DEPOSITIONAL ENVIRONMENT, SEDIMENT PROVENANCE AND OXYGEN ISOTOPE PALEOALTIMETRY OF THE EARLY PALEOGENE GREATER GREEN RIVER BASIN, SOUTHWESTERN WYOMING, USA

Depositional process, sediment provenance, and paleotopography are critical to paleogeographic reconstruction during the coeval Laramide and Sevier orogenies in the Cordillera orogenic system. Here we conduct lithofacies analysis, sediment provenance, and paleoelevation studies in the fluvial lower Paleogene Fort Union and the main body of the Wasatch formations in the center of the greater Green River Basin in order to reconstruct paleogeography. The depositional environment changed from low-energy floodplains and associated meandering river during the Paleocene to braided rivers during the earliest Eocene. The lowermost Eocene strata have more Precambrian basement-derived feldspar grains compared to the Paleocene strata. This change in sediment provenance was associated with the change to dominant northwestward paleoflow direction, suggesting major uplift of the Laramide Uinta Mountains during the earliest Eocene. The delta O-18 values of authigenic carbonates were stable during the early Paleocene, but decreased gradually from -10 permil during the late Paleocene to -18 permil during the earliest Eocene, suggesting the lack of highland precipitation in the basin catchment during the early Paleocene, and significant surface uplift and arrival of highland precipitation from the Uinta Mountains during the earliest Eocene. The reconstructed paleoelevation of the Uinta Mountains was at least 3 km, possibly greater than 3.5 km, and the paleoelevation of the greater Green River Basin floor was at most 1 km, and the paleorelief between the greater Green River Basin and Uinta Mountains was similar to 2 km during the earliest Eocene. Our record of the earliest Eocene establishment of high topography of the Uinta Mountains through renewed uplift is 4 to 6 Myr older than previously thought. If the carbonate cements in the Fort Union Formation record the isotope compositions of surface water from the Sevier orogen, their high delta O-18 values likely suggest that the Sevier orogen had rugged topography during the Paleocene.