Early Pliocene change of deposition style in the Cape Basin, southeastern Atlantic

Many effects of climate change, currents and tectonics on sedimentary deposition can be detected in seismic reflection patterns. To unravel the influence of these different forces on sediments of the southwest African margin, we established a seismostratigraphy, tied to the drilling results from Ocean Drilling Program Leg 175 Sites 1081-1087. Three depositional regions are distinguished for the Cape Basin: In the Northern and Middle Cape Basin, continuous high-amplitude reflectors characterize the upper acoustic sequences, indicating a strong, and probably climate-related, alternation in deposited matter, as well as an undisturbed deposition accompanied by high sedimentation rates due to enhanced upwelling. In the Middle Cape Basin, large reflection-free zones indicate the presence of gas. In contrast, in the Southern Cape Basin, less upwelling-related deposition restricted the thickness of sedimentary layers. Low-amplitude reflectors and slump scarps indicate a permanent reallocation of accumulated matter by mass movements on the steeper slope of the southern margin. A prominent feature in the sequences of the Northern and Middle Cape Basin is a marked increase in seismic reflection amplitudes since the Early Pliocene (ca. 4.4 Ma) in correspondence to a rise of cyclicity in reflector strength. We suggest a reorganization in the depositional system associated with enhanced upwelling as a consequence of the closure of the Central American Sea-way. The upwelling system probably became more susceptible to short pulse variations ( approximately 10 k.y.) in climate and currents. With it the composition of deposited matter severely changed, which in turn is documented in the high-amplitude reflectors.