338 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



WASATCH MTS. 



ECHO CANYON 



EVANSTON 



OYSTER RIDGE 



Green River fm. 



Adaville fm. 



Lozeart ss.-- 



Hill i ard sh. 



j-Oyster Ridge ss. 



Frontier 



Bear River fm. 



Aspen sh. 



Gannett gr. 



M. Eocene 

 "L. Eocene 



Montana 



Colorado 

 Dakota 



L.Cret. 



- U. Jurassic 



Fig. 22.11. Idealized cross section showing relations of formations from Wasatch Mountains to 

 Oyster Ridge, Wyo., restored to the close of Green River deposition. Folding of Knight near 



beds, and apparently immediately overridden by rising thrust sheets 

 both on the east and west (Fig. 22.6). The Wind River Range rose along 

 a high-angle thrust which cuts the conglomerate, and new thrusts broke 

 out in the Hoback Range on the west (Grizzly and Cabin thrusts, Fig. 

 22.9). The Cliff Creek thrust sheet is overridden by these later slices. 



The Knight formation was spread widely over erosion-beveled strata 

 in the Evanston-Salt Lake City area, and then was itself folded in large 

 open folds with amplitude of several thousand feet and fold widths of 

 5 to 15 miles. The frontal Wasatch Range north of Salt Lake City first 

 came into existence at this time. The folding was accompanied by 

 longitudinal normal faulting ( Eardley, 1944 ) , and this may have marked 

 the inception of Basin and Range faulting. 



In very late Eocene time the Park City volcanic field was formed, and 

 the related Fowkes tuff accumulated in erosional valleys in the Knight 

 conglomerate and older formations. 



The Uinta Mountains had their chief growth in late Eocene time 

 (Bridger, Uinta, and Duchesne River time). 



Late Cenozoic Phases 



The chief orogenic activity in late Cenozoic time was block or rift 

 faulting. A belt of trenches, horsts, and tilted blocks formed from 



Morrison (?) fm. 



Wasatch Mountains followed, then erosion, then deposition of Norwood tuff (early Oligocene), 

 the Basin and Range type faulting. Section 5, Fig. 22.1. 



northwestern Arizona through western Wyoming and southeastern Idaho 

 to British Columbia, and ranges and valleys came into existence such as 

 shown on Fig. 22.7. 



At the junction of the fold belt with the Wind River-Gros Ventre 

 uplift overthrusting occurred in early Pliocene time (Love, 1956b). This 

 late thrusting is unique in the Rocky Mountains and most probably 

 does not represent a part of an extensive compressional belt. We have to 

 deal with the deposition of the Camp Davis conglomerate, overthrusting 

 on the conglomerate, and normal block faulting all in a very short time. 

 The thrusts are also not traceable for any appreciable distance. These 

 observations lead the writer to the conclusion that the thrusting is a 

 gravity slide phenomenon associated with uplift. The events, structures, 

 and deposits would be interpreted as follows. Normal faulting of vigor- 

 ous nature started, and on the down-thrown block the conglomerate 

 accumulated to a thickness of 2000 to 3000 feet. Then gravity gliding 

 of large masses from the upthrown blocks occurred down over the con- 

 glomerate in places. Deposition of conglomerate continued around the 

 slide masses, and with continued normal faulting the slide masses across 

 the fault were cut and offset, and the uplifted parts removed by erosion. 

 As the fault pattern is studied it seems to fit best, if not require, this 

 interpretation. 



