INITIAL XRD ANALYSES OF SILURIAN CARBONATES FROM THE CENTRAL PEAK OF THE KENTLAND IMPACT STRUCTURE, NEWTON COUNTY, INDIANA, USA

The Kentland impact structure represents the eroded remnants of an ~13km complex crater located in northwestern Indiana (N 40° 45’ W 87° 24’). The only surface exposures present are the result of quarrying of uplifted (~600 m) Paleozoic strata in the central uplift located between Kentland and Goodland, IN. The central uplift is comprised of large megablocks of carbonates, sandstones, and shales. Megablocks hold evidence for shock metamorphism in the form of shatter cones and coesite. Shatter cones are produced by lower shock pressures (~2-10 GPa); coesite however, is indicative of much higher peak pressures (>30 GPa). The discovery of coesite has not been replicated calling into question the maximum pressures experienced during impact.

This study seeks to assess peak shock pressures across the central peak using XRD analyses of exposed Paleozoic dolostones. In order to do so, a preliminary sample set of Silurian dolostones was collected from Kentland in the northeastern central uplift and were ground into <25 μm powders using ceramic mortar and pestle for XRD analysis. Specimens were either shatter-coned target rocks or breccias. XRD spectra were then processed using the Rietveld peak refinement technique and full width half maximum (FWHM) values were calculated in an effort to identify peak broadening related to shock metamorphism. Initial results show that all shatter cone and breccia samples display peak broadening compared to unshocked dolomite. Kentland FWHM curves are consistent with those of experimentally shocked dolomite that have experienced peak shock pressures ranging from 4.6 to 17 GPa, pressures that are agreeable with those of shatter cone formation. These pressures are not high enough to produce coesite, suggesting that the remaining portion of the Kentland central peak experienced relatively low shock pressures. Future field and laboratory work in new quarry exposures will allow us to better constrain the full range of peak shock pressures across the entire central uplift.