THE ICDP-USGS DEEP DRILLING PROJECT AT THE CHESAPEAKE BAY IMPACT STRUCTURE: AN OVERVIEW

The late Eocene Chesapeake Bay impact structure, which is the source crater of the North American tektite strewn field, is among the largest and best preserved of the known impact craters on Earth. It has a diameter of 85 km with an “inverted sombrero”-shape cross section. The International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS) completed two deep coreholes to a composite depth of almost 1.8 km into the Chesapeake Bay impact structure. Field operations began in July 2005 with site preparations at Eyreville Farm in Northampton County, Virginia; subsequently, three coreholes were drilled at the Eyreville site. Eyreville hole A was cored between depths of 125 m and 941 m from September through early October 2005. Problems with lost mud circulation and swelling clays in Eyreville A led to deviation of the bit from Eyreville A to a new hole, Eyreville B, at a depth of 738 m. Eyreville B was cored from that depth to a final depth of 1,766 m from October through December 2005. Post-impact sediments were cored from land surface to 140 m depth in the Eyreville C hole (April-May 2006). The cored impactite section consists of five major lithologic units. The lowest unit consists of about 216 m of fractured mica schist, granite, and granite pegmatite, also carrying several impact breccia veins. These rocks could represent the autochthonous crater floor, or they could be parautochthonous basement blocks. Above these rocks follow about 157 m of suevitic impact breccias that are considered fallback and (or) ground-surge deposits. Above these breccias, a thin interval of quartz sand (22 m) contains an amphibolite block and other lithic clasts of centimeter to decimeter size. This sand occurs below a 275-m-thick granite megablock, which appears unshocked and thus must have been transported for tens of kilometers during the cratering process. The uppermost and thickest impactite unit consists of about 652 m of deformed sediment megablocks and overlying sedimentary clast breccia (Exmore beds). The Exmore beds contain clasts of target sediments and crystalline rock, as well as a small component of impact melt, and it is interpreted to represent late-stage collapse of the marine water column and its catastrophic flow back into the crater.