THE JEPTHA KNOB CRYPTOEXPLOSION STRUCTURE, SHELBY COUNTY, KENTUCKY: HIGHLIGHTS OF GEOLOGICAL INVESTIGATIONS, 1887-2002

The Jeptha Knob cryptoexplosion structure is located in the western part of Kentucky’s Bluegrass physiographic region and 50 miles west of the crest of the Cincinnati Arch. The rocks in this region are Ordovician in age and dip westward between 16 and 22 feet (ft) per mile. The Jeptha Knob structure towers 300 ft above the general elevation of the surrounding area. The structure is about 16,000 ft in diameter and consists of a central area that is 6,300 ft in diameter, composed of uplifted and brecciated Ordovician age strata, surrounded by a belt of concentric faults that are divided into segments by radial faults. Many fault blocks are substantially uplifted. Nearly horizontal dolostone and shale of Silurian age locally overlie the faulted and brecciated strata.

To date, the origin of Jeptha Knob has not been determined. Shatter cones and coesite, considered to be definitive criteria for determining origin by impact, have not been found. Geophysical studies indicate that there is no coincident uplift of the basement, and there are no temporal relationships of Jeptha Knob with any obvious structural trends. Nevertheless, the finding of anomalously high concentrations of iridium in breccias may suggest that Jeptha Knob is, in fact, an impact structure.