The Geology of North America Vol. G- , The Geology of Alaska The Geological Society of America, 1994 Chapter 9 Geology of southwestern Alaska John Decker Alaska Division of Geological and Geophysical Surveys, 794 University A venue, Fairbanks, Alaska 99709 Steven C. Bergman ARCO Oil and Gas Company, 2300 West Piano Parkway, Piano, Texas 75075 Robert B. Blodgett* Department of Geology, Oregon State University, Corvallis, Oregon 97331 Stephen E. Box Branch of Alaskan Geology, Us. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508 Thomas K. Bundtzen Alaska Division of Geological and Geophysical Surveys, 3700 Airport Way, Fairbanks, Alaska 99709 James G. Clough Alaska Division of Geological and Geophysical Surveys, 794 University A venue, Fairbanks, Alaska 99709 Warren L. Coonrad Branch of Alaskan Geology, Us. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Wyatt G. Gilbert Alaska Division of Geological and Geophysical Surveys, 3601 Street, Anchorage, Alaska 99510 Martha L. Miller and John M. Murphy Branch of Alaskan Geology, US. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508 Mark S. Robinson A laska Division of Geological and Geophysical Surveys 794 University A venue, Fairbanks, Alaska 99709 Wesley K. Wallace ARCO Alaska, Inc., P. D. Box 100360, Anchorage, Alaska 99510 INTRODUCTION in the central Kuskokwim Mountains, and in fault contact with Mesozoic accretionary rocks of the Bristol Bay region. Precam- Southwest Alaska lies between the Yukon-Koyukuk prov- brian metamorphic rocks also occur in the northern Kuskokwim ince to the north, and the Alaska Peninsula to the south (Wahr- Mountains and serve as depositional basement for Paleozoic shelf haftig, this volume). It includes the southwestern Alaska Range deposits. A nearly continuous sequence of Paleozoic continental the Kuskokwim Mountains, the Ahklun Mountains, the Bristol margin rocks underlies much of the southwestern Alaska Range Bay Lowland, and the Minchumina and Holitna basins. It is an and northern Kuskokwim Mountains. The most extensive unit in area of approximately 175 000 km , and, with the exception of southwest Alaska is the predominantly Upper Cretaceous Kus- the rugged southwestern Alaska Range and Ahklun Mountains kokwim Group, which, in large part, rests unconformably on consists mostly of low rolling hills. older rocks of the region. Volcanic rocks of Mesozoic age are The oldest rocks in the region are metamorphic rocks with common in the Bristol Bay region, and volcanic and plutonic Early Proterozoic protolith ages that occur as isolated exposures rocks of latest Cretaceous and earliest Tertiary age are common throughout southwest Alaska. Two major northeast-trending faults are known to traverse southwest Alaska, the Denali-Farewell fault system to the south *Present addresses: Decker, ARCO Alaska, Inc., P.O. Box 100360, Anchor- age, Alaska 99510; Blodgett, Branch of Paleontology and Stratigraphy, U. and the Iditarod-Nixon Fork fault to the north. Latest Cretaceous . Geological Survey, National Center MS 9709, Reston, Virginia 22092; Box, U.s. and Tertiary right-lateral offsets of less than 150 km characterize Geological Survey, Room 656, West 920 Riverside Avenue, Spokane, Washing- both faults. The Susulatna lineament (or Poorman fault), north of ton 99201; Murphy, Department of Geology, University of Wyoming, Laramie Wyoming 82071; Wallace, Department of Geology and Geophysics, University the Iditarod-Nixon Fork fault, juxtaposes contrasting Early Cre- of Alaska, Fairbanks, Alaska 99775-0760. taceous and older rocks and may have had significant Cretaceous Decker, J., Bergman, S. C, Blodgett, R. R, Box, S. E., Bundtzen, T. K., Clough, J; G., Coonrad, W. 1., Gilbert, W. G., Miller, M. L., Murphy, J. M., Robinson M. S., and Wallace, W. K., 1994, Geology of southwestern Alaska in Plafker, G., and Berg, H. C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G- 285 ..... .... .... ?", 286 1. Decker and others lateral displacement. The Mulchatna fault, south of the Denali- Farewell fault system roughly corresponds to a pronounced 1530 aeromagnetic discontinuity and is likely to be a major basement 1560 fault. 640 The rocks of southwest Alaska (Fig. 1) can be grouped into three contrasting assemblages: (1) the predominantly Paleozoic continental margin rocks of the southwestern Alaska Range and northern Kuskokwim Mountains (Farewell terrane, defined below), (2) the predominantly Mesozoic accretionary rocks ofthe 1620 620 610 600 , 680 650 -56' 590 DilLINGHAM AREA OF MAP 87654321 THA 100 SCALE IN KILOMETERS HAGEMEISTER ISLAND NUSHAGAK BAY 580 1 :63, mIl 360-scale quadrangle numbering system Figure 1. Tectonostratigraphic terrane map of southwestern Alaska showing the names of 1:250 000 scale quadrangles and the 1:63 360-scale quadrangle numbering system. !::.':",,::~ , " ~(', " Geology of southwestern Alaska 287 northern Bristol Bay region, and (3) postaccretionary clastic sed- stratigraphic information is now known to allow genetic relations imentary rocks and mainly calc-alkaline volcanic and plutonic to be established between previously deemed terranes and to rocks that tie together assemblages 1 and 2. In order to establish a allow a more conventional stratigraphic approach to our discus- framework for discussion of these rocks, we have consolidated sion of the geology in this area. In the Bristol Bay region, how- previously described tectonostratigraphic terranes (Jones and Sil- ever, many critical stratigraphic relations are unclear and our berling, 1979; Jones and others, 1981) in the southwestern discussion of these rocks follows the tectonostratigraphic subter- Alaska Range and northern Kuskokwim Mountains, and subdi- rane framework defined for this area by Box (1985). We believe vided the terranes of the Bristol Bay region. In the southwestern that this somewhat different format for each area allows the best Alaska Range and northern Kuskokwim Mountains, sufficient overall treatment of the geology of southwest Alaska. EXPlANATI()N OVERLAP ASSEMBLAGES Quaternary Surficial Deposits 9/J Cenozoic Deposits Kuskokwim Group TERRANES OF SOUTHWEST ALASKA Alaska Range and Kuskokwim Mountains Bristol Bay Region Farewell Terrane Nyack Terrane Mystic Sequence Togiak Terrane 1:.:~~~::1 White Mountain Sequence Hagemeister Subterrane Basinal Facies ~~~K~i~1 Kulukak Subterrane 1::.Fyyd Transitional Facies Goodnews Terrane Platform Facies ITEIJ Nukluk Subterrane GTC:' Tikchik Subterrane Platinum Subterrane r!3P:+-~ ~~9f,~ Cape Pierce Subterrane Kilbuck Terrane ADJACENT TERRANES Northern Kahiltna Terrane Innoko Terrane . - J Southern Kahiltna Terrane Pingston Terrane McKinley Terrane Ruby Terrane Peninsular Terrane Yukon-Tanana Terrane UNITS OF UNCERTAIN TERRANE AFFINITY Idono Sequence Portage Sequence Contact -? -- Fault-Dashed where approximate, dotted where concealed, queried where uncertain. ~..... Thrust fault-Dotted where concealed. Sawteeth on upper plate. ::::) ::!: 288 1. Decker and others SOUTHWESTERN ALASKA RANGE AND Previous Proposed NORTHERN KUSKOKWIM MOUNTAINS nomenclature nomenclature (Jones and Silber1ing) (this report) The predominantly Paleozoic and Mesozoic rocks of the southwestern Alaska Range and northern Kuskokwim Moun- Kuskokwim Group tains that are discussed in this chapter include parts of the Nixon Fork, Dillinger, and Mystic terranes, which Silberling and others (this volume) and Jones and Silberling (1979) originally defined as discrete, fault-bounded tectonostratigraphic terranes. Although Mystic Mystic these terranes have distinctive stratigraphies, several other authors terrane sequence have suggested that the Nixon Fork and Dillinger terranes consist -ex: instead of contrasting facies that were deposited within a single depositional basin (Bundtzen and Gilbert, 1983; Blodgett, 1983a; Blodgett and Gilbert, 1983; Gilbert and Bundtzen, 1983a; Blod- gett and Clough, 1985), and that rocks ofthe Mystic terrane were deposited on rocks of the Dillinger terrane (Gilbert and Bundtzen ...J Ijf\COf\\~ 1984). Recently, the identification of interfingering relationships ...JQ) and transitional or slope facies between the platform deposits of the Nixon Fork terrane, and the basinal deposits of the Dillinger Dillinger Nixon Fork terrane a:cn terrane have been recognized in the White Mountain area (Gil- terrane c:( c: aslna bert, 1981; Blodgett and Gilbert, 1983), in the western Lime Hills u.: S facies and eastern Sleetmute Quadrangles (Blodgett and others, 1984; Clough and Blodgett, 1985; Smith and others, 1985), and in the Minchumina basin (M. W. Henning, written commun. , 1985). :c: Because genetic relationships exist among the Nixon Fork, Dil- linger, and Mystic terranes, we herein redefine them as the Fare- Figure 2. Farewell terrane terminology and relationships. This diagram well terrane, cqmprising the White Mountain and Mystic compares the terminology proposed in this report with the terrane no- sequences (Fig, 2). menclature of Jones and Silberling (1979). The figure schematically illus- trates the genetic relations between the basinal facies (Dillinger terrane) Farewell terrane and the platform facies (Nixon Fork terrane), and the unconformable relationship between the Mystic sequence and the White Mountain se- quence. See text for discussion. Definition. The Farewell terrane (Figs. 2 and 3) consists of a coherent but locally highly deformed lower Paleozoic through Lower Cretaceous continental margin sequence, deposited,