Tokun and Stillwater Formations and similar rocks

Unit symbol: Tes
Age range Tertiary, Eocene (56 to 33.9 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks of eastern Prince William Sound
Units are restricted to eastern Prince William Sound and consist primarily of siltstone and less common interbedded sandstone (Nelson and others, 1985; Winkler and Plafker, 1981, 1993; George Plafker, written commun, 2003). Marine units are separated by the nonmarine Kulthieth Formation (unit Tkf, below). Siltstone of Tokun Formation is concretionary, and sandstone appears to be present mostly in lower part of formation. “Siltstone generally is medium to dark gray and nearly massive; locally, thin beds and lenses of lighter gray, brown-weathering calcareous siltstone and silty limestone are found within darker siltstone. Spherical calcareous concretions as much as 1 m in maximum dimension are distributed randomly or along bedding surfaces in siltstone. Interbedded sandstone in Tokun, which generally is lighter gray than the siltstone, is micaceous, feldspathic, and brown weathering” (Winkler and Plafker, 1993). Tokun Formation has a gradational to sharp contact with the overlying Poul Creek Formation (unit Top) and a gradational contact with underlying Kulthieth Formation (unit Tkf) (Winkler and Plafker, 1981). “Lithology and megafauna indicate general deposition under quiet bottom conditions seaward of the surf zone in tropical to warm temperature water” (Miller, 1975, cited in Winkler and Plafker, 1981). Unit is a transgressive marine sequence approximately 1,070 m thick (Winkler and Plafker, 1981). Fossil crabs are abundant, especially in the upper part of the formation, and occur intact in concretions (Winkler and Plafker, 1981). Siltstone of underlying Stillwater Formation is carbonaceous or calcareous; calcareous siltstone commonly contains foraminifers (Nelson and others, 1985; Winkler and Plafker, 1993; George Plafker, written commun, 2000). Lithology and microfauna of lower part of formation indicates marine deposition in neritic to upper bathyal depths (Tysdal and others, 1976; Winkler and Plafker, 1981). Grades upward into the nonmarine rocks of the overlying Kulthieth Formation (Miller, 1951; MacNeil and others, 1961). “The Stillwater Formation is complexly deformed and is characterized by tight folds and shearing in incompetent strata; hence its thickness can be estimated only crudely to be at least 1,500 m” (Plafker, 1974; cited by Winkler and Plafker, 1981). Age control derived from poorly constrained ages of foraminifers and mollusks, ranging from possibly Paleocene to middle Eocene (from the foraminifers) or to middle or early late Eocene (from the mollusks) (Winkler and Plafker, 1981). Unit also includes siltstone of Oily Lake, as defined by Plafker (1987), which consists of 100 to 200 m of thick-bedded dark-gray to greenish-gray siltstone and very-fine grained sandstone of middle Eocene age. Unit contains a small proportion of basaltic tuff

Source map information

Source map Winkler, G.R., and Plafker, George, 1993, Geologic map of the Cordova and Middleton Island quadrangles, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Map I-1984, 1 sheet, scale 1:250,000.
Symbol Tt
Unit name Tokun Formation
Description Consists predominantly of concretionary siltstone and lesser, variable amount of interbedded sandstone. Siltstone is medium to dark gray and nearly massive. Spheroidal calcareous concretions as much as 1 m in maximum dimension are distributed randomly or along bedding surfaces in the siltstone.
Lithology Sedimentary

Correlated geologic units

Label Ttf
Description Tokun Formation
Geologic age Eocene
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Siltstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental