BEDROCK-TOPOGRAPHY AND QUATERNARY DRIFT-THICKNESS MAPS OF THE OXFORD AND COLLEGE CORNER QUADRANGLES, BUTLER AND PREBLE COUNTIES, OHIO

A collaborative mapping project between Miami University and the Ohio Department of Natural Resources (ODNR), Division of Geological Survey has resulted in the generation of a revised bedrock topography and drift thickness map for the Oxford and College Corner 7.5-minute quadrangles, Butler and Preble counties, southwest Ohio. The study area consists of glaciated uplands dissected by filled valleys of the Indian Creek, Four-Mile Creek, and Seven-Mile Creek drainages. Late Wisconsinan-age moraine, outwash, and lacustrine deposits overlie Ordovician-Silurian sedimentary bedrock. As all subsurface geological units in the study area to some extent serve as residential, agricultural, industrial, and municipal aquifers, more accurately delineating the extent of these resources was the primary motivation of the study.

Building off mapping conducted by the ODNR in the 1990s, the study utilized water-well and borehole log data that is available through the ODNR Division of Water Resources and the Ohio Department of Transportation. Other sources of data was derived from the logs of municipal water-supply and monitoring wells drilled by the City of Oxford and Miami University. GPS-assisted outcrop mapping was conducted to provide accurate surface elevations of exposed bedrock across the study area. The study also incorporated horizontal-to-vertical spectral ratio passive seismic analysis to determine depth to bedrock in areas without outcrop or well control. All collected data was entered into a geographic information system that was used to construct a digital contour map of the Quaternary-Paleozoic bedrock topography. Drift thickness was determined by subtracting gridded bedrock-topography elevations from surface elevations derived from a 2.5 ft. resolution digital elevation model for the study area.

Study results generated approximately 1,500 points delineating the Quaternary-Paleozoic contact, more than five times the number used in earlier mapping efforts. The resulting higher resolution contour map of the bedrock topography indicates valley-fill deposits range from ~100 to >250 ft thick while upland areas can reach a maximum of ~50 ft. As the valley-fill deposits serve as on the primary municipal aquifers in the study area, these findings may have important implications for the future development and utilization of local groundwater resources.