Structure and Composition of the Southwestern Margin of the Buried Chesapeake Bay Impact Structure

Powars, David S., U.S. Geological Survey, Richmond, VA 23228, Quick, James E., U.S. Geological Survey, Reston, VA 20192, Bruce, T. Scott, Virginia Dept. of Environmental Quality, Richmond, VA 23240, Catchings, Rufus D., U.S. Geological Survey, Menlo Park, CA 94025, Emry, Scott R., Hampton Roads Planning District Commission, Chesapeake, VA 23320, Gohn, Gregory S., U.S. Geological Survey, Reston VA 20192, Izett, Glen, A., and Johnson, Gerald H., Geology Dept., Coll. of William and Mary, Williamsburg, VA 23187, Levine, Joel S., NASA Langley Research Center, Hampton, VA 23681, McFarland, E. Randolph, U.S. Geological Survey, Richmond, VA 23228, and Poag, C. Wylie, U.S. Geological Survey, Woods Hole, MA 02543


The Chesapeake Bay impact structure (CBIS) 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. The submarine target site consisted of a neritic to upper bathyal water column (150 to 300 m deep), an eastward-thickening wedge of water-saturated, semi-consolidated sediments (305 to 915 m), and underlying basement rocks. Borehole and seismic-reflection data indicate these main features: a 22.5-km-wide zone of fracturing outside the crater; an "inverted-sombrero"-shaped, 90-km-wide crater bounded by an outer rim with a 305- to 1220-m-high escarpment that varies morphologically from a steep wall to inward-stepping stairs; a 21- to 30.5-km-wide, flat-floored annular trough with a slumped terrace zone at its outer margin; a 38.6-km-wide, 1.6-km-deep central depression (inner basin) with an apparent collapsed central uplift; a peak ring with highly variable shape and relief (almost flat to nearly 91 m) that surrounds the inner basin; and below and adjacent to the inner basin, a bowl-shaped zone of fractured and faulted basement rocks down to about 11 km. Shocked quartz and partially melted and deformed basement clasts are present in the Exmore breccia that partially filled and covered the crater and surrounding sea floor. Outside the outer rim on the updip western side of the crater, erosion has removed the breccia except for discontinuous (<30 m thick) deposits preserved within the outer fracture zone. As much as 1.2 km of upper Eocene to Holocene deposits now cover the crater. Post-impact effects include continued structural adjustments through faulting and differential compaction contributing to high subsidence rates and, possibly, historic small-magnitude earthquakes.

The impact structure coincides closely with an anomalous region of salty ground water that presents a resource-management problem to rapidly growing population centers in the Hampton Roads area of Virginia at the structure's western and southern margins. To further investigate the geology and hydrology of the CBIS, the USGS and affiliated institutions recently drilled a 635-m-deep corehole to basement about 7 km inside the structure's outer rim and completed a tie-in 11-km-long high-resolution seismic reflection and refraction survey across the structure's southwestern outer margin. We are analyzing the lithology, mineralogy, biostratigraphy, structure, fission-track, and radiometric-age of corehole materials, as well as determining sediment permeability, pore-water chemistry, and the presence and composition of trapped gases. Preliminary analysis confirms several hypothesized features, including the presence of thick (390 m) sedimentary-clast breccia and mega-block beds buried beneath 236 m of post-impact deposits, faults in the post-impact deposits, and several faults that displace the basement near the crater's outer margin. Preliminary water-quality data show vertical variability in the salinity of ground water inside the western margin of the crater. Crater-fill deposits seen in the core appear to have been emplaced during late-stage crater modification by outer-margin wall collapse, washback processes, and tsunami resurge, resulting in a section dominated by locally derived, shallow sedimentary target rocks. The section varies from clast-supported- to matrix-supported breccia. Sedimentary-clast sizes range from millimeters to nearly 20 m.

Powars, David S., Quick, James E., Bruce, T. Scott,  Catchings, Rufus D., Emry, Scott R., Gohn, Gregory S., Izett, Glen, A., Johnson, Gerald H., Levine, Joel S., McFarland, E. Randolph, and Poag, C. Wylie, 2001, Structure and composition of the southwestern margin of the buried Chesapeake Bay Impact Structure [abs.], presented at the GSA Field Forum in Nevada and Utah on bolide impacts on wet targets, April 22-28, 2001.


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