GEOPHYSICAL INVESTIGATION OF BOUNDARIES AND UPLIFTED RIM STRATA OF A POSSIBLE IMPACT CRATER IN NORTHWEST OHIO

We are using geophysical methods to map a circular, ~1 km diameter sediment-filled hole in the Lockport Dolomite of NW Ohio. The fill, encountered as a calcite-cemented silt in a borehole near the western edge, has substantially lower resistivity and P-wave velocity than the Lockport. We think this feature, concealed by about 10 m of till, is an impact crater. Previous electrical resistivity measurements defined the crater edge in the SE, SW and NW with few constraints in the NE. A Tromino 3-component seismometer was used to analyze the Horizontal to Vertical Spectral Ratio (HVSR) of seismic noise at 45 locations in the NE quadrant. Dipole-dipole resistivity transects were also used to locate the NE edge and model resistivity near the center of the anomaly. Results agree with previously proposed boundaries based on a circular hole. A water well drilled on the NW rim encountered shale, consistent with the underlying Rochester and Dayton Formations, 70-80m above their expected elevation based on nearby oil well logs suggesting an uplifted rim. Recent dipole-dipole transects across boundaries reveal a thin (~20 m) dolomite layer near the rim underlain by less resistive material, substantially thinner than the ~100 m of carbonate documented by oil well logs. Moderate resistivity observed beneath the dolomite may represent shale from the Rochester Formation uplifted well above its expected elevation. One dipole-dipole transect across the NE boundary reveals dolomite thinning towards the rim, suggesting maximum uplift near the edge. Evidence for an uplifted rim provides additional support for an impact origin. HVSR results show clear H/V peaks around 11 Hz outside the anomaly, consistent with shallow bedrock. No clear H/V peaks were seen in measurements taken within the anomaly, which may suggest the loose fill is hundreds of meters thick. The distinct absence of an ~11 Hz peak within the anomaly makes HVSR an effective method for mapping its boundaries.