MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



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plateau valleys along the west edge of the Colorado Plateau. North of 

 the state boundary, the assemblage is known as the High Plateaus of Utah, 

 early described by Dutton (1880), and in northern Arizona the plateaus 

 are known as the Kaibab, Kanab, and Shivwits. 



In general, the faults and flexures block out ranges and intermontane 

 valleys from the horizontal sediments of the Colorado Plateau, but toward 

 the west the folded beds of the Laramide orogeny are involved. This is 

 especially true in the Wasatch Plateau of central Utah and along the 

 Hurricane fault of southern Utah and northern Arizona, previously de- 

 scribed. The map of Fig. 31.9 shows the largest faults that have been 

 attributed a post-Laramide age. Many small ones exist that are not shown, 

 and even some major ones of which the age is uncertain or which have 

 not been mapped as post-Laramide, may exist that are not shown. The 

 Hurricane fault is illustrated in Figs. 20.21 and 20.22; the Sevier and 

 Tushar faults, in Fig. 31.10. 



A large volcanic field occurs in the central part of the High Plateaus 

 and connects westward with other volcanic areas of the Great Rasin. 

 These are discussed in Chapter 36. They were mostly erupted immedi- 

 ately preceding the faulting. 



An indication of the complexity of the volcanism, faulting, and erosion 

 cycles of the region is revealed in Koons's ( 1945 ) work on the Hurricane 

 iand Toroweap faults just north of the Grand Canyon. The oldest erup- 

 tions of late Miocene or early Pliocene time preceded the earliest move- 

 ments along the Hurricane fault and antedated the cutting of the Grand 

 Canyon. They poured out on a large gently sloping pediment extending 

 iat least 16 miles north from the Colorado River. The main faulting then 

 | occurred, with displacements over 2000 feet at the Colorado River. The 

 stream held its course, a new and lower pediment was eroded, and the 

 (region was brought approximately to its present configuration, with the 

 Colorado River approximately as deep as now. The second eruption then 

 occurred; they were local, and at the Toroweap fault filled the inner 

 gorge to a height of 600 feet and perhaps 1200 feet. The lavas were 

 -entirely removed before later flows dammed the river again. These were 

 subsequently also nearly all eroded away. Repeated movements along the 

 Toroweap fault have occurred in late Pleistocene time, and in the very 



Fig. 31.9. Faults of the belt of great 

 trenches in northern Arizona, Utah, 

 Wyoming, Idaho, and southwestern 

 Montana. Hachures are on the up- 

 thrown side. Only those faults are 

 shown that have been fairly well de- 

 monstrated as late Cenozoic in age. 



MO NTANA 



