Geology of the ^Imuruk Lake Area ; Seward Peninsula Alaska * By D. M. HOPKINS \ CONTRIBUTIONS TO GENERAL GEOLOGY * GEOLOGICAL SURVEY BULLETIN 1141 C Prepared in cooperation with the U.S. Army Corps of Engineers UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1 963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.G., 20402 CONTENTS Page Abstract______-___-____-__-_______--___----__-_--_--_-____-.___ Cl Introduction ______________________________________________________ 2 Geography__ ____________________________________________________ 4 Climate ---_____-__-____--_-__--___-__-__-__--__________-_____ 4 Vegetation_ ___-.___--_-____________________._________._.__-. 4 Animals._____________________________________________________ 6 Human occupation._.-_________.__________.___.__.__._..______ 7 Physiography _______-_______-__-_--__-_--__________-_-_--_________ 7 Imuruk.Lake lava plateau.___-____--___-________.__-___..____-_ 7 Bendeleben Mountains.________________________________________ 9 Kuzitrin flats-________-_-___--____-____--_-__--_---_-__-____-_ 11 Seward Peninsula uplands---_---____--__--___-_-_-----__-___.-- 11 Regional geology__________________________________________________ 12 Paleozoic and Mesozoic rocks_____--______---___-_-___---_______..__ 13 Schist unit__-____--_---__--_--_______--____-___---___-____-_ 13 Lithology_ ______________________________________________ 14 Calcite-muscovite schist.____-____-_^___...__________.__ 14 Graphitic schist________________--___--_-_-_--_____-_-_ 14 Other rocks._____-_---__---__------___-___-____-____-_ 15 Age...----------.---------------------_----------_------. 15 Metalimestone unit.___________________________________________ 16 Lithology_ _-_________--__-_--_-_____-_----_--____-_____- 16 Age...--.-.--. -. ------------------------------------ 17 Gneiss unit--____--___--_-------_------_--__.--__-_-____-_-_-_ 17 Lithology__ _____---_---_--_--_---__-__-.___________.____ 17 Biotite-hornblende-epidote gneiss----_--------___--_-___- 18 Hornblende-microcline gneiss.______.---_.___________-_ 18 Other rocks________________ L __...._.._.__.._..__._._ 20 Grade of metamorphism.___________________________________ 21 Age..---.-----...-___-_-_-.--------_...-.-...---_-__----- 22 Granitic plutons..---_______-__-__---__--__-_-_-_-__--__--____- 22 Lithology_._-_-------__-----_----_--_--------_---_»------- 23 Contact relationships._____________________________________ 23 Origin........-.-_____-...____.___--..__-._-_.-------_-._- 24 Age___---_-----_-----_--------J-----------_------------,-. 25 Structure,._ __________________________________________________ 26 Cenozoic rocks._____-__-___-___-____;_____________--_______-_-___- 27 Unconsolidated sediments______________________________________ 29 Kougarok gravel__________________________________________ 29 Stratigraphy and lithology_______________________________ 30 Fossils .and age. ____--__.-__-_______________--_-_--____ 33 Glacial drift---.._______________.____..____.._____ 34 Nome River glaciation__________________________________ 34 Salmon Lake glaciation.____________________-_-__-__---- 35 in IV CONTENTS Cenozoic rocks Continued Unconsolidated sediments Continued Page Windblown silt____._____--..________._.__._. C35 Lithology_ T ._-_--------------------__---_-____________ 36 Mineralogy ___----___----_-----______-.________...____ 38 Age and origin_-___-___---___--_______-____._.._._.... 39 Alluvium and colluvium____________________________________ 42 Gravel.-.-.-----------------------------..__.._.._.._ 42 Stratified silt and peat.____-_ ----______________________ 43 Lacustrine sediments __--_---_---____-__-__________..____ 45 Imuruk Lake.__________-____-_-.-_-________..________ 45 Other lakes.----------------------------.-----.------- 46 Drained or filled lakes__---_--_-__--_-______________._ 46 Diatomite in valley of Andesite Creek.___________________ 46 Volcanic rocks.----------------------------------------------- 47 Stratigraphy----------------------------J_------_--__--_-- 48 Kugruk volcanics-------------------------------------- 48 Imuruk volcanics._____________________________________ 54 Gosling volcanics-_____________________________________ 57 Camille lava flow..-__---------__---______-__-__________ 64 Lost Jim lava flow__-_---_-_-_-_________________.____ 68 Brown-ash layer____-___----___----___-____--___-____-_ 71 Petrology and petrography.________________________________ 72 Olivine andesite and .basalt. ____________________________ 73 Hypersthene andesite.____--__-__----_-______.________. 76 Pilotaxitic andesite-_.___-__---____-_-________-._______ 77 Inclusion-bearing rocks___---____-____------_-_-______ 78 Composition. _ _-._____-__________________-_^______ 78 Origin______ _.____._..._...._._._._.._._._....__ 80 Structural features-.--__.-_-_____-___----_.__--._--_----_-__L-_- 83 Geologic history.__________________________________________________ 86 Permafrost- _____-______-______-_____-_____-_____-_--______-___-_- 90 Ground-water hydrology__._.___-_-_-_--_--__-_-_---____-_-______.-- 90 Alluvial gravel___________________-_-___-_-_---------------_-_- 91 Metalimestone ________________________________________________ 91 Kougarok gravel.__________-____-_____-___-____-__----___-____ 91 Imuruk volcanics-_____________________________________________ 92 Gosling volcanics, Camille lava flow, and Lost Jim lava flow__-__--- 93 Mineral resources._______________-_________ _____________'___._______ 94 Gold placers._.__-___-_-_-_--_---__-_-.-------------_--------- 94 Lignite. _________________________-______---_._-----_---_-_----- 94 Diatomite.______-______-________________-___i_-___-__-__----_ 95 Literature cited-__.___________-____-__-______-_-_----_-_-----_---- 95 Index.--_-_-----------------------------_---_---------------_--- 99 CONTENTS ILLUSTRATIONS [Plates are in pocket] PLATE 1. Geologic map of the Imuruk Lake area. 2. Volcanic and structural features of late Cenozoic age on Seward Peninsula. 3; Glaciated areas and areas covered by windblown silt on Seward Peninsula. 4. Volcanic vents, faults,, and warped surfaces in and near the Imuruk Lake area. Page FIGUBE 1. Index map of Seward Peninsula._________________________ C5 2. Physiographic subdivisions of the Imuruk Lake area._______ 8 3. Panorama of the Bendeleben Mountains___-__-__-___-__ 10 4. Porphyroblasts of hornblende and microcline_____________ 19 5. Sketch of a compound pegmatite dike___________________ 21 6. Weathered.bedrock beneath Kougarok gravel__-__--__-____ 31 7. Mechanical composition of the fine-grained mantle in the Imuruk Lake area.___________________________________ 37 8. Late Quaternary sediments in the valley of Black Gulch__ 44 9. Geologic map of the canyon of the Kugruk River_________ 50 10. Spheroidal weathering in Kugruk volcanics_.-___-__------_ 53 11. Twin .Calderas and surrounding lava dome of Gosling volcanics-_-------------------------------------_---- 60 12. Geologic map of Virginia Butte and vicinity.______________ 61 13. Comparison between the surfaces of the Gosling volcanics and the Camille flow______-_-_--_-__-_-__-_____-__-___ 66 14. Comparison between the surfaces of the Lost Jim and.Camille flows___-----------___-----__-____---__-____._______ 67 15. General view of the Lost Jim flow-_----------_-_-.-----__ 69 TABLES Page TABLE 1. -Mineral composition of fine-grained mantle in the Imuruk Lake area__.__-___ ._________._____________________________ C39 2. Chemical analysis and normative minerals of an olivine basalt lava flow__--___-----_-___---_______-__________________ 74 CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGY OF THE IMURUK LAKE AREA, SEWARD PENINSULA, ALASKA By D. M. HOPKINS ABSTRACT The Imuruk Lake area is underlain by metamorphic rocks of Paleozoic age, granitic rocks of probable Late Jurassic or Early Cretaceous age, and sediments and lava flows of late Cenozoic age. The Paleozoic metamorphic rocks include schist of several types assignable to Eskola's green-schist facies, gneiss assignable to Eskola's amphibolite facies, and metalimestone. The schist unit includes rocks that are older and younger than the metalimestone unit. The rocks of the gneiss unit are believed to be stratigraphically equiva­ lent to part of the rocks in the schist unit and to have acquired much of their present texture and mineralogy during the emplacement of the Kuzitrin Lake batholith. The granitic rocks of probable late Mesozoic age include the small Black Butte stock and the Kuzitrin Lake batholith, a large body of granitic rocks exposed in the northern Bendeleben Mountains south of Kuzi­ trin Lake that apparently extends northward beneath the Imuruk Lake lava plateau to the Asses Ears. The Kougarok gravel of late Tertiary and Pleistocene (?) age underlies the northwestern part of the Kuzitrin flats and consists of a basal gravel mem­ ber locally more than 187 feet thick, a thin middle member composed of peaty lignite, and an upper gravel member 3 to 60 feet thick. The middle mem­ ber contains a rich pollen and wood flora consisting of a mixture of conifer­ ous and deciduous trees, including representatives of several genera that now reach.their northern limits in latitudes 5° to 15° south of Seward Peninsula. Similar pollen floras in the valley of the Aldan River, Yakutia, Siberia, and in the northern Alaska Range are of late Miocene or early Pliocene age. The upper member contains poplar and either spruce or