Seismic Reflection Profiling Near Middletown, Ohio and Interpretation of Precambrian Deformational Settings

The reprocessing of four vibroseis seismic reflection lines at the AK Steel facility in Middletown, Ohio, provides new insight on the age, deposition, and structural deformation of the pre-Mount Simon sedimentary sequence below Butler and Warren Counties. Processing was focused on the pre-Mt. Simon reflections to reveal gently west-southwest dipping reflectors that make a slight angular unconformity with the overlying Paleozoic sedimentary strata. This pre-Mount Simon sedimentary sequence has been encountered in several wells from western Ohio, Indiana, and northern Kentucky and has been identified as the Middle Run Formation. Examination of the weak and discontinuous seismic character of the reflections from the Middle Run Formation on these AK Steel lines suggests that the Middle Run Basin here is apparently deep and sits above strong, continuous reflectors that are parallel to the overlying reflections from within the Middle Run. However, the gentle dip of the Middle Run exhibited at the AK Steel location contrasts greatly with the Middle Run Formation and deeper rocks to the east, as observed in the Warren County Line ODNR-1-88. There the Middle Run Formation exhibits a moderate apparent east-dip below an angular unconformity with the overlying Mt. Simon Sandstone, and the Middle Run Formation has been erosionally removed at the western end of the Warren County seismic line. The structures exhibited by these seismic lines suggest the likely presence of a fault between the AK Steel lines and the ODNR-1-88 line, with the thicker Middle Run sequence and gently dipping reflections on the AK Steel lines being best explained by preservation in a downthrown block, followed by subsequent erosion. The structural pattern of these lines suggest this fault is a Grenville reverse fault. The age and setting of the Middle Run Formation has been a subject of controversy. However, recent workers have provided new evidence that suggests that the Middle Run Formation from the core DGS 2627 in northeast Warren County is Neoproterozoic in age, deposited after peak deformation from the events of the Ottawan phase of the Grenville orogeny. Because of the sedimentologic similarities of the Middle Run to Grenvillian red lithic arenites elsewhere, the likely age of the Middle Run, the structures seen on the seismic lines, and proximity to the Grenville Front to the east, the Middle Run Formation in southwest Ohio appears to have been deposited in a Grenville foreland basin setting and subsequently deformed by the Grenville Front compressional event