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Stratigraphy, Structure, and Economic Geology of the Iliamna Quadrangle, Alaska GEOLOGICAL SURVEY BULLETIN 1368-B 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.ofDocs.no.: I 193:1368-6 1. Geology-Alaska-Alaska Peninsula. I. Reed, Bruce L., 1934- joint author. II. Title. III. Series. IV. Series: United States. Geological Survey. Bulletin ; 1368-B. QE75.B9 no. 1368-B [QE84.A34] 557.3'08s [557.98'4] 79-607115 For sale by the Superintendent of Documents, U. S. Government Printing Omce Washington, D. C. 20402 Stock Number 024-001-03263-5 CONTENTS Page Abstract ________________________________________ Bl Introduction _________________________________________ 2 Previous work ______________________________________ 3 Present investigations ______________________________ 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 in IV CONTENTS Descriptive geology Continued Tertiary System Continued Volcanic rocks Continued Volcanic rubble and breccia _ ___ __ __ _________ 55 Gibraltar Lake Tuff ______________________________ 56 Lower member _ _ _ ______ _ _ __ __ ______ __ ___ 56 Upper member ___ __ _____ ___ ___ ___ .. __ ____ ___ _ 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 ROBERT L. DETTERMAN and BRUCE L. 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 nonmarine 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 continued for about 20 m.y. Rocks formed are chiefly diorite, quartz diorite, and granodiorite with lesser amounts of hormblendite and quartz monzonite, part of a 1,300-km-long magmatic arc in southern Alaska. A Bl B2 GEOLOGY OF THE ILIAMNA QUADRANGLE, ALASKA second period, between 83 and 55 m.y. ago, produced small plutons, chiefly of quartz monzonite and granodiorite,

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