CENTRAL ROCKIES 



345 



N 

 k 





Intrusions 

 Volcanic eoneT" 



Iron Springs -/district j 





/o/u / 



/ / / 



/ QC«darCI»y 



Fig. 22.19. Structural features of southwestern Utah. Reproduced from Cook, 1957. 



the sparsity of soil and vegetation. The most recent displacement, so 

 strikingly evidenced by the offset horizontal basalt remnants, repre- 

 sents the lesser of two periods of faulting separated by a long intererosion 

 cycle. 



In Fig. 22.19 it will be seen that two structural trends intersect. One 

 which developed during the mid-Laramide orogeny along the boundary 

 between shelf and geosyncline trends northeasterly. It is marked by 

 folds, faults, and aligned intrusions. The second is a more or less north- 

 south group of faults and belongs the Tertiary Basin and Range system. 

 All the igneous masses in the Iron Springs district are intruded along a 

 single horizon in the Jurassic Carmel formation (Mackin, 1947). 



WESTERN UTAH 



Western Utah and eastern Nevada are characterized by approximately 

 north-south trending ranges separated by alluviated basins and generally 

 exhibit features of Tertiary block faulting. This is the Great Basin of 

 the geographer or the Basin and Range province of the geologist. The 

 older internal structure of the ranges is commonlv discordant with the 

 bounding block faults, and is one of strong folding and thrusting. 



In the Gold Hill district of western Utah, a complex of thrust and 

 normal faults provides a record of prolonged orogeny. See Fig. 22.23. 

 No accurately dated Cretaceous or Tertiary beds are present, and hence 

 the times of orogeny are not known. Nolan ( 1935) believes the succes- 

 sion of deformational events to have spanned the Cretaceous-Tertiary 

 boundary. 



In the East Tintic Mountains of Utah (Eureka district) north-trending 

 anticlines and synclines are superposed on a broad east-trending uplift 

 (Morris, 1957). Overthrust faults are closely related to the folds, and 

 cross sections (Fig. 22.24) suggest that the thrusts developed originally 

 as bedding plane faults which later cut the beds as the folds were intensi- 

 fied and overturned. Some of the thrusts are themselves folded. 



West of the East Tintic Mountains are the Sheeprock Mountains 

 in which an 11,000-foot thick sequence of late Precambrian meta- 

 sediments is extensively exposed (Cohenour, 1959). A tillite similar to 

 that in the Wasatch Mountains is a prominent part. All formations — 

 Precambrian and Paleozoic — are strongly folded and broken bv thrust 

 faults (Fig. 22.25). The major thrusts are the Sheeprock and Pole Canyon. 



