Kuskokwim Group, undivided

Unit symbol: Kk
Age range Upper Cretaceous to upper Lower Cretaceous (113 to 66 Ma)
Lithology: Sedimentary
Group name: Kuskokwim Group, undivided
Interbedded graywacke and shale that has local interbeds of argillite and conglomerate. Graywacke fine- to medium-grained, gray, commonly micaceous and locally silty; in places it is crossbedded or contains siltstone partings. Contains rare argillite pebbles. Originally described by Cady and others (1955), the Kuskokwim Group is widespread in southwestern Alaska and, although largely a flysch deposit, represents a range of local depositional environments. A number of workers (Patton and others, 1980; Bundtzen and others, 1992; Box and others, 1993; Miller and Bundtzen, 1994) have subdivided the Kuskokwim Group into two facies: a predominantly deep-water, turbidite-dominated facies; and a shallow-marine and fluvial facies (unit Kkn, here). Box and others (1993) subdivided the upper part of the unit in the Bethel quadrangle on the basis of provenance and depositional environment, but data are insufficient to carry those distinctions throughout the exposure area. Box and others (1993) described three depositional environments: outer-fan turbidite, inner-fan turbidite, and deltaic; and they described three packages of rock that provide material to the depositional environments—provenances that they called chert-clast, volcanic, and mixed. We have chosen to subdivide the Kuskokwim Group into two subunits that reflect the depositional environments represented (nearshore or outer fan), but we retain Box and others’ (1993) distinctions of clast provenance. Box and others’ (1993) deltaic environment, which is consists only of chert clasts, has been placed in the nearshore facies subdivision (unit Kkn) of this map. The other chert-clast provenance rocks are described as outer-fan facies and consist of thin-bedded, fine-grained, quartzose sandstone and shale with lesser thin- and thick-bedded medium-grained sandstone (Box and others, 1993). Volcanic provenance rocks consist of interbedded shale, siltstone, sandstone, and conglomerate composed of rounded clasts of volcanic, volcaniclastic, and plutonic rocks deposited in slope and inner-fan environments (Box and others, 1993). The rocks having clasts of mixed provenance represent two depositional environments: an outer-fan turbidite facies that consists of a shale-rich sequence and lesser thin- to thick-bedded, medium-grained sandstone sections; and an inner-fan-channel facies that consists of mixed shale-rich and sandstone-rich sections, and coarse sandstone and pebbly sandstone in thick-bedded, amalgamated sequences containing minor interbedded shale (Box and others, 1993). Age of the Kuskokwim Group is not well constrained because of the multiple provenances and potentially multiple depositional systems that are represented by the map unit. Cady and others (1955) assigned an early Late Cretaceous age on the basis of 38 fossil collections, but acknowledged fossil collections outside their map area that might indicate an age older than Late Cretaceous or as young as Tertiary. Elder and Box (1992) and Box and others (1993) assigned a late Cenomanian to early Turonian age based on inoceramid fossil collections in the Bethel quadrangle, whereas Hoare and Coonrad (1959a) reported Albian and Cenomanian to Coniacian fossils from the same area, and Murphy (1989) reported an Albian collection. Other age assignments have ranged as young as Santonian (Decker and others, 1994) and Campanian(?) (Miller and Bundtzen, 1994)

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol Ksu
Unit name Shale, siltstone, and sandstone, undivided
Description Chiefly dark carbonaceous, generally nonfossilferous shale, siltstone and fine-grained sandstone. Poorly exposed except where altered to a dark slaty hornfels in the vicinity of plutonic and hypabyssal intrusive rocks. Locally includes a limestone-clast conglomerate in the central Medfra quadrangle and several volcaniclastic horizons in the Ophir quadrangle. Unit probably correlative in part with unit Kss of Late Cretaceous age, but some beds may be as old as late Early Cretaceous.
Lithology Sedimentary

Correlated geologic units

Label Kk
Description Kuskokwim Group, sandstone, siltstone, shale, and conglomerate
Geologic age Cenomanian to Santonian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major