thick margin sediments extend into offsets of the continental edge? 

 Are there noncomformities in the thick margin sediments produced 

 by the initial period of uplift before rifting? Was this initial rifting 

 thermally produced or were there preexisting zones of weakness in 

 the ancient megacontinents that controlled the loci of separation? 



Obviously, obtaining answers to these questions is time con- 

 suming and expensive. Answers to some will come only after 

 intensive studies of margins that were once joined, and these studies 

 must include deep drilling (planned as one phase of the IPOD 

 program). The need for answers to these questions, however, has 

 become increasingly important in a world that is rapidly depleting 

 its land-based energy resources. Thick sediments on the passive 

 margin, numerous noncomformities, and the initial heat source 

 produced by rifting suggest the existence of significant oil and gas 

 resources. 



In conjunction with its offshore-energy-related activities, the 

 USGS has intensified studies of the geologic framework along the 

 Atlantic, Gulf of Mexico, and Western and Northern Alaska 

 Continental Shelves. To gain information on deep structures of the 

 Atlantic continental margin of the United States, USGS has 

 acquired commercial multichannel seismic reflection profiles in this 

 area. The profiles show as much as 14 kilometers of continental-shelf 

 sediment overlying basement rocks, nearly twice the thickness 

 interpreted from earlier geophysical studies in this area. To get the 

 quickest and least expensive overview of basement structure and 

 relations between oceanic and continental crust, aeromagnetic 

 studies are being integrated with the deep-penetration reflection 

 data. In addition to a much thicker sedimentary section than 

 previously suspected, preliminary conclusions from this program 

 indicate an irregular faulted basement of ancient sedimentary, 

 metamorphic, and igneous rocks, with ridges that supported coral 

 reefs during early stages of shelf deposition. 



Off the extensively studied Gulf coast, USGS, with the coopera- 

 tion of other Federal agencies, universities, and industry, is 

 compiling an extensive series of maps showing the broad geologic 

 framework of the Caribbean Sea, and in greater detail, that of the 

 Gulf of Mexico. Both maps will include geologic information from 

 adjacent land areas and will be accompanied by compilations of 

 existing geophysical data. 



Part of the data that will be incorporated into this map are NSF- 

 supported multichannel reflection data from the gulf. Seismic 

 sections show that sedimentary sequences found by the DSDP in the 

 central gulf can be traced laterally throughout much of the deeper 

 portions of the gulf. The oldest part of the section is a Jurassic salt 



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