African Atlantic Margin 



The east atlantic continental margin work of the African 

 coast was designed by K. O. Emery of the Woods Hole 

 Oceanographic Institution in 1971 to learn more about the 

 separation of Africa and South America and the subsequent 

 history and development of the African continental margin and 

 deep-ocean floor. The first major shipboard investigation to be 

 completed was the continental margin off southwestern Africa. 

 This area lies southeast of the Walvis Ridge and includes the 

 western part of Agulhas Plateau, Agulhas Basin, Cape Rise, and 

 Cape Basin. Traverses approximating 17,000 km of continuous 

 gravity, magnetic, and seismic-reflection profiles were recorded 

 to determine the structure of the continental margin from 

 Cape St. Francis to Walvis Ridge, and of the adjacent Agulhas 

 and Cape deep-ocean basins (figs. 10 and 11). 



These and previous sea-floor and land data suggest that 

 basement structures are the result of the breakup of Gondwana- 

 land and the dispersion of the fragments to their present posi- 

 tions. This breakup may have been initiated as early as the 

 Carboniferous Period, but most of the dispersion has taken place 

 since Middle Jurassic. Igneous activity during the early phase 

 may have led to the emplacement of ridges along the conti- 

 nental margin. Later, block faulting and volcanism along the 

 fracture zones that delineate the flow lines of the drifting con- 

 tinents produced Walvis Ridge, Cape Rise, and the Agulhas 

 Plateau. One of these fracture zones, the Agulhas fracture 

 zone, dominates the structural grain of the continental margin 

 and deep-ocean floor off the African southern coast. 



Sediments as thick at 7 km buried the fragmented conti- 



20° 



25° 



30° 



35' 



40' 



Figure 10. — Major physiographic units of southwest Africa 

 continental margin. 



nental basement and adjacent oceanic basement off the west 

 coast and formed a broad continental rise and abyssal plain 

 within Cape basin. This work is described by Emery and 

 others (1975). 



Work completed on the west African continental margin 

 between Angola and Sierra Leone includes about 30,750 km 

 of geophysical traverses (seismic reflection and refraction, 

 magnetics, and gravity) by the RV Atlantis 11. These tra- 

 verses, supplemented by about 50,000 km of traverses by 

 other ships, provide a basis for mapping and understanding the 

 geological structure, history, and origin of the region. 



The deep indentation in the coastal configuration of west- 

 ern Africa is paralleled by a similar bend of the Mid-Atlantic 

 Ridge, and by the prominent bulge of northeastern Brazil. 

 These sharp bends are due to left-lateral offsets by numerous 

 transform faults in a belt of equatorial fracture zones. Some 

 of the fracture zones continue eastward to intersect the entire 

 length of the east-west coast of the Gulf of Guinea and to 

 penetrate the continent at the Benue Trough or graben. Valleys 

 of the fracture zones have been sites of sediment deposition, 

 whereas the ridges have served as dams that force the sedi- 

 ment to move westward. This work is described by Emery and 

 others (1975). 



Where enormous quantities of sediment have been deliv- 

 ered to the ocean by the Niger-Benue rivers, a large delta has 

 deeply buried the irregular topography of the fracture zones. 

 The entire belt of fractured ocean floor, continental margin, and 

 continent derives its structural, physiographic, and stratigraphic 

 characteristics from lateral movement, or translation, of the 

 ocean floor with respect to the continent. Petroleum prospects 

 appear to be far greater in the Niger Delta and the region of 

 divergence south of it than in the entire region west of the delta. 

 The favorable continental margin contains thicker sediments, 

 large ancient and modern deltas, and salt and mud diapirs 

 produced by the weight of overlying sediments (Emery, 1974). 



Marine geophysical data received last year by NOAA 

 Environmental Data Service's National Geophysical and Solar- 

 Terrestrial Data Center include 24,000 nautical miles of bathy- 

 metric, magnetic, gravity, seismic-reflection profile, and 3.5-kHz 

 echo sounder data. 



Southwest Atlantic Margin 



Since early 1972 the National Science Foundation has pro- 

 vided support for a geophysical study of the continental margin 

 of Argentina and Brazil. This study is being conducted by the 

 Lamont-Doherty Geological Observatory in cooperation with 

 scientific and governmental agencies of Argentina, Brazil, Chile, 

 and the United Kingdom. The overall objective is to determine 

 the present structure of a rifted margin, and to infer from this 

 structure the early history of continental rifting. 



Research and study of the Brazilian Margin during the 

 year has made it possible to: determine the age and evolution 

 of the Amazon Cone; determine the origin and age of com- 

 pressional structures on the shelf off Brazil; acquire a better 

 understanding of the westward extensions of the equatorial 

 fracture zones, and of the consequent suggestion that the North 

 Brazilian Ridge consists only of marginal fracture ridges (a 

 hypothesis substantiated by two-ship refraction profiles); out- 

 line the sedimentary history of the Fernando dc Noronha ridge, 

 basin, and Equatorial Mid-Ocean canyon; better understand 

 the sedimentation processes on the Brazilian continental mar- 



28 



