| Popis: |
The evolution of the Southern Outeniqua Basin has been analysed within a tectono-stratigraphic framework by the interpretation of 3,700km of 2D seismic data constrained by five exploration wells, drilled on the adjacent shelf. The mapping of different fault families into distinct structural orientations, NE-SW, NNE-SSW and NW-SE, was done and the basin sub-divided into four sectors (far-west, west, central and east) based on the geographical position of these faults. Sequence stratigraphic analysis of the basin lead to the identification of seven secondorder unconformities which have been further defined into five main seismo-stratigraphic units. The variation in major structural orientations suggest the evolution of the basin developed during three phases of extension, which have been related to the broader plate tectonic context during the Mesozoic break-up of Gondwana. Early Jurassic (170-150Ma) rifting between West and East Gondwana is proposed to have created weaknesses in the crust along the Southern African margin in a NNE-SSW orientation leading to the development of small rift basins and the deposition of syn-rift sediments. A series of intra-continental strike slip faults, including the proto-Agulhas Falklands Fracture Zone, are proposed to have developed along the southern margin during the incipient formation of the Weddell Sea (147Ma) between Antarctica and West Gondwana. Later mid-Jurassic (150-130Ma) extension along the North-Falkland rift zone lead to the oblique extension along the southern margin and the development of normal faults orientated NE-SW. Continued rifting (134Ma) in the proto-South Atlantic lead to translation along the southern African margin and the coalescence of the individual segments of the intra-continental strike slip faults and ultimately the formation of the Agulhas Falklands Fracture Zone. This phase of wrench faulting (134- 130Ma) is characterised by the strike-slip deformation of syn-tectonic sediments defined by second-order unconformities BR and B - late Valanginian - Hauterivian (134-130Ma). The Diaz Marginal Ridge (DMR), which bounds the Southern Outeniqua Basin and AFFZ to the south and north respectively, shows a marked variation is seismic character along strike. In the eastern and western sectors of the basin the DMR is made of syn-rift sediments. The discontinuous and non-parallel seismic character of the DMR in the central sector of the basin is interpreted to be continental basement. Significant bevelling of the top of the DMR in this sector, along with on-lapping reflector geometries of horizon B, Hauterivian (130Ma), is proposed to signify that the continental block existed prior to the formation of the Southern Outeniqua Basin and remained a basement promontory until the development of horizon G - Tertiary (66Ma). The Southern Outeniqua Basin is therefore proposed to have formed through four periods of tectonism: 1) early Jurassic (170-150Ma) orthogonal rifting of West from East Gondwana; 2) late Jurassic to early Cretaceous (150-130Ma) oblique extension and translation along intra-continental strike-slip faults; 3) early-late Cretaceous deformation associated with transverse motion along the AFFZ and development of an active transform margin; 4) mid-late Cretaceous thermal uplift of the Southern Outeniqua Basin associated with migration of the Natal Valley spreading centre along the AFFZ. Thermal subsidence and true passive transform margin conditions developed in the Southern Outeniqua Basin from early Tertiary to present day. |
