GEOLOGY OF THE SLEETMUTE A-5, A-6, B-5, AND B-6 QUADRANGLES, SOUTHWESTERN ALASKA by John Decker, R.R. Reifenstuhl, M.S. Robinson, C.F. Waythomas, and J.G. Clough Professional Report 99 1995 Published by Alaska Department of State of Alaska Department of Natural Resources NATURAL DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS GEOLOGY OF THE SLEETMUTE A-5, Ad, B-5, AND B-6 QUADRANGLES, SOUTHWESTERN ALASKA by John Decker, R.R. Reifenstuhl, M.S. Robinson, C.F. Waythomas, and J.G. Clough Professional Report 99 Division of Geological & Geophysical Surveys Cover photo: Northwest-vergent isocline of veryfine-grained sandstone of the Lower(?) and Upper Cretaceous Kuskokwim Group. The outcrop of medium- bedded sandstone is 1.5 miles west of Kiokluk Lake in the Sleetmute B-6 Quadrangle (map unit Kkm). The hammer handle in the photo is 46 cm Fairbanks, Alaska long. Photo by R.R. Reifenstuhl. 1995 STATE OF ALASKA Tony Knowles, Governor DEPARTMENT OF NATURAL RESOURCES John Shively, Commissioner DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Milton A. Wiltse, Acting Director and State Geologist Division of Geological & Geophysical Surveys publications may be in- spected at the following locations. Address mail orders to the Fairbanks office. Alaska Division of Geological University of Alaska Anchorage Library & Geophysical Surveys 321 1 Providence Drive 794 University Avenue, Suite 200 Anchorage, Alaska 99508 Fairbanks, Alaska 99709-3645 Elmer E. Rasmuson Library Alaska Resource Library University of Alaska Fairbanks 222 W. 7th Avenue Fairbanks, Alaska 99775-1005 Anchorage, Alaska 995 13-7589 Alaska State Library State Office Building, 8th Floor 333 Willoughby Avenue Juneau, Alaska 998 11-057 1 This publication released by the Division of Geological & Geophysical Surveys, text was produced and printed in Fairbanks, Alaska by Graphics North and maps were printed in Colorado Springs, Colorado by Pikes Peak Lithographing Co., at a cost of $9.50 per copy. Publication is required by Alaska Statute 41, "to determine the potential of Alaskan land for production of metals, minerals, fuels, and geothermal resources; the location and supplies of groundwater and construction materials; the potential geologic hazards to buildings, roads, bridges, and other installations and structures; and shall conduct such other surveys and investigations as will advance knowledge of the geology of Alaska." CONTENTS Page Introduction .................................................................................................................................................................. Stratigraphy. age and structure ...................................................................................................................................... Mineral resources ......................................................................................................................................................... Description of map units ..................... ....................................................................................................................... Unconsolidated deposits ........................................................................................................................................ Volcanic rocks ..................................................................................................................................................... Hypabyssal rocks ................................................................................................................................................... Holokuk basalt .............................................................................................................................................. Sedimentary and metamorphic rocks ..................................................................................................................... Kuskokwim Group ....................................................................................................................................... Holitna group ................................................................................................................................................. Intrusive rocks ....................................................................................................................................................... Acknowledgments ......................................................................................................................................................... References cited ............................................................................................................................................................ FIGURE Figure 1. Map showing paleocurrent data for the Sleetmute A.5. A.6. B.5. and B-6 Quadrangles ................... 10 TABLES Table 1. Major oxide geochemical analysis of rock samples ............................................................................ 12 1a . Normative mineralogy ......................................................................................................................... 13 2 . Trace element geochemical analyses of rock samples ....................................................................... 15 3 . K-Ar age analyses ............................................................................................................................... 16 4 . Fossil information ........................................................................................................................... 16 MAPS [in pocket] Sheet 1. Geologic map of the Sleetmute A.5. A.6. B.5. and B-6 Quadrangles. southwestern Alaska 2 . Stratigraphic sections in the Sleetmute A-6 and B-6 Quadrangles. southwestern Alaska GEOLOGY OF THE SLEETMUTE A-5, A-6, B-5, AND B-6 QUADRANGLES, SOUTHWESTERN ALASKA by John Decker,' R.R. Reifenstuhl? M.S. Robinson? C.F. Waythomast and J.G. Clough2 INTRODUCTION as Triassic are also present within their Holitna Group. The oldest strata recognized in the unit, based on regional The Sleetmute A-5, A-6, B-5, and B-6 Quadrangles correlation with the Lone Mountain area of the McGrath are located 450 km (280 mi) west of Anchorage in the Quadrangle, are of probable Late Proterozoic age (R.B. Kuskokwim Mountains of southwesternAlaska.Elevation Blodgett, written commun., 1994). Adrain and others (in in the map area ranges from about 1,300 m (4,265 ft) in press) have suggested that the designation "Holitna the pluton-cored Kiokluk and Chuilnuk Mountains to Group" be ultimately abandoned, because it is too broadly 100 m (328 ft) along the Holitna River and Chineekluk defined. We agree with this, and use Holitna group as an Creek. Except for the glaciated peaks of the Chuilnuk informal unit designation within our report area. and Kiokluk Mountains, the physiography of the area is Most of the area is underlain by the Kuskokwim dominated by rounded, tundra-covered hills of low to Group, a >12-km-thick (39,370 ft) marine turbidite moderate relief. sequence of thick- to medium-bedded sandstone and Geologic mapping utilized 1:40,000-scale topo- shale. The age of the Kuskokwim Group has been graphic base maps, 1:63,360-scale aerial photographs, and generally considered Early Cretaceous and Late field notes and maps from unpublished U.S. Geological Cretaceous (Albian to Coniacian) (Moore and Wallace, Survey field studies. A reconnaissance study by Cady and 1985; Decker and others, 1994). The inclusion, by these others (1955) included the quadrangles mapped in this authors, of the Early Cretaceous (Albian) age-range is report. based on the rare occurrence of one species of arenaceous Preliminary 1:40,000-scale geologic maps of the foraminifera of probable Albian age from the Mulchatna Sleetmute A-6, B-5, and B-6 Quadrangles have been River area. This occurrence was reported in an in-house published, respectively, by Decker, Robinson, Murphy, ARC0 Alaska fossil report (Moore and Wallace, 1985). Reifenstuhl, and Albanese (l984), Reifenstuhl, Robinson, Numerous published reports on Kuskokwim Group Smith, Albanese, and Allegro (1984), and Robinson and macrofossils indicate a Late Cretaceous (Cenomanian to others (1984). Rock, pan-concentrate, and stream- Santonian) age and none reports ages other than Late sediment geochemistry maps of the Sleetmute A-6, B-5, Cretaceous (Cady and others, 1955; Elder and Box, and B-6 Quadrangles were compiled by Robinson 1992; Box and Elder, 1992; Elder and Miller, 1991). (1984a, b, c respectively). Compilations of previously Consequently, we will refer to the Kuskokwim Group unpublished data, mainly from U.S. Geological Survey age as Early(?) and Late Cretaceous (Albian(?) to field studies, are available in Alaska Division of Santonian). Geological & Geophysical Surveys Public-data File maps A far less extensive rock unit, superficially (Decker, Reifenstuhl, and Coonrad, 1984; Reifenstuhl, resembling the Kuskokwim Group, is the Inoceramus- Decker, Murphy, and Coonrad, 1984; and Murphy and bearing calcareous sandstone, siltstone, and shale of others, 1984). Cretaceous to Jurassic(?) age. Contacts between the calcareous deposits and other bedrock units are covered STRATIGRAPHY. AGE by surficial deposits. The Holokuk Basalt (Cady and others, 1955) con- sists of calc-alkalic volcanic rocks (table 1) that The oldest rock unit in the region is the Holitna Group unconformably overlie the Kuskokwim Group and range of Cady and others (1955). These authors reported in composition from basaltic andesite to rhyolite. K-Ar Silurian and Devonian