HISTORY OF LOESS TABLELAND FORMATION AND IMPLICATIONS FOR LOCAL PALEOCLIMATIC CONDITIONS, SOUTHWESTERN NEBRASKA

Surficial deposits of southwestern Nebraska are dominated by Late Pleistocene Peoria Loess, patchy Holocene Bignell Loess, dunefields, and sand sheets. It has been proposed that the main immediate source for both Peoria and Bignell loess in southwestern Nebraska was dunefields to the west and north, rather than glaciofluvial sources as elsewhere in the central U.S. (Peoria Loess was ultimately derived in part from Tertiary siltstones northwest of the dunefields). Wedge-shaped loess tablelands located in Chase County, Nebraska, are underlain by thick Peoria and Bignell loess, which includes paleosols and other features that record local paleoclimate. We propose that the distinctive shape of these tablelands also provides information on paleoclimate.

Spring Creek, an eastward flowing stream in the Republican River basin, appears to have influenced the pattern of loess accumulation in the study area. It is hypothesized that, during late Pleistocene deposition of Peoria Loess, the upstream limit of springs emerging from the High Plains Aquifer (hosted by the Ogallala Group) along Spring Creek remained relatively stable. Downstream from the westward limit of groundwater discharge, dense vegetation along the Spring Creek valley floor became a barrier for saltating sand migrating toward the southeast. Finer particles, dominantly coarse silt, were deposited in the immediate lee of the vegetation belt where they were protected from re-entrainment by saltating sand. Consequently, sand sheets formed north of Spring Creek, while thick loess accumulated south of the creek, forming a high tableland. During most of the Holocene, the limit of groundwater discharge was at least as far upstream as in the late Pleistocene, and dense vegetation also limited dune field migration south of the creek. Thick Bignell Loess accumulated on the north edge of the tablelands because of changes in air flow.

The shape of the loess tablelands indicates long-term persistence of northwesterly winds during late Pleistocene and Holocene eolian sediment transport, and can also potentially provide new information on late Quaternary groundwater levels in the High Plains Aquifer.