T239 Impact Stratigraphy (Posters)

X-RAY DIFFRACTION EVIDENCE FOR SHOCKED QUARTZ IN AN UPPER EOCENE SAND DEPOSIT, WARREN COUNTY, GEORGIA, U.S.A

Paper number: T239-12
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Quartz grains collected from a transgressive sand unit atop the sequence boundary at the kaolin-rich Huber Formation in the Georgia Coastal Plain were examined by X-ray powder diffraction (XRD) for evidence of crystal defect line broadening. The stratigraphic position of this sand is approximately correlative with the timing of the Chesapeake Bay impact, and thus the sand is a logical place to look for impact debris, especially since 35 Ma tektites occur in south-central GA. Sample comparisons were made between three groups of fine to medium-sand quartz separates: (1) quartz grains that display single or multiple sets of linear features, which are possible planar deformation features (PDF’s), (2) quartz grains that lack these linear features, and (3) quartz grains that were randomly selected from the sand unit. We also analyzed alluvial quartz grains from crystalline rocks of the Georgia Piedmont and quartz grains from the Crow Creek Member of the Upper Cretaceous Pierre Shale (Nebraska) that display obvious sets of PDF’s.

All samples were ground in ethyl alcohol and mounted on to zero background quartz plates. XRD peaks from the alpha-quartz (100), (-101), (2-10), (-102), (-211), (2-20), (-212) and (121) reflections were best fit using a Pearson-7 profile shape function. Instrument broadening effects were normalized using the NIST reference material SRM660 LaB6, which allows for assessment of line broadening due to crystal strain and small coherent scattering domains (CSD).

XRD peak shapes of alluvial quartz derived from the Georgia Piedmont indicate minimal defect density and crystal strain (i.e. infinitely sized CSD). In contrast, XRD data for the Crow Creek samples show peak broadening for some reflections, especially the (100). Two quartz samples, the randomly selected grains from the sand unit above the Huber and those grains lacking linear features display line-broadening characteristics similar to the Piedmont quartz. Those grains with inferred PDF’s exhibit line-broadening characteristics similar to the Crow Creek sample. These XRD findings, taken with the petrographic evidence for PDF’s in quartz grains (Harris et al, this meeting), provide the first evidence for in-place preservation of the Late Eocene impact record in Georgia Coastal Plain strata.