GEOLOGIC MAPPING AND SUBSURFACE INVESTIGATIONS OF EQUUS BEDS AQUIFER SEDIMENTS IN SOUTH-CENTRAL KANSAS

The Kansas Geological Survey (KGS) is carrying out a multi-year geologic mapping project focusing on a priority hydrogeologic unit in south-central Kansas known as the “Equus Beds” aquifer. The Equus Beds are considered the easternmost extension of the High Plains aquifer system in Kansas and are composed of currently undifferentiated fluvial, alluvial, and eolian sediments traditionally considered Pleistocene in age and younger. One of the major outstanding questions concerns the status of calcareous silts, sands, and gravels mapped along the northeastern margin of the aquifer and considered Pliocene in age based on the presence of taxa known from the Miocene–Pliocene Hemphillian North American Land-Mammal Age (10.3–4.9 Ma). This aquifer is an important source of potable water for the state’s largest city, Wichita, as well as an irrigation resource for central Kansas and a source of sand and gravel construction aggregate. Rapidly growing municipal and agricultural water demands make the quality and quantity of ground-water supplies major public issues. The primary goal of geologic mapping and subsurface investigations is to improve our basic understanding of the hydrostratigraphic architecture and chronostratigraphy of the Equus Beds. Nineteen continuous cores from various sites within the area of the aquifer are providing previously unavailable stratigraphic and geochemical information from strata below the water table. Volcanic ash deposits (2.1–0.6 Ma) from the Yellowstone supervolcano were also anticipated due to their reported occurrence in the area of the Equus Beds. Lithologic logs consistently show ~1-8 m of Quaternary loess at the top of each core. This is underlain by as much as 50 m of predominantly fine- to coarse-grained sand and thinly-bedded calcareous loam paleosols. The organic δ¹³C values of bulk sediments from below the loess 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. So far, Pleistocene ashes have not been encountered in any of the cores collected. The lack of volcanic ash deposits coupled with isotopic values implying C₃ paleoflora suggests that the Equus Bed deposits are earliest Pleistocene or much older than previously thought.