Abstract

Deposition of lacustrine sediments in the greater Green River Basin was progressively terminated in the middle Eocene as volcaniclastic detritus prograded across the basin; the lacustrine sediment of the Laney Member was replaced by deposition of the deltaic and fluvial sediments of the volcaniclastic Bridger and Washakie formations and the Sand Butte Bed of the Laney Member. The transition from the deposition of lacustrine to alluvial sediment also records a reversal in the direction of drainage across the basin that occurred between 49.5 Ma and 48.9 Ma. Prior to this transition, drainage entered the basin from the east and the arkosic detritus of the Cathedral Bluffs Member of the Wasatch Formation was deposited in the Washakie and Great Divide subbasins. At the same time evaporitic sediments of the Wilkins Peak Member of the Green River Formation accumulated simultaneously in the Bridger Basin, indicating that the clastic sediment was baffled east of the Rock Springs Arch, an intrabasinal structural high. The subsequent transition to deposition of the Laney Member is traditionally interpreted as an isochronous expansion of Lake Gosiute across the greater Green River Basin. Alternatively, the transition may have been diachronous with continued accumulation of the Wilkins Peak Member in the Bridger Basin coinciding with the earliest deposition of the Laney Member in the Washakie Basin. By 48.9 Ma the dominant drainage was clearly from the west, which reflects the capture of an external drainage network through the northwest corner of the basin that delivered extrabasinal volcaniclastic detritus to the greater Green River Basin. More broadly, these results illustrate that the geomorphic evolution of the landscape influenced the character of sedimentary fill in the nonmarine greater Green River Basin.