Abstract

The Fort Union Formation and equivalent rocks in the northern Rocky Mountains and Great Plains region were deposited by fluvial, lacustrine, and deltaic-barrier systems in basins that were progressively developed by Laramide deformation fronts, which prograded from southwest to northeast during early, middle, and late Paleocene. These basins were structurally partitioned by vertical uplifts (e.g., arches and welts) and were sites of accumulation of thin to thick coal deposits. The coals, which are economically mined, are clean (low ash yield) and compliant (low sulfur and low concentrations of trace elements of environmental concern as named in the 1990 Clean Air Act Amendments). These coals generated large amounts of coalbed methane in the Powder River Basin in Wyoming and Montana, which encouraged exploration in other basins. The characteristics of Fort Union and equivalent coals are controlled by paleotectonic, paleogeographic, paleoclimatic, and paleodepositional settings described in this paper.