Reconnaissance study of the ancient Zaire (Congo) deep-sea fan. (ZaiAngo Project)

Analysis of more than 19,000 km of multichannel seismic reflection data from the ZaiAngo Project, covering the Zaire deep-sea fan, allowed us to identify the oceanic crust and five seismostratigraphic units on the basin floor and abyssal plain. In the absence of a direct stratigraphic tie, the time frame is provided by long distance correlations. A stratigraphic model for the evolution of the Zaire deep-sea fan is proposed. A widespread major regional unconformity, representing probably the Eocene–Oligocene transition, marks a drastic change of sedimentation pattern in the basin floor: a pelagic aggradational sequence (Albian–Eocene) overlying the oceanic crust gives way to an onlapping prograding sequence of turbidite deposits (Oligocene–Recent). This change marks the onset of the ancient Zaire deep-sea fan, triggered by the climatic change related to the greenhouse–icehouse shift at the Eocene–Oligocene transition, which was responsible for a drastic increase in continental erosion and terrigenous sedimentary supply to the margin. The volume of the fan is at least 0.7 Mkm3, much broader and thicker than formerly assumed. A short-lived episode of rapid facies progradation/retrogradation is recorded sometimes during the Miocene; triggering factors for this event are likely to have a tectonic and/or climatic origin.