CHRONOSTRATIGRAPHIC INVESTIGATIONS OF THE MCPHERSON CHANNEL DEPOSITS IN SOUTH CENTRAL KANSAS: PRELIMINARY INTERPRETATIONS FROM PARTICLE SIZE AND STABLE ISOTOPIC ANALYSES

The Kansas Geological Survey (KGS) is conducting an ongoing project describing and mapping sediments within the McPherson Channel, a paleovalley underlying Harvey and McPherson Counties in south central Kansas. Channel deposits are considered the easternmost extension of the High Plains Aquifer system in Kansas and a source for agricultural waters in these counties and a potable water source for the largest city in Kansas, Wichita. The deposits are composed of undifferentiated alluvial, eolian, and fluvial sediments speculated to be Pleistocene and younger in age. One of the goals of geologic mapping and subsurface geochemical analyses is to improve our basic understanding of the hydrostratigraphic architecture and chronostratigraphy of these deposits and how they relate depositionally and geochronologically to the rest of the High Plains aquifer system. In the 2012-2013 seasons, 5 additional 30+ meter cores were collected from southern portion of the paleochannel for a total of 20 short and long continuous cores. These cores are providing previously unknown stratigraphic and geochemical data, particularly from the strata below the water table. Cored intervals are measured, described and then sampled for particle size and stable isotopic analyses. Volcanic ash deposits (2.1–0.6 Ma) from Yellowstone hotspot volcanism and Neogene volcanism in the American Southwest were anticipated due to their reported occurrence in the area. Lithologic logs consistently show ~1-8 m of Quaternary loess at the top of each core. This is underlain by up to 6 m of thinly-bedded calcareous loam paleosols and thinly bedded fine-grained sands. Paleosols give way at depth to thick beds of medium- to coarse-grained sand and gravel. The organic δ¹³C values of bulk sediments from below the paleosols range from -26 to -25‰ VPDB and suggest a predominantly C₃-dominated paleoflora while the overlying loess succession shows progressively heavier δ¹³C values and the transition to C₄ paleofloras. Pleistocene ashes have not been encountered and isotopic values implying C₃ paleoflora suggests that McPherson Channel sediments may be much older than previously thought. Acquiring radiometric U/Pb dates of volcanogenic zircons in paleosols via LA-ICP-MS is a promising method for constraining the geochronology of the McPherson Channel.