COLORADO PLATEAU 



423 



Wasatch line is contained within the High Plateaus. The dotted line of 

 Fig. 26.1 represents the boundary of the Laramide Colorado Plateau and 

 Laramide Central Rockies as designated by Hunt (1956), and separates 

 the comparatively flat beds of Colorado Plateau aspect from the deformed 

 strata of the Central Rockies. However, from the southern Wasatch Moun- 

 tains southward a gently deformed belt separates the highly deformed 

 strata from the little deformed, so the boundary is not a sharp one. Also 

 confusing the problem of boundary is the highly deformed zone in San 

 Pete Valley between the Wasatch and Gunnison plateaus which to the 

 writer and others seems to be a salt anticline structure. If this is con- 

 sidered a local structure of the Colorado Plateau and not one of regional 

 orogenic significance, then the dividing line would lie in or west of the 

 Gunnison Plateau. 



The main structural exhibit in the High Plateaus of Utah is a normal 

 fault zone of trenches and uplifted blocks. The faults, as well as known, 

 are shown on Fig. 26.16. The large mid-Tertiary volcanic field occupies 

 a central position in the High Plateaus and has been broken and offset 

 by the faults, the same as the older sedimentary rocks. The faults are thus 

 late Tertiary. In part they have continued active through the Quaternary 

 because of recent earthquakes and fresh scarps in places. See Chapter 31. 



The High Plateaus belong to the fiasin and Range system if only the 

 late Cenozoic normal faults are considered, but most geologists have 

 given attention more to the Laramide structures in designating a bound- 

 ary. 



AGE OF UPLIFTS AND VOLCANISM 



Near the close of the Cretaceous period the region probably was low 

 (Hunt, 1956). In Late Cretaceous or early Eocene time occurred the de- 

 formation that resulted in the anticlinal uplifts such as the San Rafael 

 Swell, Circle Cliffs uplift, and Henry Mountains structural basin. Evidence 

 for this date of the folding is found in the St. George basin (Gardner, 



1941), the vicinity of Escalante (Gregory and Moore, 1931), and the 







Fig. 26.16. High Plateaus of Utah. Dotted line represents western margin of Colorado Plateau 

 'according to Hunt, 1956. 



