A review of the resistivity structure of the Karoo Supergroup, South Africa, with emphasis on the dolerites: A study in anisotropy

van Zijl J.S.V. (2006)

Review

The resistivity structure of the Karoo Basin has been determined by an extensive programme of some 280 deep electrical soundings (maximum current electrode spacing = 40 km). The very conductive carbon and pyrite- rich Whitehill Formation acts as an electrical basement to deep soundings where it is well developed. Beyond its limit in the east, the electrical basement consists of granites and metamorphic rocks and is highly resistive. Notwithstanding an appreciable difference in resistivity between the sandstone and mudstone components, the sandstones are neither sufficiently massive nor continuous to be detected separately in sounding curves. Exceptions are a few, thick, massive arenaceous turbidite fans occurring under an appreciable cover in the downwarped Karoo Trough, straddling the Lower Beaufort and Upper Ecca Subgroups at the southern extremity of the basin. The average resistivity of the sedimentary pile above the Whitehill Formation increases from north to south by more than an order of magnitude substantiating geological evidence of a sympathetic increase in diagenesis and metamorphosis. The Karoo Supergroup comprises one of the rare instances of a sedimentary basin where dolerite intrusions occur interbedded with sediments. The very high resistivity contrast between the resistive dolerite and the conductive sediment, results in an exceptionally high degree of electrical anisotropy. This feature has provided a unique opportunity to study the configuration of the network of dolerite intrusions in the sedimentary succession but simultaneously has often seriously restricted the depth of investigation. Previous geological investigations have demonstrated a tri-zonal vertical variation in intrusive style. The resistivity study has shown that elements in each of these zones have their own specific geophysical signature. The largest portion of the basal zone 3 with its flat lying dolerite sills of great lateral extent is confined to well-laminated, homogeneous shales with a high degree of anisotropy in the Lower Ecca Subgroup and is characterized by a low transverse resistance on sounding curves corresponding to a concomitant low content of dolerite. The onset of zone 2, which coincides with the appearance of the first sandstones in the Waterford Formation, is accompanied by a dramatic increase in transverse resistance and hence the amount of dolerite. This dolerite-rich zone, which continues through the Beaufort Group and partially into the higher Formations is characterised by a multitude of basin structures. The variation in structure of the network of dolerites correlates with that of the nexus of lenticular sandstones intercalated with shales and mudstones. Detailed investigations, such as the one in the De Aar area, show that the average percentage of dolerite in the dolerite bearing layer of zone 2 is 45% with a high of 85% and a low of about 10%. Regionally, zone 2 thins to the south but thickens to the east by a factor of at least 5, at the expense of zone 3. The thickening is associated with a lateral increase in the amount of dolerite. Towards the upper portion of zone 2 there is a lower concentration of intrusions and a trend towards smaller basin structures in the Molteno, Elliot and Clarens Formations. Arguably, the most important contribution of the resistivity soundings is the conclusive finding that in the uppermost zone 1, which extends from the surface to a depth of about 700 m, the dominant style of intrusion is that of steeply dipping sheets and vertical dykes and that the zone is especially dolerite-poor with the amount of dolerite only making up about 0.5% or less of the sediments. The nature of zone 1 is independent of the lithology and is due to the decrease of overburden pressure. © 2006 September Geological Society of South Africa.

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