Reconstruction of the Cenozoic Development of the Angolan Continental Margin

Luc Lavier, Frederic Brigaud, Michael S. Steckler

The Cenozoic development of the Angolan continental margin has been greatly affected by the large sediment input fron the Congo River and by extensional rafting of large blocks on the Aptian salt. In addition, there are major unconformities indicating significant erosion of the continental shelf, particularly in the Oligocene. The lateral flow of the evaporites and seaward rafting of sedimentary blocks also interferes with the determination of the tectonic and thermal history of the margin. To overcome these difficulties, we have combined structural reconstruction with one- and two-dimensional backstripping, and forward modeling. These enable us to develop a simple, coherent tectonic history. Due to salt withdrawal, the reconstructed synrift section, and amount of extensi n, at the shelf is significantly larger than naive backstripping indicates. The margin had a gentle ramp morphology during the Eocene, prior to major salt withdrawal and extensional rafting. The breakaway zone for the detachment faulting over the salt developed at the hinge zone of the passive margin. We interpret the tilting of the margin at the hinge zone to have helped localize the site of extensional rafting. The growth faulting is the result of rapid salt withdrawal, and sedimentation could not initially keep pace with the subsidence created by these sedimentary tectonics. The Oligocene erosion on the shelf was a response on the shelf to the salt and raft tectonics. At this stage, the shelf edge retreated landward by 30 km. Subsidence on the shelf due to salt removal created the acc mmodation on the shelf for the thick Oligo-Miocene section. These sediments, supplied by the Congo River, have resulted in the seaward progradation of the shelf, which now has a flat shelf-steep slope morphology. This scenario, in which all of the Cenozoic tectonics are superficial, successfully reproduces the subsidence, paleobathymetry and thermal regime of the Angolan continental margin.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995