Active Turbiditic Sedimentation during Highstands of Sea-Level: The Large Mud-Rich Zaire Turbidite System (South-Eastern Atlantic Ocean)

L. Droz, F. Rigaut, P. Cochonat, and R. Tofani

The Zaire Fan developed over 900 km off the Congo, Zaire and Angola continental margins. Its activity is indicated by frequent cable breaks at the axis of the Zaire Canyon and by the occurrence of more than 10 m of Holocene muddy turbidites near the top of levees.

This present-day activity ranks the muddy Zaire Fan in an original class of fans, intermediate between the inactive large muddy fans such as the Amazon fan, and the active but smaller and sandy fans such as the Monterey or Var Fans, and provides the opportunity to characterize depositional processes, facies and rates of sedimentation during highstands of sea level.

The morphology and the architecture of the Fan are very like inactive muddy systems: morphological parameters and sinuosity of the channel are comparable to that of the Amazon Channel, suggesting that this morphology a remnant of lowstand periods.

In contrast, the highstand activity of the Zaire Fan is well expressed on the EM12 acoustic imagery and 3.5 kHz profiles; it appears to be both depositional on levees and erosional inside the channel and restricted to the axial part of the fan, on both side of the youngest channel. Depositional activity on levee includes a restricted late stage of muddy deposition and an early stage of siltier deposition. Sedimentation rates of more than 58 cm/ka characterize recent deposition on the levees. Outside the axial active part of the fan, sedimentation rates are much lower (2,3 cm/ka).

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California