Yakataga and Tugidak Formations

Unit symbol: QTgm
Age range Quaternary and uppermost Tertiary (11.62 to 0.0117 Ma)
Lithology: Sedimentary
Group name: Yakataga and Tugidak Formations
Mudstone, siltstone, sandstone, and diamictite in a diverse marine and glaciomarine clastic continental shelf deposit. Winkler and Plafker (1993) and Plafker and Addicott (1976) report that the Yakataga Formation is more than 1,670 m thick on Kayak and Wingham Islands and at least an additional 1,200 m on Middleton Island, suggesting a total thickness of about 3,000 m. George Plafker (written commun., 2002) reports that the Yakataga Formation is at least 4,000 m thick in the Mount Saint Elias and Mount Fairweather quadrangle. Interbedded gray to dark-gray and greenish-gray siltstone, mudstone, and sandstone predominate in lower third of formation. Till-like diamictite is interbedded with siltstone and sandstone in all but the lowest part of the formation and is the dominant rock type in the upper part of the formation, particularly on Middleton Island. Conglomerate is a minor lithology throughout the formation and scattered larger clasts, presumably dropstones, are present in all lithologies. In most exposures, the Yakataga Formation is conformable and gradational on the underlying Poul Creek Formation; locally there is an angular unconformity of up to 15 degrees (Plafker and Addicott, 1976). Age control derived from abundant mollusks and foraminifers, although most are identical to living species (Winkler and Plafker, 1981; Plafker and Addicott, 1976). Tugidak Formation on Trinity Island is similar and consists of 1,500 m of interbedded sandstone and siltstone characterized by randomly distributed pebbles and cobbles of glacial-marine origin (Allison, 1978). Richly fossiliferous, it contains marine fossils of Pliocene age. Allison (1978, p. 177) reported the occurrence of a diverse fauna consisting of more than 80 species that are “* * * largely composed of living, cold-water, North Pacific and Arctic taxa” and indicate water conditions colder than the present Gulf of Alaska. Allison (1978) suggested that deposition occurred in the upper part of the outer neritic zone in water depths between 91 and 145 m. The Albatross sedimentary sequence of Trinity Island (Clendenen and others, 1992) includes diamictite, sandstone, and siltstone. Conglomerate horizons contain clasts of granite, chert, mélange, and slate. Also contains distinctive calcareous shale clasts unlike any nearby exposed units. Calcareous shale clasts contain a late early Miocene fauna of foraminifers and flora of diatoms

Source map information

Source map Plafker, George, unpublished data
Symbol QTy
Unit name Yakataga Formation
Description Interbedded gray to dark-gray and greenish-gray siltstone, mudstone, and sandstone predominate in lower third of formation. Till-like diamictite is interbedded with siltstone and sandstone in all but lowest part of formation and is the dominant rock type in upper part of formation, particularly on Middleton Island. Conglomerate is a minor lithology throughout the formation, and scattered clasts-presumably dropstones-are present in all lithologies. Consists of diverse marine and glaciomarine clastic rocks along the Gulf of Alaska margin more than (?) m thick onshore and is widely distributed on the adjacent continental shelf where it is as much as (?) m thick in exploratory wells (Plafker and Addicott, 1976; Plafker and others, 1975).
Lithology Sedimentary

Correlated geologic units

Label Ty
Description Yakataga Formation
Geologic age Late-Miocene to Pleistocene
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Mudstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major