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Living Africa: A critical overview and scientific advances

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Compton, J. , Di Primio, R. and Uenzelmann-Neben, G. (2007): Living Africa: A critical overview and scientific advances , Inkaba yeAfrica workshop, 10th SAGA Biennial Technical Meeting and Exhibition, 22 - 26 Oct, 1007, Wild Coast Sun, South Africa. .
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Abstract:

Living Africa has been successful in advancing our understanding of the post-rift evolution of the western (Orange Basin) and southern margins primarily from seismic profiles. It has also been successful in our understanding of the late Cenozoic evolution of the margin and deep basins through seismic profiles and the sedimentology and geochemistry of recovered sediment cores. The results to date are clearly linked and it remains for the three subprojects to coordinate a meaningful synthesis. The principal scientific advances can be divided into post-drift (mid Cretaceous) hydrocarbon basin modelling (GFZ and AWI) and the late Cenozoic sedimentary record (AWI and UCT). The sedimentary and structural evolution of the post-rift sedimentary basin has been reconstructed for the offshore Orange Basin from the interpretation of approximately 6000 km of 2D seismic data. A 3D petroleum systems model implies an active kitchen area generating hydrocarbons in the outer western part of the basin. Gas leakage features mapped in the seismic sections relate to structural and sedimentary development with migration modelled to occur along stratigraphic horizons towards the inner near-shore part of the basin. Further modelling will constrain the pathways, timing and duration of hydrocarbon migration events and a basin-wide quantification of the thermogenic gas flux to the ocean/atmosphere as a function of geologic time. The analysis of high-amplitude seismic reflections, so called bright spots, indicates anoxic conditions between ~90 Ma and ~80 Ma for the region south of South Africa. These bright spots may correlate to worldwide Oceanic Anoxic Event 3 and represent important hydrocarbon source rocks (black shales).It has become increasingly evident that the southern Africa region and the surrounding Southern Ocean are critical components of the global Earth system and that it is essential to understand the role these areas have had and continue to have in the distribution of heat by the ocean and changes in marine productivity related to continent-ocean interactions. The late Cenozoic geologic history, so sparsely preserved on the continent, has been advanced through the analysis of high-resolution seismic reflection data (AWI) and geochemistry of sediment recovered by coring (UCT). Palaeo current reconstruction of North Atlantic Deep Water and (proto-) Antarctic Bottom Water (AABW) reveals changes in the flow paths and flow strengths since the late Eocene. The initial opening of the Drake Passage to deep water flow and establishment of the Antarctic Circumpolar Current had a large impact on the formation of AABW and the onset of upwelling in the southern Benguela region as indicated by the first major phosphogenic episode at 26-24 Ma. Seismic reflection data indicate reorganization of deep ocean circulation associated with global climate transitions of the Oligocene, middle Miocene and Pliocene. Integration of litho- bio- and Sr-isotope- stratigraphy reveals several major factors influencing the evolution of the margin: 1) neotectonic uplift in the middle Miocene and Pliocene, 2) sea-level fluctuations related to global glaciation, and 3) upwelling and the supply of terrigenous mud and nutrients to the margin as indicated by authigenic minerals such as phosphorite and glauconite. Continental slope records over several glacial to interglacial climatic cycles as well as high-resolution Holocene records from the mudbelt on the western margin indicate that terrigenous mud delivered by the Orange River has increased by a factor of 8 as a result of anthropogenic land use changes. Storage of terrigenous mud and organic carbon on the shelf during interglacial highstands alternate with resuspension and dispersal of shelf mud during glacial lowstands and thereby vary the flux of nutrients (P and Fe) to the open ocean biological pump. IyA Living Africa Publications/thesesCompton, J.S., Wigley, R. and McMillan, I. 2004. Late Cenozoic phosphogenesis on the western shelf of South Africa in the vicinity of the Cape Canyon. Marine Geology 206, 19-40.Wigley, R. and Compton, J.S. 2006. Late Cenozoic evolution of the outer continental shelf at the Head of the Cape Canyon, South Africa. Marine Geology 226, 1-23.Wigley, R. And Compton, J.S. 2007. Oligocene to Holocene glauconite-phosphorite pellets from the Head of the Cape Canyon on the Western Margin of South Africa. Deep-Sea Research II 54, 1375-1395.Compton, J.S. and Maake, L. 2007. Source of the suspended load of the upper Orange River, South Africa. South African Journal of Geology 110, .Herbert, C. And Compton, J.S. 2007. Geochronology of Holocene sediments on the western margin of South Africa. South African Journal of Geology 110, . Wiltshire, J. 2007. Late Quaternary sediments on the continental slope off Cape Columbine: Variations in off-shelf transport related to glacial driven sea-level fluctuations. Unpublished MSc thesis, University of Cape Town.Schlüter, P.; Uenzelmann-Neben, G. (2007), Seismostratigraphic analysis of the Transkei Basin: A history of deep sea current controlled sedimentation. Mar. Geol., 240(1-4), 99-111., doi:10.1016/j.margeo.2007.02.015.Schlüter, P., Uenzelmann-Neben, G. (2007). Indications for bottom current activity since Eocene times: The climate and ocean gateway archive of the Transkei Basin, South Africa, Global and Planetary Change. doi:10.1016/j.gloplacha.2007.07.002.Schlüter, P., Uenzelmann-Neben, G. (in review). Conspicuous seismic reflections in Upper Cretaceous sediments as evidence for black shales off South Africa, Mar. Pet. Geol.

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