Widespread deposition d Recent ash, and hi~torje ernptiana from Gamloi, Tanaga, and Kanaga Volmnoes, are the most recent volcanic evenh Glaciem were active on Tanaga and Amatignak during the Pleistocene, and ice patches are now present nesr dormant east Tanaga volcanoes and on the high ssEopm of Garelor Volcano.

INTRODUCTION

Nost of the western Aleutian Idands here described were mapped in reconnaissance (fig. 44) during the fieId mason of 1952 as pad of a ~eneral study of the geology of the Aleutian Islands. The Geological Survey's motorahip Eider transported the field parties during the frequent changes of base camps. Tho~e who participated in the field study were: R. E. Wilcox (geologist- ineharge), H. F. Barnett, B. H. Bieler, G. D. Fraser, E. H. Meitzner, W. H. Nelson, R. A, Robie, and G, L. Snyder.

Two-man traverses were made on foot from base campa es- tabliehed along the coasts. Coverage is summarized a8 follows: Kana~a (south of latitude 51°54') 7 camps, 34 traverses; Tanaga (south and southeast coasts) 4 amps, 8 traverses; Amatignak (east coast) and Uak, 3 travemes each; Kavalgn, 2 traverges; Unalaa, Ilak, Skagul, Ogliuga, 1 traverae each. Traverses ranged from 4 Ito 12 miles, and averaged about 7.

Contour maps prepared in 1943 by the Corps of Engineera, U. S" Army, at rr scale of 1:26,000 were available for Kanaga IaIand. The Delarof IaIandn were mapped on Coast and Geodetic Survey topographic manumript sheets, scsle 1:20,000. For southern Tanaga the only maps available were form-line Coast and Geodetic Survey topographic hydrographic charts (Nos. 9145, 9146), scale, 1:40,000. High-altitude aeriaI photographs cover most of the area.

Data included here for northern Kanaga are based upon the monnaiasances of Robert R, Costs, who also touched briefly at aome of the Delarof Islands m d part of Tanaga (Coab, 1950, 1952, 1956b, e ; written comrnunieation, 1964).

W't~c-plnnegl -1rr1xcc of Sk.sgn1 Islxmrl conrmsle w i r h \ r ~ l r : ~ n i c coac 1% mil- nortl~wwr. Sols *mall m o u n h In l o r r ~ r o u ~ ~ d . Skapu I a : d

O ~ l i u ~ a (not rhnrn) nre uniqoe Among !kc llelsrol lulnnds in Illat the ltqeal rc.1 clitl i, ntrl wc l l ~ lrrr l~ ,~rre l . \ir Frlrrc Plloto .%ir Plcolt~crirphic and C h ~ n i n : Service (If ( T S ! . 1-ni inr l S t r l ~ r 4ir Fnrcr.

GEOLOGICAL SrRVEY

X o l n ~ - a v e ~ p l a n r l l r r p * r 8 u i r f a c ~ . rr> r'lifl, pnnly corrrrd clnrm beach. and prcl-nl day rock bcucll ( t ida l O u t ) . Sn~a l l ! I I P , , I T I ~ ~ 1111 Irppcr IIITILLI.I. moy hare originnrcd nr bouldrn and irrcg~llariticc nn xhvc-cut sur1nr.r: cnrnparc bi th featvr..s nn prcscnt H.SI.C.I.LII ~ o r f n c c . .\I, Z'ureo I'hoto Air T ' l ~ n f n P a p l i i ~ ~ .inJ C)rzrl ing Semicir (31 \Tli), United Slnfrr \ir Forcr.

The 10 islands dkussed Fie along the Aleutian Islands BFC

between longitude 177"039nd 179'09' W., and between latitude 61°13' and 51°56'30" N. (fig. 44). The two large islands of Kanaga and Tanaga have areas of about 100 and 160 square miles, respectively, and are the westernmost of the Andreanof group. Four of the eight Delarof Idanda, Skagul, Ogtiuga, Kavalffa, and Unalga, constitute a narrow projection which extends 35 miles west of the larger islands and is separated from them by Tanaga Pass. Ulak and Amatignak form a spur trending southweatward from this westerly projection and from the general curvilinear trend of the Aleutian chain. El& and Gareloi are, respectively, 5 and 15 miles south mnd north of the line between Skagul and Unalga. The total area of the scattered Delarof group is about 65 square miles.

Etmm 4 I l - I n k map of the AIeutInn IslQnds, A l ~ k s . ahowln~ lacatlon of the Delnmf w d wenternmost Andmnnof Idanda.

m T E AM73 VEGETATION

Weather in the Aleutian Islanda varies locally and h related to the inbadion of ocean currents, moving air masaes, and island topography. Winds, clouda, rain, and fog generally prevail the year around. JAW heavy fogs often limit visibility to 100 feet for days at a titime, particularly in the summar, and traverses mud often be completed solely by compasa and aneroid barometer observations, Annual precipitation ia about 40150 inchee.

4 R 8 5 Q M Q - 2

In the western Andreanof s summer winds wiginate mostly from the aoutheast, south, and southwest. Average velocities are about 25 miles per hour; during sudden atoms velocities increase to 70 miles per hour. From October to March prevailing winds are from the north and west, Excessively strong, gusty down- slope winds, sometimes exceeding 100 miles per hour, result from the rapid spilling over of air masses dammed against the wind- ward sides of mountains or ridges.

The North Pacific drift from the southwest warms the cold waters brought south from the Eering Sea. Winter temperatures eeldom fall as low as 0°F. The mean temperature of the warmest month (usually August) is about 65" ; the daily temperature rangers in Bummer from 46' to 75OF. Total annual snowfall, falling mostly from December through March, averages 70 inches, but little mcumulates except at higher mountain altitudes,

Vegetation is abundant as a consequence of the temperate humid marine environment. Mosses, grasses, fungi, and a wide variety of flowering phnta (many with edible parb) blanket the sIopea t o a thickness of 2 or 3 feet. The only trees are a few stunted recumbent willow^.

Except at gullies and cliffs the plant cover ia unbroken on the flat or gently rolling uplands below 1,600 feet altitude. The irregular, hummocky, springy surface rnakea walking slow-less than 2 miles per hour is an average rate. Rock outcrops in the

- interiors of all the flat islands are rare and widely separated. Geologic investigations in wch islands were largely confined to the sewliff and rock-bench zone around the island perimeters.

The landforms among the western Andreanof blands vary greatly. Kanaga and Tanaga IsIands exhibit compound landforms. Lofty volcanic heights at the north give way southward and we&- ward to poorly drained lowlands surrounded by sea cliffs. The highest, least eroded, and most recently active volcanic area are at the extreme north of both islands. Like aeveral of the larger Aleutian Islands, Kana- has a narrow low-lying westward dension.

Gareloi fsland is rr conical volcano which rises abruptly from the ocean floor a few thousand feet below sea leveI to a summit 5,160 feet in altitude (pl. 31). Two emall glaciers hang on the s h p , slightly gullied slopea. The shoreline is precipitous along most of itEi lengkh. An eruption in 1929 opened a aeries of venter ; it spilled dark lava from a rift zone ~ o u t h ~ t of the wmmit vent

and spread pyroclaatic materiala over the southern face of the volcano. This recently active, elevated maas is north of lower landa, paralleling the arrangement on Kanaga and Tanaga.

Amatignak Island is a dissected glaciated upland marked by cirques, sharp arEtes, troughlike valleys, and small intermontane depressions. The island has 1,875 feet of relief. No constmctiond volcanic landforms were seen.

The other islands of the Delarof group are low, flabtopped featureless platforms bounded mostly by sheer cliffs and sea-level rock benches; they resemble the southern parts of Ransga and Tanaga Islands. Small ponds commonly dot the poorly drained surfaces, Ulak has a somewhat higher relief than the low Delarofs.

The flat Dekrof IaIands are dotted with small conical mounda (pls. 31,32) generally only a few feet high and too small to ahow on a topographic map. A few larger mounds are visible as dosed contours (pl. 30) ; dense vegetation, possibly the result of fertiliza- tion by birds, gives some credence to the idea that the mounds are '"bird perches," Some of the structures may have begun aa irregularities on the wave-phned surface (PI. 32).

QEOLOQY

LAYERED ROOKS

AMATIQXAX IB-

CkQmctw.-Sedimentary rocks composed of volcanic detritus make up moat of the bedrock on Amatignak Island (pl. 27). The rocla are of dark-gray, greenish-gray, olive-green, bIuiah-gray, and light-brown tuffaceous argillite and siltstone, tuffaceous sand- atone, and tuff aceous breccia. Greenish coIars are most common. Grain sizes less than one-half inch predominate, but volcanic bombs are visible in a few beds, and altered volcanic glass i~ seen in thin mtion; accordingly many of these rocks have been classified as volcanic wacke (Williams, Turner, and Gilberk, 1954). Generally the rocks are well indurated, chloritized, pyritized, and silicified. Cryptocrystalline quartz veinleta are common.

A common rock of the island is a finely laminated argiIli@ banded green and gray. The rock is brittle, breaks with a con- choidal fracture, and often shows structures recording penecon- temporaneous deformation. Similar argillite has been found on Ulak Island and elsewhere in the Aleutian Islands in association with altered pillow Iavaa and marine pyroclastic deposits.

Microscopic examination of specimens in the sand- and siIL size range confirmed their origin a tuffaceous dirnents. One

210 INVESTIGATIONS OF ALASKAN VOLCAHOEB

m i m e n had fragments (< 0.04 mm) of &reem chlorttie material, oligwlaae-andesine, and augite cemented by carbonate. Another, with zeolitic ( ?) cement had partidea 1 < 0.2 m) of chloritized volcanic glass containing feldspar latha and other crystals, and cr~ratal fragmenh of unaltered angite and albitized plagioclase (< 0.09 mm). Most of these rocks are of andesitic or basaltic composition.

Many andmitic or basaltic dikea and small d l s intrude the sedimentary rocks along the eastern and southern coast. Small dioritic or disbaaic sills are also found; a large quartz diorite ill forma Knob Point on the ead coast. Shipboard observations and study of aerial photographs suggest large areas of massive rock, perhaps plutonic, in the northwest quadrant of the island.

Age.-No precise age can be assigned te the rocks of Amatimak Island. Parts of some formations on Attu, Agattu, Adak, and Unalaska dating from late Paleozoic to early Miocene have similar lithology. The two oldest stratigraphic unib in the Rat Islands, generally coareer grained than the rmks of Amatignak, have been altered and intruded by quartz diorite, Fossils from the Rat Islands indicate an Eocene(?) to Miocene, age for the younger aequence intruded by quartz diorite, but the older unit ie not fossiliferoaa. Mesozoic fossils have been found only on Atka Island; Paleozoic fossils have been found in m e srnaIl area on Adak (Coats, 1956a). Tertiary fossils have been found on Attu, Agattu, and Unalaska in rock8 resembling those of Amatignak. On Amatignak, ~tructures reaernbling worm tubea were found in two outcropts; one unidentifiable foraminifer, probably post-Cm bceoua, was found in thin section. The rocks of Amatignak are pro~isionally assigned to the Tertiary and are probably Miocene or older.

The rocks on Amatignak have been tentativeIy wmelated with those on nearby Ulak Island, and both are probably at leaat aa old aa those on the central Delarof Islands which have not been regionally altered or invaded by plutonic rocks (p. 217).

The rmka of Ulak Island resemble those of Amatignak. They are greener and more altered than those of the central Delarof~. The layered aequence is composed of altered palagomite. breccia and tuff, basaltic pillow lavas, and tuffamua wdimentary rock, Those smka are intruded by sndeaite and hrralt dike~l and sills. In contrast with the well-bedded sedimentary mtion on Amatig- nak, the rocks of Uak are dominantly igneous and pyrocIastic,

Pillow laveas, abundant on Ulak were not obgerved on Amatignak. The rocks of UIak are chloritized, but the pillow lavae and tuff are apparently not spilitic.

A 2-foot thick layer like the distinctively banded zargi1Lit.e found on dmatignak lies between altered tuff-breccia layers on the west c a d of Ulak. This sugwts a correlation of the r& of the two islands, and possible facia change from fine-grained, wdl-bedded sedimentary rock8 on Amatigndc to more dominantly volcanic rocks on Ulak.

Bedding on the south half of Ulak Island dips toward Amatig- nak, and beds on northeast Amatignak dip steeply east, suggesting either a syncline beneath the straits between Amatignak and Ulak or drag on an interisland fault (pl. 21). If there ia a fault between Amstignak and Ulak, and if the relative altitudes indicate relative movement, then the apparent facies change may be better explained as a change upward in the aection toward Ulak. The rocks on UEak are believed to be about the same age wa the rocka on Amatignak.

CIPNTRaL PglrdROF I W A M D B AXD BO- PA.UTB OF TAHAGA AND EANAGA IBLBNDS

Chmcter-Unaltered pyroc-lastic rock8 and lava make up the bulk of the other islands of the area (pla. 27-30). Tuff-breccia and tuff are interlayered with black, gray, or brown andaitic and basaltic flows, pillow lava, and flow breccia. Tuff-breccia is mbst common, lava flows are abundant, and tuff is somewhat less common. Sedimentary rocks are least abundant, except on southeastern Tanaga. Plates 28, 29, and 30 show proportions of the rock types in selected areaa.

Volcanic centera have not been found. The relative scarcity of reworked volcanic material and the abundance of coarse debris ~uggeat that the volcanic centers were nearby, Fo~sils (table 1) indicate that m n y of the beds are of shallow marine origin and some of them are subaerial.

The composition of these rocb, as revealed by field mapping, brief petrographic study, and four chemical analysea seems to be consistently basaltic and andesitic, with no representatives of more si;iliceoua types of lava. Table 2 presents chemical analyses of nine samplee from Kanaga, Tanaga, and the Delarof Islands. The fird four analyses are of rocks considered in this section. The main differences between roeks of the sequence apparently are textural and structural rather than chemical. AII sections studied under the microscope fall in the basaltic: and andesitic groups

TAB- 1.-PoaBila j r m Kanaga and Tantlgo Zslande h3 t-r

[1dantj5@tlon of losrlls: P. 8. MaeNelL m e g a f a oca 1-9, 12); Ruth Todd, PaamlaiIm flw. 26,7 , 12): P. B. ClOu f i b 0 &a 16); 0. W. Owke, e&kd& Ow. Is); @J B. B.%sr&oom, pollan and spore not. W; R. W. Brown, mssi1 wood (loo. &)

~ C Y Idemtlfioatlon Barnarb

1. On W. side o[ Kmaga Bay 0.6 mile N. ofmoulb of b y .

2. h'ortb mast of Xaaag~ BC long 17PJ3'35". In bedded gravel dbposll about 60 L above benab.

a. In vaUa of large stream 2 1 mil= NNW. 01 bead of &s B&y. C- ~ILU%%IM ma- g10merat.e..

4, OD W. ooast N. ofcanrtd Lakes 4.5 mllcs SSW. of R a w s V o W o In geao, loUaceaul, sad- atone layom. Consplcuou~ Psckn mquha here.

~i but ~3 yds 9. o( loe. 4 I~I bedded, mmlron- solldatad pumleeoua sediment8 lntwbeddtd w l l h v o ! m l o breccia.

Lnta 'Pert-. PO* abb *mane

Lew TerLLnry (Puo- ~ n 0 b

pllOaure(7)

-----*-a---.--------.-

P d W Plloonae

P~leeypods: c w r a sp. (lqf‘3) Curdium (Caoaldama) n. dp.7 d. 0. (O dl-

durn Pabrlciru Clrrl dla:

&nw rp. (e s-, stm:lg ~ b b d ,-)

Pelwypodn: Pabm n. q. 8. (raemant * CardUa (C&oardfa) sp. sK. C. u h k u n n Dd

Cm%%# sp. (a large, -ulj ribbed apocles) ---.--.-.-.-------------.--------------------------.--.-------------.-.- Forsminllera:

2 ~ ; y m r M t LP, EYpAidJclm 3p. mpAldkUn mdlm (Pa~kw and Jones) U&a/lw urhmanl Todd h r l d d t n a limbam cushman md Eugba~?

sv.

OnldmtLBable molds of g a s t r o w and pelecypoda

Pelbcypda: Paen n. sp. B bi!. P. dbidw DnU '

Braoblo b-: 8& valr6, e.m snd m. h d e t

.--------------.---.--------------------------------- Poramlnlfam:

~ i d u l h l sp. C1Mdidfnn %p.

Oaatrowda: Buctnum 6D. Notlm up.

h l o n an age b d largely on ~ i m of oyatm. " h e m e to Recent rpww n w la

ponderom.

- P&en name er loc. 4.

Anlntsraftl moldofa P& bably Identical ~ c * m a ap. D, ~s bbE&ject iao born

8% o y Ioland nssociated wtch s k g 0 hum be wed to be Plloane. Thh Patea also m n t w aorthan Adak. B e l o w to Pcd6n WW group.

AU moUmb ara ly pr6~erpsd and axapt far Paden, &, and Mw nm I r r W molL.

Psleespoda: Pcam n. sp. C. b ~ ~ m e n t s &a& Pp. an. A. adla Dall M m a BD. Inlet. &Iw cI. S. ~ h & d b L l ~ n 6 4 ~ 1

. -. o&' nnd sp. Indet. but bevln name mlrrosoulp

turs u r-cm from ~oc. % HtmUh(vr1a larluocsa (Omell4

?' numrotphlna spp. onimellat sn.

Nonlon labradnrfcum Damn) ~ l p h l d h o l ahbk#VW 6whmm and Todd

jrloldtror, Cusbmm

FIp%Ula d i m (Prrker and Jones) Flawrim mardm& (MontRcul?

nD. UdpnCM el. cuahmanl Todd Awulogcrinn u r n Todd B L Y C ~ ~ /.Jgld! [Cushman)? Rponlde~ eolumbimsb (Cuahmnn) CbLaldullv~o cdtjorn~ Cushman a d Etughea

llrnbda Cushmna and ncc~bcs PuUnla a a L b t w g l R. E. nnd K. 0. Stmart CiHcidca lahoiulus (Wnllror and Jacabl

Pcrlrn a rp. 0 rmmhles a spdmw Mrn MlddletmIa., OUU of Alaska, tmm M beUeved to lx PU-. BeIowa to Pada

Glmllar to buna on Arnchltkn B. (Cushrnnn sod Todd 1@47) but lseklng lanktonlc9. Tha ~nnab PorarnWfera w e ~ %pmlted In &allow water potmbly not lass than b fathoms nor bare than 100 more llkely toward sh~~llower end of thk ranp. The Isuno la ehmctorlstlc of cnld wnter 8lmlle.1 tn those known ~n Alaska, northern b, and Oreonland Ln Recent was.

6. In \tme%me brecck trapmen= from v a l ~ i c Palec tang83 born ear& PUofocle to se1I-l bedS dong the beach 1.7 miles N. 01 A. Eashmd~s wh&) Cepc chonu. Macomn sp.

Myo sp. Sea fmtn?&u at end d tabla 1- f m

Tan- 1.-Poseila from Kariage and Tanaga I~lnnds-Continued

7. b sand )mu B eonslmaerale 0.9 mile N. at Pel PC&%: Cape Chunu. %,i(deot!~abk tragmant

Clrrl dla: &donLl~shle hn-nt ...................................................... .-..-...,.-..-.....---

Fmmlnlfera: Uwoldfna sp. ~!Cponddrel sp. GlMcldu sp. Cll,uldh SF. SP.

8 In masslvc, p.oorly mnsolMetBd nnbedded tug 2.2 m l l ~ NE. of Western R.

n. 1. masalve, poorly coosolldsted unberlded cull NE. shorn o t M o m Bey (bay B. of Wuv~cm J't.).

10. On west shorn of N. Ran- 1 8 rnllea S6 W. 01 Knnnga Vo lwo .

I I . soulb shbrc at wlnl (Tnmp Polnb) on 8. mar& of El06 SprIngs Bay. From bedded tu7 hmmh layer (304t thlckncss mposed) dlpplng gcotly wthwnrd.

12. Lst 6Im41'2%" N. long 177°W90 ' W., In b W ft lens ol tuI?aeeo;s COt lu lW Eoaglomcrato ow, andoslte flow.

Pelecw~ch: A& fhgmeol ~ u c l t $ onttillnmla Conrnd Macom~, Iragment

Oss~m~ods: Nalka sp.

Pelecypodn: Prqmcat oflarge shcl), unldent LBnble

Algne: Oenasslrnllarto LUhophqKam m y be new genus

or sobsenus

P e 1 e : stuicnlrofldanrS Llmaeus

Oatropodn: ACM n. BP AIL unwrmaodg lqe, heavy

ape&

Pelwypmb: Odhm n. SP?

Lgte Torllary to R e m t

Late Tertlar pmdbly hfioaens

La e henry agatcrs am mom &araeterfatIe of &hen6 than later de Its. Appannc prodambane of uo-E brms atmost preolud& s very bite Tertlnry or poab'PerLI. ary we.

A heavy amcavo o ster hav- small nrdlal rlbo rssemb tiass of some spedes ol Spondplws or%nnh.

Late Tertky

Late ~~ or Plekmcme

Ludm d W a mugw barn mid-TwUarg to Recent.

b samnatmtlgapblasequ~~~~b loa 4md 6.

la. W. eosst Cape Basmlk paplaru)e, trom blas on bscb.

PMgbdant m. Mold wit& r dsoprnmcb MU on thn 6g;pa&md abG- . -- - -- ,. cbrf=:

urn. w.7

-----......--.----------------*-------------------.. ~ommtnlfm:

EtpnidlclI'o wdkn dakwMd Jonea) Ranulbaap. Cul(cidc4 sp.

Bahtno1dsa: (Speranpoldlll) '

Two spwlrneng probably same sped&?. May be Maropncr&ra or L U l a , but la& dieg- nmtlc fu8811~~

M: L a g 177°W10" W lat 51%"06' N kam % l l e n ( N o . o f ~ ) : , cokgbmratonnd t w T ' ~ e ? j rhh in wbbbd

Late PUocm4 a Plektmne

p e t debrls., Slump block burn tas cUlG (tnLerpl&d)

Erlcaeaaa. 3

sml tmeslmfkbtbatof *fett%%?; ~1"adaf but the valve b mar8

a n -

Luaa. o m bsrvlly BnQd #-dm p"W of Ibb B. rarl.wamupU& er( e h

Pachard omloned that Dull's awdmen reslly dsme from TartlarJ & k u s e lt msambka so closely a PlehtomeMd W t sbeelaa barn the P d c C o m t . Tbers la na iroubt tha t the %went g &en was en- c- in tbe -us &merate.

The wrlters fmd no fleld evlamee tor ass Cape Bssrnlb rocks to a sequrnm ayd mnOg oldei than mckr In lorn. 12 anp14. Bemum of genezh menw augga~ted dabin may be -%ere ~s s species of ~ i n l k f a In MMioDene Mottramp formntl~n gl C W M .

7% sedlmmts w m ap arently de b& Bd nt to abdow, probRbEBd In a water. k%a w m exknslve dder z&% in the & o n . spruce and Rt 10- al m- slderabla dIsuno8. and Umltad fme corer Jn the area . d.varled bmbaceous&ra bebly e S t d ahma martdm of the sed.f%uw bftsm - -

Cllmetlc mndltlcms wem probably gomewhae mddw than l b o ~ found today in same sma Them are no trees Ln area M a g .

I . . .. .

16. Long lWS5'46" W.. let 6 lW14" N. IJI tuf- .&fu sp. a) .. . . I PcukMe#~do- I . . . .. . . . - - . lecssus aendstanu and cowlomerate. Fmmmtr or krwll wood.

- . I . , . ., . . . I . - . . ... 1 - -

1 Kmsy localltias h m acsle 1% 000 U 8 Army C o w of E n g h m maps 1% . . 3 N. sp. tor Paim dP~lgnntB4 by t e t t e r a ' ~ ' ~ , B dther let$em h sarles lor n.'s on 0th- A l e o w I)ao&). I -ga ww Liom U. 8. ~oasbend d ~ i c sump h n s 8146 a d a 1 4 , ,

Tars 2.-Chsmied aomcpo6itwn of mcka jrom ihe Dsbroj and locslsrnmosl Andranof Iabndc B. I. Tomeail ..- . , . . . . LoaaLCp, arme. .nd ooomTena d s m a

8101- ...................... ... .....--...-.. uol- ...................................... Irdk ...................................... Be0 ..............................---....... MgO -... ................................... OaO ............................-...... NC~O.. ..................................... &rO ........................................ &O- ...................................... 50+. ..................................... TI& ....................................... oob ........................................ P 01 ........................................ d .......................................... F- ......................................... 8.. - ........................................ -0- ............... .... ...................

t.m3 0 ..,.- .............-................ TO tnl-.-.... ................................ ~ m p d m dmity. .................................

' hhdea au pamsat BaO, not +own.

--

~-MOW - -

Pherlblc rooks

= um.b

A- mddte,

d m

~ ~ 4 4

a m LBN 8 78 b79 449 LOB 8.66 . d8 . 70 . b0 .m .12 . Z2 .m .a .00 -13

BBBd -01

a m - ..-.-..-

Ornnbdlorite, plutan

dg~ln-167

WOP ~b 77 L 40 L n 1.87 abd 898 8. 24 . Od .49 .a .cn .U . ae .a .01 .OB

BQBB . m QQBb

amnwm

Auddnr besal$ h w

Orm

SFS-M

a. 76 lt. 01 489 4M 4M 879 an . in , 'Kl 62

i m -01 -8) . (IB . a5

- .01 .u

lOa M .a

89.86

L E ~

ApUte Uslet 61

plotcm

RMn-ldl

nn 14 n . 81 . u

.80 1. M 4% 8.78 . OB -46 .a .m .[W .a3 .a .16

, ' .u a 8 1

.10

PB. n ............................

H ~ S E ~

Aglts . o d d &

flnw

wwdl

mm lBw 4.62 a re P70 aw aw .78 . P . a7 . BD .01 .40 .oh

. .a . b0 .la

99.98 . M m. a

.......................

OKIWB

Labredorit6. a t e

Wp$k. ew-m

4aw lase 440 b . 2 ~ 6.00 mu 188 .U -68 - 84 .ell . a9 . P -01 . m .aJ .la

lQL OB .01

I O ~ M - --.-

m.gsasga P

Dark rhwdmlta

cUkn

62go97

a m 17.16 2 U 8 87 L 16 6 06 a. n 2 01

.OR -74 .m .a . a .18 . M ..a, -143

1 M lo .Ob

la M .---.------.

a. - I l h M t e ,

0 8

twn-irn - n~ a66

.do

.a .82 .w

8.w P28 L2B I L I

-08 .(w -17 .01 . a3 . a2 . W

*WLR -ad

w. n a m

Labredmltd ddb3 .d

~ W - M

m 78 17.88 !La 8.66 1 P I a lo l.40 .61 . I7 -29 ria icre .26 -04 . oa -00 . 16

W78 . M .a 78

P n

whether the rmk is pyrocl&ic or lava, and the sedimentam rocla appear to have been derived from similar rocks. Significant variations within the basdtic and andesitic groups are indicated by a wide variation in type and frequency of mafic phenocrysts, and Et seems probable that s few of the lighter colored rocks, in particular the pyroclastic deposits, would fall in the dscite or rhyodacite groups of the chemical classification of Rittmanlr (1952). No rocks as light colored as the rhyodacite dike (specimen 52Sn-100) were found in the layered sequence, and at Feast 90 percent of the sequence is probably more basic than dacite.

The xwt names {andesite, baaaIt) used in this report f o ~ analysed aphanitic rocka are those appearing in the chemical claasifimtion of Rittmann. In the text the full Rittmann name ia given parenthetically for each analysed rock. Pigeonite appears in the calculated Rittmann name for apeclrnens 62-N-64, 52- W-64, and 62-W-70, but no pigeonite was found in the modes, Moreover, pigeonitea, as used by Rittmann (1952, p. 78) are ""mixed cryatala of diopside and clinohypersthene" not further defined by optic: angle and may, therefore, include augite and other clinopyroxenes as defined by others.

Swimen 624-64 ia a dark, greenish-gray auglb andesih (Rittrnann: pigeonite andssite). Phenocrysh of augite, magnetite, and zoned (oscillatory) calcic plagiodaae are set in a groundmass of plagioclase microfihs (#An,,) and opaque oxides. The larger pl agioclase phenocryste are often poikilitic and usually contain aeveral zones of dusty inclusions.

Specimen 52-W-64 (Rittmann : pigeonita andedte) is a light- gray porphyritic augite andesite. The phenocryste (<2 mm) are progressively zoned calcic plagioclase, augite, magnetite, and one ghost crystal of hornblende ("opacite"). The groundmass contains microphenocrysts of plagioclase, clinopyroxene, and opaque oxide in a cryptocrystEtIline matrix.

Specimen 62-W-70 is dark-gray labradorite-augite andesite (Rittmsnn: pigeonite labradorite andesite) with nurneraua black, subangular ta rounded cryptocrystalline inclusions (0.1-1.3 cm3 of unknown eompositian, but poasibly altered basaltic glass or tuff. The inclusions are opaque on the borders and are crowded with opaque dust throughout; small crystals of plagicsclase and clinopyroxene have begun to form within the inclusions. The igneous matrix for these inclusions is markedly porphyritic with zoned, poikilitic cr~rstaIs of calcic plagiocIase (wide dusty zones and areas within the cr~ratals), augite, and corroded divine in a groundmass of labradorite microlites and opaque oxides. There is a marked flow structure in the groundmass.

Spimen 6%F&94 OWbam: andmine baa3t) is a bfack, wry fine grained rock About I percent condab of scattered ghenwrysts of plasioclaae, clinopgroxene, and pseudamorphs a f k olivine. The groundmasa haa a strong flow structure and cansigh of mimlites of d c i c sndesine, cfinopyroxene, and opaque oxides.

Other rocks a2 this group tire generally similar. Augitic pyroxene, hgpersthene, oxyhornblende, and olivine are common mafic phenmrysts, Phenocrystic plsgimlase is wuaUy bbradorite or more calcic and strongly zoned. Andeaiba with both cliao- pyroxene and orthopyroxene are perhaps the most common rocks, but many fall close to the andesite-bdt boundary a d are difl- cult to classify without chemical mlyaes.

Throughout the islanda a- of bleached and ironata1ned Wka, beleved to m u l t from fumarolic wtidb, sre common; one large, poorly defined area contains abundant epidote &ringers which probably indicate a diiTerent type of alteration. mis area extends from ICltnaga Bay to Cape Chlanak and northward toward Naga Point on the east coast of Kanaga Island. An andenitit dike swam and at lea& one ~modioritic dike invade the aolcanie complex in this area and may be related to the epidotimtion.

Plates 28, 29, and 30 show typical a r d distributions of the rocks. Outcrops are confined largely to coastal areas; many major Uthologic changes occur across and along etrikes. Bedding s t t i t u d ~ vary greatly and in thick, coarse pmlast ic deposits or rnaasive tuff layers are often impo~ible to obtain; 1-1 un- conformities &st in many outmops. Key beds have not been found; consequently, mapping of h u h r e e r and marat ing even large are= into broad map units Js not poasible in reconnaismnce. Much more field and laboratory detail is necasary before an adequate generalized map of the area can be made.

Age-Fossils were found on KBnaga and Tanaga Ishnda. lRocEts of the central Delarof Islands are g-rouped with the fossiliferaus rocks h u s e of lithologic similaritiee and geographic proximity. Table 1 is a summary of the paleontologic evidence far dating rocks on Kanaga and Tanaga. The rocks are believed to be laxgel9 Miocene and Fiimene but may include Borne Pleistocene.

Similar paleontologic evidence for & older than Miocene L lacking. The epidotized r&e between Ibnaga Bay and Nags Point mwrnbk Finger Bay volcanics known h be, in part, Paleozoic (Coate, 1956a) ; but upper TertIar~r fossil8 weze found in the altered a m on Kanags (table 1, lm. 1). A ma1 contact with older m k s has not been found. The upper contact with lava of the precaldem shield ~olcanoes is apparently unmnfomabie in Rome areas and gradational elsewhere (p. 225).

Q W Q F AND WJ?&TERNMOST ANDREhNOF ISLAND8 225

-'IT la- ' P O W ~ O E a

The. radially dipping lava flow8 of partially destroyed early ahield wlcanm on northern Ranaga and Tanaga IsEanda have been mapped separately on plate 27, although the contact with mka farther south is probably transitional in part. Coats (1956b) hw dmcritied thoae m k s for northern Kamga and mapped them in more detail than i ~ l shown here. In one area on the northwest coast of Kanaga Idand, lava flows of ancient Mount Kanaton (Coata, 195633, p. 721, which predate caldera subsidence, overfie the fosdliferous beds (locs. 4,6, and 10 in table 1) with apparent angular discordance. The time intern1 belmee~ deposition of these contrasting rock types may be very short;. Because the fassiliferans sedimentary racks are characterized by variable initial dipa and local unconf ormities, an apparent unconformitg here may lack significance. Lava indintingwishable from that of the nhield volcano is intercalated farther south with p p l a s t i c debris and adiment typical of southern Xanaga, A break in topogmphie slope separates ancient Mount Kanaton from planed southern Krtnage; this critical area is covered by a thick ~ h , aoil and tundra mantle. On the north& coast of finaga Island, 6.6 rniles muth of Kanaga Volcano, lava flowa of ancient Mount ICanatonC?) are interlayered with pgr~~lastic deposits, and a madational contact ia suggested.

The mrmponding contact an Tanaga Island w h m the t o p graphic break ia even less well developed than on Kanaga, waa not vi~ited.

The upper mtmt on bdb Emaga and Tanaga is bawd largely Qn hpomphy and laation of uadimted volcanoas within caldera remnants.

A g c a t s i (1966b, p. 78) has signed a late Tertiary and H e i s m e age t.5 the lava of ancient Mount Kanatan and mso- c i a M precaldera voIcanic centera on Kanaga Island. H e baa pmtufaEed a late Pleistocene date for caldera formation. Cor- responding rocks on Tanaga Island are assumed to be about the game age. - V O ~ O CO-

Gareloi, Tanaga, and Kernaga Volcanoes, which have relatively u n d i m composite cones with records of hiatoric activib (Coats, 1950), and dormant remnants of slightly older volcanoee on Tanaga Island are of Recent age. Relations of Recent volcanw with the early shieId volcanoes on Tanaga have not been de- tmdned. The rocks composing the three active conea have initial dip8 aa deep aa 35" and reat unconformably on older rmts in

Kanaga and Tanaga. The younger cones are built within calderas formed in older volcanw.

Gareloi and Kanaga Volcanoes and parh of northern Tanaga have been studied by Coats (1950, 1952, 1966b, c; 1954, written wmmunication). The rocks of Kanaga Volcano and ancestral Mount Kanabn are basaltic or andmitic and their lime and alkali parameters show the province to be caIc-alWne (Coats, 1952, p. 485). Basaltic and (or) mdesitic rocks occur on Gareloi Volcano, and presumably exist on Tanagrr Volcano. The sbikingly similar chemical composition of these rwks and those farther south indicates that thia sector of the Aleatian a r c has been dominated by baaaltiic and anderritic rocks since Miocene time and perhaps longer.

The material which forms the present surfaces of t h e conical volcanic mountain8 ia largely postglacial. That glaciation was followed by volcanic eruptions i s evident on northera Tanaga bland where an older glaciated cone is capped by lava extruded from a reactivated volcanic vent

Depotsite, of volcanic mh and aoil covered by heavy tundra vegetation (wfthout permafrost) mantle the islanda inland from the sea diffs. LocslIy the s ~ l h an northern Kanaga, which contains thin layem of pumiceous IapilEi and reddish-brown eoil zones, reachea a maximum thickness of 20 feet and sometimes forms dunes. The ash blanket is continuous on the low, flat islands of Unalga, Havalga, Ogliugs, and Skagul, but quite thin and dis- continuoan on Ulak and Amatignak. Quaternary volcanoes nearby were probably the aource. Because of abundant loose, freah purniemus lapilli in the deposits of the central Delarofa, the ash layer there may be younger than the stratified sequences with intercalated soil layers on Kanaga and Tanaga Llsnds. A thin dark-gray soil is usually formed on the surface of the a&. Pea* deposits have f omed in some boggy areas, On several islands, notabIp Ogliuga, Skagtrl, and Kanaga, ex-

tensive sand dunes are stabilized by vegetation (pls. 28, SO). Stream-transported alluvium, mostly fine pyroclastic material, locally filIa small basins and valIeys. Deltas form at protected bay heads.

Discontinuous gravel, cobble, and boulder b a h e a fringe all the coves and bays of the islanda, commonly with a wave-cut h c h offshore. Abandoned beach deposita at higher level are sometimes preserved. Coats ( 1 9 5 6 ~ ~ p. 92) reports that gouthem Tanaga is mantled by marine gravels resting on a wavwut pIab

form, and he also reports that similar boulder gravels on Ogliuga are overlain by coarse volcanic cinders.

Glacial debris was recognized in cirques m d cola at higher altitudes on Amatignak and on northern Tanaga Island, Ulak Island presents a scoured appearance, but no morainal material was found.

Coah (1956b, pl. 16) has mapped an andesitic ash layer of caldera age on northern Kanaga Island. This layer, conspicuous as coarse and fine pyroclastic debris on the surface of Kanabn Ridge, is buried hneath more extensive layers of younger ash south of ancient Mount Knnaton. On the south bank of a large lake 6.4 miles south of Kanaga Volcano the older ash is beneath about 20 feet of the finer Recent ash as a decomposed zone of coarse pumice one foot thick. Neither the younger ash nor f ie buried ash are mapped in this report.

Mimy shallow intrusive rocks, usually rtndesitic or basaltic, either porphyritic or aphyric, cut the bedrock units nonth of Recent volcanic cone8 and older shield volcanoes. Nearly vertical dikes are usual, either as single units or as dike swarms,

The m o ~ t con~~picuous dike #warn is in the area on southeastern h g a Island extending 2% milea north of Cage Chlanak. The dikea are andesitic and contain needle hornblende. Most trend northwegt, but aeveral at Cape CRIanak trend wed. Basalt.-Many basalt dikes, most trending northwest, are on

tbe north coast of Kanaga Island about 5 miles northwest of Kmsga Bay. Several domelike masses of basalt, as much as 400 yards in greateat dimension, are visible along the coast in the same area. One basalt mass, with a radius of 150 to 200 yarda, in crosa section is steep sided, bulbous, and concentrically divided into layers about 10 feet thick. Each concentric layer has a separate set of radial columnar joints.

These rockg contain conspicuous, sporadicalIy large inclusiam of dark gabbroic rocks rich in anorthite, olivine, clinopyroxene, and chrornite. Many of the inclusione have been stretched Into vague schlieren. One of the inclusions covers a ~urface area of 400 aquare feet or more. Less abundant angular inclusions sf tuffaceoua(?) giltatone, apparently derived from the sedimentary and pyroclastie rocks, are also found.

The basalt is characterized by a strong fluidal structure of calcb phgimlase rnicrolites. A great variety of dark minerals--olivine, orthopyroxene, clinopyroxene, and 03~yhornblende-may appear

228 ~ B T I G A T I O N B OF ALASKAN VOLCANOES

in the same trlide. Some of the rnac minerals are probably xeno- c- from gabbroic hclusiona. Elongate vesicles filled with mc- ondary minerals are present in eeveral outcrops. These intrusions may be one of the many lmes of volcanic

domea usually found in and near voicanic vent8 (WiIlians, 1932), bnt mare probably they are bulbous developments in shallow sills. Evidence of doming was found at one contact with overlying tuff- breccia; basalt dikes aa much sa 60 feet wide also invade the tuff- breccia. Although the sill-like character of the Kaaaga ~tructwea is not &in, the joint patterna do suggest bbular mama (either sills or flows) and this origin seema more likely than plugs or domea within vents.

Limbwgite-A half mile inland from the coastal area which ehewa domed sills ( T } and dikes, a fragmental rock with abundant olivinerich inclusions aa much as a foot in diameter forms an is+ hted outcrop about 10 by 60 feet, The olivine inclusions, both rounded and tabular, are mixed with lw abundant, very angular inclusions of tuffaceoua lime~tone in a Iimburgite matrix, Ross and othem (1954, p. 701) examined some of these jaclasione and reported that chromium diopside, enshtik, and c h m i u m spinel are associated with dominant olivine in the ultrabasic inclusions. Possibly this outcrop is a petrographically extreme development of the incIusion-rich, domelike structures just described. The abundance, size, and angulariw of the inclueriona and their =mi- ation with gedimentary inclusions suggest a shallow nearby aource (cf. Ross and others, 1964, p. 693).

A7Edesite.--On southwestern Kana= (pl. 28) thick, emmely porphyritic eills of andesite comprise the sea cliff for long disc tances. One of these sills haa vertical columns 10 feet across and at lea& 100 feet high. Topography on the headland 1 mile southeast of Western Point sugge* s thickness greater than 300 feet Top and bottom contacts were not o b s e d , but nearly vertical end contacts al; t w o locations and one exposure of contact breccia suggests either postemplacement faulting or forceful transgres- sive intrusion.

These rocks are characterizad by ragged phenmryab of biotite and (or) oxyhornblende with '~pacite'~rima. The rocks abo contain abundant phenocrysta of zoned plagioclme (IabradoritPoli- goclase) with or without dusty zonee. Progressive, oscillatory, and reversed zoning occur, Clinopyraxene and orthopyroxene, magnetite, and a few rounded quartz grains are also present. The rmka are probably aiIiceoua andesite, but chemicer1 analyses would be required to classify them exactly.

Spee;Imen 52W-$8 (Rittmann: labradorite dacih) is frOlll a r~hbgmy columnar sill 2.6 miles northwest of C a p Tusik on southern Httnaga. This sill is finer grained and megarnopically less porphyritic than the sille on southwest Kanagk Phenocry~b, mostly lws than X miIlirneter in diameter, make up 26 percent of the rock as foIlows: abundant zoned labradorite-bJrtownite with thin rims of oligoclase-andesine and, commonly, oscillatory zoning; Iesa abundant oligmlase-andesine crystals eurrounded by a zone crowded with dark inclusions, then a rim of labradorite- oligoclam with strong oscillatory zoning (included here are maw phenmry~ts of labradorite with center parts shewing patchy ex- tinction ) ; clinopyroxene snd orthopyroxene, both rare ( < 0.6 nm) ; abundant subhedral opaque oxidea ( ~ 0 . 2 mm) ; rare sphene ; ghost hornblende (<2 mm) now composed of fine pyroxene, opaque oxide, and feldspar; rounded quartz relic8 (~0.6 mm) surrounded by rims of granular ~Einopyroxene in a brown, areakgy anistropic m a w . The strongly fluidal groundrnma is composed of: abundant lath- and blmky-ahaped crystale of zuned oligodaa~ndesine ( 4 . 2 mm) ; many stubby crystals of cline pyroxene (<0.16 mm) ; many blocky c~ystala and grains of opaque oxide (<0.07 mm) ; irregular graina and masses of carbonate, mme within pyroxene crystals and Borne within plagioclase cryatala ; abundant interstitial iaotropk, material having a low index of refraction and a fracture pattern suggestive of rrisbbalite or opal.

Both the wes- and southern Hanaga sills are complex rocks. Feldspars of several types ahowing internal zones crowded with inclusions and dilatory or reversed compositional zones, rounded qua* relics, and hornblende gh& all guggest hybridi- zation or at leaat a very complex cooling history.

RhyodQcitee-Specimen 62-Sn-100 (Rittmann : rhyodacite) is from a very light-colored podlike body about 200 yards wide, ex- paged 4.8 miles west of the mouth of Ranam Bay on the muth coast ~f Kanaga Island. This was the anJy light rhyodacite found in the Delarof-Ksnaga area. Quartz, oligoclase, and biotite are pment in the white b light-green glassy matrix InEJ 1.476) which makes up about 85 percent of the rock.

Specimen 62-Sn-37 [Rithnann : dark rhyodacite) ia of quite different composition (table 2) and would be classified microscop ically as vitrophydc labradorite dacite. This specimen is a dark- say r k from the chill border of a ~maI l djke on northwest Ka- nag& Its texture is porphyritic (,35 percent phwmrysts) with a hyd-felty ground mass and strong flow structure, Phenocrystic

pIagioclase (<4 mm) ia owmionally uniform, but u~ually shows either progressive or distinct oscilhtury zoning. Composition ranges from Anrme, with most of the determinations about An,,. A few of the crystds contain du&y mnes of glassy inclusions, Au- gite (often as an overgrowth on hypersthene), hypersthene, opaque oxidea, and rare ghost hornblende with a rim of granular pyroxene are aIso present sa phenoerysts ; ma11 grains of c r i a b balitel?) are less conspicuous, and these are d1 8et in a brown glass (rt between 1.606 and 1.510) containing numerous plagioclase (Anrp4s) miwolites. Potash, preaent in the analysis, is not expressed mineralogically and must be contained, along with oc- cult quartz, in the low-index glass.

Summ.um and age.-Most of the aphanitic intru~iom studied microscopfcally have a porphyritic texture with phenocrysta of zoned calcic plagioclase (rarely, more sodic types) and one or more of the following dark minerals : augitic pyroxene, arthopy- roxene, oxyhornblende, olivine, and (rarely) biotite. The cornpo- sitions and textures present in the dikes and dlL are more variable than those found in the flaws of the intruded sequence, but most of the small intrusive masses are andesite or barsdt,

No single period of intrusion can be assumed. Dikes found in the chloritized rock on Ulak laland are themselves altered; however, fresh-appearing andesite dikes cut the granodiorite plutan that constitutes Ilak &land, and most of the dikes in the Naga PoinbKanaga Bay altered area are fresh. If alteration is associated with subjacent plutonic invaaion, diking probably both pre- ceded and followed that event. Diking almost certainly accom- panied extrusive volcaniarn throughout the Tertiary.

One dike which cuts the fossiliferous sequence on northweat Kanaga (specimen 52-Sn-37) stops at the contaet with overlying Ranatan lava. The dike raises the lava flows slightly but does not cut them, suggesting that it failed to penetrate the massive flows ; thus aome of the dikes may be Quaternary.

Coarse-grained intmive rocks m found in several areas. A thick sill of quartz diorite makes up the outer end of Knob Point (PI. 23) on Amatignak Island. Other sills of diabase or diarite and perhaps larger plutons exist on that ialand.

Xlak Island i~s composed of medium- to coame-grained gmnodio- rite (specimen 52-Sn-167) and quartz diorite cut by numemua small aplite dikes (specimen 52-Sn-181) and a few andeaite dikw 10 to 200 feet wide. The largest andesite dike has a contact with '

the m o d i o r i t e indicating that both rocks may have been fluid at the same time. There are many rounded inclu~iona of coarse- and fine-grained material in the gtsnodiorih, but c o n k i s with the host rock are not exposed.

A granodiorite dike m u r s on Kana- Isbnd beltween Cape Chlanak and Nags Point. Possibly the alteFation (p. 224) and coarse dikes, together with the andesite dike swarm (p. 227), indicate a plutonic mass relatively close to the surf= in that a m .

ImEmhts and boulders.-Inclusions ranging En compsitiw from dunite to granite are found in the shallow intrusions and Aom on Kntnaga. Large rounded boulders of gabbroic to granitic composition and also boulders of schist and gneiss were found in association with indasiona of aimilar rmk in the lava cropping out along the nhore of an inland lake 400 feet above aea level. Near the s o u t h w t tip of Skagul boulders of diorite, marsre-grained granite gneiss, and fine-grained granite occur. Boulders of t ~ ~ s e - grained rmka, possibly of local derivation from silh h o r n to exid on the island, were also found on Amatignak Some of these boulders undoubtedly are weathered-out incln-

#ions; othm may be ice-rafted from known plutonic areas. One bou1der on krnatignak was locked in the roots of a driftw~od h. As the= are no trees in the Aleutians, this boulder may have come fmm s great distance, probably from the west. Zarge areas of metamorphic rock are not necessary to account for these boulder8 of gneirrei and achiet. On Unalaska Island the foliated w k a are confined to aheared roof pendant8 and narrow border zones on k g e plntom. No large areas of regionalIy metamorphosed rocks of even alate grade are known in the Aleutian lalands, and there is no evidence for an oldland of metmnorphic and igneoue mh However, induaions in late Tertiary aphanitea indicate that plutonic m & ~ of diverse composition are present beneath the islandy in mme areas.

Age.-All dated pIutona in the Aleutian Blands u e Tertiasg, The flak plabn and other coars~grained rmka in the Delarof- Kainaga me8 cannot b dated, nor is it c e h i n that they are all the game age. At least two episodes of comegrained in-ion are possible. The inclusions in lava of late Tertiarg age on mags indicate older rocks, and the dike there permits inference of intrusion after late Tertiary. However, it cannot be ebted d a . nihly that two episdm are required ; apophyses from one inku-

c d d cut Mme Upper Tertiarg rocks, and the same inkus ia~ could aupply inclusions for alightly younger upper Tertiarg ha 00-

A middle Tertiary e p i ~ d e of intrueion, in the gouthem parts of several ialands or groups of islands, followed by general uplift, erosion, and late Tertiary, central-type voleanicrm along the north edge of the chain has been suggested for other areas (Gatea, F r s ser, and Snyder, 1954). Possibly the Ilak pluton i8 one of several roughly synchronous inkvsione belonging to this episode. The coarse-grained sills on Amtignak may be connected with magma chamber8 that supplied andesitic Iava at different times daring the Tertiary, and may or may not be genetically related to the man- odiorite on Ilak,

mvc!Pom The Aleutian Islands may be considered aa &a of a nab-

merged mountain range rising 25,000 feet from the floor of the Aleutian Trench and 13,000 feet from the deep on the concave side. Major configurations are apparent from regional ahdies of submarine contours (Murray, 1945). The Aleutian arc ia ~ingle in ita western part and therefore is probably simpler and younger than double arcs in the churn-Pacific belt ('Urnbgmve, 1947, p. 185).

T ~ m d tines-Most of the area is covered by the sea and many ~tructures muat be presented rta interpretations (pl. 27) from fragmentary subaerial and aubmlrrine data (p1s. 2731). Struc- tures interpreted by trend lines on plate 27 are of three types: (1) linear subaerial drainage basina, (2) straight shorelines, and (3) submarine scarps, troughs, and slopes. Moat of these alinemente probably reflect faults whose precise locations are not known, Some of the linear features may reflect folda. Individual trend lines offer no positive proof of dxuctural control. Together the trend linea farm a geometric pattern which would be difficult to obtain by nonatructural methods. We believe that the submarine topography ia controlled by longitudinal, perpendicular transverse, and obIique transverrre fractures, which apparently are geometrically and genetically related to the arc aa a whole.

MAJOR ISTRUCTlfRES

In general the main ridge north of the Aleutian Trench prob ably is a complex arch bounded on both sides by faults. One thrust zone dipping north from its trace on the north aide of the Aleutian Trench i s indicated by seimic evidence and by comparison with better known arcs where the eeimic data furnish compelling evidence for such a fault (Benioff, 1949 ; 1956).

A gecond major fault bounds the arch on the north. Alinement of volcanoes aIong the north edge of the islands has been noted by all workers in the area. Gates and Gibaon (1956) and Snyder

DELhROF AND WESTERNMOST ANDREANOF ISLANDS 233

(1967) have demonstxatd by detailed submarine antouring that this alinement, in some apeas, is coincident with a high-angle, normal(?) fault downthrown on the north side. A 1,200-foot submarine scarp is evident west of the Delarofs between Little Sitkin and Semisopochnoi VoIwnw. A similar fault or fault zone ie probably buried beneath coalescing volcanic piles in the DeIarof- Andreanof area This latter fault appears to have been offset by transverse

faults. The Delaraf sector shows wutherIy displacement relative to the Rat and Andreanof sectors, ao that Gareloi Volcano lies aouth of lines connecting volcanoes on either side. Amchitka Pass, west of the Delarofs, and Tanaga Pam probably were formed by large transverse faults. The Amchitka Pass area is unique for the Aleutian chain. Ama-

tignak Ialand, east of the pass, is farther #south than any other island of the chain and is bounded on the northwest by a 6,000-foot depression (pl. 27). Amchitka Island, west of the paas, and the Southern Delarof Islands appear to be dragged south with respect to adjacent parts of the chain. West of Amchitka Pass the trend of the arc is northwest rather than we& Most significant is the 346-mile arcuate ridge, Bowers Rank (Murray, 1945), that curves north and west of Amchitka Pass. The Amchitka Pasa area, therefore, ie the gite of an intersection of two large mountain ranges, but an analysis of the structures is not possible with available data.

The trend af the main arch ia shown on plate 27. IB crat passes north of the central Delarofs and south of Tanaga and Kmaga. UnfauIted segments of the south flank of the arch show a smooth slope more gentle than that to the north, where longitudinal faulting modified by volcanic deposition gives a different profile. How much of the movement on this arch is bending and bow much is discontinuous movement on high-angle longitudinal faults similar to the north boundary fault is unknown. Abundant high-angle dikes on most of the islands may indicate a tensional environment (Anderson, 1951, p. 3). Dynamic metamorphism and tight folding are absent, possibly confirming the hypothesis of tension near the crest of the main arch. The entire arch may be ta drag feature on the main underlying thrust zone, or it may be the anticlinal lip of a *togene formed by plastic deformation (cf. Beniofl, 1966; Vening Meinesz, 1956).

Folds-In addition to the main arch, dip reversals indicate sev- emf srnlrIIer folds (pi. 27). Data are fragmentary, and 1-1

changes in dip resulting from conditions of deposition and faulb ing make all of these folds queationable.

Probable northeaahtrending folda on Ra- and Tanaga (and also on Adak Island 10 miles east of Kana-) contrast with pos- aible northwesbtrending folds on Amatignak Island. Unuauallp deep beds on northeast Kavalga are probably dragged on a etrong north-trending fault zone (pla. 27 and 29). The large and poorly exposed area of central Eanaga shows

many beds trending north and northeast; many have dips too steep or persistent to be attributed to chaotic deposition. A dome at Cape Tusik and northerly folds or faults elsewhere may explain some of these anomalous attitudes.

Fawltk-The major longitudinal and transverse faulta be1 iewd to exist beneath the sea have been mentioned. Probably some of the faulta that cut the islands have not been found becauae of the lack of lithologic marker beda, erasure of scarps by wave planation, and cover by ash and tundra. Faults that have been recognized in the field are mostly high-angle shear zones, eilicifled and stained with iron oxidea. North- and northwest-trending faut ts of this type are evident on Kavalga (pl. 29).

A small reverse fault on the south shore of Kanam Island (pl. 27) shows apparent vertical displacement of 20 feet, A steadng fissure pasaing through Kanaga Volcano atrikes N, 66"-TO" W. On western Kanaga large andesite porphyry sills commonly end in eteep side contacts that are probably faalta (pl. 28). Small faults are shown an Ulak and Tanaga (pl, 27).

A possible north-trending fault between Uak and Amatign& is suggested by the different attitudes of the beda on the two islands, and by the steepening of the beds on Amatignak. Relative altitudes af the two islands and the dragged beds suggeat that the west side is upthrown (PI. 27).

Catdems-Coats (1950, p. 43) found fncomplete calderas on both Kanaga and Tanaga Islanda, the northern sections presumably destroyed by faulting or marine erosion. These structures are approximately coincident with the east and muth margins of coastal areas of volcanic rocks of Quaternary age ~hown on plate 27 for Ranaga and Tanaga Ialands. Possibly the north r i m of these structurea were never completely formed. Arcuate normal faults, downthrown on the north and roughly alined with a larger east-weat struchra? trend, rnBy explain the calderalike rims better than the theory of partly eroded conventional calderas, (Williams, 1941, p. 242 ; van Eemmelen, 19!54, p. 88-92.)

SOXIMARZNB TOPOGB.AFXY

Submarine contours in most of the area from longitude 180" eab3t to Adak Strait, and including northern Tanaga IsIand and Amatignak, are whown on plate 27, The map was prepared from U. S. Coast and Geodetic Survey boat sheets at several larger scales. The contour interval is 300 feet. The mag ~ea le and the density and precision of location of aonndings give the contour lines the status of form lines. Figure 45 is a detailed map of the bottom topography around the central Delarof Islands.

Many lsea valleye, ridges, scarps and paasea show strong alinements miles in length. These alinements commonly trend in one of four directions : west, almost parallel with the terrestrial Aleu- tian Ridge line; obliquely northwest; obliquely northeast; or roughly north at right angles to the ridge. The linear character of ridgea, valleys, and garpa, and the presence of blmk1ike seahighe suggest that much of the bottom topography reflects geologic structure.

The islands of UnaIga, Kavalga, Ogliuga, and Skagnl emerge from a shallow submarine platform orientated jpneralIy emtrwest alow the Aleutian arc (pl, 27 and pl. 31). The platform extends west of tthe Delarofs across Amchitka Pass; east of this group of islands it appears to be offset to the north and presumably is coincident with the flat southern parts of Tanaga and Kanaga Islands.

The crest of the Aleutian Ridge between the 180th meridian and Skagul Island is bounded on the south by a steep slope at least 1,500 feet high. At the foot of the slopea just south of Ka- valga and west of longitude 179°50' W. the bottom flattens to form a ahallow trough with seaknolls alined along its outer edge. The knolls are fiat topped with a steep northern margin and a more gentle aouthern slope. Probably the trough wrta once continuous along the Delaraf Islands and across Amchitka Pass, but it is now disrupted north of Amatignak by a 2,700-foot slope which drops into a large depression whose maximum depth is 6,000 feet. The depression is a triangula~ area between the Arnatignak-Ulak Is- lands bIock, a aeahigh 16 miles northwest of Amatignak, and the crest of the Aleutian Ridge. The greatest depths are at the southeast corner of the triangle. The east-we&, or longitudinal, trend is dominant along the een-

tral Ddarofs and west to longitude 180°, but it is only generally evident east of Tanaga Pass as the boundary of tthe emergent Aleutian Ridge. The submarine topography north of the islands is only partly contoured, but additional evidence of U. S. Coast and Geodetic Survey hydrographic charts and analogy with the Kiska-

P l a m 46.-DetnUed aabmarlne t o ~ o g r a ~ h y of ocptral D d w f Idu& area

Amchitka ares {Gibson and Nichols, 1963, pl. 1) indicates the slopes are steeper than those mth of the blmb.

Tanaga Pam rrnd the pass east of Kavalga Wand (fig. 45) have features with strong northwesterly alinemenh IN. 35' W. to N. 60° WJ . The errst side of Ulak Island is the west edge of a pard- lel-sided, flabbottomed trench trending northwest; the pass between Kavalga and Unalga apparently is rm extension of this trend offset to the east. Other features with evident northwesterly trenda intersect the seahigh 16 miles northwest of Amatigxlak and bound it on the northeast; they can be seen on central Ulak and southeast Tanaga as linear subaerial features. The north margin of' Tanaga Bay trends northwest, in line with the linear mne on the island. The most obvious northwesterly alinementa of bottom topog-

raphy are in the DeIarof IsIands-Amchitka Pass area ; such aIine- ments are largely absent east of Tanaga Pass.

A zone of cbely spaced northeasterly trends (N, 26' E. to N. 60" E.) is apparent south of flak and Tanaga Islands, and may be the main structural control on Kanagrl Pass. Thk trend also out- lines the south margin of the Amatignak-Ulak Islands block, Sub- marine valleys and seaknollla oriented northeast are evident in Amchitka Pass to a lesser extent.

Northerly trends are also evident and two north-south scarps are particularly noticeable : one is south of Kanaga bland, in line with Ad& Strait; the second ia in Amchitka Paw, near the 180th meridian and intersects the Aleutian Ridge crest at the base of a s b P wesbfscing dope. Amchitka Seavalley (Gi bmn and Nichols, 1953, PI. 1) also trends north. Skagul Pass (pls. 27, 31) is an ex- ample of this trend, and many smaller alinemenh have been contoured, The muthwest peninsula of Ulak Ialand is triangular, This

shape is probably the result of the intersection of northwest-, nosthe&, and north-trending structural breaks. In general, the orienbtion of long straight coastlines and interisland passes throughout the area shown on plate 27 ier closely coincident with the four main directions of submarine alinementa (p. 235), with the directions of subaerial trends, and with the strike of dikes.

K w g a Idimd-Lmdfom on Qnaga Island illustxate the concept of constructive voleaxli~m coupled with continual erosion. Zlhe ~ymmetrical, complete cane of Kanaga Volcano at the north end of the island is the most recent constructional landform.

Steam issues from a vent near the summit, and flow8 of blocky lava in 1906 (Coats, 1947, p. 93) indicate preeent day volcanic activity. Continuing construction of the cone has inhibited diasec- tian by atreama. The shoreline is concentric around the voIcano and is scarcely eroded by the ma.

The arcuate calderalike scarp of finaton Ridge south and e a t of Kanaga Volcano confine8 the younger deposita within a half- bowl depression (Coah, 1956b, p. 76). Many branching streams flow southward from the scarp lip. They are underfrt in broad valleys which radiate from Kanaton Ridge, rand are separated by almost continuous sharp interfluves of high relief, The valleys were developed upon the slopea of the andent Kanaton Vobano, C I S recession on the west ahore has carved bluffs at least 400 feet high into the rocks of the old volcano and the underlying sedimentary rocks.

Planed surfaces on Kamga I8lamd.-About 3 miles aouth of the lip of anaton Ridge, the atream join rr transversle drainage di- rection in the flatlands below 500 feet in altitude. This northwesb wntheast drainage is the first expression of the older, wave-planed surfaces of the southern and western parts of Kanags.

Five major levels of planation are inferred from the topographic profiles (fig. 46). Each level is continuous and bounded by distinct breaks in slope many feet high. Abundant ponda and l a b on the several levels emphmize their flatness. The irregularly dashed lines on the map, which indicate the levels, were drawn from drainage patterns and slope changes shown on topographic mapa and aerial photographs.

The broad central area of the island ia topped by the highest planar surf ace, level 1, which haa an altitude of more than 500 feet on the northwest and 400 feet on the ~outheaat. (On the mag, Ievel 1 includes a minor aurface about 50 feet below the uppermost fiat- lands.) A steep scarp 100 to 150 feet high separabs level d from bvel 2, rat an altitude of 260 to 280 feet on the northwe~t and slightly more than 200 feet on the southeast side of the island. Level 2 is easily seen at the narrowed neck separating the northern volcanic highlands from the rest of the island. The high@& flat gonded area west of the central area, along the narrow pro- longation of the idand, is correlated with level 2 ; presumably Ievel 2 aha can be correlated with the higher altitudes at the extreme west end of Kanaga, north and west of Cape Chunu. Level 3, mostly about 220 feet in altitude, is weU defined near the west edge of the central area ; it makea up much of the slender we~tern extension of the island and may be present lmdIy along the steep

burlaces ot danatlon: boundulM rpprmlmtely locatad; 2 Is aldar Man 3

- --

Fmmm 46.--Sd&xs of plmatlon md drahage map. central Kamgn Island. Andreanof group, Aleutian Ialanda, Alaska

coast of the central part of the island on the northwe&. Level 4 ie at altitudes from 100 to 120 feet and is b a t preserved on, the western elowtion of the idand ; i t it^ definable, however, near the w a t coast of the narrow neck area and as small remnants elsewhere. Level 5 (not shown in fig. 46) includes the narrow benchea along the present ahoreline. Sheer clifis commonIy separate level 6 from higher levela. Minor terraces which are suggested by the topographic map conffgurations and are locally apparent in the field, particularly in the levels below level 2, are not discussed here.

The planed surfaces are almost continuous in the broad centraI region and western elongation of hnaga Island (fig. 461, but a l o n ~ the eaat c o a t north of Naga Point only levels X and 2 are present. Level 2 merges with the south slopes of Kanaton Ridge, The continuity of the eurfmes inland from Cape Chlanak, southeastern Kanaga, is less apparent because of extensive stream er+ sion parallel with a broad northwest-trending dike swarm.

The drainage divide for most of Kanaga Island, excluding the volcanic highlands, is clam to the north shore (f ig . 46). Streams flowing northward into the Bering Sea from the central rej$on drop as much aa 600 feet per mile down a mries of cliffs and scarpn ; those flowing southward into the Pacific Ocean are not so steep. The asymmetry is as marked fa~ther west, but altitudea are lower. The tendency of the drainage to take southerly courses aeroes most of the central region is commonly thwarted by structural controls, mostly bedding and diking.

Preeumably, the water age and higher altitudea of the central ares are responsible for the close coincidence of drainage direc- tion with four or five known structural trends. On level 1 the stream flow gouthwest or northeast, roughly parallel with the length of the island. Streama below level 1 commonly follow the other structural trends. Southeast drainage is dominant along the east coaat. The two deepIy incised valleys leading inta Ranaga Bay from the north reflect w north-trending structural control visible in Kanaga Bay and extend in^ north along the weat margin of northern Kanaga Island (pl, 27). The parallel southward drainage along the island's western elongation may be simply down- slope or may follow northerly strike8 in the area.

Sea-level benches now forming on the south aide ai Kanags are notably broader and more continuous than those on the north. This difference in width is consistent with the concept of a pIanation platform of marine origin tilted by upfaulting at the north. The fault w m p , subject to direct wave action, would rmde with much of its steepness preserved; the newly inundated platform areas

m a the ialand, on the other hand, would faws water-1-1 and subaerial weathering and the consequent development of a wider bench (Hills, 1949). Predominant southerly winda probably also contribute to the formation of broader benches along the south coast.

Physiographic evidence and geometric construction aug& mutheastward tilting of the entire Kanaga Island block, except- ing the volcanoes. A slope of at least 150 feet in 5.4 milea is in&- csted across centrat Kanaga. This dope is opposite to the preva- lent dips,

Tenagcl IsEarad-The physiography of Tanaga Island i~ similar ta that of Kanaga. The northernmost park of the island is, like Kanaga, a mountainous area that has been active volcanically during and since the Pleistocene, The volcanoes are alined east-west, and include relatively undissechd, ashovered cones and dissected shield volcanoes, gome heavily glaciated. The gIaciated peak near- est Gu~ty Bay has crevassed ice around the summit depression, and at leaet four recent cones to the east and aouth have poured lava

3 and ash upon radial valleys and ridges. Tanaga Volcano and ita twin to the west are located within an arcuate ~kructure, podbly a caldera. The volcano at the northwest tip of the island appears also t~ have been built up within s similar structure.

Ponded areas and abandoned beach ridges resemble those on Hanaga, and imply a related emergence. Coats (1966c, p, 92) states that mast of south Tanaga is a w a v ~ u t platform mantled by marine gravel. Correlation with the planed surfaces of Kanaga Island has not been 'attempted,

The Delarof Islands,-The low, nearly Rat D d m f Islands (pls. 31, 32) appear to be blocks planed smooth by waves and later faulted. Profiles drawn transverse to the trend of main topographic breaks on five islands (fig. 47) show the apparently un- related orientations of subaerial planar ~urfaces.

Bedding traces are well exposed on wsrve-planed surface8 around most of these isIands. On Ulak, where the ash cover is thin, a west-northwest bedding trend i~ reflected inland by dine- ment of ponds. This higher aurface probably resulted from wave planation, possibly followed by glacial scouring.

Glacial erosion has etched a north-south backbone through the south half of Amatignak Island. The ridge crests trend north- northeaat acroas the north half. A high area near the northweat- ern coast is underlain by more massive rocks. No continuoulr sarfacea of planation are evident around the island. Either the emergeme of Arnatimak was later than the benching observed on nearby islanda, or glaciation removed all trace of wave-cut planes.

S W. NE.

K AVALG A,

. W. E.

w. e .

OGLlUGA UNALGA S

B O Y i =1)1

UL AK h

mawus 47.--Belected bpogmpbtc p m h rhaaln~ ortentatbw o f pimu ncufnwa d five ManldPndr of tbe D e l d -D.

D W O F AND WESTERNMOBT ANDRFANOF 1-8 243

GEOLOGIC HXlsTORY

The pr+iocene history of this area ie unknown, but two pos- aibler interpretations are sugge~ted by available data: (I) Rocks in the Amrrtignak-Uhk area were deposited in early Tertiary in a fairly stable geosynclinal trough or on its Ranks and were invaded and altered by granodiorite plutons. The area waa uplifted and eroded during or after the time of intrusion. Later, in Miocene time, marine and subaerial volcanism began in the central Del- arof -southern h a g a area. Thew young Tertiary chaotic deposit& contrast markedly with the well-bedded older rocka farther south. (2) Another possibility i a that rocka in the Amatignak-UIak area are actually the well-bedded, deeper water fades of Miocene deposits farther north (which are nearer their source) and the alteration of the southern faciea is a reault, not of greater age, but either of proximity to late Tertiary plutons or of diff went dia- genetic conditions within the geosyncline.

The known geologic history, then, begins in Miocene time with the deposition of poorly bedded volcanic xocks. Easbwmt normal faults probably localized a chain of volcanoes near the latitude of the central Delarof~, and this zone apparentIy moved northward to ib present location. Depth contours of the present volcano belt on the north edge of the islands reveal many volcanic cones. These, and the volcanoes projecting above the sea, form a series of coalming volcanic pilea. Conditions in Tertiary time may have been simiIar to those of today; but ahifting volcanic centera, con- current and subsequent erosion of volcanic landforms, and d e p aition of Recent volcanic ash have all obscured the record. No centers have been found for the Tertiary deposits; and large areas of mame pymclastic rocks, not obviously related to volcanic centers, wggeat episodes of generalized eruption (much of i t submarine) during which mudflows and flood eruptions m y have been important. Many local unconfarmitiea record unstable conditions during

late Tertiary time and no persistent lithologic units were depm ited. Lava flows, pillow lavas, and poorly sorted tuff-breccia are the dominant rocks; many dikes and, locally, large ~shaliow sills invaded this sequence. Some of the sills were uncovwed by eroaion and large colnmlzrtr talus block8 from them were incorporated in later eedimentary and pyroelwtic beds. Relief featurea produced by faulting, erosion, and deposition were masked by subsequent floods of lava and coarae pyroclastic rocks. Pocketa of well-sorted tuff, some of which contain fossils, filled deprensiom on southwe6 ern Kanrrga

244 LNYESTIGATIONS OF h L h 8 W VOLCANOES

During the entire period of volcanism recorded here, andaitic and bmaltic rocks were dominant, with only sporadic and minor confributions of more acid rocks. S hallow-water marine life left a fragmentary record, and foregted land areas existed where today there are no trees,

Near the beginning of Pleisbene time broad ehield volcanoes formed near the aites of present volmoes. Them volcanoes were partly destroyed by caldera-forming precesses, and Recent voI- canoes of sharply conical farm grew in the old, incomplete a l - deras. At almost the same time another volcano, Gareloi, formed on the ocean floor. This late volcanism waa all confined to the northern ends of the islande.

Wave planation carved a series of platforms on the late Ter- tiary volcanic rocks during the Pleistocene; Anatignak Island, becausre it has not been planed, may have been uplifted in Pleisto- cene time. Glaciers mudifred Amatignak and the higher volcanic areas, and may have overridden She flat areas on other islands, particularly Ulak.

REFERENCEB CITED

Anderaon, E. M., 1951, The dynamics of faulting, 2d eB. r e v i d , Edinbnra, Oliver and Boyd, 206 p.

Bemmelen, R. W. van, 1954, Mountain bnildhg, The Hague, Msrtinnn Nijhoff, 1 TI p.

Benioff, W., 1049, Seismic widenee for the fault origin of oceanic deeps: Geol. &. America Bull., v. 60, p. 1837-1856.

1956, Seismic evidence for crustal atructam and k t m i c activity:, Geol. Soc. America Special Paper 62, p. 61-73.

h t n , R. R., 1960, Volcanic actfvitg in the Aleutian am: U. S. Geol. Survey Bull, 974-B, p. 96-49. 1061, Gedogy of Bnldir Island, Aleatian Islands, Al&: U. B. Geol.

SQ- Ball. 989-A, p. 1-26. -1952, Magmatic differentiation in Tertiary and Quaternary. wrlermic

rocks from Adak and Ranaga I~lands, Aleutian Islands, Alagka: Geol. Soc, America Ball., v. 63, p. 486-514.

-t966a, G e o l o g ~ ~ of northern Adak Island, Alaska: W. S. Geol. Survey Bull. 1028-C, p. 45-67.

1966b, Geology of northern Kanaga Island, Alaska: U. 8. GeoI. SUWW Bull. 1028-D, p. 69-81. 1966~~ Reconnttissanoe geobgg of mme western Aleutian Ialands,

Alaska: U. S. Geol. Survey Bull. 102&E, p. 83-100. Coshtnan, J. A,, and Todd, Ruth, 1941, A foraminifera1 fauna from Am~hitka

Ialand, Alaska: Cufihman Lab. Foram. Research Contr., v. 28, pt. 8, no. 297, p. 80-72.

Gates. O., Fraser, G. D., and Snyder, G. L,, 1954, Preliminary rep& on the- geologg of the Aleutian Eslands: Sdenee, v. 119, p. 446.

Gates, b., and Gibwn, W., 1956, Interpretation of the configuration of the Aleutian Ridge: C;eol. Soe. America Bull., v. 67, p. 127-148.

DEWROF AND WESTERNMOST ANDREANOF IBtANDS 245

G-, W, d Nichols, H., 1958, Configuration of t h Aleutian Ridge: Gml. Soe. b e r i c s Bull., 7. 64, p. 1173-1188.

Hnla, E. S., 1949, Shore platforma: Geol. Mag., v. 86, no. 8, p. 187-165. Mnrrag, H. W., 1945, Profiles of the Alutlaa Trench: GeoL Soc. America

Bull., v. 66, p. 761-782. Rittmann, A., 3952, Nomenclahre of volcanic mlts: Bulletin VoTcan01ogiq~8,

1'Aamciation de Volcanolopie. Shrie 11, tome XII, p. 75-102. Ram C. 8., Foster, M. D, and M g e r ~ , A. T., 1954, Origin of dunitas and of

olidnerich inclusions in basaltic rocks: Am. Mineralogist, v. 89, p. 698- 131.

Snyder, G. L, 1967, Oceen flmr structures, northeastern Rat Islands, blaslm: U. S. Geol. S u m BnU. 1028-6, p. 161-167.

Umbgrove, J. ET. F, 1947, The pulse of the earth, The Hague, Martin- Nuboff, 368 p.

Venting Meinem, FP. A., 1965, Plastic bnckling of the earth's crust: tba origin of geosyncIinea: &I. Soc. America Spee. Paper 62, p. 31 9-930.

WWiarns, Howel, 1932, The hiabry and character of volcanic domes: Calif. Univ, Dept, Geol. Sci. Bull., v. E l , no, 5, p. 61-186.

-1941, Calderas and their origin: California Univ., Depk Geol. Set. BulL, v. 25, no. 6, p. 239-346.

Williams, Howel, Turner, F. J., and Gfibwt, C. Y,, 1964, Petrography: S m Francism, W. H. Freeman, 408 p.

INDEX

Pant AC~OW. ts. .......................... a12 Idak Island. Finger Bas v o l d e ........... rW

~ a l ~ ~ f a s ........ n$ Ad& Btralt, submerlne topc%spby ........ 2.36, 271

................. @Eta Island TertIary M a 218 . Neutlm Ridge .... ....,...........+...+.... 235 Aleutlm Tmch ..... .................... 232 Allurinm . ................................... mf Amat- Irlsnd, moIm ................. nM1&

EIaCktlUU .............................. 241. 244 mtmdons and W d m L -.... .......... %I-m landforms ................................ 21h

Trim FQlh ...................................... a&9-234 .......... F d b . Kmw and T m m IuhL 224

submarim 6qm%mphy .................... 2 S Terttq l&ls ..................*.*...... 21 9

Amchitka island. lrnrn.... ................ m Amchitka P m . stmctnre .............

submdne to-pbp -- ...,........... 235 ~ m c b f t k s 6eavalIey .......................... n7 hndesite... ........ 2l 6. ma, ZB-E& EKb281, XJ(

Andreanof M m d s. c b w f d m l ~ 1 k l o o or rocks, Cable ....................... m

AuortMts .................................... 227 &Wits .......-..................+....... 2l3 211 ~ 8 t 1 depoai ca. fc- rms ................. rn Atks Istand . Mesosole fosfila ................. 219 A Mu Island . TertIa~ iosslls .................. 21R hudte ..-.............+........... ....... W2XI

B m b m . E . S., pohnaend up^^ Idmtlflcd bp .............................. nlcm

B&t am .................. ....-........ m-m Bowem Amk. a@Ylcsnce .................... 2U Brow. R . W.. l m U w d Meafffied bg .... 218-221

C d m W ..................................... 234 caw ~b~m&, d l b ......................... m Chedcal cornpal tlm al mb, tahls ......... X22 Chromlte ..................................... 321 Ollmete .................................... 213-214 c l b o p p m ..........*.................. m. 228 Cloud . P . E., elme identW bg ........... 21R-221 Cook& C . W., echlnolda Identified by ...... 2lR-22l

F d b ...................................... m-m Flnper Rag volcanlca ......................... XA4 Flows ........................................ 217

Kmngn nay. dlkm ...... ...............-. 234.230 a t r u c t u d c m M ........................ 240

Kanm l8lmd . n n d d t k ..................... 223 gndcsltlc ash ........ ...........,....,. Pn a m ...................................... 213 m l d m ........... ...................... a 4 chemld m m W h al ah. tab la.... ara

lnyer~d rooks ........................... 2l7-W phyuloaraphp ..................+.-.- . 237-241 planed aurfnma ......................... B.3-241 rhyodscltc .................+..-..+.-.... 229 alns 22+21B volmuIc ash d e d t s ................... S H - a n

Kaoegn Paae, gubmarlna togographg . ---.-- Z3f Kanwn Volcano ................ 2a&Z2% =.83f, 2% Kannron R i d v ............................. XU l i sva l~a Ialmd. folds ......................... 231

lwat iun .................................. 213 ............... auh.nsrlne m p h y .- S 2 3 T

...................... v o l m l c as\ depmlt 2% Knob Pofnt, nil! .............................. 230

L a M m l t a ~ u ~ l t 6 mdwita ................... Zta Landforms .. .........+..................-. 214-216 tam AOWB ......................... aaa, m, 281, US hged r o c h .............................. a15-rn LlmbnruIFe ................................... 225 LwatIon 01 ares .......................... .... 213

247

Psre MapNcdl, P. a,, me&mIh Iden- by--. 21&Xa M w a k fosalb ............................... 218 MlcrolIte .................................. 224 MLneralagp, dehl1tJm o( mma , .----, 5-23 Mloccne imatigns ........................ 214224 Mloc~ne gmloglc hlatorp ...................... 2'43 Mwut K~~ lam ..-------..---.. aaq ns

N ~ g s P h t , &HI .......................... ma30 Oalhpa k b d . o h e m i d oomtdtltnt d mkq

rsbk-+---+----------------------- Z22 md md mlmniu d e ~ m i ~ -..--....--. =am ~uatw 211 a b m d n e -hp .-.-.---. hhhhhhhhhhh pa&

O l l ~ h ..................................... m, PR Mop---------------------+------- Orybamb6enBa .,-.-.---.------ ............ mu

P.F BhW Fm. mbmariue Wrmnhpb~ .......... ap7

BtrncWm, p l @ c -...--...... ............. ZS%m Gurflcial daposfta ........................... -23

m*mlmadcddaa ---...---.----------- 2M ctdcd -darn tsble-**-** f u t e ..................................... folds .....--............................... m ............................... Imlla PI&fEn,iS ~ l ~ L 8 t l r n ................................. m landlm l.. .............................. 314 layered mb ........................... 2t7-261 ~oraz~on .................................. 2x3 phym~nphy. . ...................... .... ai

Tanwn P w . fwmd by f s u l k . . .............. 233 ablbmarlnc topography- --..-*.-...-...+ -237 ............................ Tnnnge Volcaao %226

Tertlnor formatiom .................... 2%. B12# 1.231

wmmlu lormatma ,..--.-..-----..*----+- a16 m phpslmphg- ---------------*------+*-+--- Br-Na P W w l a m ............................ 84217,243 PIeMxem lmtlom ........................

Tarllnry fpsella .............................. 219 , rRr'rnry EBoloR1a h'ntorp-- ' -.------*--**--*--

Todd, Huth, FotamInllera ldQatIilcd by.-.. 916221 .. ................... l o~l~t181, R. J., snuyllaa by Xi3 ....................

h a d d m -,*..*....**..--.-.....---....-.-. p2a

BUh Zl4%Ma .................................... BkRgU1 hlslld, boaldsl .......................

Ch*mjd -mlt'm dmh* --- ---- 1ocsUon ................................... 219

"-------+---+----------------- s13-¶4*838 . VO~CSU~C M'I &&b-. -,.--.......-,,.,, %X&'

Volcmoea, early ahiald ..-.-.._-_-,+-------- 225 w w l c h~~ - - ----- - *++* - --- - * 244 w n k -.-,,-.-.----------------------- ~ 2 %

PleWoctrne w l @ c h b m 2U Topwnphy, wbmarlaa 285-P7 ................... .................................

Pllocene formatlma ..-..--..-..-----..-------. Trnnd Ilnta

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