Stratigraphic distribution of coals and associated lithofacies in the upper Fort Union Formation (Paleocene) was investigated in outcrop and subsurface from southeast of Sussex to south of Buffalo, Wyoming. In this area, Ayers and Kaiser in 1982 proposed that upper Fort Union coals accumulated in deltas and interdeltas, and pinched out into a lake. Our study does not support these interpretations.

The upper 1,000 ft (300 m) of the Fort Union Formation in the western Powder River basin comprises interbedded conglomerates, conglomeratic sandstones, sandstones, siltstones, mudstones, carbonaceous shales, and coals. The conglomerates, consisting of pebbles and cobbles reworked from Mesozoic and Paleozoic rocks, are in scour-based bodies as thick as 25 ft (8 m). A 300-ft (90-m) thick, 12-mi (19-km) long conglomeratic channel-sandstone complex is in the lower part of the interval. In the upper part of the interval, conglomeratic single- and multistory channel sandstones reach thicknesses of 100 ft (30 m) and widths of 4,000 ft (1,200 m). These channel sandstones grade into overbank-floodplain sediments, which are interbedded with backswamp deposits of coals and carbonaceous shales. T e conglomeratic channel sandstones are interbedded with coal beds as thick as 20 ft (6 m). These coal beds probably are laterally equivalent to the 178-ft (54-m) thick Sussex coal deposit to the east.

Lithofacies associated with the coals in the western Powder River basin suggest an alluvial-plain paleoenvironment. The alluvial plain consisted of braided and meandering streams flanked by well-drained and poorly drained backswamps. These streams probably are northeasterly flowing tributaries of trunk streams.

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