Description:

The Northern Hemisphere Glaciation modified the global current system leading also to changes in the local current situation in the Cape Basin. This interaction of climate and oceanic currents left a significant imprint in the sedimentary structures along the continental margin off southwestern Africa. Additionally, tectonic movements shaped the sedimentary sequences. To gain an understanding about the influence of the different factors on the build up and composition of the sedimentary layers in the Cape Basin the

1. spatial distribution of seismostratigraphic units (event horizons),
2. physical and chemical properties of sediments at event horizons, and
3. climate information of the hinterland

need to be extracted from seismic profiles and sedimentological parameters. The combination of reflection seismic records with drill site information assisted by palynological data will provide a comprehensive information of the sedimentary regime in the Cape Basin and elucidates the effect of the Initiation of the Northern Hemisphere Glaciation on the depositional environment although located far away in the South.

As a first important result we found that the acoustic reflectors and discontinuities own a laterally marked continuity on the existing seismic lines. Thus, they can be traced about 250 km over the northern Cape Basin, along the outer shelf, slope and slope basin. We suggest that the continuity of the reflectors indicates a relatively widespread and uniform depositional environment.

A second focus of our integrated investigations of reflection seismic and core data is reflector NCB-B. Reflectors below this interface (in unit NCB-1c) are configured oblique and terminate in toplap style to it. The reflectors above (of unit NCB-1b) onlap onto interface NCB-B. This outbuilding of the slope in unit NCB-1c indicates a low relative sea level (Sangree and Widmier, 1979). Since we can identify interface NCB-B 250 km along the margin of the Northern Cape Basin we infer a large regional change in deposition regime. It coincides with a marked change of pollen assemblages. The palynology of ODP Site 1082 revealed profound changes in the terrestrial environment of southwestern Africa at 2.2 Ma. The accumulation rate of pollen suddenly drops from 50-60 pol/a/ccm in older layers to 8 pol/a/ccm in average after 2.2 Ma. This marked reduction of pollen input into the ocean is interpreted as the result of a loss of a perennial river discharge, indicating in turn a change of hinterland climate from humid to drier conditions in the late Pliocene (Dupont et al., submitted).

As a third result we can show that since the Middle to Upper Miocene boundary (~10 Ma) the deposition on the outer shelf almost starved or even erosion took place. The four seismic units NCB-1a to NCB-1c, and NCB-2 own their highest thickness on the slope and pinch out at the shelf break. This configuration documents that progradation prevailed on the slope since the Upper Miocene. Slumping features indicate a high instability of deposited material, which in turn we associate with strong increase in sedimentation rate. The age of the unit corresponds to the end of the Mid-Miocene climate optimum associated with a strong sea level drop at 11 Ma (Haq et al., 1987). Siesser (1980) dated the onset of modern Benguela Current circulation at 10 Ma, associating enhanced productivity with stepwise increase in organic-carbon accumulation observed at DSDP Site 532. Diester-Haass et al. (1992) suggested a progressively northward shift of the Benguela Current since the middle Miocene, initiating upwelling in the Walvis Ridge area at about 10.5 Ma. We found slumping traces in unit NCB-2 to confirm a high instability of deposited material during these times, and associate the establishment of the upwelling system with the sediments accumulated since unit NCB-2.

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