LATE-WISCONSINAN ALLUVIUM IN THE SOUTH LOUP AND OTHER RIVER VALLEYS IN NEBRASKA

Late-Wisconsinan alluvium, the Todd Valley Member of the Peoria Formation, previously has been identified in the Loup River basin of central Nebraska. The alluvium consists of very fine to medium, very light gray sand interbedded with silty strata, gravel lenses, and peat beds. The fill is up to 51.2 m (169 ft) thick along the lower South Loup River. Recently, radiocarbon ages of plant macrofossils (Picea, Juniperus, and Abies) and soil/sediment humates in the Todd Valley Member have been determined in the South Loup River valley. These ages range from nearly 16,000 to 14,000 14C yr B.P. These ages reveal that deposition of this alluvium occurred contemporaneously with deposition of Peoria Loess across uplands in the central Great Plains.

This research addresses the geographic distribution of evidence for similar, Late-Wisconsinan fluvial events across the central Great Plains. Was there widespread Late-Wisconsinan incision and subsequent rapid aggradation of valleys elsewhere in the central Great Plains? Although the radiocarbon-dated evidence is meager, there is some evidence of such events in valleys in eastern Nebraska, south-central Nebraska, and southwestern Nebraska. The regional nature of the Late-Wisconsinan fluvial events indicates that climate changes must have triggered these fluvial events, or, at least been largely responsible for them.