294 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



curred, the folds and faults there are all in Paleozoic strata, and therefore 

 the date of the folding cannot be fixed except as post- Paleozoic. The main 

 orogenic events in the eastern trough came in Late Cretaceous time and 

 during the Paleocene and Eocene, and therefore the folds and faults in 

 the Paleozoic strata of the geanticline immediately to the west have gen- 

 erally been accredited to these later orogenies. However, the work of 

 Nolan ( 1935 ) in the Gold Hill mining district of western Utah is espe- 

 cially significant in making clear the complexity of deformation in the 

 geanticlinal area. There the structural history is characterized by at least 

 four and possibly five phases of folding and faulting, each phase com- 

 posed of an initial stage in which compressive forces were active and a 

 final stage in which normal faulting was dominant. The first two phases 

 predate the Eocene by a long interval of erosion and are regarded pro- 

 visionally as Cretaceous by Nolan. It is probable that they are related to 



the Nevadan and post-Nevadan Cretaceous disturbances to the west (see 

 chart, Fig. 17.2 and 17.7) and to the sinking of the Utah trough during 

 the time that the Indianola, Kelvin, Aspen, and Frontier and other 

 formations were deposited in it. The map, Plate 10, shows the crust 

 intensely affected in the Sierra Nevada region in late Upper Jurassic time, 

 while the area on the east was only epeirogenically uplifted. During 

 Cretaceous time, the reverse seems to have been true. The Sierra Nevada 

 region was one of gentle emergence, and the eastern part was probably 

 orogenically deformed. 



In conclusion, there is no evidence to preclude the generalization that 

 the most intense disturbance in the landmass just west of the trough was 

 localized opposite the area of greatest subsidence, which also coincides 

 with the central part of the arcuate pattern. See especially Plates 11 and 

 12. 



