316 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 20.12. Correlation of uppermost Cretaceous and Paleocene formations of southwestern 

 Alberta. Reproduced from Tozer (1953). 



the Kishenehn beds Duchesnean. Moderate deformation and renewed uplift 

 took place in early Oligocene (Chadronian) time. 



The Eocene age of the Lewis thrusting has been demonstrated fairly 

 well by MacKenzie (1922) from an Eocene formation in the Flathead 

 Valley, back of the Clark Range. Alden (1932) also believes the Lewis 

 thrust occurred in Eocene time. 



Evans (1932) presents two arguments to support an earlier age of the 

 Selkirk system, west of the trench. The great Rocky Mountain trench 

 and the structures of the Rocky Mountains trend parallel with each other, 



but the mountains west of the trench, viz., the Purcell Range, the Selkirk 

 Range, and the mountains west of Columbia Lakes, trend nearly north 

 at an acute angle to the trench, and are truncated by it. From these 

 relations it seems that the trench is associated with the building of the 

 Rockies which were formed later than the Selkirks. 



Secondly, the Selkirk system contains many great intrusions; the Rockies 

 only a few smaller ones. See Fig. 37.1. The intrusions have been related 

 to the Coast Range batholith, and considerable evidence in Chapters 21 

 and 37 has been summarized that shows they are probably of Early or 

 Mid-Cretaceous age. 



The structural discordance and the great intrusions of the Selkirk sys- 

 tem fit into the sedimentary record very well, and all three together 

 demonstrate a fairly substantial case for the Early Cretaceous age of the 



SELKIRK SYSTEM 



NORTHERN ROCKY MOUNTAIN TftOUSN 



•LSERTt ShELF 



SELKIRK SYSYEM 



CANADIAN ROCKY MOUNTAIN SYSTEM 



FOOTHILLS 



SELKIRK SYSTEM TRENCH CANAOlAN ROCKY MOUNTAIN SYSTEM FOOTHILLS PLAINS 



Fig. 20.13. Evolution of the eastern margin of the Selkirks and the Canadian Rockies. 

 Idealized sections incorporating parts of sections B— B' and E— E'. 1, Beltian; 2, Paleozoic, 

 Triassic, and Jurassic; 3, Kootenay; 4, Blairmore; 5, Colorado; 6, Belly River; 7, Edmonton; 

 8, Paskapoo; 9, Lower Oligocene conglomerate. Upper diagram, growth of Selkirks during 

 Cretaceous time and subsidence of northern Rock Mountain trough. Middle diagram, Laramide 

 orogeny during the Eocene and the deposition of the Lower Oligocene conglomerate. Lower 

 diagram, erosion of Rocky Mountain trench and Lower Oligocene conglomerate producing 

 present aspect. 



