314 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



SENTINEL RANGE 



Or Dm Dpic ^ & r /~\ 



5T0NE RANGE 



A// J/ PauL 



IO MILES 



Fig. 20.10. Cross section along Alaska Highway, west of Fort Nelson, in northeastern British Columbia, 

 between miles 375 and 443. After Loudon and Chronic, 1949. 



North of Calgary, Alberta, and east of Jasper National Park in the 

 Upper Brazeau River foothill area, Hake et al. (1942) have mapped a 

 group of low-angle thrusts. The thrusts divide the Upper Cretaceous 

 sediments into thin sheets that have been strongly folded. See sections 

 of Fig. 20.8 and map, Fig. 20.9). The thrusts are considered noteworthy 

 ( 1 ) because they are developed in weak beds, and the fault planes lie at 

 an angle to the bedding so that the sheets themselves are not competent to 

 have transmitted the thrust which caused the displacement; (2) because 

 the faults bear an exceptionally systematic relation to the bedding. The in- 

 vestigators believe these faults developed in an asymmetrical syncline, 

 and the faulting and the attendant crumpling relieved stresses which in 

 other folds and in other stratigraphic sections are relieved by bedding- 

 plane slippage. The thrusts are thought to be confined to the Mesozoic 

 section and the major syncline from which they developed, and to die 

 out completely with depth without producing any dislocation of the 

 Paleozoic rocks. 



It is evident that this concept is at variance with the more commonly 

 portrayed one of many small high-angle reverse faults meeting a major 

 low-angle thrust at depth, but the authors think that their theory may 

 have widespread application in the foothill belt. This is confirmed by 

 Scott (1954), who describes much the same structure as Hake et al. 

 (1942) and repeats that it is found in other parts of the foothill belt 

 besides the Brazeau and Cardium areas. He thinks that two distinct 



episodes of compression occurred, first thrusting of the thin sheets, and 

 second, folding of the thrust sheets. 



AGE OF THRUSTING 



The Cretaceous and Paleocene formations of the Rockies, foothills, and 

 plains of Alberta preserve the record of orogeny in the region to the west. 

 See correlation charts, Figs. 20.11 and 20.12. Warren (1938) has sum- 

 marized the evidence, and Fig. 20.13 is an attempt to show in diagram 

 what he has said in words. Incorporated in the diagram are also Evans' 

 ideas of the origin of the Rocky Mountain trench, and in addition, the 

 concept of post-Laramide graben-type faulting. 



The Kootenay and Blairmore of Early Cretaceous age thicken west- 

 ward, and conglomerates become abundant. A basal conglomerate of the 

 Blairmore is believed to represent the first pronounced uplift to the west. 

 The formations are exposed in the Canadian Rockies in Elk River at 

 Crowsnest Pass (see Fig. 20.8), and some of the pebbles and boulders of 

 the conglomerates are medium- to fine-grained granite and granite por- 

 phyry that could only come from the Selkirks (Evans, 1932). This seems 

 adequate evidence to date the first uplift of the Selkirks and to indicate 

 that the Rockies had not yet come into existence but were a site of de- 

 position. Since the Blairmore is Aptian and Albian, it is evident that the 

 Selkirks first rose in latest Jurassic or earliest Cretaceous time. The Lower 



