AN EARLY CRETACEOUS-PALEOCENE FLUVIAL MEGAFAN IN THE WESTERN INTERIOR FORELAND BASIN, NORTHWESTERN WYOMING

Fluvial megafan deposits record the history of sediment dispersal and deposition in foreland basins and contain information on the structural and geomorphic evolution of the adjacent fold-thrust belts. This study presents evidence for the existence of an eastward prograding, Early Cretaceous through Paleocene fluvial megafan that emanated from the Sevier fold-thrust belt in northwestern Wyoming. The stratigraphic succession of this megafan is over 4 km thick and consists of a basal unit of fine-grained, anastomosed fluvial deposits, a middle unit of coarser-grained braided stream deposits and an uppermost unit of fluvial conglomerates with cobble- and boulder-sized quartzite clasts. The large-scale fluvial architecture is similar to other documented fluvial megafan successions and the northeast through southwest directed paleocurrents indicate a fan-like pattern of sediment dispersal. The uppermost conglomeratic units were deposited by transversely flowing rivers that were connected to a large drainage system located to the west in the hinterland of the fold-thrust belt. High sediment influx to this part of the foreland basin is indicated by the repeated east-southeast progradation of large, coarse-grained sediment lobes. During periods when the area was inundated by seawater, shorelines in this region continued to prograde eastward into the basin, even as coeval shorelines to the north and south were transgressed. The north-south spacing (along depositional strike) between this fluvial megafan and other documented Cretaceous fluvial megafans in the Western Interior Foreland Basin is equal to the spacing between modern fluvial megafans in the Himalayan and Andean foreland basins. The recognition of a fluvial megafan in northwestern Wyoming has important implications for resolving the depositional history of the region, as well as for reconstructing the thrust kinematics in the Utah-Wyoming-Idaho salient of the Sevier fold-thrust belt.