Xrd Analyses of Silurian Dolostones from the Central Uplift of the Kentland

Introduction: The Kentland impact structure located in northwestern Indiana (N 40° 45’ W 87° 24’) represents the remains of an ~13km diameter complex crater. Ordovician-Devonian strata have been uplifted ~600 m to the surface following impact and subsequently exposed by local quarrying operations. Large megablocks of carbonates, sandstones, and shales have been uplifted along thrust faults to comprise the central uplift [1]. Some megablocks provide evidence for shock metamorphism in the form of shatter cones [2-4], impact breccias [3,5], shocked quartz [3,4], and coesite [6]. Shatter cones are indicative of lower shock pressures (~3-10 GPa) [7]; however, coesite formation requires higher pressure conditions (>30 GPa) [8]. This may demonstrate the potential degree of hetereogeneity in the distribution of peak shock pressures across the central uplift or highlight the role in which target rock composition affects the propagation of shock waves throughout a target. This research project seeks to quantitatively analyze and constrain peak shock pressures across the central uplift utilizing X-ray diffraction (XRD) spectra from Paleozoic dolostones collected from the Kentland central uplift. In this study, we rely on more recent experimental [9-10] and empirical investigations [1012] of shock pressures in carbonate sedimentary targets and the techniques used therein to estimate shock pressures across the central uplift. This work highlights a preliminary study performed on shatter-coned, Middle Silurian dolostones collected from the eastern half of the Kentland central uplift. Methods: Seven specimens were hand collected from a single exposed megablock approximately 800 m from the geographic center of the uplift (Figure 1). Specimens were ground into <25 micron powders using mortar and pestle for XRD analysis to avoid introducing any artificial lattice strain. Resulting XRD spectra were processed using the Rietveld peak refinement technique [13] and full width half maximum (FWHM) values were calculated. FWHM values represent the level of peak broadening compared to unshocked or undeformed standards. Peak broadening has been shown to increase with increasing shock pressures in experimentally-shocked samples [9,11]. Kentland FWHM curves were thencompared to experimentally-shocked dolomite [11] and that from a known impact analog, Sierra Madera [12] to estimate peak shock pressures. Sierra Madera specimens all contain shatter cones [12] Kentland samples were compared to unshocked dolomite standards from the Mascot Dolomite in Gordonsville, TN, the Beck Springs Dolomite in Inyo County, CA, and the Peebles