REVISITING THE STRUCTURAL GEOLOGY OF NORTHWESTERN OHIO

Located at the western edge of the Appalachian Basin, Ohio is covered by low-relief Paleozoic sediments without significant folding and faulting. The Bowling Green and Outlet Faults in northwestern Ohio are the only exposed and defined structural anomalies of significance in the state. Numerous studies indicate that thick glacial deposits cover the region and act as a barrier for greater certainty and a more accurate explanation of the tectono-structural geometry and history of the region. Recent geological study of northwestern Ohio, when accompanied by review of high-quality space images, reveals numerous small-scale, low-relief anticlines in the vicinity and east of the Bowling Green Fault. These folds are oriented in a northeast–southwest direction, and alignment of the folds’ axes indicates that they are created by northwest-directed compressive stress. Structural analysis of these anticlines shows that their detachment depth is 400–450 feet (120–140 m) below ground level at the upper Ordovician Cincinnati Group shales. The Middle Ordovician Trenton Limestone, which lies beneath the decollement zone of the anticlines, is not included in the shallow folding process. 

The reduction in the length of the folded sequence requires a low-angle thrust sheet at the folds’ detachment level. This thrust sheet is created by Appalachian compressive stress in the Ordovician shale that trends northwest and upslope toward the Findley Arch axis. The thrust sheet covers the area east of the Bowling Green Fault and northeast of the Outlet Fault, and may extend eastward to the core of the basin. A thinner sedimentary sequence above the Findley Arch facilitates creation of these folds; in comparison to thicker sedimentary sequences eastward, where development of such structural features is limited. The northeast-trending axis of the anticlines indicates a northwest movement of the sedimentary sequence above the decollement zone. Thus the Outlet Fault must be a sinistral strike-slip fault, while the north–south-oriented Bowling Green Fault appears to be a sinistral transpression fault, as strata are uplifted east of the fault.