The Paleocene Tongue River Member of the Fort Union Formation supports widespread coal-mining activity and contains most of the recoverable coals in the Powder River basin. Coals are well-developed in eastern and northern Powder River basin, but the best exposures are in the northern part. Most mining is from the very thick (as much as 125 ft or 38 m) coals; however, as these coal reserves are depleted, future exploration will focus on moderately thick coals. Exploration guides for the moderately thick coals may be their facies associations. In the northern Powder River basin, 175 mi (280 km) of cross sections, constructed from closely spaced outcrop sections and drill holes, provide detailed facies associations of the coals.

One facies association is dominated by thick fluvial-channel sandstones and coals as much as 40 ft (12 m) thick and 12 mi (19 km) in extent. The coals formed in poorly drained backswamps in which uniformly thick peat bogs accumulated, subparallel to meandering channels and opposite their migration and avulsion directions. A second facies association is characterized by abundant crevasse sandstones, subordinate thin channel sandstones, and lacustrine limestones, shales, and siltstones. Facies-associated coals, as much as 8.5 ft (17 m) thick and 5 mi (8 km) in extent, formed in well-drained backswamps frequently interrupted by crevasse splays that debouched into flood-plain lakes. The best development of economic coals of the fluvial-channel dominated facies is in the lower part of the ongue River Member, and that of the flood-plain-lacustrine coal-facies association is in the upper part.

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