Reconnaissance Geol(l§f of Southern Atka Island, Aleutian Islands, Alaska I'~ 1: 1. ~ ' I ·t ·......• r I ' .f I ,. t ·: ,r., I~~~~·~~~ '• •,,, I. i r ' ' • I ,- r ' ~ i ~ •. ' ~J ~' . • r d • ~ ••t' .. _"Oc • Jo ; I ~=-I' ,10 I & I .... ~ '' I ~·. .. I: I - •.." .. '.,.. ;' •~1 11" I • Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska By James R. Hein, Hugh Mclean, and Tracy Vallier U.S. GEOLOGICAL SURVEY BULLETIN 1609 DEPARTMENT OF THE INTERIOR WilliAM P. ClARK, Secretary U.S. GEOlOGICAl SURVEY Dallas l. Peck, Director UNITED STATES GOVERNMENT PRINTING OFFICE 1984 For sale by the Distribution Branch, Text Products Section U.S. Geological Survey 604 South Pickett St. Alexandria, VA 22304 Library of Congress Cataloging in Publication Data Hein, James R. Reconnaissance geology of southern Atka Island, Aleutian Islands, Alaska. (U.S. Geological Survey bulletin 1609) Bibliography: p. 18-19 Supt. of Docs. no.: I 19.3:1609 1. Geology-Alaska-Atka Island. I. Mclean, Hugh, 1939- . II. Vallier, Tracy lowell, 1936- . Ill. Title. IV. Series: United States. Geological Survey. Bulletin 1609. QE75.B9 no. 1609 557.3s 84-600043 [QE84.A84] [557.98'4] CONTENTS Abstract 1 Introduction 1 Stratigraphy and paleontology 2 Paleogene sedimentary rocks 2 Middle and upper Miocene (synplutonic) sedimentary rocks 5 Petrography of sedimentary rocks 6 Igneous rocks 7 Plutonic rocks 8 Volcanic rocks 11 Geochemistry 11 Regional patterns of alteration 14 Structure 15 Folds 15 Faults 15 Resource potential 15 Summary and conclusions 16 References cited 18 FIGURES 1. Geologic map of Atka Island 3 2. Structural map of Atka Island 4 3. Schematic and interpretative cross section of Atka Island 5 4. Photographs showing Paleogene sedimentary rocks of Atka Island 6 5. Photographs showing synplutonic and postplutonic rocks of Atka Island 7 6. Plot of Si02 content versus FeOT/MgO ratio for Tertiary rocks of Atka Island 13 7. Ternary diagram of Sr-Ti-Zr contents of plutonic and volcanic rocks 13 8. Rare-earth-element plot for selected rocks of Atka Island 13 9. Histograms showing alteration products of Atka Island rocks 14 TABLES 1. Locations of samples of Atka Island rocks 2 2. Carbon content of Atka Island sedimentary rocks 8 3. Fossils from Atka Island 9 4. K-Ar analysis of Atka Island igneous rocks 10 5. U-Pb analyses of zircon from tonalite of the Beaver Bay pluton, Atka Island 11 6. Classification, ages, textures, and mineralogy of chemically analyzed Tertiary igneous rocks of Atka Island 11 7. Chemical analyses of representative Tertiary igneous rocks of Atka Island 12 8. Percentages of extensively altered rocks of Atka Island 15 9. Metallic-element contents in bulk and heavy-mineral separates of sediment fractions from drainages in the Beaver Bay pluton, Atka Island 16 10. Chemical analysis of silica and hematite hydrothermal deposit in Nazun Bay, Uyak Island 17 III Reconnaissance Geology of Southern Atka Island, Aleutian Islands, Alaska By James R. Hein, Hugh Mclean, and Tracy Vallier Abstract Although no metallic ore deposits of economic importance were found on Atka Island, geochemical Atka Island, one of the central Aleutian Islands indicators suggest that further exploration is is composed of two distinct parts: This report treat~ warranted for Cu, Pb, Ag, Sn, and possibly other ~he southern part, consisting mostly of Tertiary metals. Reservoir and source-rock properties of rocks Igneous and volcaniclastic rocks, and ignores the that crop out on Atka show little promise for northern part, consisting of the late Cenozoic Korovin petroleum primarily because of alteration resulting volcanic complex. The oldest sedimentary rocks from plutonism. Offshore basins, however, offer a (preplutonic), middle Eocene to middle Oligocene in greater potential for petroleum development. age, compose a sequence of bedded marine sandstone m~dstone,. and minor conglomerate that interfinge; INTRODUCTION ~Ith massive volcanic flows, volcanic breccia, and rare pillow flows. Younger sedimentary rocks, of Miocene Atka Island is situated in the central part of the (synplutonic) age, occur only on the north shore and Aleutian Islands and is part of the Andreanof Island consist of sandstone, volcanic conglomerate, black gro~p (f.ig. 1). Atka sits atop the Aleutian Ridge, an carbonaceous mudstone, and volcanic debris flows ensimatiC volcanic archipelago that separates Pacific bearing fossil logs. oceanic crust south of the ridge from older oceanic Sandstone consists of volcanic plagioclase, volcanic-rock fragments, clinopyroxene, opaque grains, crust north of the ridge that forms the floor of the and rare fragments of microfossils and quartz. deep Bering Sea. The southern part of Atka Island is Secondary minerals are pore-filling cements of about 85 km long and averages about 10 km in width. Atka is separated from adjacent Amlia Island to the ~mectite, chlorite, and zeolites. Metamorphic grade mcreases from east to west (toward the plutons) in the east by the 2-km-wide Amlia Pass. Adak Island, 175 sedimentary and volcanic rocks, from zeolite through km west of Atka, is the nearest large community and prehnite-pumpellyite to greenschist-hornfels facies, was our logistic support center. Atka Village, located characterized by chlorite, epidote, amphibole, and at the head of Nazan Bay, is inhabited by a small quartz. Overall, metamorphic grade on Atka is higher population of Aleuts. Besides the naval station on than on adjacent Amlia Island. Adak Island, Atka Village is the only community in the Volcanic rocks, mostly basalt, basaltic andesite, Andreanof group of islands. and andesite, were extruded during three eruptive Atka Island is geographically and geologically episodes. The earliest episode was submarine and of composed of two parts, which are joined by a 3-km­ middle(?) and late Eocene and early Oligocene age; the wide isthmus separating Korovin and Nazan Bays (figs. second episode extruded basalt and basaltic andesite in 1, 2). The northern part of the island consists of an middle and late Miocene time; and, finally, uppermost Tertiary and Quaternary volcanic terrane stratovolcanoes of mostly Quaternary age arose on the the informally named Korovin volcanic complex, that northern part of the island. Plutonic rocks were includes historically active Korovin Volcano and an older stratovolcano, Mount Kliuchef. The elongate intruded during the early, middle, and late Miocene. southern part of the island consists of Tertiary rocks The three plutons that were dated apparently young toward the west. Plutonic rocks are gabbro, underlying an east-west-trending mountainous ridge. Numerous elongate bays, rocky headlands, and small diorite, norite, tonalite, and quartz diorite; quartz islands mark the shoreline of the southern part of the diorite is the dominant plutonic-rock type. Most of these rocks have hypidiomorphic-granular textures, island. Atka and Amlia Islands were among a small and euhedral to subhedral crystals of plagioclase, number of Aleutian Islands that had not previously mafic minerals, and Fe-Ti oxides occur with anhedral been mapped geologically, even on a reconnaissance quartz. scale. Former work on southern Atka consisted of a Plutonic and volcanic rocks show both calc­ brief description of fossil mollusks from shore alkaline and tholeiitic trends. Differences in the state exposures near Martin Harbor by Grewingk (1850), who of differentiation can explain the two series. visited the area during the period of Russian Folds are generally gentle and open; however, occupation. The Quaternary volcanic terrane of dips of rocks locally exceed 70° where drag folds northern Atka Island was mapped by B. D. Marsh result from faulting. Faults of small displacement are (Johns Hopkins University) in 1977 but not published. common. Faulting produced a pronounced east-west­ trending topographic lineament that extends from the We did reconnaissance mapping, mostly head of Egg Bay to Atka Village. Most faulting shoreline geology, and sample collecting of Tertiary appears to be extensional, related to the emplacement rocks on Atka Island in 1977 and 1979 (table 1). This of plutons and unrelated to compression associated report presents data from our work and some broad with plate convergence along the Aleutian Trench. interpretations based on them. Introduction 1 Table 1. Locations of samples of Atka Island U.S. Geological Survey, provided technical assistance rocks in the laboratory. Radiometric ages were provided by J. M. Mattinson, University of California at Santa Barbara, and M. L. Silberman and L. B. Gray, U.S. Geological Survey. Paleontologic ages were Field Latitude Longitude determined by D. Bukry, J. A. Barron, K. McDougall, No. N. w. J. A. Wolfe, R. A. Scott, and L. Marincovich of the U.S. Geological Survey, and F. H. Wingate of Cities Service Co. Chemical data were provided by H. T. 877-6-3a2,a4 52°15 1 174°15 1 Millard, D. McKown, R. Knight, J. Budahn, M. Taylor, 877-7-3A,B,F 52°10 1 174°(5 1 M. Cremer, L. Espos, J. S. Wahlberg, J. Taggart, J. 877-7-4C 52°1 o• 174°25 1 Baker, J. Kent, R. Lerner, and S. Neil, all of the U.S. 877-9-1 52°15 1 174°15 1 Geological Survey. 877-9-2A 52°15 1 174°15 1 877-10- LA ,b 52°05 1 174°05 1 877-10-2 52°05 1 174°10 1 STRATIGRAPHY AND PALEONTOLOGY 877-H1-5A,R 52°05 1 174°15 1 877-11-1A 52°05 1 174°15 1 Southern Atka Island consists of a sequence of 877-1?-1 52°05 1 174°25 1 middle and upper Eocene and lower Oligocene 877-12-4 52°0 5• 174°35 1 (preplutonic) sedimentary rocks, interbedded with 877-12-H 52°05 1 174°40 1