Petrographic Investigation of Selected Samples from Drill Cores Eyreville A and Eyreville B: Chesapeake Bay Impact Structure, Virginia

Abstract

The meteoritic impact process is expressed uniquely in marine, sedimentary target impact structures versus hard, dry crystalline targets. Wet sediments accommodate extreme pressures and temperatures differently than crystalline rocks. The Chesapeake Bay impact event took place on the continental shelf of the passive Atlantic margin during late Eocene, enabling exceptional preservation of the feature. In 2005, the Eyreville A and B cores were drilled in the central zone of the Chesapeake Bay structure. These cores were acquired through the International Continental Drilling Program/United States Geological Survey Chesapeake Bay impact structure deep-drilling project. Fifty-two samples were allocated to the Auburn University research group in 2006.

The sample set was photographed as hand samples and most hand samples were then made into thin sections, which were studied and imaged. After a sample sheet was developed for petrographic analysis, the samples were analyzed including hand-sample description, thin-section investigation with a petrographic and universal-stage microscope, and, in rare instances, analysis of thin sections using an electron microprobe and scanning electron microscope. This work produced a modal analysis and inventory and description of shock-related features for each sample.

The stratigraphic assemblages of the Eyreville core established by the USGS were used in this study. The results of sample analysis were evaluated with respect to their stratigraphic position. Assemblage 1 is the basal section and includes broken pieces of schists and pegmatites that locally grade into granite. Assemblage 2 is a zone of lithic breccias, suevite, and impact melt rocks. A relatively thin layer of gravelly sand forms Assemblage 3. The mixed granites of Assemblage 4 are allochthonous blocks with variable textures and compositions. Assemblage 5 contains impactoclastic sands and breccias and sedimentary megablocks and boulders.

Shock features were noted within Assemblages 1, 2, 3, and 5, but traces of shock may also be present in the mixed granites of Assemblage 4. Shock features include kink-bands, mechanical microtwins, planar microstructures (PMs), toasted quartz, mosaicism in quartz, recrystallized minerals, and quenched silica glass. Diagnostic methods used to evaluate PMs yielded results consistent with previous work, which explained a variation of PM orientation with depth. Traces of L- or LL-chondrite meteorite material may have been found within altered melt from Assemblage 2; this section experienced the highest shock regime, more than 35 GPa locally, inferred from its greater volume of melt material.