ROCKIES OF NORTHERN MEXICO 



443 



NORTH 



SIERRA OE LA PAILA 



SOUTH 



MELCHOR OCAMPO 



COAHUILA 

 PLATFORM 



^^rz jl°- - *-- 2 



Fig. 28.2. Parras trough at close of Cretaceous sedimentation. After Imlay, 1939. 



The exogenic belt of western Sonora continued active during Late 

 Cretaceous time and crowded the Mexican geosyncline eastward. At two 

 different times, the land rose sharply, and thousands of feet of sediment 

 were deposited along the western edge of the trough (now in eastern 

 Chihuahua); first shale and sandstone, and later conglomerate. In north- 

 ern Zacatecas, southeastward from Sonora but still along the western 

 margin of the geosyncline, thousands of feet of tuffaceous beds were de- 

 posited. 



An east-west trough subsided over 15,000 feet in southern California. 

 Figure 28.2 shows the sediments in it and the Coahuila platform to the 

 north. The depression has been named the Parras trough from the present 

 Parras basin in which the Upper Cretaceous sequence crops out (Imlay, 

 1944). 



SONORAN REGION 



Very little is known about the Paleozoic history of Mexico. The Per- 

 mian of the Mexican state of Coahuila has been treated in Chapter 14. 

 On the west side of the country in Sonora, additional Permian beds have 

 been noted (R. E. King, 1939). They consist of limestones with abundant 

 crinoid stems and fusulinids. Reefs of massive limestone about 1500 feet 

 thick grade laterally into lesser thicknesses of well-bedded, darker lime- 

 stone. Permian strata may have occurred, originally, east of the central 

 Sonoran outcrops, because cobbles of fossiliferous Permian limestone are 

 found in the basal conglomerate of the Cretaceous there (R. E. King, 

 1939 ) . Present data are not sufficient to outline the basin in northwestern 

 Mexico, and the tectonic significance of the Sonoran exposures is, there- 



