Abstract

The Tertiary Fort Union and Wasatch formations in the Powder River Basin contain economic coal and uranium deposits that are targets for exploration and development. These activities have provided considerable subsurface and surface data that were used to analyze the evolution of depositional systems in the basin. The Paleocene Fort Union Formation consists, in ascending order, of the Tullock, Lebo Shale and Tongue River Members. The Eocene Wasatch Formation consists of the conglomeratic Kingsbury and Moncrief Members and laterally equivalent finer grained deposits. Both formations contain sandstone, conglomerate, siltstone, mudstone, limestone, carbonaceous shale, and coal.

The high proportion of sandstones in the Tullock Member and combined Tongue River Member of the Fort Union Formation and Wasatch Formation generally occurs in interconnected east-west and north-south belts. East-west belts probably represent deposits from alluvial fan and braided and meandering tributary streams. North-south belts probably represent deposits from meandering and anastomosing trunk streams fed by basin margin tributaries. The sandstones of the Lebo Shale Member generally show east-west trends and probably represent deposits of fluvio-deltaic systems that fed a closed lacustrine basin. Lake formation may have been promoted by localized subsidence along the Buffalo Deep Fault. These contrasting styles of fluvial deposition were largely controlled by extrabasinal and intrabasinal tectonics during the Laramide orogeny.

Comparison of the distribution of sandstone belts in the Powder River Basin with modern fluvial analogues indicates similarities with the following trunk-tributary fluvial systems: the Mahakam River in the Kutai Basin, Borneo; Rio Paraiba do Sul in the State of Sao Paulo, Brazil; and Lower Saskatchewan River, Canada. These fluvial systems consist of mixed-load meandering and anastomosing streams that drain alluvial plains in an intermontane or continental setting.