--> Abstract: Stratigraphy and Depositional Environments of Tongue River Member of Fort Union Formation (Eocene), Powder River-Clear Creek Area, Northeastern Wyoming, by Romeo M. Flores; #90961 (1978).
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Abstract: Stratigraphy and Depositional Environments of Tongue River Member of Fort Union Formation (Eocene), Powder River-Clear Creek Area, Northeastern Wyoming

Romeo M. Flores


Stratigraphic environmental analysis of the Tongue River Member of the Fort Union Formation in the Powder River-Clear Creek area in northeastern Wyoming suggests deposition of the Canyon-Anderson coal sequence in upper alluvial-plain fluvial channels and Previous HitfloodNext Hit basins, and the overlying Anderson-Roland coal sequence in lower alluvial-plain fluvial channels, Previous HitfloodNext Hit basins, and lakes.

The upper and lower alluvial-plain deposits differ in the characteristics of their channel-sandstone, crevasse-splay, lacustrine, and coal deposits. Upper alluvial-plain channel sandstone is multilateral, thick, up to 4 km in lateral extent, and has a deep erosional base. Lower alluvial-plain channel sandstone is thin, up to 1.6 in lateral extent, and has a shallow erosional base. Both sandstones contain lag deposits and accretion beds with planar and festoon cross-beds grading upward into convolution, ripple, and parallel laminations. Crevasse-splay and lacustrine deposits are common in the lower alluvial plain. Crevasse-splay deposits include fossiliferous sandstone, siltstone, and shale. The crevasse-splay sandstone is cross-bedded, convoluted, rippled, and parallel laminated. Lacu trine deposits, which contain abundant fossils of freshwater mollusks, include interbedded sandstone, siltstone, shale, and limestone.

The upper alluvial-plain coal beds are thick and erratic whereas those of the lower alluvial plain are relatively thin and laterally pesistent. Upper alluvial-plain coal deposition occurred in Previous HitfloodNext Hit-basin backswamps adjacent to meandering fluvial channels. Lower alluvial-plain coal was deposited on crevasse-splay fillings of Previous HitfloodTop-basin lakes adjacent to fluvial channels that were confined in narrow linear belts.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma