ARCHAEOLOGICAL GEOLOGY OF THE PLEISTOCENE-HOLOCENE BOUNDARY IN STREAM VALLEYS OF THE CENTRAL GREAT PLAINS, U.S.A

Despite a long history of geoarchaeological research in the Great Plains, few Paleoindian sites have been documented in the interior of this region. To address the paucity of recorded early sites and direct future archaeological surveys, a detailed study of late Quaternary valley fills and associated soils was recently conducted in Kansas and Nebraska. Nearly 600 radiocarbon ages provide a chronological framework. Emphasis was placed on the spatial pattern of alluvial deposits dating to the Pleistocene-Holocene boundary (ca. 11,000-9,000 B.P.) throughout all levels of drainage hierarchies. Hence, this study provides information that not only explains the absence of Paleoindian sites in some areas, but identifies segments of drainage systems that are likely to contain early cultural deposits. In sum, alluvium dating to ca. 11,000-9,000 B.P. is stored in large valleys (> 4th order) across the Central Plains and in small, dry valleys (draws) on the High Plains; only late Holocene deposits are preserved in the valleys of most small streams (< 5th order). It is also stored in alluvial fans along the western margin of the Central Plains. Slow alluviation in large valleys, draws, and alluvial fans from ca. 11,000 to 10,000 B.P. was accompanied by the development of a thick, organic-rich Mollisol with a cumulative profile. This paleosol was deeply buried by alluvium soon after ca. 10,000 B.P. Accumulation of fine-grained alluvium between ca. 10,000 and 8,000 B.P. was punctuated by multiple episodes of floodplain stability and pedogenesis. Hence, there is high geologic potential for Paleoindian cultural deposits in large valleys, draws, and the westernmost alluvial fans of the Central Plains, but these deposits will generally occur at depths > 2 m. There is low geologic potential, however, for Paleoindian materials in the valleys of most small streams (excluding draws) because alluvium dating to ca. 11,000-9,000 B.P. is no longer stored in them.