Chignik Formation and similar units in southern Alaska

Unit symbol: Kcs
Age range Upper Cretaceous (83.6 to 66 Ma)
Lithology: Sedimentary
Group name: Nonmarine to shelf sedimentary rocks
Consists of the Chignik Formation of the Alaska Peninsula, the Summit Island Formation of southwest Alaska (Hoare and others, 1983), and the Saddle Mountain section of Magoon and others (1980) in the Cook Inlet region. Named by Atwood (1911, p. 41–48), the Chignik Formation is a cyclic, nearshore marine, tidal flat, and nonmarine floodplain and fluvial deposit (Fairchild, 1977; Detterman, 1978) as much as 600 m thick between Port Moller and Chignik Bay. Unit is dominantly light-olive-gray to olive-gray sandstone that has interbedded olive-gray to olive-black siltstone and conglomerate. Conglomerate is composed of multicolored chert, white quartz, felsic plutonic, and minor volcanic clasts. Nonmarine part locally contains coal beds as much as 2 m thick. Marine fossils, mainly pelecypods, indicate late Campanian and early Maastrichtian age (J.W. Miller, written commun., 1983–85). Chignik Formation unconformably overlies all older units on the Alaska Peninsula and conformably interfingers with and underlies its deep-water-facies equivalent Hoodoo Formation (Khk) (Mancini and others, 1978; Molenaar, 1980; Detterman and others, 1996). As shown here, unit also includes rocks mapped as undivided Chignik and Hoodoo Formations in Ugashik 1:250,000-scale quadrangle by Detterman and others (1987a). Summit Island Formation of southwest Alaska consists of lenses and inter-tonguing beds of nonmarine conglomerate, sandstone, carbonaceous siltstone, mudstone, and shale that contain abundant plant detritus and a few coal seams (Hoare and others, 1983). In the type section, the unit “consists of about 200 m of massive and thick-bedded pebble [to] cobble conglomerate overlain by about 650 m of interbedded sandstone, siltstone, and mudstone and a few conglomerate beds” (Hoare and others, 1983). At the reference section of the unit, its character changes to consist of “about 75 percent carbonaceous mudstone, siltstone, and shale and 25 percent interbedded sandstone and pebble grit” (Hoare and others, 1983). Conglomerate clasts are well-rounded, generally not larger than 10 cm, and primarily derived from flows, tuffs, and sedimentary rocks of Jurassic and Early Cretaceous age (Hoare and others, 1983), as well as white quartz and sparse schist and plutonic clasts; they suggest the schist clasts are derived from tectonically metamorphosed rocks that occur locally along major faults in the region. Noting two contrasting sandstone compositions, Box (1985) divided the unit into two subbasins along the trace of the Togiak-Tikchik Fault; both subbasins contain more than 1 km of section. Sandstone of Summit subbasin (southeast of Togiak-Tikchik Fault) is composed mostly of slaty sedimentary lithic fragments, similar to underlying Jurassic sedimentary rocks. Rocks in this subbasin consist of a lower conglomeratic member (200 m) that probably originates from alluvial fans and braided streams, and an upper carbonaceous shale member that probably originates from meandering streams and floodplains and has channelized sandstone bodies that become finer grained and thinner upward. Sandstone of Hagemeister subbasin (northwest of Togiak-Tikchik Fault) is composed of nonfoliated volcanic detritus similar to underlying Jurassic rocks and consists of a lower channelized sandstone member (600 m) that probably originates from meandering streams, and an upper sandy member (400 m) that has cycles that coarsen and thicken upwards in section, beginning with laminated siltstone-coal intervals that are possibly lacustrine delta deposits. Plant fossils from the base of the reference section indicate a latest Cretaceous or early Tertiary age (J.A. Wolfe, oral commun., 1974, cited in Hoare and others, 1983). A dike that cuts the formation yielded a K/Ar age of 64.6±2 Ma, which suggested to Hoare and others (1983) that a Late Cretaceous age for the Summit Island Formation was more probable. Subsequent dating of a dike cutting the formation yielded an older age of 76.6±4.5 Ma (Box, 1985), further supporting a Cretaceous age assignment for the unit. The Saddle Mountain section of Cook Inlet consists of dominantly nonmarine fine- to medium-grained sandstone that becomes finer grained upward and lesser conglomerate, minor siltstone, and coal in a section 83 m thick (Magoon and others, 1980) found northeast of Chinitna Bay in the Seldovia quadrangle. Unit is soft and friable except where cemented by calcite. Conglomerate contains volcanic and plutonic rock boulders as large as 30 cm in diameter in a sandy matrix. Coal beds, which tend to occur in the upper part of the section, are as much as 2.7 m thick and locally have underclay (Magoon and others, 1980). Sporomorphs Cranwellia striata (Couper) Srivastava, Balmeisporites spp., Wodehouseia spinata Stanley, Proteacidites spp., Aquilapollenites bertillonites Funkhouser, A. reticulatus Mtchedlishvili, and A. delicatus Stanley (Magoon and others, 1980) indicate Maastrichtian age. The Saddle Mountain section was assigned to Kaguyak Formation by Bradley and others (1999), but is distinctly different than Kaguyak Formation (unit Khk). Unit overlies the Upper Jurassic Naknek Formation with angular unconformity and is overlain by West Foreland Formation with angular unconformity

Source map information

Source map Wilson, F.H., Detterman, R.L., and DuBois, G.D., 2015, Geologic framework of the Alaska Peninsula, southwest Alaska, and the Alaska Peninsula terrane: U.S. Geological Survey Bulletin 1969-B, scale 1:500,000, 2 plates, 34 p.
Symbol Khc
Unit name Hoodoo and Chignik Formations, undivided
Description Poorly exposed dark siltstone and sandstone near Imuya Bay.
Lithology Sedimentary

Correlated geologic units

Label Khc
Description Chignik and Hoodoo Formations, undivided. Also includes Late Cretaceous rocks called Kaguyak Fm. on the west side of Cook Inlet in the Seldovia quad
Geologic age Campanian to Maastrichtian
Geologic setting Undivided
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Lignite < Coal < Sedimentary Bed Incidental