DETERMINING THICKNESSES OF LATE PLEISTOCENE AND HOLOCENE FILLS IN THE PLATTE RIVER VALLEY USING OPTICALLY STIMULATED LUMINESCENCE DATING

Sediment cores were drilled in late Quaternary-aged deposits in rough transects along the Platte River floodplain near the towns of Hershey and Kearney Nebraska. Late Pleistocene and Holocene sediments recovered in drill core generally consist of slightly calcareous tan-brown fine-grained silt and sand with interspersed gravels. Sediments recovered in cores from the valley show no readily distinguishable differences with depth in the fill. Optically Stimulated Luminescence (OSL) dating was used to determine burial age of recovered sediments and to quantify the thicknesses of Pleistocene and Holocene age fill in the two study areas. OSL ages were determined using the single aliquot regenerative procedure on quartz sand grains. Analyses were conducted using small aliquots to limit the impact of partial bleaching on these samples. Even with this approach many of these samples show some degree of partial bleaching, suggesting that these ages should be considered maximum ages for these deposits. Based solely on results from OSL dating, burial ages ranged from approximately 1 to 68 ka. Five additional samples had saturated OSL signals and in these cases we could only assign minimum ages that are estimated to be greater than approximately 90 ka. Holocene age alluvium in both study sites is present to depths of up to 8 meters below the present floodplain. Datable late Pleistocene age sediment is found at depths of up to 15 m below the present floodplain. Our geochronology indicates there are considerable differences in age with depth in these two portions of the valley. Locally late Pleistocene sediment is found within three meters, and sediments that are too old to be dated are found within 4 meters of the present ground surface. Further analysis of this data set will allow us to characterize changes in alluvial behavior since the last glacial maximum.