150 



STRUCTURAL GEOLOGY OF NORTH AMERICA 







D 













D 



1.83) 



1.83, 



1.83 k 



1.83, 2.77? 





WATER 





1.74:-. v;: I.83-V-'?-. :"i". . 



■••.'• 2.6I.V 



315 2.78 



J-? 





. I.83-. 

 4.11 





3.84 3 87 3.80 



2 





Ir ^_ 



4.89 



1 



4.45 ' 4 . 6 







A 



6 



/■" 5.5 



55 62Z 







552 



1 



1 



4 • 



1 



1 



1 — - 

 \ 









*•* *~ 







fl 





— b.85 





639 S, / 

 • X / 



6.21 







12 

 14 















ToV 



MANTLE 



10 







KILOMETERS 



200 



300 



100 KILOMETERS 



Fig. 10.15. Crustal structure sections D-D' and G— G' of Fig. 10.6. After Hersey ef a/., 1959. Numbers are 

 velocities in kilometers per second. Stippled layers are interpreted as unconsolidated and consolidated 

 sediments. 



The results on the continental shelf are correlated with adjacent continental 

 geology. The deepest horizon traced along the shelf is interpreted as granitic 

 basement, which has compressional velocities of 5.82-6.1 km/sec. At the 

 southern extremity it is at a depth of 6 km., shoals to 0.86 km. near Cape 

 Fear, and deepens north of Cape Hatteras to more than 3 km. North of 

 Charleston, South Carolina, there is excellent depth correlation with granitic 

 basement in coastal wells; to the south all deep wells are inland. Age correla- 

 tions are based on well data near the coast, which indicate to us that most of 

 the observed section is Cretaceous. 



On the Blake Plateau, several layers (1.83-4.5 km/sec.) are interpreted 

 as sedimentary. A 5.5-km/sec. layer is found only south of a line from 30°30' 

 N., 78°W. to Cape Canaveral. Velocities higher than 5.5 km/sec. have been 

 measured on six profiles on the Blake Plateau. The 5.5-km/sec. layer and a 

 6.2-km/sec. layer appear to form a positive feature to the south of the above- 

 mentioned line [indicated as fault on Fig. 10.6]. Higher velocities, 8.0 km/sec, 

 and 7.28 and 7.3 km/sec, which are probably not the same horizon, are 



found at markedly different depths. Possibly these represent the M layer and 

 ultrabasic material, depending on relations not now known. 



[The Outer Ridge along section G— C] is underlain by thick low-velocity 

 layers (1.83-2.96 km/sec), interpreted as sediments, and higher-velocity 

 layers which form a distinct linear structure having the same general trend as 

 the ridge. At its northwestern end this trend treminates against a thick 

 lower-velocity section interpreted as a sediment-fuled trough (Hersey et ah, 

 1959, p. 1). 



An attempt is made on Fig. 10.6 to contour the base of the interpreted 

 sedimentary layers (velocities less than 4.5 km/sec) from the profiles of 

 Hersey et al. The results are to be taken simply as pictorial. There seems 

 little doubt, however, that a major fault transects the Blake Plateau, but 

 of a date preceding the deposition of the upper two velocity layers of sedi- 

 ments, because they bury the escarpment. This fault, extended south- 



