192 C. SCHUCHERT THE NORTH AMERICAN GEOSYNCLINES 



SONORAN EMBAYMENT 

 (See Maps, Figures 1, 5 to 11) 



The Sonoran embaymeut was an east-west trending seaway in con- 

 nection with the Cordilleran geosyncline during Paleozoic time. It 

 appears to have come into being during the Proterozoic. There is, how- 

 ever, no region of North America less known geologically than north- 

 western Mexico, and therefore the history of this embayment can be 

 given only in the light of the strata known in southern Arizona and 

 N^ew Mexico and southwestern Texas. There may, therefore, be in this 

 area many more horizons than are now recognized. AYe have already 

 given the Cambro-Ordovician history and pointed out that the thickness 

 of these deposits may attain a maximum of 3,000 feet (page 182). 

 The uppermost Ordovician and the Middle Silurian appear to be well 

 represented by about 1,500 feet of sediments, mainly limestones. The 

 Upper Devonian and the Lower Mississippian are also well represented 

 by about 1,500 feet of elastics and limestones, while the thin Upper 

 Mississippian is known onl}'' in southwestern Texas. In middle Penn- 

 sylvanian time the western end of the Sonoran embayment was blotted out 

 by mountain-making, and then the eastern part of the Cordilleran sea 

 opened out southeastward across Chihuahua and Coahuila into the Gulf 

 of Mexico. 



Then the thickest stratal development in this trough appeared, in 

 middle and late Pennsylvanian and earlier Permian times ; these deposits 

 in southwestern Texas may exceed 10,000 feet of elastics and limestones. 

 It is by all means the best marine section of these times anywhere in 

 North America. With the middle Permian, this earliest appearance of 

 the Mexican geosyncline vanishes and the region remains dry land until 

 the Lower Cretaceous, when the Comanchian seas of the Gulf of Mexico* 

 spread westward across Chihuahua into eastern Sonora and northward 

 across eastern New Mexico into Kansas. Of these Lower Cretaceous 

 strata the thickness may not exceed 3,000 feet. All in all, it appears 

 that no part of the Sonoran embayment sank in Paleozoic times over 

 20,000 feet, and it is more probable that the maximum thickness of de- 

 posits in any one place is not over 15,000 feet. 



Development of the Franklinian Geosyncline 



(See Maps. Figures .'^, 5 to 13) 



AVe will now study the stratal history of the extreme northern end of 

 North America. Explorers have been bringing scattering geologic knowl- 

 edge out of Arctic North America for nearly a century, but, due espe- 



