RE-ASSESSMENT OF AN IMPACT ORIGIN OF THE JEPTHA KNOB STRUCTURE, SHELBY COUNTY, KENTUCKY

The Jeptha Knob structure near Shelbyville, Kentucky (38.177401°, -85.111613°) is a proposed, but not confirmed, meteor impact site. Previous geophysical and geochemical studies have provided only circumstantial support for an impact origin for this structural disturbance. Evidence for shock metamorphism has yet to be detected in the deformed Ordovician limestones and carbonate-rich shales that comprise the structure. A systematic search of two drill cores from the uplifted center has not revealed macroscale evidence of shock (i.e. shatter cones), however, cores are extensively faulted with monomict and polymict breccias, some of which appear to have been injected into the local strata. Large, internally micro-faulted, meters-scale megablocks (locally bound by slickensides) have been observed, similar to those present in other confirmed complex impact craters. Samples from one of the limestone-dominated cores is being collected and analyzed using X-ray diffraction to determine whether or not peak broadening occurs in calcite diffraction patterns as a result of shock metamorphism, similar to that observed at the Ries crater in Germany. If peak broadening is detected, this may lead to Jeptha Knob’s confirmation as the second impact structure in the state of Kentucky and prove a focus for some of the Ordovician seismites present in the central part of the state.