EOCENE HISTORY OF MEXICAN CULF COASTAL AREA 485 



CRETACEOUS-TERTIARY CONTACT 



South of the Salado River the exposures of the Cretaceous 

 shales along the line of contact with the Tertiary indicate that 

 they have been subjected to more or less folding and to erosion 

 prior to the incursion of the waters of the Eocene, so that the 

 contacts from Rodriguez south show very decided unconformities. 



On the Rio Grande the unconformity is shown partly by differ- 

 ence in direction of dip and partly by the fact that the Midway is 

 found resting upon different members of the Escondido. 



Northward from the Rio Grande the uppermost beds of the 

 Cretaceous present, as a whole, a gentle slope toward the Gulf, 

 and when we find contacts between these beds and the basal Ter- 

 tiary the discordance of stratification is usually very small, even 

 if it be present at all. There is, nevertheless, ample proof of fold- 

 ing in this area also, for logs of wells drilled in the Tertiary belt 

 of western Louisiana and eastern Texas give positive evidence of 

 broad folds in the underlying Cretaceous beds far to the seaward 

 of their present outcrop. 



The most striking feature of the orogenic movement in this 

 area is, however, found in the numerous submerged hills of the 

 salt domes that now occur as inhers in the Tertiary belt. These, 

 as shown by the discordance of dip between the Cretaceous rocks 

 and those of the surrounding Eocene, are pre-Tertiary. 



LOWER EOCENE 



Midway. — The initial sediments of the Eocene were marine and 

 are fairly uniform in character throughout their entire extent within 

 the area of the western Gulf Coast. While east of the Brazos they 

 occupy a belt which has at times a width of several miles, with 

 a total thickness of between 300 and 400 feet, west of that stream 

 they are found in disconnected exposures due generally to the 

 overlapping of later deposits. 



This is clearly shown on the Rio Grande, where excellent 

 contacts have been found by Kennedy, and where this substage 

 attains a thickness of 300 feet. Here the Midway is frequently 

 covered both by the Wilcox or Lignitic and by the Carrizo sands 

 or basal Claiborne. 



