Abstract

The Zaire (Congo) turbidite system developed in an east-west direction off the Congo, Zaire, and Angola continental margins in the southeastern Atlantic Ocean. It includes an incised canyon that deeply penetrates into the Zaire River estuary, and a deep-sea fan that extends over ≈900 km toward the Angola abyssal plain. Cable breaks at the canyon and channel axis and occurrence of Holocene muddy turbidites on the fan attest to the present-day activity of the Zaire system. This activity is linked to the connection between the canyon and the river, which allows a permanent feeding of the deep basin. Being the only large mud-rich fan known to have been active during the Holocene highstand of sea level, the Zaire fan is unique among other fans and constitutes an original model, intermediate between the large inactive muddy fans and the smaller active sandy fans.

The morphology as well as the architecture of the Zaire fan are not strongly different from inactive mud-rich systems. The fan is characterized by nine tightly meandering channels at the axial part of stacked-up, lens-shaped channel/levee bodies with typical high-amplitude channel facies and stratified levee facies on seismic profiles. These channel/levee bodies are suggested to belong to two superimposed turbidite systems. The Zaire system includes the youngest channel and the southern fossil channels, while the ancient Kouilou/Niari system includes the northern channels. Morphological parameters and sinuosity of the youngest channel are comparable to that measured for the presently inactive Amazon fan. This observation suggests that the present-day morphology is mainly a remnant of the lowstand period, the highstand period being, in the case of active systems, a period of reshaping by erosion.

The highstand activity of the Zaire fan is well expressed on the EM12 acoustic imagery and 3.5 kHz profiles. This activity is restricted to the present-day axial part of the fan, on both sides of the youngest channel, and appears to be both depositional on the levees and erosional inside the channel. Depositional activity on the levee-overbanks includes a restricted late stage of fine-grained sediment deposition and an early stage of coarser-grained sediment deposition. The two depositional stages are separated by a subtle unconformity and are associated with strong backscatter variations, from very low close to the channel (late fine-grained turbidites of Holocene age) to very high (early coarser-grained turbidites) laterally away. Areal distribution of these recent deposits suggests a 60-km-long zone of sediment by-passing or reduced deposition at the canyon mouth during the Holocene.

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