31. 



MIDDLE AND LATE 

 CENOZOIC SYSTEMS OF 

 THE CENTRAL CORDILLERA 



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 l 



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GENERAL DIVISIONS AND THEIR CHARACTERISTICS 



For structural purposes it seems best to treat the middle and late Ter- 

 nary mountain systems in the central part of the great Cordillera of 

 Morth America in three divisions, namely, the Basin and Range system 

 )f southern Oregon, eastern California, Nevada, western Utah, and north- 

 vestern Arizona; the Sonoran-Chihuahua system of desert ranges in 

 vestern and southern Arizona, New Mexico, and central Mexico; and 

 he system of great trenches in central Utah, eastern Idaho, western 

 Wyoming, western Montana, and British Columbia. The first two di- 

 osions are generally included by the physiographers in the Basin and 



Range province, and the third has generally not been distinguished From 

 the Laramide Rockies on whose folds and thrusts its fault-made trendies 

 are superposed. 



The Basin and Range system is one generally of north-south-trendin.: 

 basins and ranges, with the majority of the ranges probably blocked out 

 by high-angle faults. The distinctive features of the province, accordin'j; 

 to Fenneman (1931), are "isolated, nearly parallel mountain ranges (com- 

 monly fault blocks ) and intervening plains made in the main of subaerial 

 deposits of waste from the mountains. These deposits, although locally 

 absent, are often very deep and are generally unconsolidated." 



The boundaries of the Basin and Range system are shown on the map 

 of Fig. 31.1. The Great Basin of internal drainage, the Mojave Desert, 

 and the Salton trough are the chief regions here included. The Basin and 

 Range province is bounded on the west by the Sierra Nevada, on the 

 east by the Wasatch Mountains and High Plateaus of Utah, and on the 

 north by the Malheur plateau and Snake River lava plains. The nar- 

 rowing south end has been arbitrarily defined by Nolan (1943) to have 

 the San Andreas fault on the west and the Colorado River on the east. 

 The physiographic section of the Great Basin, called the Sonoran Desert 

 by Fenneman, includes large areas on both sides of the Colorado River; 

 and the Basin and Range system is probably continuous across it to the 

 desert ranges of southern Arizona. 



The desert ranges of Arizona, New Mexico, and part of the Mesa Cen- 

 tral of northern interior Mexico are somewhat similar to those of the 

 Great Basin in being rudely parallel and separated by basins filled in part 

 or completely by alluvium. Those in southern Arizona stretch northwest- 

 ward across the southern and southwestern part of the state. They con- 

 verge haphazardly, in the southeastern corner, with basin ranges of New 

 Mexico which extend northward through the central part of the state. 

 Together, the ranges of block-fault character of Arizona and New Mexico 

 extend southward into Mexico through the state of Chihuahua to Du- 

 rango, Coahuila, Zacatecas, and San Luis Potosi. The sources of 

 information on this great region are a few detailed studies of wide sep- 

 arated areas. Large parts of it have never been reported on, so the con- 

 cept is not secure that it is everywhere a mountain system whose present 



493 



