PRELIMINARY RESULTS OF LITHO- AND CHRONOSTRATIGRAPHIC INVESTIGATIONS OF THE MCPHERSON CHANNEL DEPOSITS IN SOUTH CENTRAL KANSAS

The McPherson Channel is a paleovalley underlying south central Kansas and contains undifferentiated alluvial, eolian, and fluvial sediments speculated to be Pleistocene and younger in age. These deposits comprise the primary water-bearing units of the Equus Beds Aquifer and are the easternmost extension of the High Plains Aquifer in Kansas. Equus Beds groundwater is used for agricultural irrigation and as the primary potable water source for Wichita. The Kansas Geological Survey has retrieved twenty (20) long and continuous cores along three latitudinal transects of the McPherson Channel in an effort 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. 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 interbedded calcareous loam paleosols and 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 below the paleosols range from -26 to -25‰ VPDB and suggest a strongly C₃-dominated paleoflora while the overlying loess succession shows progressively heavier δ¹³C values (-15‰ VPDB or greater) and the transition to C₄ paleofloras. Pleistocene and latest Pliocene volcanic ash deposits (2.1–0.6 Ma) from primarily the Yellowstone volcanic center were anticipated due to their reported surface occurrences in McPherson Channel outcrops. However, these ashes have not been encountered in any of the cores retrieved. Their absence, along with isotopic values implying C₃ paleoflora, suggests that McPherson Channel sediments may be Pliocene in age. Recent efforts to acquire radiometric U/Pb dates of pristine volcanogenic zircons in paleosols via LA-ICP-MS are a promising method for constraining the geochronology of the McPherson Channel.