AN ASSESSMENT OF SHOCK METAMORPHISM IN THE JEPTHA KNOB STRUCTURE: A SUSPECTED COMPLEX CRATER IN NORTH-CENTRAL KENTUCKY

Impact craters in carbonate rock account for ~30% of all impacts on Earth yet little research has been conducted to study the effects of shock metamorphism on such targets. Unlike quartz, shock metamorphism in calcite does not produce planar deformation features but results in an increase in the density of mechanical twins, a feature that also results from non-impact related processes. However, under low to moderate shock pressures, calcite develops mechanical twins while dislocations dominate at higher pressures. With increasing shock pressures, x-ray diffraction peaks in calcite and dolomite broaden and peak intensities decrease as their crystal structures are affected by the shock pressures. Petrographic observations and XRD analyses of carbonate minerals have the potential to identify shock metamorphism in suspected structures and to constrain peak pressures in confirmed terrestrial impact sites all over the world such as the proposed impact site in north-central Kentucky, Jeptha Knob.

The Jeptha Knob structure is an approximately 4.26 km diameter circular structure situated just east of Shelbyville, Kentucky. Although Jeptha Knob has been proposed to represent the eroded remains of an impact crater, there is insufficient evidence to support this. This study examines the carbonate rocks at the Jeptha Knob structure for evidence of shock metamorphism and, if shock metamorphosed, to determine peak shock pressures.

A total of 65 samples were collected from two different drill cores, one from the deformed and uplifted interior of the structure (JK78-3) and another from an analog (non-deformed) section 21 km away (C-204). Analysis of the JK78-3 core, hand samples, and thin sections has shown that there is ~500ft of vertical displacement between of the same geologic units in each of the two cores. Carbonates in JK78-3 have been extensively fractured, faulted, and brecciated in a fashion similar to target rocks exposed in the central peaks of confirmed complex impact craters; however shatter cones were not identified. Ongoing XRD analysis of diffraction powders of analogous geologic units from both cores is being conducted in an effort to identify shocked dolomite or calcite in Jeptha Knob structure.