328 



STRUCTURAL GEOLOGY OF NORTH AMERICA 









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Fig. 22.1. Index map of Central Rockies. Lines of cross sections are indicated by numbers. 

 Intrusive igneous bodies are indicated by dotted lines. 



OROGENIC DEPOSITS 



A number of coarse conglomerates and thick sequences of sandstone and 

 shale mark the eastern border of the central Rockies, and in connection 

 with thrust faults and unconformities define a succession of orogenic 

 phases. The various formations with which we are mostly concerned from 

 southwestern Montana to southwestern Utah are shown in the correlation 

 chart of Fig. 22.2. 



The coarse conglomerates have generally been taken to record the 

 chief phases of mountain building immediately to the west, but thick 

 sequences of sandstone, siltstone, and shale may be equally significant. 

 The conglomerates record settings where a mountain front rose pre- 

 cipitously from a plain, as might have been the case of a vigorously 

 advancing thrust front. Rut a 10,000-foot section of sandstone, siltstone, 

 and shale of limited time range also records a substantial uplift in the 

 hinterland, possibly less vigorous but sustained, and without an immedi- 

 ately nearby thrust front. 



The example of the clastic deposits of Colorado time may be considered 

 ( see Fig. 22.3 ) . The lower 3,000 feet of the Indianola group in the Cedar 

 Hills is coarse conglomerate, but eastward and upward it becomes more 

 sandy and shaly. The thick conglomerate has been considered to mark 

 the Cedar Hills orogeny (Chapter 18). An associated thrust sheet rode 

 over part of the conglomerate in the Canyon Range but finally the thrust 

 front was buried by the last of the coarse deposits. Now, going north 

 to the Evanston area an accumulation of more than 8000 feet of sandstone 

 and shale occurs. Conglomerates are insignificant, yet the volume of 

 sediments appears almost as much as in the Indianola area, and the 

 adjacent uplift, therefore, almost as significant. 



Where a thrust sheet overlies a coarse conglomerate two orogenic 

 phases might be interpreted; the first to form the conglomerate and the 

 second by the riding of the thrust sheet over the deposit. However, the 

 conglomerate exposed may be simply an early part of the orogenic deposit 

 which was overridden as the thrust sheet advanced, in which case the 

 conglomerate and thrust are manifestations of the same orogeny. Local 

 settings have to be studied individually, and isopach maps such as shown 



