Basin. It also showed that the continental shelf along most of 

 southwestern Africa is due to simple progradation of sediments 

 and that no tectonic barrier underlies the shelf. 



Two geologic features of the continental margin may be 

 possible sites of oil and gas accumulation with potential eco- 

 nomic significance: a huge ancient Delta off the Orange River 

 and a field of numerous diapirs extending, from Angola to 

 Nigeria. 



The ancient Orange Delta, shown by bathymetric con- 

 tours to be convex seaward, contains sediments having a total 

 volume of about 1.9 million cubic kilometers beneath water 

 depths of 100 to 3,600 meters. Relationships with earlier sedi- 

 mentary accumulations suggest that the Delta began to be 

 deposited soon after Africa rifted from South America and 

 that deposition continued into Tertiary time, but essentially 

 ended before the middle Tertiary. 



The second feature is a large area of diapiric structures in 

 water depths of 800 to 300 meters off Angola. Some 3.000 

 kilometers of seismic profiles indicate that the diapir field ex- 

 tends from latitude 13°30'S to north of latitude 5°00'S. 



All data, with the exception of some deepwater gravity 



data from the field work accomplished in 1972, arc now avail- 

 able from the National Geophysical and Solar-Terrestrial Data 

 Center. Microfilm copies of the seismic profiles as well as 

 digitized and plotted bathymetric, magnetic, and gravity data 

 can be obtained by contacting the Marine Geology and Geo- 

 physics Group of NGSDC. 



Data in similar format from the 1973 cruise (fig. 27) are 

 expected to be available in early 1974. 



The broad objectives of a 3-year plan to study the conti- 

 nental margin of Argentina and Brazil are to find the true edge 

 of the South American continent and determine the nature of 

 the transition zone, to determine the nature and extent of 

 diapirism observed on the continental margin with particular 

 attention to its economic potential, and to test previous hy- 

 potheses on the formation of marginal fracture ridges and on 

 the effect of these ridges on the structural framework of conti- 

 nental margins. Additional investigations will be made on the 

 Malvinas (Falkland) Plateau and Scotia Ridge structures to 

 formulate a model of continental breakup and subsequent mo- 

 tion. The total areas to be surveyed will extend along the western 

 margin of the South Atlantic from the Scotia Ridge to the 



50° 



Figure 27. — Geological and geophysical survey lines of VVHOl's RV ATLANTIS II, January 20 June 30, 1973. 



28 



