MACROSCOPIC AND PETROGRAPHIC MODAL ANALYSES OF COASTAL PLAIN BASEMENT ROCK IN THE USGS BAYSIDE CORE, CHESAPEAKE BAY IMPACT STRUCTURE, VA

Macroscopic and petrographic analyses of drill core from the late Eocene Chesapeake Bay impact structure (CBIS) are helping define the geologic and tectonic history of the crystalline basement rock underlying the Atlantic Coastal Plain. In 2001, the USGS recovered granitoid rock from the Bayside #2 corehole drilled into the inner part of the annular trough of the CBIS in Mathews County, VA. The drill site field log described the rock as granodiorite, and USGS reported a SHRIMP U-Pb zircon age of 625 ± 11 Ma.

Northern Virginia Community College (NVCC) students are studying samples from the bottom of the core to the uppermost basement. Modal analyses have been performed on six samples from the bottom 12 feet of the core. Normalized QAPF percentages for two samples plotted as monzogranite, three plotted as granodiorite, and one fell directly between the two classifications. These six samples do not appear to indicate that QAPF percentages are a function of depth. Feldspars are heavily sericitized. Original hornblende (?) and biotite are mostly to entirely replaced by Fe-rich chlorite; relict biotite is rare. Quartz shows strain features that are common in rocks of the Appalachian orogen. Calcite, epidote, and other secondary minerals suggest hydrothermal alteration and/or lower greenschist facies metamorphism, but we found no evidence of shock metamorphic effects which would directly implicate the bolide impact. Mineralogy is consistent throughout the bottommost 12 feet.

Petrographic analysis of the uppermost 14 feet of the Bayside core to assess mineralogy and chemical weathering as functions of depth is in progress.

Students will also attempt to place the Bayside core intrusive body into a wider geologic context by sampling and conducting petrographic analyses of plutons of similar age in the Piedmont for comparison.