AN ASSESSMENT OF PEAK BROADENING IN XRD SPECTRA FROM LIMESTONES OF THE JEPTHA KNOB STRUCTURE, SHELBY COUNTY, KY

For nearly 90 years, debate over the Jeptha Knob structure, Shelby County, Kentucky (38.18°, -85.11°), USA has focused on its origin by either an impact of an asteroid or comet or by a cryptoexplosive mechanism. Although Jeptha Knob (morphologically a monadnock) bears most of the characteristics of an eroded central peak of a large complex impact crater, evidence of shock metamorphism (e.g. shatter cones, shocked silicates) has remained elusive in Ordovician-aged, carbonate-dominated host rocks. Our study is an attempt to assess whether or not limestones from Jeptha Knob have experienced pressures indicative of an impact event (>2 GPa).

Samples from Middle-Upper Ordovician limestones were collected from surface exposures at varying distances (0.77-11.60 km) from the center of the structure. Most specimens were taken from pre-deformation event strata (autochthonous and para-autochthonous sedimentary rocks); however, one specimen was collected from a Middle Silurian limestone breccia superposed on Jeptha Knob (post-event). Samples were cleaned, and ground by ceramic mortar and pestle (to minimize potential lattice strain) into <25 μm powders for XRD analyses. Peak matching was used to identify primary mineral phases and Rietveld peak refinement analyses were utilized to identify peak broadening, a phenomenon which has been documented in experimentally and naturally-shocked carbonates.

Preliminary results indicate that 10 of 14 potential target rock specimens show evidence of peak broadening compared to unshocked calcite. The Middle Silurian breccia capping Jeptha Knob also displays relatively high FWHM values. There was no evidence of a systematic increase in peak broadening relative to the distance from the center of the structure for the samples collected in this study. FWHM values are consistent with those from impactites from terrestrial impact structures, but are also consistent with values from tectonically-deformed calcite specimens. Circumstantial evidence of impact (uplifted center, extensive fracturing/faulting with proximity to knob, disturbed bedding, etc.) and the lack of any localized tectonic mechanisms during the Late Ordovician-Middle Silurian in this region supports a shock metamorphic interpretation for peak broadening in limestones from the Jeptha Knob structure.