Abstract

Multichannel seismic data newly acquired during two ZaïAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood.

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