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 Csejtey, Bela, Jr., Nelson, W.H., Jones, D.L., Silberling, N.J., Dean, R.M., Morris, M.S., Lanphere, M.A., Smith, J.G., and Silberman, M.L., 1978, Reconnaissance geologic map and geochronology, Talkeetna Mountains quadrangle, northern part of Anchorage quadrangle, and southwest corner Healy quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-558-A, 60 p., 1 sheet, scale 1:250,000.
Symbol Tsu
Unit name Sedimentary rocks, undifferentiated
Description Fluvatile conglomerate, sandstone, and claystone with a few interbeds of lignitic coal. Looks similar to the sedimentary rocks of the southern Talkeetna Mountains but lacks fossil evidence for definitive correlation. The sequence is over 160 m thick. It resembles the Chickaloon Formation, lithologically.
Lithology Sedimentary

Correlated geologic units

Label Tch
Description Chickaloon Formation
Geologic age Paleocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Coal < Sedimentary Bed Indeterminate, major
Bituminous < Coal < Sedimentary Bed Indeterminate, major