ABSTRACT

The Eocene Wasatch Formation consists of deposits of intermontane alluvial fan and trunk- tributary stream systems. These deposits include alluvial fan facies that consist of proximal and distal deposits of the Kingsbury and Moncrief Conglomerate Members of the Wasatch. This facies passes laterally into alluvial plain facies of meanderbelt, floodplain, lake, and backswamp deposits.

The dynamics of coal deposition include three modes that operated in the alluvial plain. The lower interval, the Arvada coal bed and related sediments, formed in backswamps associated with a floodplain that was locally drowned by lakes. The middle interval, the Felix coal bed and related sediments, accumulated in backswamps formed on abandoned meanderbelt deposits resulting from stream avulsion. The upper interval, the Lake de Smet coal bed and related sediments, accumulated in backswamps that were probably located on an elevated site that may have been formed by syndepositional faults superimposed on a generally subsiding or downwarping axial basin. The Lake de Smet backswamps were immediately bounded at the east by stream channels and at the west by alluvial fans. These backswamps were probably sustained by a rise of ground-water table resulting from interaction of shallow and deep basin ground waters that operated in the dynamic, compacting Powder River Basin.