342 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 22.16. Diagram of the southern Wasatch Mountains showing the Nebo thrust and the later 

 normal block faulting. After Eardley, 1933. Section 14, Fig. 22.1. 



uplift. It is not clear what relation the Evanston formation on the north 

 bears to the early Uinta uplift. The Cottonwood dome and east-west 

 folding north of it also probably formed at this time. See Fig. 22.12. In 

 north-south cross section the Cottonwood intrusion appears to have had 

 some effect on the doming, but it is quite discordant eastward and may 

 be later. It is discussed further in Chapter 36. 



Eocene Phase (Late Laramide) 



The Knight conglomerate of early Eocene age was spread widely 

 over deformed and eroded strata of the east front of the central Rockies 

 and marks another superior uplift on the west. See Fig. 22.6. A large 

 fresh-water lake formed on either side of the early Uinta uplift in which 

 the Green River formation was deposited under quiet conditions and at 

 about 1000 feet above sea level. The principal and sharp Uinta uplift 

 then occurred with high-angle border faults, and the uplift was recorded 

 by the deposition of the overlapping Uinta, Duchesne River, and Bridger 

 elastics. 



The Knight conglomerate in the Salt Lake-Evanston area was then 

 cast into broad folds extending north-south, as shown in section C-C of 

 Fig. 22.14. 



CENTRAL UTAH 



The stratigraphic relations of central Utah as worked out by Spieker 

 (1946) are idealized in Fig. 22.17. This is the physiographic region of 

 High Plateaus of Utah, described colorfully 80 years ago by Dutton. 

 The formations are flat-lying and conformable but faulted on the eastern 

 flank of the Wasatch Plateau (upper cross section of Fig. 22.18) which 

 stands above the Colorado Plateau to the east, but on the west flank 

 and adjacent San Pete Valley the structural relations are very complex. 

 A monoclinal flexure is prominent on the west slope and is interpreted 

 by Spieker as having been originally a truncated anticline following the 

 deposition of the Price River conglomerates and the North Horn forma- 

 tion. The Flagstaff limestone was then deposited over the truncated 

 anticline and then flexed downward to the west at a later time. Uncon- 

 formities exist at the base of the Price River, North Horn, Flagstaff, 

 Colton, Green River, and younger formations, and Spieker and students 

 have postulated numerous orogenies extending from the Colorado epoch 

 through Paleocene, Eocene, and later time. Stokes (1952), however, has 

 pointed out the similarity between this structural complex and the salt 

 anticlines in the Colorado Plateau, and contends that the thick Arapien 

 shale of Jurassic age with its salt and gypsum beds has moved upward 

 in an anticlinal core, has suffered extensive erosion, has permitted over- 

 lying beds to sag or collapse, and possibly has moved upward again on 

 several occasions. The localized, numerous unconformities are thus 

 explained. Hardy ( 1952) has shown that the central core of Arapien 

 shale is a tight anticline, so the shale cannot be said to have flowed 

 upward like a viscous intrusive salt body. 



Farther west the remarkable Canyon Range thrust ( Chistiansen, 

 1952) occurs and is shown in the third from the top section of Fig. 

 22.18. According to Christiansen a first episode of thrusting preceded the 

 deposition of the Indianola conglomerate. The sheet probably came from 

 the west but its roots are not evident. A second episode followed the 

 deposition of the conglomerate. 



An extensive volcanic field was built in the central part of the High 

 Plateaus province of Utah, after the main phases of Laramide orogeny. 



