served as early sediment traps. Extensive work on the deep-sea 

 sediment accumulation produced from the Amazon River suggests 

 these deposits began to form only about 25 million years ago. Off the 

 southern coast of Argentina, the Falkland Plateau and Falkland 

 Islands have been shown to be a fragment of continental crust that 

 was fractured from Africa as the southernmost portion of the South 

 Atlantic began to form. 



Work on the western African continental margin suggests a 

 somewhat earlier opening history of the South Atlantic. Sediments 

 as thick as 7 kilometers buried the fragmented continental basement 

 and adjacent oceanic basement off the west coast and formed abroad 

 continental rise and abyssal plain. The source of much of this clastic 

 debris is believed to be the Orange River. Sedimentation has 

 apparently been much more extensive off the northern portion of 

 West African coast than to the south. 



The South Atlantic is only one area of paleo-oceanographic study. 

 Numerous DSDP drill cores in the equatorial Pacific have allowed 

 the history of that portion of the ocean to be traced over the last 50 to 

 60 million years. Upwelling of nutrient-rich bottom water along the 

 equatorleads to high biologic productivity and, hence, high sediment 

 deposition on the underlying ocean floor. For the oldest sediment, 

 this zone of high deposition is presently displaced to the north of the 

 equator, a natural consequence of the northward component of 

 movement of the crust of the Pacific Ocean beneath the zone of high 

 equatorial productivity. An abrupt change in the type of sediment 

 preserved on the sea floor is also noticeable approximately 40 

 million years ago. Prior to that time, the sediments are composed 

 predominantly of the remains of siliceous organisms, and younger 

 sediments are dominantly produced by calcareous organisms. Large 

 changes with time in the width and sedimentation rate of these 

 calcareous sediments point to major variations in depositional 

 conditions during the last 40 million years. 



As in the South Atlantic, numerous gaps are present in the Pacific 

 and represent several major phases of erosion. The earliest phase, 

 approximately 45 million years ago, can be related to the northward 

 movement of Australia away from Antarctica. Cold, deep bottom 

 water in the Indian Ocean began at the time to spread into the Pacific 

 through gaps left by the separating continents. This influence of the 

 Australian-Antarctic separation or deep water flow into the Pacific 

 has been suggested by earlier work supported by both NSF and 

 ONR. A major erosional event in the equatorial Pacific about 12 

 million years ago may be attributed to enhanced bottom water 

 production in the Antarctic as a result of a major increase in 

 Antarctic glaciation. 



The analyses and syntheses of sedimentary data from the DSDP 



45 



