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Reconnaissance Geology of Admiralty Island Alaska

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Reconnaissance Geology of Admiralty Island Alaska Reconnaissance Geology of Admiralty Island Alaska LI By E. H. LATHRAM, J. S. POMEROY, H. C. BERG, and R. A. LONEY ^CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY-BULLETIN 1181-R ^ A reconnaissance study of a geologically -t complex* area in southeast-..--.. Alaska.... UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary X1 GEOLOGICAL SURVEY Thomas B. Nolan, Director Y. For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS Page Abstract_____________________---_-__-_-__--__-_ _..___.__ Rl Introduction... _______ __-_____-_---_-_ __________ ______ 2 M Geography______________---__---___-_-__--_-_---_ ________ 2 ;._. Previous investigations---..-. ________________________________ 4 Present investigations..-______-_--__ _ ____________________ 6 ^ Tectonic aspects of the geology__-_-__--_--__-_-_______._____.____ 7 Stratigraphy...... _____-___-__--_^---^---^ ----------------------- 10 "-< Silurian(?) rocks_________ . _ 10 Devonian and Devonian(?) rocks--___________-___-______________ 10 J Retreat Group and Gambier Bay Formation___-____-_._ __ 10 ^ Hood Bay Formation..... ............'............ 13 Permian rocks..___-----------------------------------------_-- 14 4 Cannery Formation...__________ _________________________ 14 Pybus Dolomite____---_-_-_-_---_---_---_--__ __________ 16 '>* Undifferentiated Permian and Triassic rocks._____________________ 17 Triassic rocks.__-______________---_---_-_-_---_-----_-________ 18 Hyd Formation._________-__---__-_---_-_-___-__-__-_-____ 18 ; Jurassic and Cretaceous rocks_-__---_---_--__-_--_____--_____._- 22 N Stephens Passage Group__________________________________ 22 Seymour Canal Formation_________-_-_---_-_-____________ 24 Douglas Island Volcanics----------------------------------- 27 y Brothers Volcanics _______________-_-__--_-_-__-.___-____ 28 Tertiary rocks___.._.___.___._-.-___.___._.-__________.______ 28 > Kootznahoo Formation...___-___-___-________-___-__--___-_ 28 Admiralty Island Volcanics------------------------ ____-. 31 Quaternary sedimentary depositS-_____-_-_-___---_-___-_-_-_____ 33 Undifferentiated metamorphic rocks__---____--____-___-______- 33 ,"*' Migmatite, gneiss, and feldspathic schist_________________________ 34 Plutonic rocks_________________-____-_____-___-___-___-_-_-. 35 Felsic plutonic rocks-___-------------_------_-------.---_-- 35 ,-y Intermediate plutonic rocks..____-_-__-___-__-____-________. 35 Mafic and ultramafic plutonic rocks__--_--__--_-_-_-__-___. 36 4 Structural geology._____________-___--______-------_-_-__---___---. 37 Admiralty anticlinorium_____ ___________________________________ 37 Juneau synclinorium.___________----_______-_-___-__--___-_-_-. 38 Tertiary basins.-____________-__-_____--_-_-_---_-_-_---_-_____ 38 ^ Faults .._____.____.___._________________. 39 Age of folding, faulting, and plutonism--_____________________________ 40 Glaciation and changes in sea level-_-_---_-___---------------------- 42 j Economic geology__-__-_-___-_-_-_-_-_--_--_--_------------_---__- 43 Selected references.._______________-____.---_-___-___--___--_---___ 46 * ni A H t IV CONTENTS ILLUSTRATIONS [Plates are In pocket] PLATE 1. Geologic maps of Admiralty Island and Kootznahoo Inlet area, Alaska. 2. Map and chart of mineral and fossil occurrences on Admiralty Island, Alaska. Page FIGTJBE 1. Index map showing location of Admiralty Island, Alaska. __ _. R5 2. Distribution of formations of the Stephens Passage Group.___ 22 TABLES Page j». TABLE 1. Summary of stratigraphic units on Admiralty Island.__ _ R9 2. Results of X-ray spectroscopic examination of sulfide, oxide, and 7 rare earth mineral-bearing samples.__-____--..____.______ 44 x V- v y- X CONTRIBUTIONS TO GENERAL GEOLOGY By E. H. LATHRAM, J. S. POMEROY, H. C. BERG, and R. A. LONEY ABSTRACT Admiralty Island, one of the major islands of the Alexander Archipelago, lies adjacent to the Alaskan mainland south of Juneau. Silurian (?) to Quaternary sedimentary, metamorphic and igneous rocks crop out in the largely uninhabited, moderately rugged area. The layered rocks of Admiralty Island comprise six depositional sequences, five of marine (eugeosynclinal) and one of nonmarine origin, each separated .by unconformities. Graywacke, slate, and limestone of Silurian (?) age are the oldest sequence. Schist and marble, representing metamorphosed graywacke, siltstone, limestone, and volcanic rocks, form the Retreat Group and the Gambier Bay Formation. These rocks, together with the apparently coeval unmetamor- phosed black argillite and chert of the Hood Bay Formation, make up the next younger sequence (Devonian and Devonian(?)), which is of unknown thickness. The Cannery Formation, consisting of more than 2,000 feet of graywacke, agril- lite, chert, and conglomerate, and the overlying Pybus Dolomite, as much as 1,000 feet thick, form the next younger sequence (Permian). The Hyd Formation (Triassic), composed of thin-bedded impure limestone, black chert, and black slate interflngering with and overlain by basalt and andesite pillow flows, forma the fourth sequence. The fifth sequence, the Stephens Passage Group (Jurassic and Cretaceous) is more than 5,000 feet thick; it comprises the Seymour Canal Formation, composed of slate, graywacke, and conglomerate, the Douglas Island Volcanics, which includes augite-bearing andesitic and basaltic flow breccia, and the Brothers Volcanics flow breccia. The formations of the fifth sequence are equivalent in age and intertongue along Seymour Canal and Stephens Passage. The youngest sequence is nonmarine, late Paleocene through Miocene in age, and includes the Kootznahoo Formation, composed of about 5,000 feet of con­ glomerate, sandstone, siltstone, shale, and coal, and the overlying Admiralty Island Volcanics, which consists of basaltic andesite flows more than 10,000 feet thick. A major orogeny, post-earliest Early Cretaceous to pre-Paleocene in age, prob­ ably culminated in middle-Cretaceous time and formed the major structural fea­ tures of the Island. These are the Admiralty anticlinorium, which forms the central and western part of the island and is a part of the Prince of Wales geanticline, and the adjoining Juneau synclinorium to the east. Folding of at least two stages resulted in superposition of the folds. Most of the folds are overturned those in the anticlinorium largely to the northeast and those in the El R2 CONTRIBUTIONS TO GENERAL GEOLOGY synclinorium to the southwest. Earlier deformation may be the reason for dif­ ferences in folding and metamorphic grade observed in the older depositional sequences, but lack of critical exposures has prevented recognition of earlier orogeny. The emplacement of felsic to ultramafic plutonic rocks may have been largely posttectonic, although one pluton, that at Point Retreat, is clearly pre- or syntectonic. Large bodies of contact-metamorphic rocks and migmatite border some of the plutonic masses. Low-grade regional metamorphism accom­ panied the orogeny and affected most of the island, particularly the central and western parts. Faulting along northwest trends, with both vertical and hori­ zontal movement, accompanied and followed the folding. The Tertiary sedi­ mentary rocks were deposited in two unconnected intermontane basins. Al­ though there is no evidence of post-Miocene folding or plutonism, post-Miocene faulting, largely with vertical movement, occurred along north and northeast trends. The area was glaciated to an elevation of about 3,000 feet during Pleistocene time. No glaciers remain at present. Fossiliferous marine beaches at eleva­ tions of 475 feet attest to a higher sea level or a lower elevation of the land surface during Pleistocene time. Metamorphic rocks west of Hasselborg Lake and around Slide Lake contain local concentrations of sulflde and oxide minerals. Aside from intermittent exploration at the Funter Bay nickel-copper deposit, there is currently (1963) little mining activity on the island, although some of the now inactive mines have produced modest amounts of gold and silver. INTRODUCTION GEOGRAPHY Admiralty Island, one of the major islands of the Alexander Archi­ pelago of southeastern Alaska, lies between lat. 57° N. and 58°25' N. and long. 133°50' W. and 135° W. (fig. 1). It trends in a northerly direction, is approximately 96 miles in length, and ranges in width from less than a mile to 30 miles. The northern shore, along Stephens Passage, is 10 miles south of Juneau, the State capital. The island is bounded by Stephens Passage, Frederick Sound, Chatham Strait, and Lynn Canal, all inland waterways. Admiralty Island is shown on parts of three 1:250,000-scale U.S. Geological Survey reconnais­ sance topographic maps the Sitka, Juneau, and Sumdum quadrangles. The population is small, and the few settlements are located on the west coast. The only permanent village is Angoon with a population of about 300. Seasonal settlements are located adjacent to the can­ neries at Hood Bay and Hawk Inlet. As a rule the canneries are closed during the winter. The villages of Funter, Killisnoo, Neltushkin, and Tyee have been abandoned. Admiralty Island is accessible only by airplane or boat. Alaska Coastal-Ellis Airline flights from Juneau or Sitka make daily stops at Hood Bay, Angoon, and Hawk Inlet during the summer. Charter service is needed for other areas. Most of the inlets and bays provide good anchorages for boats; however, utmost care is needed when RECONNAISSANCE GEOLOGY
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