BRECCIA OBSERVATIONS WITHIN THE JEPTHA KNOB STRUCTURE, KENTUCKY, REVEAL CHARACTERISTICS SIMILAR TO STRUCTURES HAVING AN ORIGIN BY IMPACT

Previous reports of polymict breccias within the Jeptha Knob structure, a postulated impact crater in north-central Kentucky, suggest similarities with polymict breccias at the Sierra Madera impact crater, Texas. At both structures, polymict breccias consist of fragments that have moved both upward and downward. Polymict breccias at Sierra Madera are not along faults, but are in the vicinity of the central uplift and form tabular sheets that cut the country rock at steep angles and contain shock-metamorphic features. Polymict breccias at Jeptha Knob are associated with faults outside the vicinity of the central uplift with some having the appearance, or geometry, of tabular sheets.

Polymict breccias within the Lockne impact structure in central Sweden are sheet-like and cut the country rock as dikes and propagate sideways as sills between weaker bedding planes. The Lockne breccias contain shock-metamorphic features and were injected simultaneously into the country rock during the cratering process. Breccias observed at Jeptha Knob also appear to emanate from a feeder dike, or fault, that cut through the limestone succession and propagate normal to bedding planes as sills.

Breccias observed at Jeptha Knob have characteristics similar to the Sierra Madera and Lockne breccias and therefore may also have formed as a result of impact. In the rock formations that compose the Jeptha Knob structure similar breccias are not found outside the limits of this structure.