NEW RESULTS FROM THE KENTLAND IMPACT STRUCTURE, INDIANA: SYNTHESIS AND INTERPRETATION

The Kentland impact structure (40°45’N, 87°24’W) is ≤13 km in diameter and consists of highly deformed, deeply eroded, and weakly shocked Paleozoic sedimentary target rocks. We studied quarry exposures in the central uplift and also used cores from around its periphery. A <97 m.y. old impact age had previously been determined using paleomagnetism. To better constrain impact age, we studied fission tracks from apatite grains in the St. Peter Sandstone. To constrain shock pressures we studied grains of sphalerite (ZnS), galena (PbS), and quartz (SiO₂) picked from polymict breccia dikes that cut steeply dipping beds in the central uplift. We studied the effects of shock-metamorphism on the crystal lattice of the sulfide grains using X-ray powder diffraction (XRD). Sphalerite and galena unit cell lengths are unchanged within XRD measurement uncertainties, indicating no mineral polymorphism. Sphalerite samples showed strong systematic increases in % d-value variance, whereas galena grains showed no such changes. Microscopically, the quartz grains extracted showed PFs and possibly incipient PDFs that intersect and create "feather structures". Apatite fission tracks from the central uplift and periphery were thermally reset during a Jurassic (185±13 m.y. ago) regional cooling event and do not constrain the age of the impact.