STRUCTURE AND COMPOSITION OF THE SOUTHWESTERN MARGIN OF THE BURIED CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA
To further investigate the geology and hydrology of this structure, the USGS and affiliated institutions recently drilled three deep coreholes in the western portion of the structures annular trough. We completed high-resolution seismic reflection/refraction, gravity, and audio-magnetic-telluric surveys across the structures southwestern margin. Two of the coreholes are located about 7 km inside the outer rim, and one is about 22 km inside the outer rim and 8 km outside the inner basin. The lithology, mineralogy, biostratigraphy, structure, isotopic-age, sediment permeability, pore-water chemistry, and the presence and composition of trapped gases are being analyzed. Preliminary analyses confirm the presence of a thick (at least 390 m) sedimentary-clast breccia and megablock beds buried beneath 225 to 280 m of post-impact deposits, faults in the post-impact deposits, and several faults that displace the top of basement near the structures outer margin. Rapid infilling of the annular trough occurred during late-stage crater modification by outer-margin wall collapse, resurge processes, and tsunami washback, resulting in a polymict diamicton, which overlies authochthonous megablock breccia. Shocked quartz and deformed basement clasts are present but sparse in the diamicton.