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Within the Inkaba yeAflica project, magnetotelluric (MT) data were acquired along three profiles across the Karoo Basin in South Africa. The entire Karoo Basin and its sedimentary sequences are intersected by a number of deep boreholes, which were drilled during exploration for coal, oil and uranium. One of the most consistent and prominent features in the electrical conductivity images is a shallow, regionally continuous, sub-horizontal band of high conductivity that seems to correlate with the Whitehill Formation. The Whitehill Formation in particular is known to be regionally persistent in composition and thickness and can be traced throughout the entire Karoo Basin. In the region of our experiments, the Whitehill Formation varies in thickness between 50 to 70 m and consists of pyritic black shales with up to 14 weight % carbonaceous matter. Rocks can exhibit high electrical conductivity if electronically conducting minerals, such as graphite, are interconnected. To study physical properties and the maturity of the carbon present in the Whitehill Formation, impedance spectroscopy and vitrinite reflectance were applied to core samples from the SA1/66 borehole, taken from this formation at a depth range between 2750 m and 2800 m. Vitrinite reflectance analysis of the pseudo-coal and oil shale rocks indicates that most of the carbon is in the meta-anthracite maturity field, approaching, in one sample, the "graphite window". Impedance spectroscopy reveals that the highest electrical conductivities are associated with the pyrite rich portion of one sample, showing conductivities well above 1 S/m. Recordings on pyrite-poor samples are less conclusive, with observed conductivities varying in the range of a few S/m to mS/m. Measurements on samples from the Whilehill Formation, on the other hand, show consistently low resistivities in the range between 4 to 6 Ωm. This result suggests strongly that the sub-horizontal conductivity anomaly in the MT models is linked with the carbon-rich sequences of the Whitehill Formation [ABSTRACT FROM AUTHOR]

Abstract: The origin of the approximately 1000 km-long Beattie Magnetic Anomaly (BMA) in South Africa remains unclear and contentious. Key issues include the width, depth and magnetization of its source. In this study, we use uniformly magnetized spheres, prisms and cylinders to provide the simplest possible models which predict the 1 km-altitude aeromagnetic measurements along a profile across the BMA. The source parameters are adjusted by forward modeling. In case of a sphere, an inversion technique is applied to refine the parameters. Our results suggest that two similarly magnetized and adjacent sources, with a vertical offset, can explain the observed magnetic anomaly. The best fitting model corresponds to two highly-magnetized (>5 A m−1) sheet-like prisms, extending from 9 to 12 km depth, and from 13 to 18 km depth, respectively, and with a total width reaching 80 km. Other less-preferred models show thicker and deeper magnetized volumes. Associated magnetizations seem to be mostly induced, although a weak remanent component is required to improve the fit. We also compare our results with the interpretation of independent magnetotelluric and seismic experiments along the same profile. It suggests that the geological sources for the BMA are mostly located in the middle crust and may be displaced by a shear zone or a fault. Contrary to previous models suggesting a serpentinized sliver of paleo-oceanic crust within the Natal–Namaqua Mobile Belt, we propose that granulite-facies mid-crustal rocks within this belt may cause the BMA. [Copyright &y& Elsevier]