Some Physical Properties of Suevites from the Bosumtwi Impact Crater, Ghana

Suevite is a polymict breccia of clastic material derived predominantly from the crystalline basement. It is an impact-derived rock usually found at meteorite impact crater sites. In Ghana, suevites have been found at two locations at the Bosumtwi meteorite impact crater. The suevites are located in the Northern and Southern parts outside the crater rim. Due to the presence of different rock clasts of various sizes, the suevite exhibits physical properties that are quite different from those of other rocks such as granites, gneisses, etc. Suevites found in the North and South locations have some characteristic differences. In this paper, we report on the anisotropic behaviour of the compressional wave velocity Vp with pressure and azimuth for suevite samples collected from the North and South locations. The effect of pressure on Vp for the sample from the South is more pronounced than that from the North because of the high porosity of the sample at the South location. Also, the seismic velocity anisotropy is more pronounced in the samples from the South probably due to the distribution of rock inclusions in the matrix. Vp-minimum directions determined for some samples indicate that the Vp-minimum axes seemed to point toward the center of the crater. This supports the reasoning that after the impact, the ejected material on the ground might have assumed a preferred orientation with respect to the center of the crater. It was also found that suevite samples require higher saturation pressures (650 MPa and above) than solid rocks such as amphibolite which reaches velocity saturation at 100 MPa. Key words: suevites, impact crater, compressional wave velocity, anisotropy, velocity saturatio

Results of pre-drilling potential field measurements at the Bosumtwi crater

Gravity and magnetic measurements were carried out at the Bosumtwi crater to determine the geophysical signature of the crater. Land gravity data was acquired at 163 locations around the structure and on the shore of the lake. The separation between the gravity stations was 500 m for radial profiles, but 700-1000 m along roads and footpaths that ran parallel to the lakes shore. Additionally, a marine gravity survey was carried out along 14 north-south and 15 east-west profiles on the lake. Magnetic data was also acquired along 14 north-south profiles on the lake. In all marine surveys, the line spacing was 800 m, and navigation was provided by a Garmin 235 Echo Sounder/GPS. The gravity signature of the crater is characterized by a negative Bouguer anomaly with an amplitude of about 18 mgal. Using the seismic results as constraints, the gravity model obtained indicates the central uplift at a depth of 250 m. The negative anomaly is the contribution of the gravity deficiencies due to fractured and brecciated rocks in the rim area and below the crater floor, the impact breccias within the crater, and the sedimentary and water infilling of the lake. Magnetic modeling yielded a model for the causative body, which is located north of the central uplift: the model has a magnetic susceptibility of 0.03 S.I. and extends from a depth of 250 to 610 m. The causative bodies have been interpreted as impactites.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Integrated 3-D model from gravity and petrophysical data at the Bosumtwi impact structure, Ghana

The Bosumtwi impact structure of central Ghana was drilled in 2004 as part of the International Continental Scientific Drilling Program (ICDP). A vast amount of geoscience data is available from the pre-site surveys and the actual drilling phase. A 3-D gravity model was constructed and calibrated with the available data from the two ICDP boreholes, LB-07A and LB-08A. The 3-D gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data, and with the petrophysical data from the LB-08A and LB-07A core materials and the two borehole logs. Furthermore, the model exhibits lateral density variations across the structure and refines the results from previous 2.5-D modeling. An important new element of the 3-D model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite, monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

The Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?

The Bosumtwi impact structure (Ghana) is a young and well-preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact-melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact-melt rock magnetic source. A new 3-D magnetic model for the structure was constructed based on a newly acquired higher-resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact-melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Monsoon Multidisciplinary Analysis (AMMA) : an integrated project for understanding of the West African climate system and its human dimension

Countries in Africa are the most vulnerable to the impacts of climate change but lack the capacity to manage effectively climate-related environmental problems. Meteorology and climate education programmes are offered at West African universities, but the poor job market and insufficient funds from governments are impacting negatively on the running and sustainability of master's and PhD programmes. This situation compels students to pursue further studies and seek their fortune abroad, a practice which leads to brain drain. One of the AMMA initiatives is to develop international postgraduate programmes and to seek funding to provide students and lecturers with grants and fellowships. Copyright