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Development of the Neogene current system in the Cape Basin as recorded in the seismostratigraphy

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  • Archaeology
  • Earth Science

Abstract

The sedimentary sequences of the continental margin off southwest Africa have been shaped by different currents. At the surface, the Benguela and Agulhas Currents are active, in greater depths the Antarctic Intermediate Water (AAIW), the North Atlantic Depth Water (NADW) and Antarctic Bottom Water (AABW) take effect. Our aim is to unravel the development and modifications of those current systems. Especially we want to concentrate on the onset of the Benguela Current Upwelling System, which represents one of the largest upwelling regions worldwide and is closely related to the developement of the climate in southern Africa. Significant events as incursions of warm or cold waters, terrigenous input, climate changes and slumpings are recorded in the deposited material. The sedimentary properties are expected to be reflected in the signal structure of the seismic data from this area.In this contribution we will present a seismic stratigraphy for the Neogene sedimentary layers in the entire Cape Basin on the base of results of the ODP Leg 175 Sites 1081-1087 which are correlated with seismic data. With a maximum depth of 610mbsf (Site 1085a) the sites span the time period between Holocene and late Miocene (0-15 Ma). Corelog parameters as for example velocity, density, lithostratigraphy, carbon and carbonate content are linked to the seismic sections via synthetic seismograms. This procedure enables an interpretation of the origin of reflectors and a spatial extrapolation of sites informations.The depositional enviroment in the Cape Basin can be divided roughly into three major areas and is reflected in a change in stratigraphy from north to south: Deposition in the northern regions is influenced by eroded sediments from the Walvis Ridge and by upwelling sediments. Sediments of the middle Cape Basin are dominated by the presumable most productive coastal upwelling cells. The depositions in the southern Cape Basin contain the most imprint of incursions from the south as the Agulhas Current.All seismic sections show a typical structure of a prograding shelf edge. For the northern Cape Basin (the area of Sites 1081-1083) at least three units can be defined according to the reflection pattern. The upper unit consists of closely spaced high amplitude and high frequency reflectors. The second unit is characterised by lower amplitude and lower frequency reflectors. Below the reflection pattern changes to wide spaced reflectors of high amplitude but low frequency.Two major units can be identified for the middle Cape Basin at Site 1084. The upper one consists of a sequence of numerous closely spaced strong amplitude reflectors. The unit below is characterised by low frequency reflectors in general with weaker amplitudes. This could be an expression of a lithological change from organic rich material above to more carbonaceous sediments in unit 2. The transition between those two units is marked by an interval of hummocky to chaotic reflectors indicating mass movements.Towards the south at Sites 1085-1087 four units CB-1 to CB-4 were defined for the Neogene and Quaternary sediments. The upper unit CB-1 (covering the last 1.5 Ma) represents global cooling and glacial-interglacial cycles, characterised by continuous high-amplitude reflectors. Unit CB-2 (~14 Ma) we associate with the onset of the Benguela Current Upwelling System. It shows a seismic pattern of low-amplitude reflections and numerous traces of sliding events. The lower units CB-3 and CB-4 (~17-35 Ma) again are characterised by continuous high-amplitude reflections, indicating carbonate dissolution events.The deeper structure on all seismic sections reveals a band of high amplitude reflectors similar all over the Cape Basin. This sequence is tentatively interpreted as reflector D. An uppermost Paleocene age was assigned to this reflector a DSDP Site 360. Extrapolations of sedimentation rates of Site 1085, 1086 and 1087 reveal a decreasing age for this feature northwards.

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