| Popis: |
Late Mesozoic sedimentation off the south coast of South Africa records a history of initial continental rifting (?Middle-Late Jurassic to latest Valanginian), followed by a transitional episode (latest Valanginian to Early Aptian) and a drifting episode (Early Aptian to present day), as Africa separated from South America. Rifting appears to have been initiated by separation of East and West Gondwana during the Middle to Late Jurassic. Sediments associated with rifting are now confined to four major basins —Bredasdorp, Pletmos, Gamtoos and Algoa, which are underlain and bounded by rocks of the Ordovician-Devonian Cape Supergroup that form prominent arches between the basins. During the rifting phase when half-graben basin styles were typical, sediments accumulated in a wide range of environments (non-marine to slope). In the Bredasdorp Basin, where major bounding faults are less well developed, sediments were laid down in non-marine and marginal marine environments resulting in widespread development of red and green claystones overlain by clean, porous glauconitic littoral sandstones. In contrast, transitional early drift sedimentation, which began after a major regional unconformity (seismic horizon 1At1) in the latest Valanginian, is characterised by deep-marine, poorly oxygenated conditions. The pre-1A, 13A and 14A sequences are of considerable economic significance for hydrocarbons, particularly in the Bredasdorp Basin where commercial gas and condensate production began in 1992. Late drift sedimentation since Late Aptian has occurred in generally well-oxygenated environments, and has led to the steady southwards development of the continental shelf and the formation of an elongate basin parallel to the relict shelf break. This basin, the Outeniqua Basin, is composed of essentially mid-Aptian to Maastrichtian deposits, and overlies the pre-existing rift basins with a transverse structural grain. |
