ROCKIES OF NORTHERN MEXICO 



147 



intense shortening, few thrust faults developed. The Sierra Madre 

 Oriental, west and northwest of Tampico for a distance of about 100 

 miles, is made up of several ranges, the Sierra Cucharras, the Sierra Tan- 

 chipa, and the Sierra del Abra. These also consist mostly of folds, but at 

 a number of places Kellum ( 1930 ) has interpreted a thrust structure 

 along their east front. His drawings are reproduced in Fig. 28.5. This belt 

 of thrusting is approximately in line with the El Paso— Rio Grande thrust 

 belt farther north. Several large thrusts along the west side of the Sabinas 

 basin seem to connect the northern and southern thrusts and to form a 

 belt about 800 miles long. See Tectonic Map of the United States and Fig. 

 28.1. 



The region formerly occupied by the Coahuila peninsula is one, accord- 

 ing to Kellum et al. (1936a): 



... of broad, gentle folding and includes the great brachyanticlines or periclinal 

 folds of the Sierra de la Paila, the Sierra de los Alamitos, and the Sierra de 

 Garcia, in the east. It also includes the Sierra del Venado, the Sierra del 

 Sobaco, the Sierra del Tlahualilo, the Sierra de Campana, and related ranges, 

 in the west. Undoubtedly, it takes in many mountain ranges lying to the north 

 of the western group, but these have not been studied in sufficient detail to 

 demonstrate the regional structural plan. The eastern group of ranges also has 

 never been studied in detail, but the general structure, as seen from the south 

 and as reported by Bose, Kane, and others who have crossed them, is a gentle 

 uplift. The western group is essentially the same but differs in that erosion has 

 progressed much further and divided the broad, gende uplift into numerous 

 ranges, more or less separated by valleys filled with alluvium. 



These ranges are composed, in large part, of the gypsum facies in the 

 Cuchillo formation. This is an easily eroded unit, and, where the gypsum and 

 marl predominate in the section, the mountains have been cut down more 

 rapidly than where limestone predominates. 



The structure of the Cretaceous rocks in the mountains bordering the valleys 

 of Las Delicias and Acatita illustrates the type of folding characteristic of the 

 central province ( Coahuila Peninsula ) . The major structure between the two 

 valleys is a broad, composite, anticlinal uplift, trending northwest-southeast 

 and plunging in both directions. Superimposed upon it are many sharp, per- 

 sistent folds, parallel to the central axis. Minor cross folds, ordinarily non- 

 persistent and with gende dips, appear to reflect topographic irregularities in 

 the basement rocks. The main axis of the major anticlinorium extends along the 

 western margin of the range, in its northweastem part, but to the southeast 

 the axis crosses the central part of the mountain area. Limited observation on 

 the minor anticlines southwest of this axis indicates that they tend to be asym- 

 metrical, with the steeper dip on the southwest. The major structure of the 



Cretaceous rocks in the Sierra del Tlahualilo, west of the Acatita Valley, is .i 

 broad, gentle fold, almost perpendicular to tin's major trend, cross Ok- rang! — 

 one, at its north end; the other, about 15 miles farther south. These are believed 

 to reflect topographic features, or zones of displacement, in the underlying 

 basement rocks. 



Figure 28.6 shows an example of the structure that developed over the 

 site of the former Coahuila peninsula. 



Tamau//pas 



Goaya/ejo Cc/nyOn 



Southwest of 

 3an Lucas ranch 



Tamou/zpas /s 



Va//ey north of Monte. Cr/sto 

 ranch 



ei fli° r "liX 



Gomez Farias 



£/ fibre, /s 



Car/7 ton F'ass 



Atascacfor 



Theor/zed development of /I bra- Tanch//oo 

 moun ta/n fro n t 



Fig. 28.5. Sierra Madre Oriental front west of Tampico. After Kellum, 1930. 



