The Tongue River Member of the Fort Union Formation, the major coal-bearing unit in the Powder River basin of Wyoming and Montana, records a history of paludal, fluvial-deltaic, and lacustrine sedimentation. Tongue River deltas filled the basin primarily from the eastern margin as they prograded into a lake (Lebo Shale Member) that occupied the basin axis. Major streams entered the Fort Union coastal plain from point sources, resulting in areas of broad interdeltaic coastal plain isolated from major clastic influx.
We mapped the clastic framework facies and the regional distribution of thick Tongue River coal seams using approximately 1,400 induction-electric logs. A detailed study of a thick coal seam in the center of the basin shows that coal occurrence is facies dependent. Peat accumulation began in interdeltaic and interdistributary areas at the loci of regional ground-water discharge. Upon delta abandonment, peat swamps overspread the abandoned lobes. The result is a thick, strike-parallel, interdeltaic coal seam bounded by fluvial-deltaic framework facies. The depositional model and coal-occurrence maps provide a guide for coal exploration and a framework for interpreting individual deposits in the Powder River basin.