LATE HOLOCENE DUNE ACTIVITY IN THE EASTERN PLATTE RIVER VALLEY

Eolian sand sheets and dunes are commonly found on alluvial terraces that lie between the Loup and Platte River Valleys in east-central Nebraska. This study focused on the stratigraphy and geochronology of dunes and sand sheets located near the present-day Loup-Platte confluence at the town of Duncan, Nebraska. Optical (aka OSL) dating methods were used to constrain when sediments found within both the dunes and the underlying alluvial deposits were deposited. Samples from both types of deposits were collected with hand augers, and a total of 14 optical age estimates were generated for eolian sediments and one for the alluvial deposits that underlie the dunes. The latter age estimate indicates that the alluvial sediments within the terraces were deposited around 4,900 ± 400 yrs ago. Three periods of dune activation were identified from the eolian sediments. These include periods of dune activity that occurred ~ 5,000 years ago, and between 4,000-3,500 and 850-500 years ago. The latter two dune activation periods correspond reasonably well with so-called ‘mega-drought' events identified in studies from the nearby Nebraska Sand Hills. It is therefore likely that dune activity during these time periods was related to drought conditions. However, the period of dune activity dating to ~ 5,000 years ago is more likely related to the sub-aerial exposure of abandoned alluvial sediments, which may or may not be related to changes in aridity. Future work will focus on the importance of changes in sediment availability to late Holocene dune mobilization events in the region.