TIMING FOR THE INITIATION OF LATE PALEOGENE LOESS DEPOSITION AND ASSOCIATED ENVIRONMENTS IN THE WESTERN UNITED STATES

The timing and spatiotemporal pattern of the initiation of eolian deposition are crucial for understanding ancient land surface processes. The upper Paleogene (~36-30 Ma) loess in the western USA remains poorly studied. This loess has long been viewed as recycled volcanic ash, and its initiation was thought to be at the apparent change from stratified fluvial to massive eolian units. Here, we present new mineralogy and grain-size data collected from the White River Formation or Group at the Flagstaff Rim in central Wyoming and Toadstool Geologic Park in western Nebraska. Our results show that the loess deposits (n=15) have less volcanic glass, but ~6 % more potassium feldspar and ~6 % more calcite than the interbedded volcanic ashes (n=9), thus it is unlikely derived wholly from recycled volcanic ash. At the Flagstaff Rim section, the grain-size data from the fluvial unit show significant variability, including typical loess bimodal distribution, modified bimodal distribution with enhanced very-fine silt and clay fraction, and modified bimodal distribution with an additional fine sand mode. Granulometry indicates that deposits vary from primary loess, pedogenically modified loess in floodplains, and reworked loess by fluvial processes. Field observations in tandem with available geochronology indicate that loess deposition started at ~36 Ma, ca. 2 Myr earlier than the lithofacies change. This initiation was synchronous with that in the Beaver Divide to the west, suggesting more extensive loess deposition prior to the drastic cooling at the Eocene-Oligocene transition. Renewed uplift of the North American Cordillera may have triggered enhanced river erosion and silt production, and subsequently increased the dust loading by wind. The initiation of loess at the Toadstool section to the east was at the lithofacies change (~32 Ma), consistent with the previous observation of the eastward younging trend of loess initiation.