AN ASSESSMENT OF SHOCK METAMORPHISM IN BRECCIAS FROM THE HOWELL STRUCTURE, LINCOLN COUNTY, TENNESSEE, USA

The Howell Structure is an anomalous circular area (~2.5 km diameter) of brecciated Middle Ordovician sedimentary rocks centered on Howell, TN (35.230485°, -86.611497°). These rocks are capped by over-thickened, flat-lying Upper Ordovician strata, constraining the deformation event to Middle-Upper Ordovician, a notion that has been disputed. The Howell Structure was first mapped and described as a feature of cryptovolcanic origin, while others have speculated about an impact origin. Definitive evidence of shock metamorphism however, has been lacking. In this study, we have performed XRD analyses of matrix-supported limestone breccias from the Howell Structure in an effort to identify diffraction peak broadening indicative of shock metamorphism.

Breccia samples from multiple surface exposures (by R. G. Stearns, Vanderbilt Univ.) and several 2.7 cm diameter drill cores (courtesy of J. W. Bensko) were utilized in this work. An additional sample of unshocked, optically clear calcite from Taos Co., NM (from the Ohio Univ. collections) was used as a standard of comparison. All specimens were cut into small pieces with a Hillquist SF-8 trim saw and reduced to <25 μm powder in isopropyl alcohol using a mortar and pestle (to avoid the introduction of lattice strain). Samples were analyzed using a Rigaku Multi-flex X-ray diffractometer at 30 kV and 15 mA over a 2θ ranging from 20-120°. Rigaku PDXL software was used to perform a Rietveld peak refinement for each spectrum to identify peak broadening. Full width half maximum (FWHM) values were calculated for all samples over 20-120°. Those from Howell were compared to unshocked calcite values as a means of identifying peak broadening that can occur in the XRD spectra of shocked carbonates. Initial results indicate that FWHM values for at least 3 of the 7 samples deviate significantly from those of unshocked calcite. The degree of peak broadening in those 3 samples is consistent with the magnitude of peak broadening observed in limestones from confirmed terrestrial impact structures (e.g. Kara, Sierra Madera, Steinheim) and in tectonically-deformed limestones. With the lack of apparent evidence of tectonism in the immediate vicinity, shock metamorphism of carbonate breccias at the Howell structure is favored.