GULF COASTAL PLAIN 



CRUSTAL STRUCTURE OF GULF OF MEXICO 



867 



Fig. 41.14. Configuration of surface of pre-Mesozoic rocks in Florida and southern Georgia, 

 and distribution of pre-Mesozoic rocks. Precambrian consists of granite, diorite, and meta- 

 j morphic rocks; Paleozoic (?) and Precambrian (?) consist of rhyolite, tuff, and agglomerate. 

 After Applin, 1951. 



ever, suggests a structural high offset to the southeast, with intrusive 



igneous rocks, probably Pre-cambrian, exposed in the core. These are 



| flanked on the northwest and southwest by volcanic rocks which may be 



the equivalent of the Unicoi formation (basal Chilhowee) of the southern 



' Appalachians. Then in nortiiern Florida a basin of Ordovician and 



! Silurian sedimentary rocks occurs, fairly flat-lying and unmetamorphosed 



(Applin, 1951). These undisturbed Paleozoic strata are southeast of the 



Appalachian orogenic belt, and pose a rather mysterious problem in 



tectonics and the evolution of the southeastern margin of the continent. 



Geophysical Data 



Refractive seismic traverses by Ewing et al. (1955) and a magnetic 

 intensity survey by Miller and Ewing (1956) serve as the principal evi- 

 dence for sediment layering and crustal structure under the Gulf of 

 Mexico. The seismic data are given in Fig. 41.15, and the magnetic 

 data have been used in constructing the geologic cross section of the 

 same figure. Another seismic refraction profile by Antoine ( 1959 ) across 

 the Colombian basin from western Cuba to Colombia continues the Gulf 

 of Mexico section to South America. Although the two sections are 

 offset from Yucatan to Cuba, the Yucatan-Cuba tectonic element may 

 be visualized as shown in Fig. 41.15, and the effect of a continuous section 

 obtained, which helps in understanding the constitution and history of 

 the great mediterranean region. Cuba and the Caribbean region will be 

 discussed in Chapter 42. 



In making the geologic interpretation the rocks indicated by the 

 various seismic velocities are taken as follows. These are generally the 

 ones suggested by geophysicists in previous references on the Atlantic 

 continental shelf and ocean floor, and in the above articles. 



1.8-3.7 km/sec 



4.5-5.2 km sec 



5.2-5.5 km sec 

 4.5—5.5 km/sec 



5.6-6.1 km sec 

 5.8-6.1 km sec 

 6.5± km/sec 

 7.0-7.5 km, sec 

 8.0-8.3 km sec 



Shelf of Gulf Coastal Plain 



Unconsolidated and semiconsolidated sedi- 

 ments 



Semiconsolidated and consolidated sedi- 

 ments 



Limestone and dolomite 



Extruded porous volcanic rock. Lower 

 values probably indicate porous rock 



Intrusions in volcanic rock 



Crystalline basement of continent 



Gabbroic or basaltic subcrust 



Transition layer, mantle to subcrust 



Mantle (periodotite or eclogite) 



It may be seen in Fig. 41.15 that the wedge of sediments of the Gulf 

 Coastal Plain thickens nearly to the shelf slope where a total thickness 



