5° 



Figure 13. — African Atlantic Margin Project. 



Using seismic reflection and refraction data collected dur- 

 ing cruises over the past 3 years (fig. 14), Lamont-Doherty 

 scientists determined that a diapiric zone extends along the 

 Brazilian margin for approximately 18'' of latitude (fig. 15). 

 At its southern end, the zone is approximately 350 miles wide 

 where it forms the Sao Paulo plateau. A comparison of the 

 Brazilian diapiric zone boundaries with the west African zone, 

 which extends 12 of latitude between Equatorial Guinea and 

 Angola, suggests that in early Cretaceous time the two zones 

 were contiguous and the salt deposits resulted from restricted 

 circulation of marine waters that were enriched in minerals by 

 volcanism during the development of the South Atlantic Ocean. 



Investigations of the continental margins of the South 

 Atlantic under the Seabed Assessment and other programs 

 served as excellent site surveys for subsequent Deep-Sea Drill- 

 ing programs. Three legs were drilled in the South Atlantic. 

 Leg 59 drilled along the coast of Brazil (Sao Paulo and Rio 

 Grande Rise ) and the Argentina basin. Leg 60 drilled off south- 

 west A.frica (Walvis Ridge and Angola Basin). Leg 61 drilled 

 a series of sites olT northwest Africa. Much of the geophysical 

 data is now being correlated with the drilling data. Perhaps the 

 most significant discovery was the widespread occurrence of 

 black shale in the lower part of the geological section, evidence 

 that the nascent Atlantic was cut off from circulation with the 



open ocean. The Walvis Ridge and Rio Grande Rise appear to 

 have formed a continuous barrier that separated the Cape- 

 Argentine Basin from the Angola-Brazil Basin and produced 

 wide-spread stagnant water conditions (fig. 16). These basins, 

 separated by the ridges, were in eflfect suffocated by lack of 

 oxygen for a period of 20 million years. Similar environments 

 exist today in the Black Sea where black oil shale forms on the 

 ocean bottom. Such conditions appear to have produced vast 

 deposits of black oil shale in the early Atlantic basins. Geologic 

 processes, however, failed to provide the heat necessary to 

 reduce the oil shale to usable oil. 



Studies from the sea floor and land area of South Africa 

 suggest that basement structures are the result of the breakup 

 of Gonwanaland and the dispersion of the fragments to their 

 present position. Most of this dispersion has taken place since 

 the mid-Jurassic. Block faulting and volcanism along the frac- 

 ture zones that delineate the flow lines of the drifting continents 

 produced the Walvis Ridge, Cape Rise, and Agulhas Plateau. 

 Igneous activity during the early phase of opening of the South 

 Atlantic may have led to emplacement of ridges parallel to the 

 coastline. These served as dams behind which thick deposits of 

 sediments accumulated. These sediments, which form the con- 

 tinental margins, had their sources in the major west-flowing 

 rivers that drained the interior of Africa: the Orange River in 



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