Reconnaissance Geology of Some Western Aleutian Islands, Alaska

Home , Agattu, Great Sitkin Island, Kagalaska Island, Kasatochi Island, Little Sitkin Island, Shemya

By ROBERT R COATS

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956 UNITED STATES DEPARTMENT OP THB INTENOR

Fred A. Seaton, Secret-

Thomas B. Nolan, Dtrectw

For deby the mu-t of Doarmentq U. S. GmmcatRIatlng ma W-tm 25, V. C. PREFACE In Ocwber 1945 the War Department (now Department of the Army) mqmted the Qeological Survey to undertsko a pmgmn of volcano inveetigations in the AIeu tian blanda-Alaska Peninads area. Theht field studies, under gsnerd d~ti~nof G. D. Robinson, were begun aa soon as weat.hsr permitted in the ~pringof 1'946. The resulta of the first yeas% fidd, laboratory, and lib- work were aa- sembled as two dminktmtive reports. Part of the date was published in 1850 In Qeol+d Survey Bulletin 9744, Volcanic activity in the Aleutim arc, by Robt R. Coats. The remainder of the data has been revised far publication in BulEeth 1028. The geologic and geophysical inveatiptiom oovdby thi~report were reconnaissance. Tbe factual information presented is believed ta be accurate, but many of the tentative interpretations and conclu- sions wiZt be modified ae the invtlstigstiona continue and knowledge P-. The inves@ptiona of 1946 were supported alrntrat entirely by the Military Intelligence Division of ths Office, Chief of Eqgineorm, U. S. Army. The Geological Survey ia indnbted to the Ofice, Chief of Enginem, far its carly rocopition of the value of geologic studies in the Aleutian region, which mdo this roport posihle, arid fox its mntpinuingsupport. m

Aei~de--.,.,,,,,,,,,------,--,----,,,,-,----*-,,------,-,---,--

Abatrad -,,,,,,,,,,,-,---,,,-,,,,-,,,------.I Int~uetim,,~,,,--,,--~-~,,,,,,~~~~~~--~~~~~--~-~~----.-----~* Geo~ph~,,,,,-----,,-,---~~~~,,-,,,,~------~-~------~-~----

Location and siae of are^ ,,-,-,,------I-- Terssint~,----,,--,,,,,ww-,------,------,,,---- h~-llying~&*--,-,,,,,,,-,,------,,,----,- MountaiaouaLalauda,,,,~~-,-,-,,---,,,------,------,,--- V+tation-,,,,,,----,,,,-,,-.,------,,,------Ueotogp------+--,---,-- *,*,---- ,,,,,, --,-,,-----,,,-,,-----

Granitlo and mehmorpMo rock_- -,,-,---,,,,,orporp ---- orporporp~orporporpJ-~- Gdk(?),-,,,,,,,--,,,,,-,-,,,------,,-I-----.---,,------Rdtia rooks, argillih, and graywncke ,-,,,,, -- ., -- -,,, , ---, ,-+ - Finger 'Bey voloanica -,,--,,---- ,,- ,------.- --,,*------Graywacko and paywacke breccia ,--,,,,,,,-,------a Early Terbiary basmltio rocks------,,-,,,,- --- ,------,+ --- -

Lett! Tertiary and Quaternary blticmckm ----,-,----,----tt!tt!-,tt!- Tertiary and Quaternary sedimentary mka. ,,------,,-- - - ,- ,, - Gtruct~re,-,,,------,~,-+----,,-,,,,--,,,----,,----,,---,,,,, Pre-Tertiarg- -,,------.-, ., , , , ,, -, -, ,- -, - ,, ------,-, Tertiq mdQnsternaty,,,,,,,,,,,------*----- GmIogic Mhry---- ,---,, --.------,------+------Futum volcanicaativiZy,,-,,,,,,,,,,,,------,----,+-- Selected bibliography- - ,,, --,,,,-,,--,-,-,,------,,.,-- -,,,---- - Xndex ------,------,------

ILLUSTRATIONS

Pwe PLATE17. Geologic monnaiaaance map of eomc weetern Aleutian IsIands, Alaaka -,,,--,------* ------Inpocket 18. A, Ilnk Island from the northenet; B,Angular granitic bulders

on Arnchitka Island -,,,--,,,,-,- -,,------*- Faces 92 19. Gmeloi IeEand from the muthwmt,.-. , . ------I'M 93 R~URE6. Map of the Alaska Pcninsul~snd Aleutian Idand8 --,,,-,,-,- 85

IWETIGATIONS OF ALASKAN VOLCANOES

R~cONNATSSANGE GEOLOGY OF SOME WESTERN ALEUTIAN ISCATsTDS, ALASKA

Tbe general featurea of the westam Aleutian Islands, from Kwtochi to dttu, were studied in L9P6 in mnn(~ctlonwith inveetigationa of northern Adah I~Iand, amthem Kanlrga Idand, and other dleutien imhnda. The geology of the w&rn Aleutian Ialnnda i~ complex. Its deducible history b~nain the PaIeozoio(7) era, when a land made up of rnctamorphjc rocks, in- t&ed by granite, occupied thc region. Buccessive additions of basaltic volcanic rocks and rsedirnonts of voloanic derivation were mde during Pdmzoic(?) an$ ewly Tertiary time, producing a hightan& mouth of the present ielands from about the longitude of wastern Tanqa inland b that of eastern Amchltks Is- land. Submergence of mat of the land resulted in the narving of wide wave-cut benchw tb't lator emerged from the wn.tp:r. In late Tertiary time, glaciers acaumuluW In the highland and rnovod northward over the uplifted bnchce. In very late Terti~ryand in Quaternary time many basaltic shield volcanoee and oompoaito oonea were built on the wave-out and lmalIy glaciated remnants of the older volcanoea. The new volomoe~were alind on a great arc along the northcrn margin of the prprwnt &It of idands. The eruptiom that produccd many of than volcanoes began at a tima when ees level waa higher, relative to the Iand, than it ie nou*; mnrino sediments wem demiited off~horeand are now expoeed above rpes level. A period of domnfa~~ltingfoUowed, and most of the ancient hi~hlandain the wuthern p~rtof the belt disappeared beneath the Pacific Ocean. Mtsny of the idands wcru agtlin bevelcd by marine eroaion. Later, the ialrtnda mrncrgcd in about their prcsent positions. In Quakrn~time, new glncieta dev~lopedin the hi~herparts of the idands, except on the vcry active volcanoes nnd on aomo Lobtcd prsks. Some of the volcano~abccame extinct before and during the most recent pcrid of glaciation, but many are dill sct'ive. Mild volcnnlc activity, such ae eruptions of ash, can he expected to occur lrequcntly in the northern part of the island belt; violent actir-ity can be expected ta occur infrequently. INTRODUCTION In the summer of 1946 several of the Aleutian islands west of A&kaldmd wero mapped geoTogidy. In addition to areas mapped in detail on Adak and Kanaga Islands, a number of the islands from Great Sith to eastem Attu were examined briefly on the ground, and athem from Kasatmhi westward were studied from Ghe air. Tho purpose of thia mnneisssnce investigation wm to choose areas for later, more detail4 work, and ta acquire ganerel howlcdge of the place of the wmteFn Aleutian Islands in the volcanic history of the Aleutian arc. In ordcr to present a mare newly somplets picture, this report summarizes not only the reconnaissance investigations made from Tanaga Island to Attu Island, but dso the more detded work done on Adak and Kanaga Islands. Tho map accompanying this report (pl. 17) is therefore generalized, oven in the are= that have been co~eredby more detailed maps. The geologic units used in the mapping snd description me ~lsobroad and generalized. This report is based almost entirely on fidd work done during the summer of 1946 by Robert R. Coats, assisted during most of the season by Will F. Thompson, Jr. Eighteen dap wsrs spent on Tanwa Island, 4 days on Ga.reloi Island, half s day each on Oglinga, Attu, and Amatignak Islands, 4 d~pon Arnchitka Island, ssnd 2 days each QU Shemya and Great Sitkin Islands, in addition to time spent on Adak and Kanaga Islands. The Scmichi Islands, Bddir Island, and parts of Attu, fiks, Rat, Amchitka, and the Delmf Islands were obso~vehfrom the air. Most of the islanda from Adak to Amchitka were also obsertred from shipbosrd. Aerial photographs of many of the islanda, taken by both the U. S. Navy and U. S. hy, mataridly aided the study. Acknowledgment is made of the mopetation of officers and men of tho Army find Navy, especially Lt. Col. R. E. Warm, poet engineer, Adak; Lt. Col. C. E. Johnson, port commander, Adak; Commander S. 0. Cole, chief staff oflicer, Ad& Sector; Lt. Co1. Leonard Cox, acting post wmmander, Shemya; Lt, Col. Ivan Cunningham, post engineer, Shemya; and Lt. John Rogers, Attu.

The area dacuased herein includea that part of the Aleutian chain bm Gatocbi Idand watward to Attu Island, md is refarred to EM the weetern Aleutian Islands (fig. 6). It extends from loktuds 175'30' W. to 173' E., and from Patituda 51'15' N. to slightly more than 53' N. The length of this part of the chain is about 500 statute dm,and the greahst width, which is in the vicinity of Semisopochaoi Island, is about 50 miles. The largmt islands are Ad~k,284 squwo miles, and Attu, 318. The rtrerts of the 10 lmgest of tbe western Aleutian Islande (west of Atka, Island) mgiven in the tabla on page 86. The meaaurementa were made by Harold Drewes, using a planimeter on mqsof a ~de of 1: 250,000. 17W 170' SO' - -

USSR

. , ALA BKA

64'

BERING

Aow.!"~~ .I Yll".. rn

1*~~~1,~,.!..t1!~,,,r,.m.!,,~~t,~~~f~~,,1~~,~1~5O~ 160' IIQ 180' 170" 160. 150" P1~0ag6.-Eda~ Of the AkkA PdrdSEUd Mouth khds, M %z= %E r- %z' Attu,--,------317.8 Amchitka- ,------114.1 Agsttu , 77.5 Ad& -,------284. I Kish---- l, - +, ,-, 109.8 G-t Sitkin ,+,, ,, 01.6 Tmags------1% 8 Semisopochnoi- - - - 78 3 Kagalmka,,, ,- .- , 45.4 Kan~ga-,------134 6

Great topgmpbic diversity exist@-among the western Aleutian Islands as s dtof numerous dserencea in their p1Oa;o histories, Broadly, the islands are of two terrain types-low-lying ialands land momhinow islands. The Iow-lying islands are wave-cut platforms whose surfaces are nearly featureless. The relief of the mountainom islands is ~~1lstfc1ctions1,bemuse of volcanism, or dmtructiond, became of dissection oi an upland surface, or s combination of the two. The topography of many amas of each terrain type has been modi6ed by glaciation. Both tcrrain typrs are present on some of the larger islands. fAEw-LYMQ ZBm The eIevated wave-cut platfm that constitute the. low-lying klmbnda and par@ of aome of the mountainova islands am of three types, The platforms that emerged moet recentIy retain the initial surface form, pmctically unmodified by fluvial erosion. &me of these.aurfacea are cav~r~dby thin layers of boulder gavel, perhaps of glacial dorivation, that wcro romrked by marine abrasion before the emergence of the platforms. The second type of platform emerged earlier, and the topography has bcen modified to a greater, extent by ~ubaerialeroaion; distinctions may be made &thin this group on the baais of the relative age. The third type of platform bas been eroded by ice aheeta, formed by the confluence of valley glaciem that originated in nearby highlands, ~incethe latest emexgence of the island. The platforms that emerged most recently retain the &noat feature- laas surfaces produced by marine abrasion, md are either horimntal or are gently tilted. A few stre8me haad near tha crenulated margins of the platforms, which tirey breach and descend in short, steep can- yons. The steep rn-~ of the pIatfoms are fringed at their bottoms by nmwboulder beaches. A few Iarge, round or elliptic~llakes are present on the platform surfaces. Islands of this type include Ogliuga, Ilk (pl. 18 A), Skd,KavaIga, and Unalga in the Dclttrof Islands, and Shemya and Oubeloi in the Semichi Tslrcnds. The second t-vpe of platform emerged earlier than the htsnd haa a more nature topography. Several successive marine banehw are commody present, and the daceof the highest one generally forms the principal interst.ream divide. The bench surfaces are poorly drained md many have laka of different size and irregular ~hape, Shsm yalleye me longer and drainage patterns better developed than on the platforma that emerged most recently. Drainage patternm me poorly ad jutted td bedrock structure. An emmple of this type of physiography is eastern Amchi tka Island, whwe glacid boulders occur benath marine gravel st altitudes of more than 100 fast (pl. 18B). On the oldest platforme the interstream divides are narrow, and lib are few, large, and of pimple outline, except where the encroach- ment of vegetation b&s made them irregular. Nmwduvjal plains are pr-t along the larger streams. With increasing aItitude the ancient sea cliffs between tsueccsaive levels of marine abrasion have progressively more gentla slopes. Tho wavs-cut platforms of sont.h- western Kanaga Island and southern Tanaga Idand are of thh type. Wave-cut platform on soveral islanda were glaciated during the Pleistocene epoch. The effect of glaciation haa been to emphasize the bedrock structure. Streams and chains of lakcs lie dong fault zones and weak beds, preferentially excavated by glacial erosion. The topography in general is minutely kegdm or hummocky, and numerous bedrock knobs, small depressions, ponds, and lakes are present. Glacial erosion has gendybeveled off the abrupt alloulders botween uplands and sea cliffs. In place cMs have been cut in the slopm by poetglacial marine erosion, but them cliff a are generally low. The platform of Ulak Island is of this tge as is that of the extreme muthwestcrn part of Ad& Idand and tho adjacent part of Kanaga Idand. Tho sources of the eroding glaciers were the highland@of Amatignak and Adak Islands, respectively. MOUNT~~INO~mm Two typea of mountainous or hilly islands have bem distinguished. The relief of one t-vpe, herein referred to as an oldland, is destructional, being due te differences in the resistants of th~mcks to erosi,on, although uplift of the entire landmnss was probably 813 accessory factor in producing the topography. The relief of the other type is cunskuctiona1, owing i ta height, to upbuilding by volcanic activity. The oldlarld islsnd~of tll I\ Ale~!utinnchain comprise three pup: those that ltav~bccrr dissected by rlormal subaerinl crosion but not by glaciars, those that formerly had small glaciers near t.1~0higher sum- mih, and thome that were axt,ensively glnriatd Thc highlands of Artid Island seem not to hwe been glacint ed. A few relatively smalI glaciers were preaent in the highlands of western Amchitka Island. Islands extenively covered by glaciers include At.tn, Eska, Agattu (Sharp, 1946), Amatignak, Adak, and the soutbern Andreanof Islands east of Adak. These islmds are characterized by knifeedge ridges at high altitudes, glaeiallp atristed and rounded ridges at lower altitudes, numerous rock-basin lakes, flat-floored passes, and wide, U-shaped ~dej-s. Thn highland8 bnat by ~okmicactivity include those that have been dtssaoted by at- and glaciere, and those, either lower in altitude or of relatively recent constmction, that have undergone little or nno diamction and stilI retain their original cbndika f0m4. The volcanic conea on Tanaga Island, except the lqh triple peak of Tanaga Volcano, have been dissected by glaciation; and all but small remnants of the mutheam flanks of the low~rpeaks dong the emtam part of the north coast have been removed by vigorow marine erosion. The two volcanoes on Adak Idand-Mount Moffett and Mount Adtagdk-have been eomewhat diqsected by subaerial erosion, and by marine truncation on the sidea exposed ta the ma; Mount Moffett hna been locally glaciated. The low cone on Bohrof Island has been doeply disaecbd and largo parts have been removed by marins erosion. The cone on Gsreloi Island (pl. 19) hm been only dightly dissected, even though it supports two glaciers. The lmge island of Semi- sopochnoi consists of a great postglacial caldera and several young, undia~ectedvolcanic cones. The symmetrical cones on Segula and Riska and Buldir Islands htnve not baen deeply gkiated or dissected by streme. VBOETATIOFZ The low-lying arcas of the wostern Aleutian Islanda support a denac growth of grass and moss during the late aumrner. Narcissus, anamone, and lupino are abundant, and denso stands of wild sgo occur near beaches. Heath plants tassociated with a very thick, spongy carpot of moss and lichena are common on poorly drained ridgetopa, A species of prostrate willow grows on some exposed slopea. The principal plant associations have baen described by Collins, Clark, and Walker (1945, p. 64-68).

Eight geologic uni ta have been distinguished in tho western Aleutian Islanda. Six of theso have been mapped, and their distributia~lia sl~ownon ptatc 17. The unitg and thcir distribution am discussed bclow, but no attempl is made to describe separately the geology of each faland. I)wTIII dlQD XICTkmbORPEIO BOCX Some smoothly rounded edoouldem in grad beds on the wavecut platform of wcstern Tannga Island and on Oghgs Island appeaf to be dcrived from tho oIdest rocks in the western Aleutians, The contrast betwccn these boddcrs and the rough anguI&r boulders derived from the exposed bedrock of these idands is mmked. The rounded boulders are of severd rock types, induding harnfeIs, hornblende gneiss, slate, schist, granulite, granodiorite, and several parietia of granita, including biotite granite and hornblende granite. The bedrock source of the boulders was not seen and may not be exposd. Therefore, the relative ages of the granitic and metamorphia 6 cornpasing the boulders could not be definitely determind. The granitic rocks are believed b be younger, however, becauae most of them lack directional tkxtww, indicating that they h~e not been metmorphosed, although some are slightly gneissic. The boulders may reprswnt rocks of many ages and more than one inhwd of dn.The boulders of metamorphic rock have undergone more thorough recrystallization than is known in any rocks now expoaed in the western Aleutians, suggesting that the parent rocks axe older than the Eier Bay volcmia (p. 89), and that they may be of Paleozoic age. The presence of crystalline rock8 dearly of clonhmtd type indicates that a large land mass must have existed in the vicinity in Mesozoic or Paleozoic time.

The U. S. Caaat and Geodetic Survey (1944, p. 1201, reports that Ilak Island is composed of granite. [In the summer of 1952 landings were made on the island and the rock was found to be a light-colored granular rock, resembling the quartz diorite that forms intrusions in many of the islands.]

BASALTIC ROCKS, bBGtILLITE, AND @RAYWACm A sequence of dark-peen to black basalt flows and basaltic tuff, gray hard argillits, and gray-green coarse graywacke, containing small lenses of brown chert and brown limestone, was seen on Attu and Shemya islands. The rocks on Attu, described by Capp (1934, p. 162), svidently are part of this sequence. Most of the rocks have bean intensely sheared and fractured. The age of the rocks is not known. They ha50 been found in contact only with early Tertiary volcanic rocks which intrude them, Because of their greater degree of shearing and defomtion, they are bnbtivdy considered older than the Finger Bay ~oIcanicsand the graywacke and gaywacke breccia sequence of Amatignak Island (see beEow). BAT VOLQblQICS A sequence of black, dark-gray, purplish-gray, greenish-gray, and grayi~hyellow green volcanic rocks crops out along the north shore of Finger Bay on Adak Island and hna been named Finger Bay volcmics. These rocks, which uridcrlie most of Adak Island, consist of bwdtic flows, tuff-braccia, agglomerate, md tuff, intruded by Iarga rn-s of grsbbro and smalI masses of rhyolite. The volcanic rocks and many of the intrusive massea included with them have W greatly deformed md hydrothermally alterd. Similar mh am present on southeastern Great Sitkin Island, and they probably corn- pow most of the Andremof Islands east of Adak. The wcb ham been studied on Great Sitkin and Adak Islands and are diacuased by Simons and Mathewson (1956) and Coats (1958a). The deformed and altered Finger Bay volcanim were Cu~~eIatd, on lithologic grounds, with volcanic rocks of Mesozoic age on the Alaska Peninsula (Simons and Mathemon, 1955). Because a late Paleozoic plant hw been identified in the Finger Bay volcmicrs exposed in an isolated outcrop an Adak Island, these roch are now regarded as Pdeozoic(?) in age (Coats, 1956s). GBAYWACKE AMD GRAYWACKE BRECOIA Beds of graybh-green graywacke and graywacke breccia, containing subordinate mudstone, were seen on Amatignak Island. They ara exposed in aea cMs dong a smd bay on the northeast side of the island, and between the bay and the higheat point of the ihd. Tbe rocka me composed of fragments of chloritized, albitized, and epi- dotized basalt. The rocks are well bedded and well indurated. They we intruded by sills of medim-grained granophyric, partly uralithed gahbro, as much aas several hundred feet thick. The relative age of this rock sequence has not been dekitely deter- mined, a~ iits contact with another Boquence haa not been seen. All fragments appear to bc thoso of basaltic rocks; no metamorphic or granitic fragments are present. The sequence codd be the sdi- rnentary equivalent of the Finger Bay volcanics, or it might be regarded ma consisting of material derived from Finger Bay volcanics that wme eroded and deposited in shallow water. Sediments that are samewhat similar to these, oxcept that they contain a great proportion of chert, occur on Agattu Island (Sharp, 1946, p. 193-1 94). &Y 'Pm'PIARY BABALTXC BOCKS Volcanic rocks of probahIa onrly Tertiary age have been mamined on Tanaga, Ogliuga, Amchitka, and Shemya Islands. Rock types include hwdt and hornblende basalt tuff, tuffaceous agglomorata, tuff-breccia, agglomeratic tuff, flows, and dikes. Many of the fmg- ments in the tuff-breccia are large, soma as much ae 7 fwt in length. The flows are generally thin and in placos are interbedded with scoria. On sastern Amchitka Idand these volc.mic rocks are intruded by a stack of quartz diorite. The earIy Tertiary volcanic rocks include five mahlses of andesite porphyry near Zeta Point on the east coast of northern Adak Island (Coats, 1956a), md a similar mam of hornblenrle andesite porphyry on Shemya Island. . . Evidence for age deterrnimtion is inconclusi~a. Bemuse the rock& are much lem altered than the Finger Bay volcsanics, are but slightly deformed-the greatest measured secondary dips being only 10' to 15O-aad have been glaciated, they are believed to be of Tertiary age. They have bean beveled during one or more atages of wave cutting, and are therefore assigned an early Tertiary age to distinguish them from other glaciated volanic rocka that have not been similarly beveled and that are assigned a late Tertiary or Quaternary age.

The northern pmta of Greet Sitkin, Adak, Kanaga, and Kisb ,Idah&, and most. of Kasatochi, Bobrof, Gmeloi, Semisopochoi, .Little Sitkin, Segdn, and Buldir Islands are composed of prtwticnlly omdissected volcanoes. All the volcanoes aeem to be gtratovolcanaes (composite. cones) made up of int~xbedded besdtic and andesitic pymlastic rock^ and lava flows, and of mudffows and volcanic sedimentary rocks on their lower slopes. The oldest of these rocks appear -to r~tunmnformably upon a plane of marine abrasion that bovels the early Tertiary volcanic rocks. Assuming the period of marine erosion to be middle l'ortiary, tho extrusion of theso rocks is believed to have begun during tho late Tert,iary and to have continued inter- mittently to the present. On many of the islands, the oldest mch of this aquenw mi well-beddad volcanic sandstone and conglomerate, interbedded with or grading lahrally into tuff-breccia and overlain by thick flows of basalt that retain low initial outward dips. Around Kana@ and Tmaga Volcanw and on northern and western Scmisopwhnoi 181md, the low-dipping flows and pyroelastic rocks form tho radial ridgea and arcuate rim^ of hrge calderas. Within those calderas, single or multiple composite canes have been built. Tho conm are made up 'of lava and scoria whose initial dips are as much as 30". The extrusion of Mack to dark-gray vesicular-to-compact lava has beon occasionally interrupted by the explosive ejection of much fine basaltic and andesitic lapiUi and ash, and smaller quantities of pumice. A blanket of fine ash covers largo swaa on Kana@, Tanaga, Ogliuga, and other islands at least as far west as Amchitkn. Tha thickn ms of this blanket variea considerably, depending an the length of time since the land surface was last scoured by sea or glaciers, the distance from active volmnoes, the direction of prevailing win&, and the rate of removal of the ash after it was deposited. The maximum thickness iswmorethan 5% feet on Adak Island, more than 7% feet on northern hapIdand, and more than 19 feet on northern Tanaga Island. The ash on each island probably accumulated to its present depth over a period of several thousand years and as a r~ultof the eruptions of various volcanoe~. The aah on OgEuga Island is abuk @l.&$ deep, but its fmdmsss suets t.ht almost dl of it may hva been emphd recently, perhapa by Mount Gareloi.

Tertitrry and: Quaternmy aedirnentav mka occur on moat af the islands in minor quantity. The outcrop areas &re tm smdl to be conveniently indimtad on plate 17. The only fmilifarous Tertiav or Quaternmy sedimentary roch recognhed are exposed in mall areas on Adak Island near the ,ibgee of the awt side of Mount Adagdak, md on Amchitks Island at the head of a mall bay 2.6 miles west of East Cape. Other areas bava been mentioned by Dall (1896). The rocks consist of poorly con- solidated fm~ilifemussandstono and mglomerate. The precim age ol the fomils is not known, but they am Tertiaq in aspect. On Adnk Island these rocka ovlprlia beda of bodder conglcixnersbs. On Ilmchih Island, west of tha head of Constantine Harbor, a sequence of regularly interbeddod, unfossiliferous, yellow-py, poorly con- solidated smdstone and ~iltstone,about 60 feet thick, is expoeed in borrow pita, The aandahone and siltstone are overl~inby till rand unconformably undor1nin by early Tortimy voleanie rocks. . On Adak Idand, glwial drift forms a tatberod rnande covering much of the northern part of the island. It condsta chiefly of gmund moraine, but indudm subordinate mounts of lateral mamipe and fluviodacial deposits. Glacial deposits are less conspicuous on other islands; small deposita were obserped on Tanaga, Am~tipak, Amchitka (pl. 18B), and Attu I~lands. On bchitka Idand, on the nor-th s1lore of Constnntine Harbor, three aucmive layem of till md interbedded fl uvioglacial ~ledimenlaare exposed. Wave-cut hmces, covered by a thin veneer of marine boulder gravel, are present at wvtlral places along the shores of the bigher islands. On nors.hem Ad& Island are rmnantsl of the temcm, thohighest about 100fcct ahovespalovel. Most of southern Trtrtaga Island is mantled by marine gravel, resting on a wave-cut platform. On Ogliuga Island come boulder gravel veneers a wave-cut pla.t,form end i~overlain by mama voIcanic cinders. Unwnsolidated aand and gravel as much an 10 fcet thick rests on a darwave-cut platform on Shemya Island. Small beacha are present on most of the ialands; most. of the material composing them ranges in size from that of cobbles to that of bould~rs. Streams emptying into baya protected by bay-mouth ham have built delt.as, 8 few tens or hundreds of scree in extent. Most of the delta sediments are reworked pyroclastic or glacial mnterial. Similar

RECONNAfSSNCE GEOuXr*Y, WESTERN ALEUTIAN 18-S 93 matefid foma mdl patcbe~of alluvium dong some of the streams, particul~rtywhem atreams empty into glacial lakm. On Adelr Idand, a few scres are owupid by mud dunes on both sides of the entrance to Clam hpn. Dune amas are &o present on Tan* Tdmd. The maximum height of the dunes is about 100 feet. All appear to have been stabilixcd, except those from which the cover of vegetation has been recently removed. On Ogliuga Island, isolated hillocks, rising a few feot above the featunless plain of volmnic ~h,are apparently cumpomd of wind-blown ash.

&owledge of the structurfi of the western Aleutim falands is iacomplete bemuse of the absence of information about many of the islands, Mrnma2ment of msny critical amas beneath the see, kck of fogs& that indicate the age of the rocks, 8nd lack of key horizons that reveal atmctnral datails. As now conceived, the atructur~lhistory of the western Aleutians begins with an oldland, made up of metamorphic rocks intruded by pdte. The structural trends in this oldland cannot be inferred, but the younger graywacke of Amatignak Island and, locally, the Finger Bay volcanics on Adak Island have fold axoa that trend northward. LocdIy, the Finger Bay volcanies and the carly Tertiq socks, have bean folded along axes trending eastward. All the rocks formed before late Tertiary time are mrt by faults that trend roughly parallel to the. trend of the Aleutian arc. The Fate Tertiary and Quaternary volcanoes am evidently rels~tedto a fault or frwture zone extending the length of the Aleutian arc. Too littlc is known about the distribution of the early Tertilary and older rocks to pmit a scatment of tbeh relation to the present arc; it is inferred that t.he Aleutian Islands werc established in about their present form and distribution late in Tertiary time. PRE-TEFLrnY The aeguencs of older basaltic volcanic rocks, argillib, and gray- wacka of Attu rand Shemya IsEands is intensely ~hearedand faulted. Detemhing the age of tho faults is difficult because no rocks of definitely known a.g~are involved. In rs quarry 2%miles north of the Attu ahbase, many faults are expc~ledwhich strike N. 10'-30" W., and dip 65°4380 S'rV. It is not known whether the faults were formed em1ier than a vertical fadt that trends west-northweatad and cuts early Tertiq rocks on Shemya Island, or were pdueed simul tan& oualy tas. mrnplemcntary shears. The attitudes of the Finger Bay volcdcs are diverse. On the southern shore of Adak Island, racks that from the bo8t were judged to be prt of the Flager Bay volcmics occur as a gr~topen syncdine : trending mstd;the dips on the flsnks of this fold approach. 20". . Comparable open folding and trends are suggested by th8 low dip j, the series of Finger Bay volcanica between Andrew and Clam La-, goons, on Ad& Idand. Farther ~outh,between .K:uIuk Bsay and Shagak Bay, and between Finger Bay and Sweeper Cove, ateeper dips and northward trending folds sre present. This discordance in

tren& on Adak Island may be due ta the prmenca of two unconform- , able sequences in the nrcke mapped as Finger Bay volmnics. , ,.< , , Many faults, wllich appear to be ateeply dipping normal faulta, occur in the Finger Bay volcanim on Adak Island. The predominant btranQ of these faults range from N. 60" E. to N. 60' W. and from N. 20' E. to N. 16' W., but faults of intomediato trend are present, bd some faults curtre through s wide range of trmda. Their age h not definitely determinable. They do not appear to cut my of the Ter: tiaxy rocks of the island, but they cut gabbm, which inhd& th6

Finger Bay volcanica and is probably the youngest prs-Tertiary rock. ' Beds of grqwacke and greywacke bmia on Amatignak Mand have been tilted and now trend about K. 8' W., and dip 30" E. These Beds and the ebbro sills that intrude them have been faulted; tho fhults, apparently reverse faults, trend N. 35'-50* E. and dip 50'-70" SE. The age of these faults is uncertain, &s no younger Tertiary whentwy or volcanic rocks appear to have bmn cut by them. -Y AND QUATBRHART In addition to the cones built of Tertiary and Quaternary ~olcsnic: rocks, Tertkry and Quatmary rstrmct~of tmtonic wigin occur on swme weatem Aleutian Islands. Early Tertiary volcanic rocks on Amcitka Island, and tbs d- mentq rocks associated with them, are gently folded or tilted. The beds trend weatward and dip from 10'-27" S. Laym of Pleistowne glacial till on drnchitks Ieland are mrt by normd fault%,ranging in trend from N. 80' E, to N. 45O E. Some of them faulh outIine downfaultd block8 in which the younger ~edi- menta are praemd; one such block forms Constantine Harbor, the . floor of which hae berm dropped about 130 feet on its north Bide. U Some of the numerous faults cutting the basaltic volcanic rocb, mgdlite, and pgwwke sequence on Shsmya ~ndAttu Jh& Beem ta be reIahd ta a fault which cuts early Tertiary volcanic rocks on Shsmya Idand. This fault, exposed on the north share of Shemya Island a few hundred feet from the eastern end, cuts rr t~~aons basaltic conglomerate, evidently pwt of the early Tertiq volcanic rocks. The fad t ia vertical and stdmN. 60° W. A mylonitkd zone is deviloped dong it. Faults on Attu Island that art! possibly re- ~CONlUABMNCEGEOLOQT, WESTERN lated to tbis fault trend K. 85" W. and dip from 50" 8. to vertical. The trend of the fault striations is northeast, and the south ides of the fault appear to have moved up and eastward relative ta the north sides. The young& faults found pbre thoae om the flanks of Mount Adagdak on Ad& Island. They are probably Recent in age, md they appear bo be norm J,the domthmwn sidn being towd the mountain sum- mit. They were probably mused by subsidence which wcmed when megms withdrawn from beneath Mount Adagdak.

The earlimt event that can be infed in the geologic history of the western Aleutian Tsllande ia the deposition, perhaps in Pdmzoic he, of dientary and volcanic rocks in the vicinity of what are now Tmaga, Ogliuga, and Adak Islands. These racks wm deformed and metamorphosed, then intruded by granite. After a long interval of erosion, ~olcanismWM resumed in Mesozoic(?) tine. Stratovolcanoes were built on Shemya mdAttu and perhaps on other idands. Around the rn* of these volcanoes, mrtrine deposita of argillte and gray- wscke were made between intervals of extrusive activity. Gmyw~cke and graywacko breccia were. deposited in quiet water on what ia now Amatignak I~land,probably either at the same time or slightly after the Finger Bay volcaniw were extruded. 731s Finger Bay volcaniea and the older rocks were comprm~ed,i~ part by for- acting nearly normal to the pment trend of the chain, and extensively faultdl. Deformation wse accompanied by hydm- thermal alteration. G~bbrointruded the rocks, md some of the intrusive massea were faulted and hydmthemndly altered dong with their host mks. After a period of deformation and emsion, badtic and andeaitic mlcanic rocks were erupbd in early Tertiarg time to fom new Bequences on the dissected foundation of older volcanic and intrusive rocks. The early Tertiary val~noeswere beveled by marine erosion at a hewhen sea level was aa much as 150 feet higher, relative to the land, thm it ia at present. It is not known whcther this difference indimtm that the level of the sea wns actually higher or that a lab upward movement of tho land od.The torraces produced by thin marine hve1ing are not of uniform height",and some me tilted. Volcanic activity was medin late Tertiary time. The new venb were, in geneml, north of the Meeozoic and early Tertiary vents. The early flows from soma of the late Tertiary venta rest upon sediments that apparently accumulated under marine conditions, suggesting that, when the building of the late Tertiary con- began the level of the sea was again relatively higher &m at present; fluctuations of relative aea level appluently ocmmd ae~eralthm during later Twtimy tima, Fine-gmined marine sedimmts, bdyfoeilifemus, which amumulated during periods when *sea level was relatively high, are now exposed. Dhga period of relatively low sea level, perhap in early Heist+ cene, gleoiers advanced northward from a highland, presumabIy devd- oped on the older volcanic, metamorphic, and intrusive rocks. This highland is inf~rredto hetve lain south of the present islands md to have ex tonded from about the longitude of wmtern Tanaga Island to that of eastern Amchitka Ialand. Glaciers moved northwd acm the emergent platforms, than retreated. Subsequent submergence permitted waves to work tho gIacid tiUs on Ogliuga, Tanaga, and perhaps other islands. me disappemmee of moet of the, highland is thougbt ta be due &idly ta downfaulting below ~ealev4 In Quaternary time, glaciem born in the highIands of many of the present islands greatly modified the topogmphy of the uppar mom- bin slopes before retreating. Narrow marine terraces, now covered with thin lsyera of boulders, ware cut during one or more jnbrglacial stages. Since the moat recent glmiation, the rocks on some islands have bees cut by steep-dipping faultg of mall disp1-ent. Several. cones, auch aaa Mount Moffett and hlount Adagadak on Adak XsIand, became inactive before or during tho last interval of glaciation, but volcanoes on most of the islands from Kasatochi to Buldir ham baoa active since then.

Many of the wmtern Aleutian Idsnds have active volcanoes. The ~olcaniccones on other islands are deeply eroded, and early renewal of activity i~ mely. Volcanow believed or known b be active indude those of K~atochi,Great Sitkin, Kanaga, Tanaga, Gareloi, Semisap- ochnoi, Little Sitkin, Segula, end ICiska Islands. Inactive volcanoes include that on Bobmf Island, Mount Adagak and Mount MofFett on Adak Island, md the mountains of eaatern Tanaga Ialmd. Recarded activity-years ' for Kiska, Little Sitkin, Semisopochnoi, Garebi, Tanaga, Kmag#, and Grcat Sitkin Islands total 30 during the 185 years ainm 1760. None of the recorded eruptions can be cbdea major. Many other eruptions have undoubtadly occurred but have not been recorded. Oa the baei~of the fragmentmy records, a mild eruption can be expected, on the average, once every 6 yesre in the western Aleutian Islands. One great calderbforming eruption ha8 occumed on theae islands in postglacial time, but such an eruption, analogous to the one that

1 ~ctlvlty-mB &hod hmh m r mlannkr ym~du- which we vohk m. For -, U &me ~0-w Wive .-Is hdupear, thm tbm actlv*y-M sre crmted Okmok Caldera on Umnak Island, could occur on any of the active volcanic islands. Caldera-forming ermptiona are least to be expected on the islands on which cddmas have already been formed- Ranraga, Tmaga, and Semisopochnoi. No evidence has been found suggesting that wch violent activity is in pro~pect for the near futuro, except possibly at Great Sitkin Island (Simons and Mathewson, 1955, p. 4041). In the zono of greatest voIwic activity hm moved northward in the western Aleutian Idands. The most recently active and ,tho leaat eroded volcmoes are on the northern parta of the large idands or on the more northerly amall dm&. Activity in the near future can be expwted to be limited to the northern part of tbe western Aleutian arc. SELECTED IB~WOCfXt~

Cappa, R. R., I9M, Noh on the geology of the Alaska Peninsula and Aleutian Ielands: U. B. GeoL Survey Bvll 857-D, p. 141-153. Cmb, R. R., 1968a, Geology of northern Adsk Ialand, Alaska: U. 8. Gwl. Rurvsp Bull. 1028-C. -I960b, Geology of northern &mga Idand, Ahks: V. 8. hl. bey Bull. 102GD. Collins, R. R,, Jr., Clark, A. H., end nTdker,E. H., 1945, The AI~nt.im11lands: Their people and natural hietory: Smithaonian Inst. IY~hinmn,War Background Studics, no. 21. Dd, W. H., 1896, Report on the coal and lignito of Alaska: U. S. Gml. Sumey* 17th Am. Rept., 1895-98, pt. I, p. 311-814. Grewingk, constantin, 1850, Beitrag zur Kentnia~ dar omgraphiflchen uod pgmstiachen Beechaffenheit der Nordwwt Kbte Amedkas, mit den anlimnden Tnseh: Rum. IC min. G&eedl. Verh., St. Petereburg, IW9, p. 1 Z&2?S. Emarp, R, P., 1948, Note on the geology of Agatta, an Aleutian bland: Sour. GeoIogy, v. 54,p. 193-200. &mom, F. S., md Mathewson, D. E., 1056, Geology of Great Sitkin Island, Alaska: 0. S. Geol. Survey Bull. 1028-B. U. 8. CDaet and Metic Burvcy, 1941, 11. 8. Coast Pilot, Alsska, v. 3.

INDEX

Page Page Abrsion, marine -.------.--.------.86 ~elarof1slands,'tyW ..-.------86 ....~.-...-.-lllllllll----- Achow1edgement ~~.-llllllllll------... 84 Deltas 92-83 A&& Island, ash -----...------.------91 Dunes------.--- .------.93 dunes ...... 83 geologic history- 95 Erosion- -..-~~--.~.~9999999999----- 88,91,96 faulting~-~~------.~~~~-~~------94.95 Faulting --..-.-..-.CSCSCSCSCSCSCSCSCSCSCS------93-95 Finger Bay voldCS~~~~~--~-~CSCSCSCSCS 93-94 Field work .-----..--..--..------84 93-94 ...... Finger Bay ----.--.------89

~_~~-----~--~--OUSOUSOUSOUSOUSOUS-- 92 fassilifeFOUS rock Finger Bay volcanics, faulting.--.-----.------94 glaciation ..-..-.------87,92 features. -..-....-.------.------89-90 marine! ees------92 terra folding------93-= r& types ...... geologic history --.-...------95 sir&.-. -.- . - .------..------Folding ..-.----.-.-.-.------93-94 type. - - - - .- - - - -.------.------

volcanoes ..-~-~~~~~.oeoeoeoeoeoe.-.-.--.. 91,96 Qareloi Island, glaciation -.----..------88 Agattu Island, glaciation------volcanoes ...-----.---.------.91,W mktspes .--.-..-....------...Geologic history ------.------95-96 size. .---. ------.------.------Glaciation..---.--.------.------8687-88,91,W hmatt igna Island, grsywacke brecca------2 Granite ---.--.-.--.----.-..------88-89 fault 94 Graywacke------89,90 foldiog 93-84 ...... Great Sitkin Island, Finger Bay volcsnies.--- 90 glacial deposits. ------.--.------.--.------a future volcanic activity-.--_.------97 physiography..----..---.------87 e .-86 ms 89 volcmes...~~.~~-.~-~~-~------91,W mehitks Island, foss,,i ferous mk -_---_-_--- 92 glaciation...... 87, a nak Island, granite.--..--.------89 rook types ....--.-----.----..------.-.-.. type ...---.------.------86 size ------..- - - .-. . ------.------.------Island types .-...----.-.-.------.------8648 type------.- -..-. - -. ------Islands' Finger Bay volcanics----- Kagalaska Island, size .-----.----..------86 glaciation.---. ------KB"aga Islsnd' *h------91 Areas of islands...---.-.------. -- 86 adera.------*------91~w7 k ..89 U, postglacial ...... 91-92 mpe------87 AttU Idsn& esrly geologichistorg------95 91*98 faulting...... 83,w5 vO1mm------Kasatochi Island, volcanoes.. .-----.-.-.-----91,W daciation ...... 87, CJZ Kavalga Island, type ------.------86 rock types .----.------..-..-.------.----- a Island s 86 size.. - - - - -..------.------volcano..-----.------88,91,96 tirocks - ..89 es------&j,g2 Lakes. .----..-..-...-.-.....------86,87 Bobmf Island, mineerosion- -.---..--.----- 88 Little Sitkin Island, volcano.-.----.-.---..... 91,W vdeanws------~---.~.~..~~..-~~~ .---91,96 ~~~ld~gravel, merine ------.-.-._- 92 Metamorphic rocks ------..------88-89 Bods,exotic ...... 88 Mt. Adagdak, erosion------88 faulting 95

@wacke ~~-~~.~-~J*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*wJ*w 90 -- ~uldb~~l~d, dissection------_-_------88 fossiliferousrock -...-.----.-.------92 v01canicactivit~-----.---.------96 volm~.~~~~...~...... OBSOBS.OBSOBSOBSOBSOBSOBS..OBSOBSOBSOBS..OBS 91 Mt. Gareloi.-----.------92 Oalderas..----..------...... -.----- 91,9&97 Mt. Moffett, galciation --..---...-.---..------88 Constantine Harbor --.----...... -...----....-94 volcanic activity...... -----.-.----.------98 Pam hm ogliuga Island, gab ...... 81,~ Iabnd,a&sh...... PI

dUTLBS +.... 63 caldem ...... rn earlygnoEoglo bworg ...... 85 dmps ...... 93 mktypa W-W.qW2 early w1w hlUto?Y-...... 95 ...... 88 sl~latl~...... Okmok CeJ- ...... 87 rock typ...... 8&B9,8o,m oab$oiIm,- ...... se she...... A0 . *SF9 ...... B1 PlatrOTmU,I*8d-.- ...--..-.---....-....-.e6 .,I,- ...... 88 Rat l'w VQwo...... a91 IdIMla...... &L Topmphy...... %+a drepoFt ...... m-84 88guIa Island, volcatlo...... *..-...... g@,,Pl,ge rnkT!JImd,tgpa...... m hmlchl XsLsnd...... $4 t'nalgRfslsnd,type ...... Bs &amhnwchnolIdand. slse...... U8 I!- 8. nd* 8-P A-

Shemy&Iald, early gwloglc hk4W...... P5 IRultIlW..-+.+---.------.- ...... 93,W V*tatbn ...... lonlonlonlonlonlonlonlonlonlon.lon.....lon~~.~ rock mpee ...... 88. W2 ~o!cadonatlvlty, luhup ...... 88 Mt ..*...... *-...---...... gB s&ld Esland, type .....*.*...... 8R amdhhtm-y...... g3 Zeb POW.,.....*...... *..*....