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Abstract: Seismic profiles extending from the southern South African shelf into the deep sea reveal a strong erosional activity, which affects large parts of the continental margin. Quaternary to Oligocene units and, in places, the complete sedimentary column appear to have been removed. Mass movements were previously considered as the origin of this erosion. However, structures indicating slumping can be identified in only a few places. The erosional activity is confined to specific water depths, which correlate well with the activity levels of water masses observed here. We thus suggest that the Agulhas Current, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottom Water have intensively shaped the sedimentary units over a long period. This indicates a stable circulation comparable to that at present day since the Neogene. [Copyright &y& Elsevier]

Abstract: The Upper Cretaceous is commonly associated with greenhouse climate, Oceanic Anoxic Events (OAE), and the ongoing break up of Gondwana, which resulted in strong variations of the ocean''s currents flow paths and climate change. Little is known about these changing conditions, in particular south of South Africa. A high-resolution seismic reflection data set from the submarine Transkei Basin off South Africa revealed various depositional stages for this region for the past ∼90Ma. In these seismic sections, a recurrently appearing high-amplitude horizon within weak to homogeneous Upper Cretaceous reflections was observed. Due to the inaccessibility of any drill hole data from the Transkei Basin and adjacent regions, the origin of this reflector is difficult to specify. It is roughly dated at ∼80–∼85Ma, which falls within the last big OAE in Upper Cretaceous (OAE 3). A seismostratigraphic analysis led to an exclusion of several features as the origin for this reflector, such as gas hydrates, opal bearing horizons or biogenic oil. Other possible features, such as lithologic variations, intrusions or black shales, are discussed in more detail. According to its appearance and reflection characteristics it could possibly be the first report of black shales in this region. [Copyright &y& Elsevier]

The gateway south of South Africa allows the exchange of water masses between the Indian/Pacific Oceans (warm surface water masses) and the Atlantic (cold deep and bottom water masses). This heat transfer maintains the global conveyor belt. Hence the South African continental margin represents an important gateway, whose detailed evolution is not yet fully understood. A compilation of the evolution of the oceanic circulation within the South African gateway since the Early Eocene on the basis of seismic reflection data is put forward and discussed. The effect of a proto-AABW can be found in the southern Cape Basin and south of South Africa its early its Early Eocene-Early Oligocene. In the period Early Oligocene-Middle Miocene a current: equivalent to AAIW/Agulhas Retroflection leaves its oldest traces identifiable on the eastern Agulhas Plateau. Indications for the Benguela Current can be found in the Cape Basin in Middle Miocene times. With the onset of NADW in the period Middle Miocene-Early Pliocene the branch of AABW flowing through the Agulhas Passage is weakened and finally deflected to the south in Early Pliocene-Holocene times. [ABSTRACT FROM AUTHOR]

The southern margin of South Africa developed as a consequence of shear motion between the African and South American plates along the Agulhas-Falkland Transform Fault during the Early Cretaceous break-up of Gondwana. The Agulhas-Karoo Geoscience Transect crosses this continental margin, and includes two combined offshore-onshore seismic reflection/refraction profiles. We present results from the western offshore profile. Using ocean-bottom seismometers and a dense airgun shot pattern, a detailed image of the velocity -- depth structure of the margin from the Agulhas Passage to the Agulhas Bank was derived. Modelling reveals crustal thicknesses of between 7 km and 30 km along the profile. The upper crust has P-wave velocities of between 5.6 and 6.6 km/s and the lower crust has velocities that lie between 6.4 and 7.1 km/s. Uppermost mantle velocities range from 7.8 to 8.0 km/s. The 52 km wide continent-ocean-transition zone, where the Moho rises steeply, occurs at the Agulhas-Falkland Fracture Zone. Beneath the Southern Outeniqua Basin and the Diaz Marginal Ridge, a zone of relatively low velocities (~5 km/s) with a thickness of tip to 3 km, can be discerned in the upper crest. We interpret this zone as an old basin filled with pre-break-up metasediments, which may be related to the Cape Fold Belt. We suggest that the origin of the Diaz Marginal Ridge is bound up with the tectonic history of this basin as it exhibits a similar velocity structure. Almost no stratified sediment cover exists in the Agulhas Passage because of strong erosion due to ocean currents. The velocity structure and seismic reflection results indicate the presence of alternating layers of volcanic flows and sediments, with a mean velocity of about 4 km/s. We suggest these volcanic flows were an accompaniment of possible re-activation events of parts of the Agulhas-Falkland Fracture Zone. Tectonic motion seems to be sub vertical instead of strike-slip along the re-activated part of the fracture zone. Therefore, a relation to the uplift of the South African crust may be possible. [ABSTRACT FROM AUTHOR]