GULF COASTAL PLAIN 



669 



across the steep slope argues against the fault theory, as does also the 

 lack of seismic activity there (Miller and Ewing, 1956). 



Mexican Basin 



The seismic data indicate that some 30,000 feet of unconsolidated and 

 consolidated sediment under the Mexican basin rests directly on a gab- 

 1 broic subcrust which in turn is about 25,000 feet (8 kilometers) thick. 

 This condition indicates that the Gulf of Mexico crust is of the oceanic 

 type, but that sediments have been accumulating in the large amounts 

 characteristic of continental borderlands on the gabbroic layer from at 

 least the beginning of Mesozoic time. 



Yucatan Platform 



The north side of the Yucatan peninsula or platform, the Campcche 

 Bank, is believed to be underlain by limestone or dolomite with only a 

 thin veneer of unconsolidated sediments. The velocity of 5.6 (Fig. 41.15) 

 is regarded by Miller and Ewing (1956) to indicate limestone, dolomite, 

 or crystalline basement, but the exposed geology suggests the presence of 

 carbonates rather than a crystalline basement. The carbonates are be- 

 lieved to be sufficiently lithified and strong to hold up an exceedingly 

 steep slope, which in turn is interpreted to be an escarpment built up 

 by sedimentary processes and not a fault scarp. The uniform magnetic 

 field over the escarpment points to the sedimentary origin. 



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