Named as basal member of Duchesne River Formation for Brennan Basin on the Green River and in Halfway Hollow to north, Uintah Co, UT in the Uinta basin. Type section extends from near center sec 17, T7S, R21E north to sec 13, T5S, R19E. Crops out from UT-CO state line west to Duchesne Co, UT. Is 1,949 ft thick at type measured section. Thins west to 888+ ft at Rock Creek. Is 422+ ft at Red Wash, UT. Pinches out 14 mi east of Red Wash. Predominately sandstone that varies from yellow-gray to red brown, very fine, fine, medium, to coarse grained, thin to thick to very thick bedded, generally poorly sorted, and may be nodular, silty, pebbly, and cross stratified. Minor beds of green-gray to red brown, silty to sandy, thin to very thin bedded, poorly sorted claystone, red brown, thin-bedded, nodular, poorly sorted mudstone, yellow brown, very thin bedded, nodular, poorly sorted, horizontally stratified siltstone, and red-purple, sandy, clayey, thin-bedded, medium to coarse crystalline limestone. Some beds burrowed. Overlies Uinta formation. Underlies Dry Gulch Creek Member (new) of Duchesne River. Has a diverse vertebrate fauna (listed). Of late Eocene age. Cross sections.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Adopted as basal member of Duchesne River Formation. Mapped for the first time in central and eastern parts of Uinta basin in Duchesne and Uintah Cos, UT in the Uinta basin. West of Rock Creek, western Duchesne Co, member loses identity and is mapped as part of Duchesne River Formation undivided. Geologic map. Generally overlies Uinta Formation. At northeast basin margin Uinta, Green River, and Wasatch Formations have all pinched out and Brennan Basin is on Mesozoic rocks. Age changed from late Eocene to late Eocene and early Oligocene?

Source: GNU records (USGS DDS-6; Denver GNULEX).