PROVENANCE-CALIBRATED FLUVIAL RESPONSE TO WARMING DURING THE EARLY EOCENE CLIMATIC OPTIMUM, BRIDGER BASIN, WYOMING

The Bridger subbasin of the Greater Green River Basin contains some of the thickest and most continuous early Eocene strata in the world. These strata record the early Eocene climatic optimum (EECO), the hottest portion of the most recent sustained greenhouse climate. The well-dated lacustrine Green River Formation in the study area is laterally equivalent to fluvial strata of the Cathedral Bluffs Member of the Wasatch Formation adjacent to the Uinta Uplift. We conducted provenance analysis to constrain the paleocatchments which fed these Eocene streams, and measured fluvial morphologies across the EECO to obtain a detailed record of flood hydrology from before, during, and after the EECO. The different bedrock units of the Uinta Uplift source distinct sediment types: red Precambrian quartzite; Paleozoic carbonates and eolian sandstone; and predominant sandy and muddy units of the Mesozoic. We gravelometrically sampled modern catchments draining characteristic Uinta Uplift bedrock units to calibrate their provenance signature for comparison to Eocene sedimentary strata. Composition and grain size distributions were obtained via XRF scanning, photogrammetry, and point counting using a 10 cm net. Provenance signatures will aid reconstruction of Eocene catchment geometries, which is a major unconstrained variable in most paleohydrologic studies. Bedform height and grain size of fluvial forms (point bar deposits, incised channels) were measured to compare changes to flood magnitudes. We used GNSS-GPS-calibrated photogrammetry to enhance fluvial bedform quantification of large and hard to reach outcrops. Fluvial strata were correlated to the well-dated central core of the Bridger subbasin with measured sections and handheld gamma ray and XRF logs. Provenance and bedform analysis, together with existing geochronology, will form the basis for a time series of flood hydrology across the EECO that accounts for paleocatchment geology. Preliminary findings suggest that higher magnitude flood events occurred during the EECO than in preceding and subsequent time intervals, which produce deeper channels and larger grain sizes.