HIGH-RESOLUTION PROVENANCE MAPPING OF CYCLIC EOCENE FLUVIAL FANS THAT FED ANCIENT LAKE GOSIUTE, WYOMING

Closed lake systems often experience base level changes that act to intercalate fluvial and lacustrine facies. In the Wilkins Peak Member of the Green River Formation of southwest Wyoming, nine named siliciclastic marker intervals are interbedded with lacustrine strata at the 10-30 meter scale and mark intervals of lowered lake level and enhanced basinward clastic advection. To better interpret a paleoclimate record from a new basin-center core that contains these units, we conducted lithofacies analysis, handheld XRF analyses and sandstone petrography on the Wilkins Peak Member. Comparison of sandstone petrography with XRF results using principal component analysis shows a clear correspondence between sedimentary lithofacies, grain composition, and element concentrations. Results indicate that clastic marker beds are composed of predominantly of arkosic sediment rich in Al, K, Rb, Fe, and several other transition metals was delivered to the basin by the Aspen paleoriver that drained the west slopes of the Rocky Mountains of northern Colorado. Another significant stream that drained the east side of the North American Cordillera entered the basin from the west from the Sevier fold and thrust belt delivered a sub-lithic petrofacies to the western Bridger basin. Quartzose sediment delivered by steep streams which drained the Uinta Uplift were largely confined to the basins southern edge. Repeat high spatial-resolution XRF-based clastic provenance for all nine clastic marker beds documents subtle changes to an evolving distal fan that ramped down into the Bridger sub-basin from the SE. The terminus of this fan was composed of multiple distributary channels, and appears to have shifted gradually north and east across a million years of Wilkins Peak deposition, mirroring the changing position of an evaporite depocenter. Over time, clastics derived from the west and south prograded into the basin center, displacing parts of the Aspen paleoriver fan, potentially tracking a slowdown in thrust-generated accommodation and sediment-trapping outboard of the Sevier fold thrust belt and Uinta Mountains and/or a reduction in sediment input from the Aspen paleoriver.