STRATIGRAPHY, GEOCHRONOLOGY, AND GEOARCHAEOLOGY OF LATE QUATERNARY ALLUVIAL FILLS IN DRAWS ON THE HIGH PLAINS OF WESTERN KANSAS

Thick deposits of alluvium with multiple buried soils dating to the Pleistocene-Holocene transition are stored in dry valleys of low-order intermittent streams, or draws, in areas of the High Plains with a thick loess mantle. The loess is a major source of silty alluvium, accounting for the large volume of sediment in the draws. The draws lie high in drainage networks and the alluvial fills are inset into late Pleistocene loess or older Neogene deposits. Waves of entrenchment post-dating the Pleistocene-Holocene transition did not extend into these draws until the late Holocene, generally after 2000 ¹⁴C yr B.P. Consequently, there has been insufficient time for complete removal of the vast quantity of terminal Pleistocene and early Holocene alluvium stored in the draws. The soil-stratigraphic record preserved in the draws indicates aggradation punctuated by episodes of landscape stability and soil development beginning as early as ca. 11,600 ¹⁴C yr B.P. and continuing as late as ca. 8200 ¹⁴C yr B.P. However, most of the radiocarbon ages determined on organic carbon from buried soils in the alluvial fills range between ca. 11,000 and 9000 ¹⁴C yr B.P. Based on the analysis of gastropods, phytoliths, and δ¹³C values of soil organic matter, a general trend of warming and drying that characterized the climate of the High Plains at the end of the Pleistocene continued and intensified during the early Holocene (ca. 10,000-8000 ¹⁴C yr B.P.). From an archaeological perspective, documenting the stratigraphic and paleoenvironmental record preserved in the draws is important for interpreting the early record of human occupation in the region. Many of the buried soils in the alluvial fills of draws represent former Paleoindian landscapes that may harbor cultural deposits. This is underscored by the recent discovery of stratified, Early Paleoindian archaeological deposits at the Kanorado locality in upper Middle Beaver Creek, a draw in northwestern Kansas.