LATE-WISCONSINAN ALLUVIUM IN THE LOUP RIVER BASIN, NEBRASKA

The Holocene alluvial history of the Loup River basin, the largest tributary to the Platte River system in the central Great Plains, is generally well understood. However, little is known about late-Wisconsinan alluvium in these valleys. Did climate changes trigger widespread fluvial adjustments? How did the deposition of thick, late-Wisconsinan Peoria Loess and dune sand affect these rivers? To begin to answer these questions I have described and dated in natural cutbanks what traditionally has been classified as the Todd Valley member of the Peoria Formation. I have also studied logs of test holes and irrigation wells to attempt to determine the spatial distribution of this alluvium. Very fine to medium, very light gray sand interbedded with silty strata, gravel lenses, and peat beds comprises this valley fill in the Loup River basin. The fill is up to 51.2 m (169 ft) thick along the lower South Loup River. Radiocarbon ages of plant macrofossils (Picea, Juniperus, and Abies) and soil/sediment humates in this fill range from 15,730 to 14,080 14C yr B.P.; deposition of this alluvium occurred contemporaneously with deposition of Peoria Loess. In Todd Valley, an abandoned valley of the lower Platte River, the Todd Valley member is older than 20,900 14C yr B.P. The difference in ages may be the result of complex adjustments of the drainage network to changed sediment loads, differential preservation of the alluvium in different parts of the drainage basin, or two, discrete late-Wisconsinan depositional events with different causes. This study indicates that deposition of late-Wisconsinan alluvium occurred rapidly in the lower reaches of the North, Middle, and South Loup river valleys. The cause here may have been a sudden breaching of a dune dam in the Sand Hills portion of the drainage basin. Subsequent incision of Todd Valley alluvium throughout the Loup River basin occurred after 14,080 14C yr B.P. and before 10,850 14C yr B.P.