Stratigraphy, Structure, and Economic Geology of the Iliamna Quadrangle, Alaska By ROBERT L. DETTERMAN and BRUCE L. REED GEOLOGY OF THE ILIAMNA QUADRANGLE, ALASKA GEOLOGICAL SURVEY BULLETIN 1368-B A comprehensive study of Mesozoic to Holocene sedimentary, volcanic, and plutonic rocks of the Iliamna quadrangle UNITED STATES GOVERNMENT PRINTING OFFICE. WASHINGTON : 1980 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Detterman, Robert L. Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska. (Geology of the Iliamna quadrangle, Alaska) (Geological Survey bulletin ; 1368-B) Bibliography: p. B82 - B86. Supt. of Docs. no.: I 193:1368-B 1. Geology-Alaska-Alaska Peninsula. I. Reed, Bruce L., 1934- joint author. 11. Title. 111. Series. IV. Series: United States. Geological Survey. Bulletin ; 1368-B. QE75.B9 no. 1368-B [QE84.A34] 557.3'08s 1557.98'41 79-6071 15 For sale by the Superintendent of Documents, U. S. Government Printing Office Washington, D. C. 20402 Stock Nllmber 024-001 -03263-5 CONTENTS Page Abstract ........................................ B1 Introduction ____------------------------------------ 2 Previous work . ......................................... 3 Present investigat~ons---------------------------------.----- 4 Acknowledgments ___---------------------------------->-- 5 Geologic. setting _-----___-------------------------------5 Descriptive geology -~______-----------------------------6 Permian(?), Triassic, and Jurassic Systems _____-_---____------------------6 . Kakhonak. Complex ....................................... 6 Triassic System ........................................ 9 Cottonwood Bay Greenstone ........................................ 9 Kamishak Formation ___-___-__--------------------11 Bruin Limestone Member _ _ _ ____-_-___ __---- -- - ---- - ---- 11 Middle Member ........................................ 14 Ursus Member _----_-----_----------------------------16 Jurassic System ........................................ 17 Talkeetna Formation --____--__------------------------------18 Intrusive rocks_--__---____---_------------------- 20 Mafic and ultramafic plutonic rocks ___-_------__-----------------21 Pyroxenite ............................. 21 Hornblendite -__------------------------------------- 22 Gabbro ___----_-_------------------------------22 Diorite ......................................... 23 Quartz diorite _---____--------------------------------24 Quartz .monzonite . --__------------------------------------25 Trondhjemite ......................................... 28 Tuxedni group______________--^---___-------- 28 Chinitna Formation ........................................ 32 Naknek Formation_----___---_------------------------ 34 Jurassic or Cretaceous Systems. --------_----__------------------------ 38 Sedimentary rocks, undivided__---__----~------------------------- 38 Cretaceous System -_~----_____--___--__-----_----------------41 Kaguyak Formation ...................................... 41 Quartz monzonite ........................................ 42 Cretaceous or Tertiary Systems --------_---___---------_---------------43 Granodiorite and quartz monzonite ----_-_-_-------_-----------------43 Tertiary System ........................................ 44 Copper Lake Formation ........................................ 45 Lower conglomerate member --_-----_--------------------------- 45 Sandstone and siltstone member ---__---------------------------46 Upper conglomerate member .................................... 47 Sedimentary rocks, undivided ---------_--_-_-----____----------------48 Volcanic rocks -_-__----__-----------------------------49 Basalt and andesite ........................................ 50 I11 CONTENTS Page Descriptive geology-Continued Tertiary System-Continued Volcanic rocks-Continued Tuff-~------_---_----------------------------------------------B54 Volcanic rubble and breccia ..................................... 55 Gibraltar Lake Tuff ........................................ 56 Lower member _-----_-___-----------------------------56 Upper 57 Intricate Basalt ........................................ 58 Intrusive rocks ...................................60 Quartz diorite ----____--------------------------------60 Intrusive rocks, undivided ..................................... 60 Volcanic necks, sills, and dikes ................................. 61 Tertiary or Quaternary Systems................................. 62 Volcanic rocks, undivided ........................................ 62 Quaternary System ........................................ 63 Volcanic rocks, undivided ........................................ 63 Augustine Volcanics ........................................ 64 Surficial deposits --.~_~_~---------------------66 Structural geology --_-----.____-____---------------------66 Regional setting --_---.~_~_-~-~~__----------------------66 Folds ----------------.__-_-------------------68 Faults----------------.----------------------- 68 Bruin Bay fault --.-_~__--~-_-__------------------------68 Geologic history ........................................70 Economic geology --____---____---__--_-------------------71 Petroleum ........................................ 72 Copper ........................................ 75 Gold and silver --------____----------------------------77 Iron ........................................ 79 Limestone ........................................ 80 Pumice ........................................ 81 Miscellaneous minerals ..............................81 References cited ........................................82 ILLUSTRATIONS Page PLATE 1. Geologic map of the Iliamna quadrangle, Alaska ----------In pocket FIGURE 1. Index map showing location of Iliamna quadrangle ------------ B3 2. Generalized stratigraphic sections of Kamishak Formation, Bruin Bay and Iliamna Bay, Ursus Cove, and Kirschner Lake ------ 13 3. Stratigraphic section of Tuxedni Group and Chinitna Formation, Iniskin Peninsula ........................................ 30 4. Stratigraphic sections of Naknek Formation, Iniskin Peninsula and Kamishak Bay ........................................ 35 CONTENTS V Page FIGURE 5. Type section of Copper Lake Formation, Upper and Lower Copper Lake and Kakhonak Lake area --------__------46 6. Map showing location of natural resources in the Iliamna quadrangle _-_____-__-----______-------------------74 TABLES Page TABLE 1. Potassium-argon ages and analytical data for some intrusive rocks in the Iliamna quadrangle _~----_____---__------------B26 2. Geographic and stratigraphic distribution of fossils in Naknek Formation _______-______-_____----------------------- 39 3. Chemical analysis of selected Tertiary volcanic rocks ---------- 53 4. Chemical analysis of selected rocks from Augustine Volcanics -- 65 GEOLOGY OF THE ILIAMNA QUADRANGLE, ALASKA STRATIGRAPHY, STRUCTURE, AND ECONOMIC GEOLOGY OF THE ILIAMNA QUADRANGLE, ALASKA By ROBERTL. DETTERMANand BRUCEL. REED ABSTRACT The Iliamna quadrangle, located near the north end of the Alaska Peninsula, is part of the magmatic arc that characterizes the peninsula. Bedrock in the quadrangle con- sists of sedimentary, metamorphic, and intrusive and extrusive igneous rocks of Per- mian(?) to Holocene age. The named rock units include one group, 11 formations, and one complex; some are newly named, others revised. The oldest rocks are part of the Kakhonak Complex (new), a metamorphic rock preserved mainly as roof pendants in the Alaska-Aleutian Range batholith. The com- plex contains chloritic greenschist, quartz-mica-garnet schist, gneiss, quartzite, mar- ble, and amphibolite. Metamorphism is mainly of greenschist and low amphibolite facies; unmetamorphosed equivalent rocks are included in the Cottonwood Bay Greenstone (new), Kamishak Formation (revised) of Late Triassic age, and Talkeetna Formation of Early Jurassic age. A basin east of the early Mesozoic magmatic arc received at least 6,000 m of highly fossiliferous marine clastic sediments during the late Mesozoic. During Middle Juras- sic time, volcanogenic graywacke and conglomerate with thick interbeds of siltstone and shale were deposited. These rocks are divided into the Red Glacier Formation, Gaikema Sandstone, Fitz Creek Siltstone, Cynthia Falls Sandstone, Twist Creek Siltstone, and Bowser Formation of the Tuxedni Group and the overlying Chinitna Formation, subdivided into the Tonnie and Paveloff Siltstone Members. By Late Juras- sic time, the Alaska-Aleutian Range batholith had been emplaced as the core of a volcano-plutonic complex that supplied most of the detritus to the flanking basin on the southeast. The conglomerate, siltstone and shale, and arkosic sandstone of this basin are mapped as the Chisik Conglomerate, Snug Harbor Siltstone, Pomeroy Arkose, and upper sandstone members of the Naknek Formation. Deposition into this basin con- tinued throughout the Cretaceous, but within the quadrangle, Lower Cretaceous rocks were eroded before deposition of the sediments forming the Kaguyak Formation in Late Cretaceous time. A Tertiary basin, developed in what is now Cook Inlet, received about 6,500 m of nonmarine clastic rock. A small part of the basal sequence is preserved as the Copper Lake Formation (new). The greater part of the nonrnarine Tertiary sequence underlies Cook Inlet. Three periods of intrusive activity are recorded. The oldest began in late Early Jurassic time, about 180 m.y. ago, and