PROPOSED SCIENTIFIC DRILLING AT THE BOSUMTWI IMPACT STRUCTURE, GHANA, WEST AFRICA

The 10.5 km diameter Bosumtwi impact crater in Ghana (06°32'N and 01°25'W) is almost completely filled by Lake Bosumtwi. The crater has an age of 1.07 Ma and was excavated in lower greenschist facies metasediments of the 2.1-2.2 Ga Birimian Supergroup. The Ivory Coast tektites and microtektites originated from this crater. A first high-resolution aerogeophysical survey was conducted in early 1997. Since then several projects have dealt with land- and lake-based geophysical measurements and surface geological and geochemical investigations regarding the subsurface topography of the structure. The results from these studies provided all the background work necessary to characterize the subsurface structure of the Bosumtwi crater that are necessary to define the targets for a deep drilling program. Such a deep drilling project, proposed by the authors, has recently been approved by the International Continental Scientific Drilling Program (ICDP). Drilling is desirable for several reasons, including 1) to obtain a complete 1 million year paleoenvironmental record in an area for which so far only limited data exist; 2) to study the subsurface structure and crater fill of one of the best preserved large young impact structures. Regarding the paleoenvironmental record, Bosumtwi is in a perfect location to reconstruct and investigate interannual through orbital-scale variations in 1) the West African monsoon, 2) hydrologic variations of the Sahel, 3) dust export from SW Africa, and 4) sea-surface temperature variations in the tropical East Atlantic. Understanding the full range of climate variability in this region over the last 1Ma will thus fill a major hole in our understanding of global climate dynamics, and thus also lead to an enhanced climate prediction capability over a broad part of the earth. In terms of cratering studies, Bosumtwi 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, maybe similar to that of the Ries crater in Germany. Drilling allows to correlate the geophysical studies and will provide material for geochemical and petrographic correlation studies between basement rocks and crater fill in comparison with tektites and other ejecta.