32 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



a great elongate basin generally north-south and parallel to the Pacific 

 margin of the continent, and that the basin is discordant with the older 

 orogenic belts, across which it lies. This, if true, is of great significance. 

 It suggests that following the Mazatzal orogeny that a major part of the 

 western margin of the continent was removed, because the older belts of 

 orogeny now extend at nearly right angles to the continental margin. 

 It also suggests that in Beltian time the processes of sedimentation and 

 orogeny first became established along and parallel to the present con- 

 tinental margin. 



The discordant relation of the Beltian trough and orogenic belt to the 

 older belts emphasizes the concern that must be attached to the uraninite 

 date. It is almost as old as the Mazatzal orogeny, and presumably should 

 be separated from it by considerable time. 



Not only is the Beltian orogenic belt discordant with the Mazatzal 

 orogenic belt, but also are the Antler and Shuswap belt and Nevadan belt 

 which lie west and parallel with the Beltian ( see Chapters 6 and 17 ) . If 

 the theory is held that the nucleus of the continent has been added to by 

 successively younger orogenic belts, then some major change occurred 

 to the southwest margin of the North American continent in Beltian or 

 pre-Beltian time. Perhaps a major part of the southwest margin as it ex- 

 isted in pre-Beltian time is missing, but no plausible theory of translation 

 or foundering has been thought of to restore the missing part. It is con- 

 ceivable that a major change occurred in the constitution and assembly 

 of the continents in the interval of time immediately preceding the 

 Beltian. 



Purcell Orogenic Belt 



Following the Beltian orogeny in the southern British Columbia and 

 northeastern Washington region a thick conglomerate was deposited, and 

 then extensive volcanic rocks were spread all the way from the Columbia 

 River in Washington to Waterton, Alberta. These were followed by sand- 

 stones and argillites, particularly in a main trough in the Purcell Range 

 area. After this depositional and volcanic cycle another disturbance oc- 

 curred in which, in the Purcell Divide area, the entire series was removed 

 together with a considerable thickness of the underlying Belt series 



(Weiss, 1959). This unconformity attests the removal of a greater thick- 

 ness of strata than the one at the base of the Shedroof-Toby con- 

 glomerates, according to Weiss. See Fig. 4.6. 



The overlying Lower Cambrian quartzite appears to have been derived 

 from the west, like the basal Huckleberry-Shedroof-Toby conglomerate, 

 and, if so, indicates that the major axis of orogeny lay to the west. The 

 zone from the Purcell Range to the front of the present Rockies was a 

 broad geanticline across which the Early Cambrian seas failed to spread. 

 The Middle Cambrian seas, however, probably transgressed much of the 

 geanticlinal area (Campbell, 1959). 



The orogeny of post-Monk and Three Sisters age, yet of pre-Early 

 Cambrian age, will here be called the Purcell. 



In dealing with Precambrian formations distant correlations are gener- 

 ally questionable, and this is especially so when assuming that the 

 Mineral Fork tillite and Mutual strata of northern Utah are equivalent 

 to the Upper Purcell group. If valid, however, an orogeny can be said 

 to have occurred after the close of Mutual time and before the late 

 Lower Cambrian sands were spread across the beveled edges of these 

 formations as well as those of the Big Cottonwood series. It is not clear 

 how discordant the tillite and Mutual are to the underlying Big Cotton- 

 wood strata because of limited exposures, but Crittenden et al. (1952) 

 note that the tillite occupies broad smooth-bottomed basins scooped out 

 of the upper part of the Big Cottonwood series. 



Both the Beltian and Purcell orogenies may be combined in one angular 

 unconformity in the Grand Canyon of the Colorado in northern 

 Arizona. It is evident that information on the extent of the Beltian and 

 Purcell orogenies in scanty and that the pronouncements of the preceding 

 paragraphs are postulates of fairly tenuous nature. 



Keweenawan Belt 



The Keweenawan series of the Lake Superior region is the youngest 

 of the Precambrian rocks there and is well known because of the great 

 value of its copper mineralization. An imposing sill dated 1100 m.y. by 

 Goldich (personal communication) and believed to be part of the Ke- 

 weenawan series, crops out along the northwest shore of Lake Superior. 



