A GEOPHYSICAL SEARCH FOR THE FLOOR OF A CONCEALED IMPACT CRATER IN NORTHWEST OHIO

We are using geophysical methods to map a sediment-filled gap in the carbonate bedrock under Sections 17, 18, 19 and 20 of Liberty Township, Seneca County, Ohio. No evidence of the 100 m thick Lockport dolomite documented in oil well logs north and south of this anomaly has been detected inside an approximately circular feature about 980 meters in diameter, concealed by about 10 m of drift. Our working hypothesis is that we are studying an impact crater. Electrical resistivity measurements used to map the crater suggest that the edge of the carbonate does not everywhere mark the edge of this hole. A well-drilling attempt outside the northwest edge encountered only a thin layer of Lockport dolomite before penetrating shale that should lie at least 80 meters deeper. Rocks of the crater lip are uplifted. A Schlumberger sounding east of the crater’s center discovered 8 Ohm-m material extending from 57 m under the surface to at least 200 m. Trytten (1995) reports crater fill 600 m to the northwest in the 17 – 20 Ohm-m range. The P-wave velocity of the refractor under drift east of the crater center is about 2100 m/s, significantly less than the 2800 m/sec measured in the northwest quadrant. Cuttings from an 80 m deep borehole drilled near this seismic line in an unsuccessful search for groundwater are calcite-cemented silt, pinkish gray when dry and dark reddish gray when wet. Another well drilled into the crater encountered salt water. We speculate that brine from the oil-bearing Trenton limestone just over 400 m down migrated into the crater, forming a lake that was mostly salt water except for the northwest quadrant. We have not encountered electrical resistivity values this low, under 10 Ohm-meters, elsewhere in northwest Ohio except locations involving landfill leachate. The 2100 m/s refractor shows evidence of relief. Glaciers that swept limestone from the uplifted crater rim may have carved grooves into the lacustrine sediment’s surface.