GULF COASTAL PLAIN 



653 



Triassic sediments have not been recognized under the Coastal Plain, 

 but during the Jurassic the Gulf waters invaded the continent and a 

 succession of formations was deposited. From top to bottom they are the 

 Cotton Valley, Buckner, and Smackover. Underlying the Smackover are 

 the Werner gypsum and Louann salt formations which according to some 

 authors are Permian (?), as in Fig. 41.2, and according to others Jurassic 

 (Eagle Mills) as in Fig. 41.3. The Jurassic sediments were everywhere 

 overlapped by the Cretaceous. The Lower Cretaceous sea and deposits 

 extended across Texas and Oklahoma to connect with the vast epeiric 

 sea of the Great Plains and the Rocky Mountains (see Plate 11). Upper 

 Cretaceous seas probably spread over most of the Lower Cretaceous 

 deposits in Texas, but their sediments have subsequently been stripped 

 back so that the Lower Cretaceous now occurs farther inland (see Fig. 

 41.1). 



The Upper Cretaceous deposits overlap the Lower in Mississippi, 

 Alabama, Georgia, and South Carolina (see Fig. 41.3). After Late Creta- 

 ceous time the seas began a persistent retreat and the younger sediments 

 are spread generally in successive belts toward the present Gulf of 

 Mexico. Exceptions may be noted in the Mississippi embayment where 

 on the west side the Eocene sediments overlap the Upper Cretaceous, 

 and in Georgia and South Carolina where the Eocene sediments reach 

 just beyond the Cretaceous in places and rest on the crystalline Piedmont. 



The Cretaceous and Eocene seas especially extended up the Mississippi 

 Valley, and their sediments reflect a transverse downwarp known as the 

 Mississippi embayment. The evolution of the embayment is shown in four 

 stages in Fig. 41.4. 



The Rio Grande embayment is a gentle transverse downwarp and ex- 

 tends approximately from Corpus Christi northwestward for 200 miles up 

 the Rio Grande. The axis of the downwarp lies somewhat northeast of 

 the present river and close to the Nueces River. 



The embayment is due strictly to Eocene downwarp, as only the Eo- 

 cene sediments produce the embayed pattern. The Cretaceous strata 

 cover large areas inland and merge with the widespread deposits of the 

 Cretaceous seas in Mexico and the Rocky Mountains and Great Plains of 

 the United States. See paleotectonic maps of Plates 11 and 12. The Oligo- 



Fig. 41.3. Cross section of Gulf Coastal Plain through Mississippi, after Paul Weaver, 1951. 

 Vertical scale is in thousands of feet. 



cene, Miocene, and Pliocene deposits of the outer margin of the coastal 

 plain continue around the Gulf without an embayment at the Rio Grande. 

 A large part of the sediments of the Rio Grande area from Eocene to 

 Pleistocene is of deltaic origin and was carried from the interior of the 

 continent by rivers ancestral to the present Rio Grande, Pecos, and 

 Nueces (Storm, 1945). 



Concept of the Gulf Coast Geosyncline 



Recognizing the existence of 20,000 to 30,000 feet of sediments in the 

 thick part of the wedge from surface and well studies of the coastal plain 

 formations, and confirming the figures by geophysical data. Barton ( 1936) 

 realized that the floor of the wedge was at least 10,000 feet below the 

 floor of the Gulf of Mexico. In addition, he believed that the layer of sedi- 

 ments at the bottom of the Gulf of Mexico is only a few thousand feet 

 thick at the most; and so he depicted a great elongate downwarp which 

 he thought should rightfully be called a geosyncline. A number of papers 

 by Barton and others have established the name Gulf Coast geosyncline 

 firmly in the literature. The great accumulation of sediments along the 

 sinking continental margin, however, has not yet been deformed — it has 

 not been cast into folds and thrust sheets — but on the other hand, it is 



