PROGRESS REPORT ON GEOPHYSICAL INVESTIGATIONS OF A (PROBABLE) IMPACT CRATER CONCEALED BY GLACIAL SEDIMENTS, LIBERTY TOWNSHIP, SENECA COUNTY, OHIO

A newly acquired 28-channel automated switchbox and cable deployed with AGI’s SuperSting R1IP has significantly increased the efficiency of collecting electrical resistivity data useful in mapping a suspected impact crater site. Bedrock is concealed by ~10 m of glacial sediments that lies mostly under fields inaccessible to geophysicists when muddy or when planted in row crops. Post-harvest expeditions in 2009 and 2010 collected a 680 m long dipole-dipole profile (a = 10 m) crossing the southeast edge of the crater and a 410 m long dipole-dipole profile crossing the south edge. Results support our hypothesis that 100 m of Lockport Dolomite (Silurian), documented in nearby water wells and oil exploration boreholes is missing not only from the crater proper, but also from up to 100 m outside the crater. We think the top of bedrock between the crater edge and the surrounding dolomite consists of the Rochester Formation and Cabot Head Formation, units identified directly under the Lockport Group in a continuously cored borehole (Wickstrom et al., 1985) 6 km to the east. These units, which include layers of green plastic shale, were uplifted by the event that formed this crater. Shale collected in cuttings from a 30 m deep borehole drilled northwest of the crater provides direct evidence of at least 70 m uplift. Although 7 of the 8 electrical resistivity surveys conducted in search of the crater edge show abrupt lateral changes consistent with a circular feature 980 m in diameter, these lateral resistivity contrasts are not the same. Electrical resistivities of crater fill range from under 10 Ω-m to over 100 Ω-m, and electrical resistivities of the country rock adjacent to the crater range from 40 Ω-m to over 1000 Ω-m. Wide variation in the properties of crater fill, coupled with the likelihood that Paleozoic shale rims sections of the crater, explains why most prior geophysical measurements were unable to tightly constrain the crater edge.