LIMESTONE BENEATH THE MIDDLE RUN FORMATION IN THE GRENVILLE FORELAND OF SW OHIO AND EASTERN INDIANA (Invited Presentation)

Borehole and seismic reflection data in southwest Ohio and eastern Indiana suggest that limestone may be regionally present beneath the Middle Run Formation. Three boreholes at two locations in eastern Indiana (Binegar #1 and #2), and western Ohio (Mattison #1) have encountered limestone beneath the basal transgressive Mt. Simon Sandstone. In Jay County, Indiana, two crossing seismic lines collected by Wright State demonstrate that there the limestone together with an overlying lithic arenite like that of the Middle Run Formation, are subhorizontal and parallel to the overlying Paleozoic strata. In Clark County, Ohio, the borehole encountered several hundred feet of dark limestone beneath and associated with lithic arenite like the Middle Run Formation (Wasson, 1932). However, Wright State seismic lines at that locality so far do not adequately image pre-Mt. Simon strata perhaps because they are steeply dipping in proximity to the Grenville Front.

Seismic reflection data in SW Ohio, including the ODNR-1-88 seismic line at the type locality of the Middle Run Formation (Shrake, 1991), as well as a Wright State seismic line to the east and a grid of lines at the AK Steel facility to the west, reveal Grenville Foreland reverse faulting and increasing imbrication nearing the Grenville Front. A significant reverse fault likely is present in the gap between the AK Steel lines and the ODNR-1-88 line due to the large offset of the pre-Middle Run strong reflectors of these lines. On the Wright State seismic line east of ODNR-1-88 these strongly reflective layers appear imbricated by a series of Grenville reverse faults which bring the pre-Middle Run layered rocks close to subcrop at the basal Mt. Simon unconformity. Hauser (2000) previously suggested that the strongly reflective layers beneath the Middle Run as seen on the ODNR-1-88 seismic line may be other than mafic flows/sills, as has commonly been interpreted, due to the lack of distinctive potential field anomalies associated with the dipping reflectors. Consequently, the thick dark limestone encountered in Clark County farther north along the strike of the Grenville Front may sample these pre-Middle Run strongly reflective layers. The implication is that a poorly understood limestone may underlie the Middle Run Formation throughout the region.