Coal-bearing sedimentary rocks

Unit symbol: Tcb
Age range Tertiary, Pliocene to Eocene? (38 to 5.333 Ma)
Lithology: Sedimentary
Group name: Coal-bearing sedimentary rocks
Widely exposed around the state, unit locally bears formal names, such as Healy Creek, Sanctuary, Suntrana, Lignite Creek, and Grubstake Formations of the Nenana coal field; more typically, though, the unit is mapped as coal-bearing sedimentary rocks. Located largely north of the Alaska Range, unit includes the sedimentary rocks of the Jarvis Creek coal field of Nokleberg and others (1992a) and similar units in other Tertiary basins;. The following quote from Csejtey and others (1992) can be generally applied to this unit: “The coal-bearing rocks comprise terrestrial cyclic sequences, in varying proportions, of siltstone, claystone, mudstone, shale, generally cross-bedded and pebbly sandstone, both arkosic and quartz-rich, subbituminous coal and lignite, and minor amounts of dominantly quartz- and chert-pebble conglomerate.” Locally, volcanic ash is present. Unit is nonmarine and may be faulted and (or) folded. Coal most commonly is lignite but locally is subbituminous. In the Livengood quadrangle consists of poorly to well-consolidated conglomerate, graywacke, siltstone, shale, coal, greenstone, basalt, and tuff in Schwatka-Rampart area of central Alaska. Unit is nonmarine, friable, calcareous, and contains some nodules, lenses, and thin layers of ironstone. Conglomerate has locally derived well-rounded pebble- to boulder-sized clasts of greenstone, black chert, rare white quartz, and colored chert; siltstone and shale less common. In the McGrath quadrangle (Bundtzen and others, 1997a), includes fluvial gravel, silt, sand, and carbonaceous shale interbedded with coal seams as much as 12 m thick. On Saint Lawrence Island (Patton and others, 2011), poorly consolidated sandstone, grit and conglomerate, carbonaceous mudstone, ashy tuff, volcanic breccia, and seams of lignitic coal as much as 60 cm thick is found in poorly exposed and badly slumped outcrops. Unit contains abundant plant fossils of Oligocene age (J.A. Wolfe, written commun., 1968). Patton and others (2009) reported poorly consolidated nonmarine deposits also occur in several areas of the Yukon-Koyukuk Basin, some associated with the Kaltag Fault. Pollen samples from these deposits range in age from Oligocene to Pliocene

Source map information

Source map Weber, F.R., Foster, H.L., Keith, T.E.C., and Dusel-Bacon, Cynthia, 1978, Preliminary geologic map of the Big Delta quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-529A, scale 1:250,000.
Symbol Tcb
Unit name Coal-bearing formation
Description Sandstone, siltstone, claystone, and conglomerate, light-yellowish-gray to light-reddish-brown, poorly consolidated, easily eroded. Conglomerate particles mostly well-rounded quartz and chert as much as 4 cm in diameter. Lignitic coal layers as much as 30 cm thick are rare. Limonitic sandstone concretions common.
Lithology Sedimentary

Correlated geologic units

Label Tcb
Description Coal-bearing rocks, Miocene? to Eocene?
Geologic age Late-Eocene to Late-Miocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Claystone < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Lignite < Coal < Sedimentary Bed Minor
Sub-bituminous < Coal < Sedimentary Bed Minor