Abstract

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 2 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 N.-central part of the Wind River Basin. The member is 2,830 ft. 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 ft. 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 ft. 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.

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