ICDP-USGS DEEP DRILLING PROGRAM IN THE CHESAPEAKE BAY IMPACT STRUCTURE, VIRGINIA, USA

The International Continental Scientific Drilling Program and the U.S. Geological Survey conducted deep drilling operations in the central part of the late Eocene Chesapeake Bay impact structure during September-December 2005 and April-May 2006. Cores were recovered continuously from land surface to a composite depth of 1,766 m in three holes at the Eyreville site in Northampton County, Va. The drilling target was the deep part of the structure, the moat, between the central uplift and the margin of the 38-km-diameter collapsed transient crater. The project's research program includes hydrologic investigations, deep biosphere studies, and studies of Cenozoic continental-margin and climate history in addition to studies of impact products and processes.

The recovered geologic section consists of 1,322 m of impactites and 444 m of overlying post-impact sediments. Major units, from base to top, are: 1) variably brecciated schist and pegmatite containing minor impact-breccia veins, 216 m; 2) suevitic and lithic impact breccias, 157 m; 3) quartz sand with lithic pebbles, cobbles, and boulders, 22 m; 4) a granite megablock, 275 m; 5) megablocks of Cretaceous sediments overlain by allogenic sedimentary breccia containing target-sediment clasts and crystalline-rock ejecta clasts, 652 m; and 6) post-impact sediments of late Eocene through Pleistocene ages, 444 m. We interpret unit 1 to represent parautochthonous blocks of the crater floor and impact injection dikes. Unit 2 represents relatively melt-rich fallback deposits and (or) ground-surge deposits. Units 3 and 4, and the megablocks of unit 5, consist of slump blocks emplaced during late-stage gravitational collapse. The allogenic breccia of unit 5 is a diamicton produced by the resurge of ocean water and debris into the collapsing crater. The post-impact section consists of thick upper Eocene, thin Oligocene and lower Miocene, and thick middle and upper Miocene and Pliocene fine-grained shelf sediments overlain by a thin interval of Pleistocene paralic deposits.

Lessons learned or reinforced include the need for deep drilling programs in large impact structures to clearly characterize their drilling target, to expect structural complexity and lithologic variability, and to establish contingency funding to address the inevitable drilling problems.