BRECCIATION AND DEFORMATION IN EXPOSURES AND CORE AT JEPTHA KNOB, KENTUCKY: A POSSIBLE IMPACT STRUCTURE

Jeptha Knob is a circular area of intensely deformed Ordovician strata, approximately 5 km in diameter. Deformed strata are overlain by undeformed carbonates and shales of Early Silurian age. These stratigraphic relationships constrain the time of deformation to a relatively narrow interval near the Ordovician-Silurian boundary.

Geologic mapping by previous investigators in the 1920's and 1970's shows folded and block-faulted Upper Ordovician strata exposed around the periphery of the structure, and exposures of breccia in the central part and around periphery of the structure. Exposures of Middle Ordovician megabreccia blocks in the central part of the structure suggest uplift of approximately 250 m. Core observations reveal that deformation decreases with depth. These cursory observations suggest a possible impact origin for Jeptha Knob, but diagnostic evidence of impact remains to be documented.

Based on field relations, there are two broad categories of breccia associated with the Jeptha Knob structure. The first type of breccia occurs as massive or irregularly-shaped bodies found within the highly deformed Middle and Upper Ordovician rocks of the structure itself. The second type of breccia occurs as a stratiform deposit found lying upon the central area of the structure above the disturbed Ordovician rocks and below the undisturbed Silurian rocks. Such contrasting field relations suggest genetic differences between the two types of breccia. The first type of breccia originally was identified as fault gouge. The texture also is comparable to injection breccias that have been reported from accepted meteorite impact structures. The second type of breccia originally was interpreted as reworked sedimentary clasts. This type of breccia may compare favorably with resurge or crater-fill breccias reported from other impact structures. Ongoing study of the texture, composition, and field relations of these breccias is aimed at testing these and other genetic hypotheses.