156°45' 156°40' 156°30' 156°22'30'' 61°15' 61°15' 1 1 6 5 3 500 2 1 6 5 4 500 3 2 Spl 500 40 2 SOsl 500 500 Qal 12 7 8 9 10 11 12 7 8 9 10 11 500 Qs 15 SOsl 13 18 17 16 15 14 13 18 17 Qal 16 15 14 W W W H O DZwp T013N,R042 T013N,R041 H 500 T013N, R041W R040 T013N, 24 500 19 20 O 21 22 23 24 19 20 21 22 23 L I 3 T 500 Bedrock GeologicN Map of the 15' Sleetmute A-2 A 500 Quadrangle, Southwestern Alaska 25 30 29 28 27 26 25 30 29 28 27 26 750 90 500 Qs By Robert B. Blodgett, Frederic H. Wilson, Nora B. Shew, and James G. Clough SOsl 750 Dd Spl cb 36 Pamphlet31 to accompany32 R 33 34 35 36 31 32 33 34 35 I V E Scientific InvestigationsR Map 3450 750 61°10' T013N, R042W T013N, R041W T013N, R041W T013N, R040W T013N, R040W T012N, R042W T012N, R042W T012N, R041W T012N, R041W T012N, R040W 61°10' 4 35 750 4 3 2 1 6 5 4 3 2 1 6 5 Spl 4 SOsl 1000 75 750 SOsl Qs Spl Door Mountains 9 10 11 12 7 8 9 10 11 12 7 8 9 750 Qal 30 Spl 750 500 Qs 750 16 15 14 13 18 SOsl 17 16 15 14 13 18 17 16SOsl 5 500 T012N, R041W T012N, R040W T012N,R042W T012N,R041W 1250 21 21 22 23 24 19 20 21 22 23 24 19Qal 20 Dab 1500 6 1250 750 1000 27 26 25 30 Qal 26 25 500 30 29 28 28 DSwc 29 28 27 SOsl Dab 70 SOdl 84 11 1000 750 H 10 DSwc O H Dls 1000 O L Spl 1250 DZwp I T 7 Dab N 1000 9 12 A Dls 750 Dab 33 34 35 36 32 34 35 36 31 32 33 31 33 61°05' 61°05' 750 R T012N, R042W I V T012N, R040W 20 T012N, R041W E 8 T011N, R042W T011N, R041W R T011N, R040W 4 3 2 1 6 5 4 3 2 1 6 5 4 SOdl 25 13 Qs 500 Qs SOdl 14 60 37 40 SOdl 750 SOdl SOdl 15 80 9 31 9 10 11 12 7 8 1000 9 DZwp 10 11 12 7 8 500 SOdl 750 Ols Oab 30 Ols Ols x i s A dlo 1000 u m 16 202015 14 13 18 17 16 r i 15 7514 13 18 17 16 Ols DZwp n o l i c Ols i 750 t Zls 750 n 011N, R041W 011N, R040W 011N, 011N, R042W 011N, 011N, R041W 011N, T T T U.S. Department of the InteriorT A 16 56 U.S. GeologicalZls Survey dlo dlo Qal dlo 20 21 21 22 23 24 19 20 1000 21 22 23 24 19 25 1000 Qs 750 s 20 Obm 34 1000 65 750 17 20 35 24 25 50 1000 DZwp 750 Ols 18 bu 28 27ls 26 25 30 29 28 23 27 26 25 30 29 28 20 14 cp 21 750 19 Oab 500 SOdl 1000 cu Qs Zls 750 22 dlo 750 Ols 61°00' 33 34 35 36 31 32 33 34 35 36 31 32 33 61°00' 156°45' 156°40' 156°30' 156°22'30" U.S. Department of the Interior DAVID BERNHARDT, Secretary U.S. Geological Survey James F. Reilly II, Director U.S. Geological Survey, Reston, Virginia: 2020 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Blodgett, R.B., Wilson, F.H., Shew, N.B., and Clough, J.G., 2020, Bedrock geologic map of the 15' Sleetmute A-2 quadrangle, southwestern Alaska: U.S. Geological Survey Scientific Investigations Map 3450, 18 p., 1 map sheet, scale 1:63,360, https://doi.org/10.3133/sim3450. ISSN 2329-132X (online) ii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Geologic Setting .............................................................................................................................................3 Structure..........................................................................................................................................................3 Paleontology ...................................................................................................................................................4 Petroleum Potential .......................................................................................................................................4 Description of map units ...............................................................................................................................5 Unconsolidated deposits .....................................................................................................................5 Bedrock ..................................................................................................................................................5 Platform facies .............................................................................................................................5 Basinal facies ...............................................................................................................................9 Undescribed bedrock ................................................................................................................10 Acknowledgments .......................................................................................................................................11 References Cited..........................................................................................................................................11 Figures 1. Map showing location of the Sleetmute A-2 1:63,360-scale quadrangle in the Holitna Lowland of southwestern Alaska ...............................................................................................2 Tables 1. Fossil data from the Sleetmute A-2 quadrangle, southwestern Alaska. ...........................14 Conversion Factors International System of Units to U.S. customary units Multiply By To obtain Length centimeter (cm) 0.3937 inch (in.) millimeter (mm) 0.03937 inch (in.) meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) kilometer (km) 0.5400 mile, nautical (nmi) meter (m) 1.094 yard (yd) Datum Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). Horizontal coordinate information is referenced to the 1927 North American Datum (NAD 27). iii iv Bedrock Geologic Map of the 15' Sleetmute A-2 Quadrangle, Southwestern Alaska By Robert Blodgett1, Frederic H. Wilson2, Nora B. Shew2, and James G. Clough3 Abstract Blodgett and others [1999a]) for a geologic bibliography of the region). Overviews of the geology of the Sleetmute 1:250,000- Twelve unnamed, bedrock stratigraphic units are recog- scale quadrangle and adjoining areas of the central Kuskokwim nized within the Sleetmute A-2 1:63,360-scale quadrangle of Bay region are found in Cady and others (1955), Miller and southwestern Alaska. These units range in age from late(?) Pro- others (1989), Decker and others (1994), Wilson and others terozoic through Devonian and can be divided into two distinct (1998), and LePain and others (2000). The bedrock units of the facies belts: 1) a southern facies of dominantly shallow-water map area provide a nearly complete record of late Proterozoic platform carbonate and minor siliciclastic rocks (including through Devonian sedimentation in southwest Alaska. Both Early Ordovician–Early Devonian platform edge algal build- shallow-water platform facies and age-equivalent deeper-water ups) with subordinate transgressive tongues of deeper-water basinal facies rocks can be recognized in the map area. The platy carbonates; and 2) a northern facies belt of approximately relatively low degree of thermal alteration of these rocks made age equivalent deep-water carbonate and siliciclastic rocks them an attractive target for petroleum exploration during the deposited in slope and basinal environments. Both facies belts early and mid-1980’s. The excellent preservation of megafossils belong to the Farewell terrane of Decker and others (1994). and much of the carbonate rock fabric make the area useful for Two structural provinces are also recognized, which correspond future detailed stratigraphic investigations. The time scale used directly with these belts. The Farewell terrane is interpreted as in this report is based on the U.S. Geological Survey time scale a continental margin sequence that rifted from Siberia. Many of (USGS, 2018) except where noted. the bedrock units recognized in the Sleetmute A-2 quadrangle The geologic data presented here for the Sleetmute A-2 are equivalent to units previously recognized to the east and 1:63,360-scale quadrangle are based on field studies conducted northeast in the Lime Hills, McGrath, and Medfra quadrangles. in the mid-1980s by the petroleum industry and the Alaska Shallow-water carbonate platform rocks make up the major- Division of Geological & Geophysical Surveys (ADGGS), in ity of the southern facies and occur primarily along the crest 1998 by ADGGS, and in 1999 by the U.S. Geological Survey. and north side (and to a lesser degree along the south side) of a The authors of this report were involved at various times and in prominent crescentic-shaped, east-west trending anticlinal axis differing capacities in the above-mentioned field investigations. exposed in the southern part of the Sleetmute A-2 quadrangle. These studies were primarily focused on geochemical sampling Because of the relatively low thermal alteration indices of the for petroleum potential and reconnaissance geologic mapping rocks of this area and the presence