EARLY OLIGOCENE UPLIFT OF THE ROCKY MOUNTAINS BASED ON U-PB DATING OF FLUVIAL CONGLOMERATES FROM SOUTHWESTERN WYOMING AND NORTHWESTERN COLORADO
Maximum depositional ages (MDAs) calculated from the U-Pb ages show that the conglomeratic units have similar ages (34-28 Ma). In southwestern Wyoming, the Gilbert Peak erosion surface bevels upland areas of the Uinta Mountains as well as basin-fill strata along the north flank of the uplift. The overlying Bishop Conglomerate yields MDAs of ca. 34 Ma. Stratigraphic reconstructions across the Rock Springs Uplift coupled with the new age estimates for the Bishop Conglomerate indicate that ~400-500 m of erosion occurred ca. 47-34 Ma, probably due to late Eocene reactivation of the Rock Springs Uplift.
In northwestern Colorado, post-Laramide conglomeratic units include the Bishop Conglomerate and the basal unit of the Browns Park Fm., which yield MDAs of ca. 30-28 Ma. These units also overlie regional surfaces that record significant ca. 45-30 Ma erosion of Laramide ranges and adjacent basin-fill strata. Similar fluvial conglomeratic units elsewhere in the Rocky Mountain region overlie the Rocky Mt erosion surface, and include the Telluride Conglomerate (31.5-28 Ma) of southwestern Colorado and the Tallahassee Creek and Castle Rock Conglomerates (<37 Ma) of south-central Colorado.
The broad similarity in the ages of these fluvial conglomerates and their associated erosion surfaces, their broad areal extent, and the magnitude of erosion associated with these fluvial systems (hundreds of meters), support broad-wavelength, post-Laramide (Oligocene) uplift in the Rocky Mountain region.