668 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



GALVESTON 

 SHORE 



GULF COAST 



OULF OF MEXICO 



BASIC SEISMIC REFRACTfON DATA 



CUBA YUCATAN BASIN 



NICARAGUAN RISE 



— « 



COLOMBIAN BASIN 



MEXICAN BASIN 



SPECULATIVE GEOLOGICAL INTERPRETATION 



YUCATAN-CUBA TECTONIC ELEMENT YUCATAN BASIN CAYMAN TRENCH NICARAGUAN RISE 



COLOMBIAN BASIN COLOMBIA* 



UNCONSOLI- Vl- 



oateo Lr 



SEDIMENTS 



_-_-) SEMI- TO -1-,-! J 

 -li-3 CONSOLIDAT- 13 — P LIMESTONE 

 ED SEDS. 



l-*-*-*lvOLCANICS 



AUT1CAL MILES 



Fig. 41.15. Crustal structure of Gulf of Mexico (Ewing ef al., 1955; Miller and Ewing, 1956) and western 

 Caribbean (John Antoine, 1959). The speculative geologic interpretation is slightly altered and somewhat 

 more detailed than given by the authors cited. 



of about 45,000 feet of combined consolidated and unconsolidated 

 sediments appears to exist. Refractions from the base of the "consoli- 

 dated sediment" layer could not be obtained, and it is inferred by 

 Ewing et al. (1955) that either a limestone or salt layer of about 5.28 

 kilometers per second velocity overlies lower velocity sedimentary rocks. 

 The great thickness of the "consolidated" layer may be the result of con- 

 solidated carbonate facies, and the boundary shown on the geologic 

 section of Fig. 41.15 may therefore not be a systemic or time boundary. 

 This seems a more logical interpretation than one involving vertical move- 

 ments of the ocean floor. Offshore carbonate deposition in the form 

 of barrier reefs could have affected the semi-isolation of extensive la- 

 goonal seas for the precipitation of salt and gypsum. Some such retaining 



form or structure is necessary to produce the Jurassic or Permian evap- 

 orite conditions of the Gulf Coastal Plain. 



It will also be observed that the sediments near the shelf slope rest 

 directly on the gabbroic subcrustal layer, and that the continental crystal- 

 line basement layer does not make an appearance until about the shore- 

 line. This arrangement is concluded by Miller and Ewing (1956) to exist 

 because the magnetic intensity field is remarkably uniform and without 

 conspicuous anomalies from the basin across the shelf slope onto the 

 shelf. 



The shelf slope has been considered to be a fault scarp and in ad- 

 dition to indicate that the Mexican basin is a down-faulted depression 

 (Gealy, 1953; Jordan, 1951; Eardley, 1954). The uniform magnetic field 



