Abstract-Physical properties from rocks of the Bosumtwi impact structure, Ghana, Central Africa, are essential to understand the formation of the relatively young (1.07 Ma) and small (10.5 km) impact crater and to improve its geophysical modeling. Results of our petrophysical studies of deep drill cores LB-07A and LB-08A reveal distinct lithological patterns but no depth dependence. The most conspicuous difference between impactites and target lithologies are the lower bulk densities and significantly higher porosities of the suevite and lithic breccia units compared to meta-graywacke and metapelites of target lithologies. Magnetic susceptibility shows mostly paramagnetic values (200-500 × 10 -6 SI) throughout the core, with an exception of a few metasediment samples, and correlates positively with natural remanent magnetization (NRM) and Q values. These data indicate that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite. The paleomagnetic data reveals that the characteristic direction of NRM has shallow normal (in a few cases shallow reversed) polarity, which is in agreement with the Lower Jaramillo N-polarity chron direction, and is carried by ferrimagnetic pyrrhotite. However, our study has not revealed the expected high magnetization body required from previous magnetic modeling. Furthermore, the LB-07A and LB08-A drill cores did not show the predicted high content of melt in the rocks, requiring a new interpretation model for magnetic data.

An unconformity has been observed along the Black Sea shelf on seismic reflection profiles and wells which is broadly similar to the one associated with that formed during the Messinian salinity crisis (MSC) in the Mediterranean. Therefore, this intra- (or Middle) Pontian unconformity has been traditionally interpreted as the manifestation of the MSC in the Black Sea Basin. However, the magnitude of the sea-level fall associated with this erosive surface does not appear to be nearly as significant as was assumed previously. Also, the inferred MSC surface itself cannot be easily followed into the palaeo-deep-water basin as a regional unconformity in the same manner as in the Mediterranean. Moreover, around the Black Sea, there is no evidence of major river incisions during the MSC, unlike the well-documented cases in the Mediterranean region. If the MSC evaporites in the Mediterranean indeed deposited in a subaerial setting at the basin floor, the lack of a major drawdown in the Black Sea explains why there are no Messinian evaporites in the Black Sea. Owing to the approximately 500 m MSC sea-level drop the Black Sea basin system, this basin did not even get close to the conditions required for the formation of evaporites in the basin centre. As the magnitude of the sea-level drop and the overall impact of the MSC in the Black Sea is interpreted to be less significant than in the Mediterranean, the risk of breaching pre-existing hydrocarbon traps during the MSC is less than has been suggested before.

Abstract-Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock-forming and rockchanging processes, e.g., deformation or sedimentation.To characterize the rocks in drill cores LB-07A and LB-08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite.The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta-graywacke). It was possible to correlate layers of high (shale component), intermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB-07A and LB-08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta-graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB-07A towards core LB-08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure.