330 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 22.3. Orogeny and sedimentation in Colorado time in the Central Rockies. 



Cordilleran geanticline, as it did in Idaho and Utah, and the sediments of 

 these two periods are only about 2000 feet thick altogether. They thin 

 eastward. Some gentle epeiric movements occurred in the shelf in Jurassic 

 time, as outlined in Chapter 18. 



In early Cretaceous time the western geanticline was raised sharply, 

 and the Kootenai conglomerate and arkosic sandstone were washed 

 eastward. It is not thick in most places, but very persistent in western 

 Montana. No structures have been segregated from the structural com- 

 plex of the west-lying ranges that were formed during the uplift respon- 

 sible for the Kootenai conglomerate. See paleotectonic map, Plate 15. 



A second conglomerate, the Dakota or basal Colorado, is much like the 

 Kootenai; it is perhaps not so uniform in distribution, but it is taken to 

 represent another uplift of the eastern margin of the geanticline. 



The radioactivity dates of the Idaho batholith appear to show it a little 

 later than the Dakota conglomerates. At least the first flood of Reltian 

 quartzite boulders so far identified appeared in late Montana time, and 

 these may have arrived at their present destination sometime after the 

 doming of the Reltian strata, consequent to the intrusion of the batholith. 

 The conglomerates in question make up the Harebell formation of the 

 northern Jackson Hole country. These will be considered in a later para- 

 graph. 



Montana Phase 



Little can be said about events in Montana time in southwestern Mon- 

 tana except that about 5000 feet of sandstone, siltstone, and shale accumu- 

 lated. These sediments make up an undifferentiated series near Monida, 

 and they undoubtedly attest uplift to the west. See Fig. 22.4. 



Paleocene Phase (Mid-Laramide) 



In Paleocene time a broad arch of about the size and shape of that of 

 the Rig Horn Mountains rose, and extended in a northeast direction ( Fig. 

 22.5). Its southeast flank in part was marked by a thrust fault; its north- 

 west flank was a fairly gentle flexure where observed. The Reaverhead 

 conglomerate seems to be localized around this great arch and to be 



