DUNE FORMATION SOUTH OF THE NIOBRARA RIVER VALLEY, NORTH-CENTRAL NEBRASKA SAND HILLS

The Nebraska Sand Hills (NSH) are a vast (19,300 square mile) area of grass-stabilized sand dunes. Many of the larger dunes in the NSH formed by eolian deposition primarily during the Late Pleistocene, although widespread reactivation during the Holocene has been recorded. Recent studies utilizing Optically Stimulated Luminescence (OSL) dating indicate these dunes were last active around 800 years ago, during the Medieval Warm Period. This project is focused on investigating the formation of linear dunes located in northwestern Brown County, Nebraska along the southern margin of the Niobrara Valley. These dunes range from 300-600 meters in length, and their relief ranges from 6-10 meters. Recently acquired 2-meter resolution LiDAR imagery revealed that many of these dunes were paired, a phenomenon which had not yet been identified in other types of dunes in Nebraska. Interestingly, LiDAR showed each dune has a smaller counterpart, formed upwind of the larger one. The northerly crests are 1-2 meters lower than their southerly counterparts. The distance between the paired crests ranges from 80-90 meters. Both ground-penetrating radar (GPR) and subsurface investigations were used to study these dunes. We obtained five GPR lines, created with frequencies ranging from 100 mHZ to 450 mHZ, which penetrated to the base of the dunes. The lines depict beds that dip either entirely to the north or south in each dune, but not in both directions within one dune, as previously suggested for linear forms. OSL samples were collected from twelve sediment cores and five hand-auger holes. Preliminary eolian ages show these dunes stabilized between 1100 -1500 years ago for samples extracted from 1-3 meters below ground surface. These ages are similar to the last period of dune activity recorded in the NSH. Several of the deeper (35-40m) cores contain alluvium that underlies the dunes, which dates to around 24,000 years old. Through a combination of GPR, OSL and particle size analysis we intend to improve our understanding about the development of the paired dunes and how these unique dune morphologies were formed.