GULF COASTAL PLAIN 



661 



i 



Jurassic (undifferentiated) 



Smackover (?) 



Limestone, dolomite, anhydrite 



Pre-Smackover 



Gray crystalline anhydrite and at bottom 

 1 foot of clear white rock salt 



1620 feet 

 30 feet 



A well near the front of the Mississippi delta penetrated to a depth of 

 22,570 feet and ended in Miocene strata (Paul Lyons, personal communi- 

 cation). In relation to the Vasen's well on the Wiggins anticline, 120 

 miles to the north, a marked southward dip is evident, which is re- 

 ported as 7°. The steepening of southward dip in the Mississippi delta is 

 prominent on the maps of Fig. 41.4, and for the Miocene beds a trough 

 axis has been discerned extending east-west through the delta. 



IGNEOUS ROCKS 



Moody (1949) has summarized the igneous rocks of the Gulf Coastal 

 Plain, both pre-Cretaceous and Cretaceous in age. The greatest concen- 

 tration of igneous activity centers in the Monroe uplift and Jackson 

 dome areas (as designated on the Tectonic Map of the United States) in 

 the tristate area of Arkansas, Louisiana, and Mississippi. There, in well- 

 drilling operations, alkaline and ultrabasic intrusive rocks have been 

 drilled into, and also volcanic rocks in the form of flows or sills; pyroclas- 

 tics are abundant, both alkaline and basic. Some of the intrusive bodies, 

 possibly dikes and stocks, are definitely intrusive into Upper Jurassic 

 strata. Some are older and believed to be related to the Triassic diabase 

 of the Atlantic piedmont. 



Throughout the entire northern part of the Coastal Plain in the Upper 

 Cretaceous sediments fragments of volcanic rocks are found in associa- 

 tion with the common sedimentary detrital minerals. 



TAMPICO REGION, MEXICO 



The Tampico region has a somewhat different Cretaceous geology 

 from the rest of the Gulf Coast, but a similar Tertiary. Instead of an 



overlap from the Gulf, the Cretaceous beds are continuous with tliose of 

 the interior Mexican gcosyncline and the Parras basin. The Cretaceous 

 beds of the geosyncline are intensely folded, and along the cast front of 

 the Sierra Madre Oriental they are thrust eastward in places. The zone 

 from the Sierra Madre front to the coast, 60 to 100 milts wide, may be 

 regarded as the coastal plain where the sedimentary rocks are fairly flat; 

 but several anticlinal mountains (or hills) formed of Cretaceous rock 

 interrupt the plain. The Tertiary sediments were deposited in seas that 

 invaded the coast from the Gulf and buried unconformably a number of 

 relief features. 



The anticlinal or domal mountains that rise from the plain are. from 

 north to south, the Sierra Burro, Lomerio Peyotes, Sierra Lampazos, Sierra 

 San Carlos, and Sierra Tamaulipas (Muir, 1936). See maps, Fig. 42.1 and 

 35.1. The Sierra San Carlos has already been described in Chapter 28 

 and is fairly representative of the mountains east of the Sierra Madre 

 front. Some of the ranges have gentle dips on the flanks from 3 to 10 

 degrees. The Sierra Papagayos is steeply folded, with dips up to 40 de- 

 grees and more. The doming of the Sierra San Carlos has been accentu- 

 ated by the intrusion of a stock of nepheline syenite, and the folding of 

 the Sierra Picochos has been influenced bv intrusions (Muir, 1936). 



All the ranges just mentioned in the coastal plain are parts of a contin- 

 uous structural element and hence related genetically. The Sierra Tamau- 

 lipas anticline plunges southward, and the so-called northern oil fields are 

 on its prolongation. See cross section of Fig. 41.12. Near the termination 

 of the Sierra Tamaulipas on the southern flank is an offshoot named the 

 Sierra de Buenavista. The Tamaulipas limestone in the core is intruded by 

 a laccolith. Muir concludes that the forces that produced the mountains 

 and oil-field structures of the coastal plain are due to vertically acting 

 forces, in contrast to the Sierra Madre and interior structures which are 

 due to horizontally acting forces. 



The northern oil fields are in an area of Cretaceous rock that reaches 

 nearly to the coast at Tampico. Immediately south of Tampico. beds of 

 Eocene, Oligocene, and Miocene age lap 50 miles inland across the Cre- 

 taceous and bury an arcuate ridge which lies west of Tuxpam. Albian- 

 Cenomanian reef limestones were probably laid down on a late Aptian 



