REVISED BEDROCK TOPOGRAPHIC MAP FOR THE OXFORD AND COLLEGE CORNER QUADRANGLES, BUTLER AND PREBLE COUNTIES, OHIO

A collaborative bedrock/surficial deposit 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 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 and range in thickness from 0 to >200 ft. 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 on mapping conducted by ODNR in the 1990s, the present study utilized water-well and borehole log data available on-line through the ODNR Division of Water Resources and the Ohio Department of Transportation. Additional Quaternary thickness and depth-to-bedrock data were derived from well logs for municipal water-supply and monitoring wells drilled by the City of Oxford and Miami University. The study also incorporated field mapping of exposures of the Quaternary-Paleozoic contact and horizontal-to-vertical spectral ratio (HVSR) passive seismic analysis. HVSR data were calibrated at sites adjacent to wells with bedrock penetrations.

The study generated elevations for > 600 individual Quaternary-Paleozoic contact points, more than twice the number used in earlier mapping efforts. The resulting higher resolution contour map of the bedrock topography indicates filled valley deposits in some parts of the study area are narrower but deeper than previously mapped, and thus likely contain steeper basal contacts with underlying Paleozoic rocks. As valley-fill deposits serve as the primary municipal aquifers in the study area, these findings may have important implications for the future development and utilization of local groundwater resources.