444 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



5 MILES 



Fig. 28.3. Sections illustrating structures and stratigraphy in central Sonora, Mexico. A, Clasita 

 area; B, Urro Cobachi area; C, Zubiate area; D, El Trigo area; E, Arroyo Arenosa area; F, 

 Southern Sierra de San Javier; G, vicinity of Guamochil. pal, Paleozoic rocks; JTb, Barranca 

 formation; Kv, Cretaceous volcanics; Tv, early Tertiary volcanic rocks; Tbl, lower member of 

 Baucarit formation; Tbu, upper member of Baucarit formation; Gr, granite; Di, diorite; Sy, 

 syenite. After R. E. King, 1939. 



fore, obscure. Not helping to clarify the obscurity is the average eastward 

 strike of the younger rocks. See map, Plate 8. Late Permian or early Trias- 

 sic folding is indicated by generally greater metamorphism and folding 

 in the Permian than in nearby Upper Triassic and Cretaceous rocks, as 

 well as by the divergent strikes. The immediate impulse is to relate the 

 Sonoran east-west trends to the Coahuila, but then the belt of volcanics 

 that supplied much material to the Coahuila Permian basin apparently 

 lay between, and it would therefore seem that the two did not form a 

 single continuous tectonic system. The peculiar thing is that the volcanic 

 archipelago type of sediments is on the east in this region and the inland 

 basin type on the west, just the opposite from the distribution to the north 



in the western United States and Canada. Because the Sonoran Permian 

 is of the inland basin type, it might be supposed that the Pacific orogenic 

 belt lay considerably west. Since the peninsula of Baja California is made 

 up in large part of Cretaceous batholithic intrusions, evidently a continu- 

 ation of the great Sierra Nevada batholithic belt, and since the batholithic 

 belt coincides strikingly with the Permian trough in the United States, 

 British Columbia, and southeastern Alaska, it can also be supposed that 

 the Permian orogenic belt paralleled the peninsula and perhaps in part 

 lay west of it. It is evident that this is supposition, but possibly a reason- 

 able guess in the absence of factual information. The interpretation 

 rendered on the Permian tectonic map, Plate 8, does not produce a mean- 

 ingful tectonic pattern and is probably not correct, but various other 

 arrangements seemed even less tenable. We must await more field work 

 in western Mexico. 



Paleozoic rocks older than Permian are rare in Mexico. In central 

 Sonora, Ordovician limestone, sandstone, and conglomerate have been 

 identified (R. E. King, 1939). See cross sections, Fig. 28.3. In southern- 

 most California, an outcrop of marble is Mississippian. Farther north in 

 the San Bernardino Mountains, the Furnace limestone may be Mississip- 

 pian ( Woodford, 1939 ) . It is bounded above and below by quartzite for- 

 mations. Very similar massive dolomitic limestones and somewhat similar 

 quartzites are widely distributed in the Perris and San Gabriel moun- 

 tains. The Arrastre quartzite of the San Bernardino Mountains underlies 

 the Furnace limestone, and is so far below the fossiliferous horizon that 

 it is probably pre-Mississippian (Woodford, 1939). 



The earliest formation of the Mesozoic in Mexico is the Barranca. It is 

 Late Triassic and Early Jurassic in age, and crops out extensively in north- 

 western and central Sonora (R. E. King, 1939). There are also isolated ex- 

 posures in southeastern Sonora and western Chihuahua. In the ranges 

 bordering die Rio Yaqui, the formation is wholly of continental origin, 

 widi a diickness of more than 3300 feet. Three members are recognizable. 

 The upper and lower divisions are massive sandstones with some inter- 

 bedded dark shale. The middle member consists of shale and thin-bedded 

 sandstone with layers of coal and graphite. Farther east a short dis- 

 tance, the formation is 4250 feet thick. In northwestern Sonora, a clastic 



