PROBABLE SHOCKED QUARTZ AS EVIDENCE FOR AN UPPER EOCENE IMPACT HORIZON IN COASTAL PLAIN STRATA, WARREN COUNTY, GEORGIA, U.S.A

Although 35 Ma tektites are found in younger Coastal Plain alluvium throughout southern Georgia, a discrete impact horizon has never been identified in the upper Eocene section. Albin and Wampler (LPSC 27, 1996) argued, based on glauconite ages and biostratigraphy, that any ejecta layer associated with the Chesapeake Bay impact (35 Ma) should be located at the base of the Twiggs Clay Member of the Upper Eocene Dry Branch Formation.

In east-central Georgia, the Twiggs Clay overlies the Middle Eocene kaolin-rich Huber Formation. The contact between the two is a sequence boundary and transgressive surface overlain by a patchy coarse-grained sand layer up to 10 cm thick. Approximately 3-5% of fine to medium-grained quartz examined from the unit contain planar elements similar to planar deformation features (PDF’s) indicative of impact shock. Most of the quartz grains display one set of planar features, but some exhibit 2 or 3 intersecting sets. Many of these grains show anomalous patchy extinction patterns.

Two grains merit detailed description. The first is a rounded, 600 x 350 mm grain, displaying one prominent set of lamellae. The lamellae are 2-3 mm wide and are spaced about 5 mm apart. Spindle stage measurements indicate that the pole to the lamellae lies 32º off the quartz c-axis. The lamellae appear to be parallel to {10-12}, a common PDF orientation in strongly shocked quartz. The second grain is more angular, 350 mm wide, and exhibits two sharp sets of planar features forming a chevron pattern. Spindle data show that their poles lie 31º and 64º off the c-axis; they are possibly parallel to {10-12} and {11-21}. The linearity, spacing (<10 mm), multiplicity, and apparent coincidence with rational crystallographic planes suggest that the observed fabrics are PDF’s and not tectonic lamellae. Moreover, the features are invisible except when immersed in oils with refractive indices close to quartz, suggesting a structural or compositional difference between the lamellae and host grain. X-ray diffraction results support the interpretation that some of the quartz grains are shocked (Schroeder et al., this meeting).

If these quartz grains are indeed shocked, the sandy lag at the base of the Twiggs Clay may record the Chesapeake Bay impact in the stratigraphy of Georgia and may be related to the parent stratum for georgiaites.