The Fort Union Formation (Paleocene) was deposited in structural and sedimentary basins which developed in the Rocky Mountain states in response to Laramide structural disturbances. Exposures of the Waltman Member in the eastern Wind River basin reveal an interaction of alluvial fan and lacustrine depositional processes. Fan deltas developed where the distal reaches of alluvial fans, which issued from the tectonically active Granite Mountains to the south, prograded into an isolated or restricted body of fresh water that occupied the rapidly subsiding basin.
Facies of the Waltman depositional system, in basinward progression, include distal alluvial-fan, proximal fan-delta, delta-front, and prodelta. This sequence of facies exhibits an increasing influence of lacustrine over fluvial processes. Depositional environment of each facies is interpreted through examination of outcrop and subsurface characteristics, and supported by analogy with other ancient and modern depositional systems.
Sedimentation was largely controlled by the tectonic behavior of the source area and receiving basin. Rapid subsidence of the basin relative to the source area resulted in three vertically stacked fan-delta lobes in the study area. Uplift of the source area relative to the basin subjected preexisting formations and facies to erosion and redeposition. Climate and basin morphology (which controlled storm activity), stream runoff, waves, currents, basin slope, water depth, and water salinity were also factors which influenced sedimentation within this depositional system.