Beschreibung
This is a multidisciplinary proposal to drill the Bosumtwi impact structure in Ghana, West Africa. The Bosumtwi impact crater in Ghana (centered at 0632'N and 0125'W) is almost completely filled by Lake Bosumtwi. The crater has a rimto- rim diameter of about 10.5 km, while the lake has a diameter of about 8 km and maximum depth of about 80 m. 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. Only recently, detailed geophysical studies were done. A first high-resolution aerogeophysical survey was conducted in early 1997. Measurements included: total magnetic field; aeroelectromagnetic; gamma radiation data for U-, Th- and K-contents, and total radiation. These data provide a much more detailed image of the crater structure than any previous data. Several projects have dealt with land- and lake-based geophysical measurements and surface geological and geochemical investigations regarding the subsurface topography of the structure. A shallow drilling project studied the near-surface composition of ejecta and weathered surface rocks for correlation with the aerogeophysical data (Austria/Ghana). Groups from Germany, the US, and Ghana have performed gravity measurements around the crater and over the lake, as well as refraction seismics with the goal to obtain a 3D tomographic model of the crater interior. Highresolution seismic studies were done that led to the discovery of a sediment-covered central uplift. Some shallow cores of lake sediments were also recovered. 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 the deep drilling program proposed here.