The Fort Union formation of Paleocene age in the Wind River Basin of central Wyoming consists of interbedded sandstone, conglomerate, and carbonaceous shale of fluviatile origin overlain by fine-grained clastic strata of lacustrine or marine origin. The latter sequence consists of two units, one deposited in the marginal areas surrounding the Paleocene lake or sea during the period of maximum expansion and subsequent regression, and the other deposited in the offshore areas.

Strata representing the marginal unit are named the Shotgun Member from exposures near Shotgun Butte in the north-central part of the Wind River Basin. The member is 2,830 feet thick at the type section and characterized by uniformly bedded soft claystone, siltstone, and shale with minor amounts of sandstone and carbonaceous shale. Typical colors are gray, olive-drab, buff, brown, and tan, but some zones are pale red and purple. Vertebrate fossils indicate a middle and (or) late Paleocene age, except for the uppermost part which possibly is earliest Eocene in age. The fossils include remains of marine-type sharks.

Strata representing the offshore unit are named the Waltman Shale Member from exposures near Waltman in the eastern part of the Wind River Basin. The type section consists of 645 feet of chocolate-brown and gray silty micaceous claystone interbedded with a few thin beds of ledge-forming sandstone. In subsurface sections the member is a remarkably homogeneous dark brown to black silty micaceous shale and covers a region of several hundred square miles. The maximum thickness is about 2,600 feet. The shale is commonly glauconitic and pyritic, and contains appreciable amounts of organic material, some of which converts to oil upon distillation. Preliminary spore and pollen studies indicate a middle and late Paleocene age.

Where thickest, the Shotgun and Waltman Shale members are essentially contemporaneous units, and there is a nearly complete lateral gradation between the two lithologic types. At many places, however, both members are present and thin transitional zones of interbedded sandstone, black micaceous shale, carbonaceous shale, and coal directly underlie and overlie the Waltman Shale Member. The lower zone is a transgressive series and is included in the lower part of the Fort Union Formation, and the upper zone is a regressive series and is included in the overlying Shotgun Member. Many of the sandstone beds in the transition zones are glauconitic.

The marine-type sharks, specimens of hystrichosphaerids, and glauconite formed in place suggest a marine origin for the strata in the upper part of the Fort Union Formation. Analytical data, however, suggest that deposition was in waters that were less saline than normal marine waters. It is tentatively concluded that these upper Fort Union strata originated in a restricted body of water that had limited access to the Cannonball sea which extended from the Gulf of Mexico to South and North Dakota during Paleocene time.

Intertonguing of the Waltman Shale Member with porous sandstone beds of the enclosing strata, and the organic content of the shale, provide favorable conditions for the generation, migration, and accumulation of oil and gas in the lower Tertiary rocks of the Wind River Basin.