84 Stratigraphy I: Impacts and Extinctions

Tuesday, 6 November 2001
1:30 PM - 5:30 PM

STRUCTURE AND COMPOSITION OF THE SOUTHWESTERN MARGIN OF THE BURIED CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA

Paper number: 84-14
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The Chesapeake Bay impact structure is a complex, 135-km-wide feature created about 35 million years ago when an asteroid or comet slammed into the continental shelf near the present-day mouth of Chesapeake Bay. Initial borehole and seismic-reflection data revealed an “inverted-sombrero”-shaped, 90-km-wide crater. At the center is a nearly 38-km-wide, 1.6-km-deep inner basin surrounding a central uplift. A peak ring with highly variable shape and relief surrounds the inner basin. Below and adjacent to the inner basin is a bowl-shaped zone of faulted basement rocks down to about 11 km. A 21- to 31-km-wide, flat-floored annular trough has a slumped terrace zone at its outer margin, and is bounded by an outer rim, which has a 305- to 1220-m-high gullied escarpment. A ~23-km-wide concentric zone of faulting surrounds the crater.

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 structure’s annular trough. We completed high-resolution seismic reflection/refraction, gravity, and audio-magnetic-telluric surveys across the structure’s 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 structure’s 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.