Nearshore and nonmarine sedimentary rocks

Unit symbol: Ttk
Age range Tertiary, Eocene to Paleocene (59.2 to 23.03 Ma)
Lithology: Sedimentary
Group name: Nearshore and nonmarine sedimentary rocks in southern Alaska
Unit includes several similar formations in southern Alaska. These include the Tolstoi Formation of the southwestern Alaska Peninsula, the West Foreland Formation of the Cook Inlet region, the Wishbone, Arkose Ridge, and Chickaloon Formations of the Matanuska Valley, and scattered other occurrences. Sandstone is the dominant lithology of the Tolstoi Formation; sandstone intervals grade upward from light-gray to olive-gray and tend to become more thin-bedded. Siltstone intervals in section are consistently thin bedded and are usually light olive-gray. Plant debris, including well preserved leaves, is present throughout unit, whereas megafauna are only reported from the lower 280 to 290 m of type section. Lithic clasts in conglomerate and conglomeratic sandstone are dominantly granitic and arkosic detritus but also include as much as 30 percent volcanic clasts. Most volcanic clasts are altered or weathered, which is in sharp contrast to most overlying units (Detterman and others, 1996); additionally, presence of granitic and arkosic detritus suggests a Mesozoic source rather than derivation from contemporaneous magmatic activity. Rocks of Tolstoi Formation are characteristic of a shallow marine environment succeeded northward and stratigraphically upward by rocks characteristic of nonmarine delta-plain and fluvial deposits, mainly reflecting braided streams (Detterman and others, 1996). West Foreland Formation is exposed only on west side of Cook Inlet, where unit consists of tan to light-yellow-brown cobble conglomerate interbedded with lesser sandstone, laminated siltstone, and silty shale (Detterman and Hartsock, 1966). Thin coal beds are interbedded with the siltstone and shale. Conglomerate clasts are mainly rounded to subrounded quartz diorite, volcanic rock, argillite, sandstone, siltstone, quartzite, tuff, and coal fragments. Intrusive and volcanic rock fragments each make up about 35 percent of the clasts in conglomerate. Medium- to coarse-grained arkosic sandstone forms the conglomerate matrix and forms distinct lenticular beds. Siltstone and shale interbedded with conglomerate is very fine-grained subarkosic equivalent of the sandstone. Unit assigned Oligocene age by Kirschner and Lyon (1973) and later reassigned early Eocene and late Paleocene age by Magoon and others (1976), presumably on the basis of plant fossils. Zircon from an interbedded tuff about 1 m thick yielded an age of 43 Ma, middle Eocene (P.J. Haeussler, written commun., 2008). Unit may be equivalent to Arkose Ridge Formation and represent a transtensional basin along the Castle Mountain Fault System. Arkose Ridge Formation is fluvial and alluvial feldspathic and biotitic sandstone, conglomerate, siltstone, and shale that contains abundant plant fragments (Csejtey and others 1977; Winkler, 1992). Coarsening-upward sequence was deposited on alluvial fans and by braided streams carrying sediment derived from rapid erosion of uplifted mountains to the north (Winkler, 1992). Thickness is as much as 700 m. Age control is largely based on late Paleocene fossil plants and radiometric ages on locally associated volcanic flows and dikes. Age is considered broadly coeval with Chickaloon Formation and is based on whole rock K/Ar ages from volcanic rocks that range from 56 to 46 Ma in lower part of formation and a whole rock K/Ar age on basalt dike from the middle of formation of 46.1±2.8 Ma, as well as the presence of late Paleocene plant fossils (Silberman and Grantz, 1984; Winkler, 1992). Chickaloon Formation is more than 1,500 m thick and is predominantly fluvial and alluvial carbonaceous mudstone, siltstone, conglomeratic sandstone, and polymictic conglomerate (Winkler, 1992). Locally, upper and middle parts of unit contain numerous beds of bituminous coal (Winkler, 1992). Upper part of Chickaloon Formation contains a diverse fossil assemblage of Eocene age, including abundant leaf imprints, petrified tree trunks, and large mammal track ways (Barnes and Payne, 1956; Winkler, 1992). Lower part of unit is largely conglomerate and lithic sandstone derived from erosion of the Talkeetna Formation (Winkler, 1992). Unit includes a strongly deformed, “southerly derived, green-weathering, noncarbonaceous basal sequence of poorly sorted, massive to crudely stratified cobble and boulder conglomerate ”* * * which grades “upward into well-stratified, thick-bedded sandstone and conglomerate with a chloritic matrix” (Winkler, 1992). Little (1988, 1990, cited in Winkler, 1992) interpreted this sequence as a prograding alluvial fan derived from uplift and erosion of the Chugach accretionary complex (Chugach terrane) to the south (Winkler, 1992). Age control for the Chickaloon Formation is derived from the presence of Paleocene fossil leaves (Wolfe and others, 1966; Triplehorn and others, 1984) and from K/Ar and fission-track determinations on ash partings within coal beds, ranges from 56 to 52 Ma (Triplehorn and others, 1984; Winkler, 1992). Unnamed rock units from other areas also included within this unit include an intercalated fluvial sequence of conglomerate, sandstone, siltstone, and mudstone, and a few thin, interlayered flows of basaltic andesite found in the Healy quadrangle (Csejtey and others, 1992), fluviatile conglomerate, sandstone, and claystone with a few interbeds of lignitic coal in the Talkeetna Mountains quadrangle (Smith and others, 1974; Csejtey and others, 1978), and brown-weathering sandstone and shale, pebble to granule conglomerate associated with the Holokuk Basalt of southwest Alaska. Unit also includes a similar unit in the McGrath quadrangle (unit Tvs, volcaniclastic sandstone and lacustrine silt of Bundtzen and others, 1997a)

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 Tt
Unit name Tolstoi Formation
Description In type section, sandstone is dominant lithology of formation; sandstone intervals grade upward from light gray to olive gray and tend to become more thin bedded. Siltstone intervals in section are consistently thin bedded and are usually light olive gray. Plant debris is present throughout type section, whereas megafauna is only reported from lower 280 to 290 m. In reference section, proportion of sandstone and siltstone thickness is sub-equal; additionally, conglomerate and conglomeratic sandstone intervals make up roughly 15 percent of section. Sandstone intervals tend to be massive or thick bedded, medium grained, and various shades of gray and brown. Siltstone intervals in reference section are usually thin bedded and consistently dark gray. Conglomerate and conglomeratic sandstone intervals are typically pale yellowish brown and massive. Lithic clasts in conglomerate and conglomeratic sandstone are dominantly granitic and arkosic detritus but also include as much as 30 percent volcanic clasts. Lithology is characteristic of shallow marine environment that is succeeded northward (stratigraphically upward) by rocks characteristic of nonmarine delta-plain and fluvial deposits, mainly of braided-stream character. Much thicker (1,319 m) reference section has lithology that is typical of nonmarine fluvial flood-plain, delta sequence, which is common in main part of Tolstoi Formation. Marine rocks are not known in Tolstoi north of Ivanof Bay (C.M. Molenaar, written commun., 1991).
Lithology Sedimentary

Correlated geologic units

Label Tt
Description Tolstoi Formation
Geologic age Late-Paleocene to Middle-Eocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Minor
Lignite < Coal < Sedimentary Bed Incidental