USING OPTICALLY STIMULATED LUMINESCENCE DATING TO STUDY THE EVOLUTION OF THE PLATTE RIVER VALLEY OVER THE PAST 100,000 YEARS

Underlying the Platte River Valley in Nebraska, alluvial fills record the geologic history of the Platte River in the late Quaternary. Sediment cores were recovered from these alluvial deposits in rough transects along the Platte River floodplain near the towns of North Platte, and Kearney, Nebraska. Optically Stimulated Luminescence (OSL) dating was used to determine burial age of recovered sediments, and to quantify the thicknesses of Pleistocene and Holocene age alluvial fill in the two study areas. OSL ages were calculated using the single aliquot regenerative procedure on quartz sand grains. Based on results from OSL dating, burial ages ranged from approximately 1 to 68 ka. Holocene age alluvium in both study sites is present to depths of up to 8 meters below the present floodplain. Late Pleistocene age sediment that can be dated with OSL is found at depths of up to 15 m below the present floodplain. Our geochronology indicates there are considerable differences in age with depth at the two study sites. Evidence suggests that the North and South Platte Rivers at the North Platte site experienced aggradation in both the late Pleistocene and Holocene, followed by entrenchment of the alluvial fills during the late Holocene. Evidence from the Kearney site indicates the Platte River valley aggraded during the late Pleistocene and Holocene.