obtained by the DSDP must be used. Drilling in the southern oceans 

 has produced evidence of continental glaciation in Antarctica as 

 early as 20 million years ago. Drilling in the North Atlantic has 

 suggested that continental glaciation in northern latitudes may have 

 begun 5 million years ago. 



With the advent of plate-tectonic theory, the geologist's formerly- 

 held view that the ocean basins were stable has been radically 

 altered. The movement of the continents is now accepted to be 

 closely related to the creation, destruction, and modification of the 

 ocean basins. One of the most interesting studies to understand the 

 evolution of the oceans was the work connected with DSDP legs 39 

 and 40. These legs were designed to probe the early history of the 

 South Atlantic recorded in the sediments off Africa and South 

 America. Scientists now theorize that the South Atlantic was 

 initiated as a very narrow crack when Africa began to separate from 

 South America. Fossil remains of plants and animals indicate that 

 this crack was first occupied by deep, freshwater lakes similar to the 

 present rift-valley lakes of East Africa. As the rift broadened, the 

 lakes deepened and changed from fresh to salt water. Because of 

 sluggish circulation, the deep waters stagnated, a condition that 

 lasted for over 20 million years. Thousands of meters of thinly 

 laminated organic-rich sediments were deposited during this period. 



As the continents split slowly apart, stagnant marine waters from 

 the south seeped northward across ridges into the newly forming 

 deep basins. The rate of evaporation exceeded the rate of water 

 inflow, and a massive layer of salt, 2,500 meters thick, was deposited 

 between offshore Angola and Nigeria. This layer of salt, which is 

 equivalent to about 10 percent of all the dissolved salt in the rest of 

 the oceans, was deposited in a few million years. 



As South America finally separated from Africa, the period of salt 

 deposition ended abruptly as cool marine water invaded the South 

 Atlantic from the south. Numerous gaps in the sedimentary record 

 are found throughout the subsequent history of the South Atlantic. 

 These gaps may represent changes in the rate of supply and 

 dissolution of sedimentary material, or increases and decreases in 

 erosion produced by the deep bottom water circulation. 



Additional knowledge of the separation of Africa and South 

 America has resulted from several NSF studies of the western and 

 eastern margins of the South Atlantic that have recently been 

 completed. On one 4-year program, scientists studied the geologic 

 history of the continental margin off Brazil and Argentina. The data 

 gathered in this study indicate that the early phase of the opening of 

 the South Atlantic occurred between 127 and 84 million years ago. 



Movement along fracture zones of the separating South American 

 and African continents produced compressional ridges, which 



44 



