ROCKIES OF NORTHERN MEXICO 



449 



LOMAS 

 COLORADAS Kaa 



SIERRA DE SARNOSO 



2 



7000' 

 6000' 

 5000' 

 4000' 

 3000' 



Fig. 28.7. Cross sections of the mountains west of the Laguna district, after Kellum, 1936. Tc, Late 

 Cretaceous or Early Tertiary conglomerates; Kct, Cenomanian-Turonian sh. and Is.; Kaa, Aptian-Albian Is.; 

 Klv, Torcer-Las Vigas series; Jrr, Red Rock series; Tig, igneous intrusives. 



against the buttressing and less deformed Coahuila peninsula. To the 

 west of the Sierra de Hispana thrust, a high mountainous mass consists 

 of tight folds overturned toward the northeast. This zone has not been 

 traced in ranges to the northwest, but it undoubtedly continues in that 

 direction. 



The east-west belt of folding veers south-southwesterly at Saltillo and 

 merges with and forms the Sierra Madre Oriental. Quoting from Kellum 

 et al. (1936a) again: 



In this region, intensive compression has developed a series of overturned 

 or fan-shaped anticlines and synclines in Cretaceous and Jurassic rocks, with 

 enormous horizontal shortening. The axes of these folds trend, in general, east- 

 west and pass eastward by a rather short curve to a southeast direction. At the 

 west end, where they close and plunge into the Parras Basin, they become sym- 

 metrical, and then are overturned westward, parallel to the strike of their axes. 

 No important faulting has been recognized. 



East of the Sierra Madre Oriental, this prominent zone of east-west folds is 

 present in central Tamaulipas in the San Carlos Mountains, which rise out of 

 the coastal plain about midway between the Cordilleran front and the Gulf 

 Coast. The San Carlos Mountains are a broad, arcuate geanticline [arch as de- 

 fined in this book], trending in a general easterly direction, with the convex 

 side to the south. Superimposed upon this major structure are numerous, low 

 flexures, parallel to it, and also a number of domes produced by igneous in- 

 trusions. The axes of folds in this geanticline, are not the continuation of axes in 

 the Sierra Madre Oriental, but are the continuation of axes which lie east of, 

 and parallel to, the Cordilleran front, farther northwest, and are turned east- 

 ward in the zone of cross-folding. 



West of the Sierra Madre Oriental and south of the Parras Basin the structure 

 of this belt has been studied in a number of areas. In the region of Mazapil- 

 Concepcion del Oro, in northern Zacatecas, the mountain ranges are anticlinal 

 and trend generallv eastward; their structure is complicated by several faults 

 and by the presence of intrusive masses. The Sierra de Santa Rosa and the core 

 of the Sierra de la Canutillo show a slight tendencv toward fan structure. The 

 Concepcion del Oro anticline, a southeast continuation of the Siena de la Caja 

 structure, is overturned toward the northeast and is crossed bv a fault. 



