THE BOSUMTWI IMPACT STRUCTURE - PETROPHYSICAL AND PALEOMAGNETIC INVESTIGATIONS

Deep drillings of continents has become a powerful tool to investigate impact craters - to study their structures, lithologies, age; and to test the validities of the geological and geophysical models. The Solid Earth Geophysics group of the Division of Geophysics, University of Helsinki is working currently on four ICDP-projects: (i) Chicxulub impact structure in Mexico; (ii) Bosumtwi impact structure in Ghana; (iii) Chesapeake impact structure in Virginia; and (iv) Outokumpu formation in Finland. The purpose of the project of the Solid Earth Geophysics group is to investigate physical properties (such as density, porosity, magnetic susceptibility, etc.) of samples from deep drillings. Here we present petrophysical and paleomagnetic data from deep drill cores of Bosumtwi impact structure, Ghana, Central Africa. Bosumtwi, the youngest (age of 1.07 Million years) of the large impact craters known on the Earth, is very well preserved and accessible, and is almost totally filled by Lake Bosumtwi. Our core samples (with only up-down orientation) come from the hard rock (impactite) cores LB-07A and LB-08A. Results show no depth dependence of petrophysical parameters. Only average of bulk density of drill core LB-08A shows slight increase with depth. Most of the samples hold low ferromagnetic component of magnetic susceptibility. Few metasediments, however, show some magnetic susceptibility highs (≤ 1600 10⁶ SI). Susceptibility data correlates positively also with NRM values and q-ratio. Data from paleomagnetic measurements reveal that remanence component is characterized by fairly high coercivity (removed between 50 and 100mT, in some cases its more stable than 130mT). This component has shallow normal (in few cases shallow reversed) magnetization and is carried out by pyrrhotite.