Reconnaissance Geology of Some Western Aleutian Islands, Alaska 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 vol- canic 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