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The Geological and Geophysical Framework |
Robertson Research International Ltd, Llandudno, LL30 1SA, North Wales, UK
Findings drawn from the Tertiary pre-rift, rift and post-rift sequences of Africa and the Red Sea are applied to the South Atlantic margins with a view of aiding hydrocarbon exploration. In relation to gravity and magnetic prediction of top basement, template rifts tend to follow pre-existing lines of weakness and the upper crust underlying these rifts may have geophysical characteristics which are different from those of the rift flanks. Template rifts may or may not be associated with volcanicity. In sparsely volcanic rifts, syn-rift volcanism is located on rift axis structural highs. Extensive volcanism is accompanied by regional doming and erosional loss of the pre-rift section is likely in rift sectors preceded by volcanism. Consequently, it is doubtful whether pre-rift sediments are preserved beneath the major volcanics offshore from the Parana-Etendeka areas in the South Atlantic. The Kalahari Group of southern Africa is identified as a good analogue for pre-rift reservoir objectives in the South Atlantic, and facies and thickness criteria are discussed that help discriminate pre-rift sequences from rift sequences. Where sedimentation has kept pace with rift subsidence, so that fault scarp facies have not developed, then the Okavango Delta provides a useful analogue. Parallels are drawn between major sand dykes interpreted from seismic in the deepwater deposits of Lake Tanganyika and similar units within similar facies in the Recôncavo Basin of Brazil. Direct analogue links between the Red Sea and the South Atlantic are less strong, mainly because marine incursions, and salt deposition, occurred at a later structural stage in the South Atlantic. The onset of salt deposition in the Gulf of Suez-Red Sea occurred whilst rift block faulting was still in progress, and the post-rift sag phase, though partly salt filled, rapidly turned to fully-marine deposition. The earliest part of the post-rift sag phase in the South Atlantic is represented by laterally extensive lacustrine source rocks. Salt deposition in the South Atlantic occurred only after full development of the sag phase.
