ROCKIES OF NORTHERN MEXICO 



451 



also occurred on the eastern (hinterland) side. In Late Jurassic time, 

 movements of the southern part of the peninsula adjacent to the geosyn- 

 cline are reflected in the sediments; and these in turn reflect the Nevadan 

 orogeny. In the Early Cretaceous, minor and local movements in the 

 Coahuila peninsula furnished some coarse sediments to the seas. The 

 peninsula was finally submerged in Aptian time but continued to act 

 as a relatively high and stable mass. In the Late Cretaceous, considerable 

 thicknesses of sediments were deposited along the southern border of 

 the peninsula in the Parras trough, whose position coincides with the 

 margin of the Late Jurassic seas. 



In the Early Tertiary, the deposits adjacent to the peninsula and its east- 

 ward and southward extension were deformed into long narrow folds by 

 forces acting about normal to the western and southern border of the 

 buttressing mass, which itself was only slightly deformed. Along the 

 east side another belt of folds was formed. This belt is overthrust east- 

 ward along its east margin, and a foothill belt of structures was formed 

 in front. 



FOOTHILL BELT 



In places along the inner margin of the Gulf Coastal Plain and not far 

 east of the east front of the Sierra Madre Oriental are several low ranges. 

 About 60 miles southwest of Del Rio on the Rio Grande is the Serranias 



del Rurro; to the southeast of this is the Sierra Lampazos; still farther to 

 the southeast is the Sierra de San Carlos; and then northwest of Tampico 

 is the Sierra Tamaulipas. These are generally broad folds and domes that 

 rise from the nearly horizontal beds of the Coastal Plain and expose 

 gently arched Lower Cretaceous limestones along their crests. Some of 

 these mountains, notably the Sierra de San Carlos, contain igneous pings 

 and laccoliths (Fig. 28.8) in part of alkalic composition. The San Carlos 

 Mountains also contain gentle, east-west trending folds superposed on 

 the dome; and these, Kellum (1936, 1937) believes, reflect the eastward 

 trend of the Parras basin folds. The folds, in fact, are found, although ill- 

 defined, still farther east in the Sierra de Cruillas in central Tamaulipas. 

 where they have a northeast trend, and there disappear under the 

 Coastal Plain. 



About 60 miles west of Tampico is the Sierra del Abra, which there 

 forms the east front of the Sierra Madre Oriental. It is an uplift in part 

 of eastwardly overturned Middle and Upper Cretaceous beds (Kellum, 

 1930). East of the Sierra del Abra is a Cretaceous and Tertiary basin and 

 then the buried Tamasopo ridge about halfway between the Sierra Madre 

 and the coast. Kellum believes that this buried ridge has had a historv 

 similar to the Sierra del Abra. It is the site of the remarkable "southern" 

 oil fields of Tampico-Tuxpam district of Mexico. Rasic intrusions and 

 extrusions are present in both the Tamasopo buried ridge and the Sierra 

 del Abra. 



