by Christian Koeberl, Kenneth G. Miller

The late Eocene Chesapeake Bay impact structure is among the largest and best preserved of the known impact craters on Earth. We propose a multidisciplinary and international drilling project at this crater, involving an international research team. Research topics include studies of impact processes, regional basin evolution (comparing impact effects with “normal” effects produced by tectonics, global sea-level, and sediment supply on a passive continental margin), hydrogeology, borehole and regional geophysics, and the deep biosphere. The subsurface structure of the Chesapeake Bay crater is constrained by several shallow coreholes, over 2,000 km of marine seismic-reflection surveys, and gravity analyses. Major subdivisions of the structure are a circa 38-km-diameter central crater enclosed by a 24-km-wide annular trough. Several characteristics make the Chesapeake Bay structure unique among subaerial and submarine impact craters on Earth because: (1) it is associated with the North American tektite strewn field, (2) it had a multi-layered, (rheologically varied) marine target, (3) it is a well-preserved and relatively young structure compared to most large terrestrial craters, (4) its location on a passive continental margin has prevented the tectonic disruption that is typical of many large terrestrial craters, (5) its original location on a relatively deep continental shelf allowed marine deposition to resume immediately and bury it rapidly and completely, thereby preventing subsequent erosion; (6) the upper part of the breccia section inside the crater was derived from resurge currents and impact-generated tsunami waves, (7) the breccia body contains a large volume of impact-generated brine, and (8) the crater underlies a densely populated urban corridor, whose two million citizens are still affected by crater-related phenomena, specifically the presence of salty ground water within the structure. Thus, we propose to drill a 2.2-km-deep corehole near the central uplift within the “moat” of the structure’s central crater (as defined from seismic and gravity data), to obtain as thick and undisturbed a post-impact succession as possible, and a thick section of impactites (hopefully including impact melt rocks), and to reach the sub-crater basement to study the shock barometry and fracturing of these rocks.

• Impact Studies: Drilling in the central Chesapeake Bay crater will provide important constraints on cratering processes in multi-layered marine targets in general and for comparison with results from the larger Chicxulub crater. The drilling will provide unique constraints on (1) the crater structure, depth, morphology, and formative processes, and (2) the crater materials, including impactite and basement lithologies, stratigraphy, mineralogy, chemistry, fractures, and physical properties, and (3) numerical models of impact processes. Hydrogeologic Studies:
• (1) determine salinity and other chemical attributes of ground water for water-resource managment purposes,
• (2) determine the post-impact hydrogeologic history of the crater including any hydrothermal system. Post-impact Studies:
• (1) document the impact-produced local biotic crisis and recovery and the physical transition from high-energy impact environments to the normal shelf environment,
• (2) evaluate the effects of impact cratering relative to other tectonic, sea-level, climate, and sediment-supply effects on the long-term evolution of the mid-Atlantic continental margin. Deep Biosphere Studies:
• (1) examine subsurface microbial communities at significant depths in the crust,
• (2) examine the numerous thermodynamic gradients provided by multi-component, multi-layered crater and post-impact materials and ground-water salinity variations for exploitation by subsurface microbial communities.

CBAY, Cenozoic Sequence, Climate Change, Eocene, Global Environment, ICDP-2004/06, Impact Crater, Paleoclimatology, Sea-Level Changes, U.s.a., Virginia

Latitude: 37.3217, Longitude: -75.9757