Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers

The Fort Union Formation (Paleocene) was deposited in 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. Alluvial fans issued from the basin margins and developed fan-delta deposits where their distal reaches prograded into a restricted body of fresh water that occupied the basin.

Facies of the Waltman depositional system, in basinward progression, include fan plain, braided stream channel, distal fan, and prodelta/lacustrine. Subdivisions within the fan-plain facies include sheetflood, stream channel, abandoned channel, and undifferentiated lower alluvial fan deposits. The distal fan facies is also subdivided into distributary mouth bar, distal delta front, and destruction bar deposits. This sequence of facies exhibit a basinward increase in lacustrine over fluvial depositional influences.

Distribution of these facies within strata of the Waltman Member suggests three major depositional events, each related to the tectonic behavior of the basin and surrounding uplifts. Lowermost is preserved a thick, widespread squence of lacustrine shale with fan-delta deposits encountered at a relatively great distance from the basin axis. This configuration is indicative of deposition during a period of rapid basin subsidence relative to marginal uplift. Overlying this is a thin, laterally continuous, calcareous siltstone deposited during a period of tectonic quiescence. The concluding event is characterized by episodic progradations of vertically stacked fan-delta lobes into the lacustrine environment as far as the basin axis. This uppermost sequence was deposited during a period of greater marginal uplift relative to basin subsidence which persisted into the Eocene.