Geologic Mapping and Subsurface Investigations of Equus Beds Aquifer Sediments in South-Central Kansas

John Smith, Brian Platt, Richard Sleezer, Greg Ludvigson, and Russell Harlow

We are reporting on a multi-year geologic mapping project of a priority hydrogeologic unit in south-central Kansas known as the "Equus Beds" aquifer. The aquifer is considered the easternmost extension of the High Plains aquifer system in Kansas and are composed of fluvial, alluvial, and eolian sediments traditionally considered Pleistocene in age and younger. 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. Growing municipal and agricultural water demands make the quality and quantity of ground-water supplies major public issues. The primary goal of the subsurface investigations is to improve our basic understanding of the hydrostratigraphic architecture and chronostratigraphy of the Equus Beds. Nineteen 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) were also anticipated due to their reported occurrence in the area, though none have been collected from core. 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 around -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. 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. 

AAPG Search and Discovery Article #90176©AAPG Mid-Continent Meeting, Wichita, Kansas, October 12-15, 2013