GEOPHYSICAL METHODS FOR MAPPING THE EDGE OF A SHALLOWLY BURIED IMPACT CRATER

Although a circular residual gravity low over 1 km in diameter and 1.6 mGal in amplitude clearly shows that a crater in the dolomitic bedrock is concealed under glacial till in northern Ohio, even densely spaced (30 m interval) gravity measurements cannot delineate the lateral contact between crater fill and the deformed target rocks. Uplift of bedrock under the crater rim and its subsequent removal (probably by glaciers) are responsible for extending that gravity low beyond the crater rim, similar to the pattern at Barringer crater reported by Regan and Hinze (JGR, 80(5), pp. 776 – 788) in 1975. We used Schlumberger resistivity soundings to measure till thickness and bedrock resistivity along a profile extending from normal dolomitic bedrock subcrop to inside the crater. Although soundings outside the crater and in the transition zone generate A- or H-type curves, those located further from the crater exhibit greater contrasts between till and subcrop resistivities than soundings conducted in the transition zone or inside the crater. We think that uplift due to the enormous explosion which excavated this crater has elevated a shale unit approximately 100 meters and that this shale is the subcrop in the transition zone. This model, consistent with a well drilled into uplifted rocks of the rim, also explains perplexing results of seismic refraction profiles shot in attempts to delineate the crater rim. Electrical resistivity models consistent with our soundings will be used to model apparent conductivities measured with the Geonics EM-34, an instrument system with which we can cover several profile kilometers per day when it is mounted on sleds and pulled across snow-covered fields.