Coastal Plain province - The Geology of Virginia
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Generalized Map of the Chesapeake Bay Impact Structure

T

he immense Chesapeake Bay impact structure lies hidden beneath the shallow waters of the Chesapeake Bay and a thin veneer of Coastal Plain sediments. This complex 85-km-wide (~50 mile) impact crater is the largest in the United States, yet was not recognized until the early 1990’s. The crater is centered under the Chesapeake Bay approximately 8 km (5 miles) west of the town of Cape Charles on the Eastern Shore. It includes an inner basin surrounded by a ring of raised basement rock, encircled with a flat-floored terrace zone and bounded along the outer rim by a zone of concentric faulting.

The crater is cut into ~650 m (~2000 feet) of Early Cretaceous to late Eocene sedimentary material and at least a kilometer (~3000 feet) of the underlying granodioritic basement rocks. Much of the crater is filled with a chaotic sedimentary deposit known as the Exmore breccia. The Exmore breccia contains angular clasts of older sedimentary material, and granitic to metamorphic basement rocks in a sandy matrix.

Shocked quartz grain sampled from Hampton, Virginia
Shocked quartz grain sampled from
NASA Langley at Hampton, Virginia
by Dr. Glen Izett

Shocked quartz grains and impact derived glass also occur in the Exmore breccia. This deposit is interpreted to be impact ejecta that partially infilled the crater immediately after impact. Isotopic dating of impact glass indicates the crater formed 35 million years ago when a meteorite or comet, a few kilometers in diameter, collided with the Earth. At the time of impact, a shallow sea covered the Virginia Coastal Plain and the coastline lay to the west near the present day Fall Zone.

Since the formation of the crater, younger marine and nonmarine sediments deposited on the Coastal Plain completely buried the structure. Although geologists had long recognized anomalous features associated with Coastal Plain sediments in southeastern Virginia it was not until seismic surveying under the Chesapeake Bay and detailed examination of deep sedimentary cores that the crater was revealed. Differential movement along the outer crater rim affected later sediment deposition. The exposures of large cross-bedded biofragmental sand exposed in the bluffs near Yorktown are interpreted to be shoal deposits that formed in response to rotation of underlying slump blocks along the outer crater rim 4 to 5 million years ago.

Although hidden at the surface the Chesapeake Bay impact crater is still affecting the region as briny groundwater associated with the crater is a problem for many deep water well in eastern Virginia.

References

Poag, C. W., Powars, D. S., Poppe, L. J., Mixon, 1994, Meteoroid mayhem in Ole Virginny: Source of the North American tektite strewn field: Geology, v. 22, p. 691-694.

Johnson, G. H., Kruse, S. E., Vaughn, A. W., Lucey, J. K., Hobbs, C. H. III, Powars, D. S., 1998, Postimpact deformation associated with the late Eocene Chesapeake Bay impact structure in southeastern Virginia: Geology, v. 26, p. 507-510.

(11/4/98)