Geochemical evidence for an eolian sand dam across the North and South Platte rivers in Nebraska

Geochemical and geomorphic data from dune fields in southwestern Nebraska p rovide new evidence that the Nebraska Sand Hills once migrated across the N orth and South Platte rivers and dammed the largest tributary system to the Missouri River. The Lincoln County and Imperial dune fields, which Lie dow nwind of the South Platte River, have compositions intermediate between the Nebraska Sand Hills (quartz-rich) and northeastern Colorado dunes (K-felds par-rich). The most likely explanation for the intermediate composition is that the Lincoln County and Imperial dunes are derived in part from the Neb raska Sand Hills and in part from the South Platte River. The only mechanis m by which the Nebraska Sand Hills could have migrated this far south is by complete infilling of what were probably perennially dry North Platte and South Platte river valleys, Such a series of events would have required an extended drought, both for activation of eolian sand and decreased discharg es in the Platte River system. A nearby major tributary of the North Platte River is postulated to have been blocked by eolian sand about 12,000 C-14 YT B,P, We propose that an eolian sand dam across the Plattes was construct ed at about this same time. (C) 2000 University of Washington.