CONTINENTAL SCIENTIFIC DRILLING AND THE STUDY OF TERRESTRIAL IMPACT CRATERS: BOSUMTWI (GHANA) AND CHESAPEAKE BAY (USA)

Drilling allows obtaining information on the subsurface structure of impact craters, provides ground-truth for geophysical studies, and delivers samples of rock types not exposed at the surface. Recently the International Continental Scientific Drilling Program (ICDP) has supported projects to study impact craters. The first ICDP study of an impact structure was at the subsurface Chicxulub impact crater, Mexico, from late 2001, which reached a depth of 1511 m and intersected 100 m of impact melt breccia and suevite. Between June and October 2004, the 10.5 km Bosumtwi crater, Ghana, has been drilled with ICDP support. Bosumtwi is associated with one of only four known tektite strewn fields. It is a well-preserved complex impact structure with a pronounced rim and is almost completely filled by the 8 km diameter Lake Bosumtwi, which is a closed-basin lake that has wide paleoclimatic significance and a detailed paleo-environmental record. In terms of impact studies, it is one of only two known young craters of this size, and may have a crucial diameter at the changeover between a traditional “complex” crater with a central peak and a crater structure that has a central peak-ring system. Drilling also allows correlating all the geophysical studies and provides material for geochemical and petrographic correlation studies between basement rocks and crater fill in comparison with tektites and ejected material. Sixteen different cores were drilled at six locations within the lake, to a maximum depth of 540 m. Borehole logging as well as vertical seismic profiling (to obtain 3D images of the crater subsurface) were done in the two deep boreholes. About 2.2 km of core material was obtained. This includes ca. 1.8 km of lake sediments and 0.4 km of impactites and fractured crater basement (in the deep crater moat, and on the central uplift). Analyses of samples and geophysical data are currently under way by several different research groups. From September – December 2005, another ICDP-funded project, at the 85-km-diameter Chesapeake Bay impact structure, eastern USA, will involve drilling to a depth of 2.5 km, to recover impact breccias, melt rocks, and a detailed post-impact record. Acknowledgments: for the Bosumtwi project, thanks to ICDP-CoPIs B. Milkereit, J. Overpeck, and C. Scholz; Funding by ICDP, NSF, FWF, OeAW, and NSERC.