SEISMIC PROFILING NEAR MIDDLETOWN, OHIO: AN INTERPRETATION OF PRE-MT. SIMON DEFORMATIONAL HISTORY IN THE EASTERN MIDCONTINENT

The reprocessing of four seismic reflection lines at the AK Steel facility in Middletown, Ohio, provides new insight on the age, deposition, and structural deformation of the Middle Run Formation in southwest Ohio. A new residual statics solution improved the resolution and coherency of pre-Mt. Simon reflectors. Processing focused on pre-Mt. Simon reflections to reveal gently west-southwest dipping reflectors that create an angular unconformity with the overlying Paleozoic strata. Examination of the weak and discontinuous seismic character of the reflections from the Middle Run Formation reveals that the basin here is deep (approximately 670m to 1,130m) and sits above strong, continuous reflectors that are parallel to overlying reflections within the Middle Run. The thicker Middle Run sequence and gentle antithetically dipping reflections in the AK Steel lines (Butler County) contrast the structure observed on a seismic line to the east in Warren County. In the seismic line ODNR-1-88, the Middle Run exhibits a moderate apparent east-dip below an angular unconformity with the Mt. Simon Sandstone, and has been removed by erosion at its western margin. The variable thickness of the Middle Run and the opposing orientations of pre-Mt. Simon reflectors at these two localities suggest the presence of a Grenville reverse fault between them. Subsurface structure, sedimentologic similarities of the Middle Run to Grenvillian red lithic arenites elsewhere, the likely Neoproterozoic age of the Middle Run, and the proximity to the Grenville Front imply that the Middle Run Formation in southwest Ohio was deposited in a Grenville foreland basin setting. Deformation from the Ottawan Phase of the Grenville Orogeny uplifted the source of the Middle Run sediments that were later deformed by the Grenville Front compressional event. This was then followed by erosion and the subsequent deposition of the Phanerozoic sedimentary cover.