PRELIMINARY INTERPRETATION OF THE USGS–NASA LANGLEY COREHOLE, CHESAPEAKE BAY IMPACT STRUCTURE, YORK-JAMES PENINSULA, HAMPTON, VA

The USGS and cooperating institutions* drilled a 2,083.8-ft-deep corehole through the Chesapeake Bay impact structure at the NASA Langley Research Center during July-October 2000. The core site is within the annular trough of this complex, 53-mile-wide crater, about 4 miles inside the trough’s outer rim. Beneath 774.3 ft of post-impact upper Eocene, Oligocene, Miocene, Pliocene, and Pleistocene sediments, 1,280.4 ft of sedimentary-clast diamictons and moderately disturbed sediments fill the crater. Fractured, weathered metagranodiorite below these deposits (2,083.8 to 2,054.7 ft) appears to lack shock features, suggesting minimal impact disruption of basement near the outer rim. Relatively undisturbed sediments of the Cretaceous Potomac Formation are present from 2,054.7 ft to about 1,470 ft. In this interval, the sands are massive or horizontally laminated with minimal fluidization, and most clay-silt deposits appear to be primary beds and intraclasts. These features suggest that the deeper crater fill underwent limited translation and rotation, perhaps coupled with the basement rock in large, coherent slump blocks. From about 1,470 ft to about 878 ft, a clast-supported diamicton having minor muddy, quartz-glauconite matrix contains Potomac-Formation clasts that range from feet to tens of feet in apparent maximum diameter and display high-angle bedding, contorted and fluidized layers, and inclined boundaries. This interval likely represents extensive rotational slumping, fluidization, and general disaggregation of the sedimentary section but without significant excavation of sediments or addition of ballistic projectiles from the crater center. From about 878 ft to 774.3 ft, the crater fill consists of a matrix-supported, muddy-sandy (quartz-glauconite) diamicton containing granule- to foot-sized clasts of pre-impact Tertiary marine units as well as Potomac-Formation clasts. Shocked quartz and brecciated crystalline-rock clasts are sparse in this upper diamicton and extremely sparse in lower crater sections. The upper diamicton likely represents tsunami-backwash deposits that buried the trough-margin slump deposits. *NASA Langley Research Center, Hampton Roads Planning District Commission, Virginia Department of Environmental Quality, College of William and Mary.