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Reconstruction of the Neogene and Quaternary Depositional System in the Northern Cape Basin as Recorded in the Seismostratigraphy

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Citation:
Weigelt, E. , Uenzelmann-Neben, G. and Dupont, L. (2005): Reconstruction of the Neogene and Quaternary Depositional System in the Northern Cape Basin as Recorded in the Seismostratigraphy , ICDP und IODP/ODP Kolloquium, 14-16 Mar., Potsdam, Germany. .
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Abstract:

The sedimentary sequences of the continental margin off southwest Africa have been shaped by different forces: So, the interaction of climate, oceanic currents and sea level fluctuations left a significant imprint in the sedimentary structures along the continental margin. Additionally, tectonic movements affected the sedimentary sequences. Our aim is to unravel the development and modifications of the depositional system in the Northern Cape Basin. Especially we want to concentrate on the effect of the Initiation of the Northern Hemisphere Glaciation on the depositional environment.As a first step we present a seismostratigraphic model for the Neogene and Quarternary sedimentary layers in the Northern Cape Basin based on a combination of reflection seismic lines with drill site records of the ODP Leg 175 Site 1082 assisted by palynological data. The drill site data were converted from depth domain to two-way traveltime domain by sonic logs calculated from P-wave velocities of the site data. This procedure provides the mapping of seismostratigraphic units and reflectors (event horizons) and enables an interpretation of their origin.We defined seven seismic units NCB-1a to NCB-5 for the Cenozoic sediments in the Northern Cape Basin according to the reflection pattern. These units, their acoustic base reflectors as well as some 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 striking observation is an unconformity dated at 2.2 Ma between units NCB-1b and NCB-1c. The reflectors above onlap onto this interface (named NCB-B) whereas the internal reflectors of unit NCB-1c show toplap termination. The outbuilding of the slope in unit NCB-1c indicates a low relative sea level. In contrast the sigmoid reflection configuration in unit NCB-1b is an indication for a gentle rise of relative sea level which indeed corresponds to a rise in eustatic sealevel. 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.As further point we found 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. The onset of modern Benguela Current circulation is dated at 10 Ma, associating enhanced productivity with stepwise increase in organic-carbon accumulation. 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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