PRECAMBRIAN TECTONIC PROVINCES 



widely distributed, and locally have been given different names. Lead-uranium 

 ratios indicate that the age of the Pikes Peak granite is approximately 1 billion 

 years and that of the Silver Plume granite approximately 940 million years 

 (Lovering and Goddard, 1950). 



The lead-uranium ratio age determinations for the granites are younger 

 than those yielded elsewhere by the potassium-argon and rubidium- 

 strontium methods, and it seems probable that these will be recognized 

 as too young and replaced by new age determinations. 



Utah. The Precambrian rock succession in central Utah is shown in 

 the correlation chart of Fig. 4.5. The Farmington Canyon complex is the 

 basement rock and consists of gneisses, schists, and granulites, about 

 20,000 feet thick, once a stratified sequence of arkose, calcareous shale, 

 impure dolomitic and tuffaceous beds, and very pure quartz sandstone. 

 Metamorphism is of the lower amphibolite facies and therefore medium- 

 grade (Larson, 1957; Bell, 1951). The metamorphism is dated as 1590 

 m.y. (Gast and Long, 1957). 



Another sequence of beds, the Willow Creek and Harrison, seems to 

 be of intermediate age, and it is not clear yet whether they were in- 

 volved in the Mazatzal orogeny. The Farmington Canyon complex is 

 overlain unconformably by the Big Cottonwood quartzite and argillite 

 series and did not participate in the metamorphism of the older gneisses 

 and schists. 



The Big Cottonwood and Uinta series are generally correlated with the 

 Belt series of western Montana which is very thick and widespread. These 

 will be referred to under the next heading. 



Beltian Orogenic Belt 



A major trough or geosyncline of sediments and volcanic rocks of post- 

 Mazatzal age, yet pre-Paleozoic age, extends north and south from the 

 Mexican border through Arizona, Utah, Idaho, western Montana, eastern 

 Washington, western Alberta, and eastern British Columbia to the 

 Yukon, and possibly into Alaska. Its stratigraphy is complex, and much 

 remains to be discovered and worked out. Two major divisions appear 

 to stand out, namely, a lower one, the Beltian, and an upper one, which is 

 typified by a thick and well-described succession in the western Purcell 



Range (Reesor, 1957) and in northeastern Washington (Park and Can* 

 non, 1943). It has been referred to as the Upper Purcell b) 1 

 (1957) and also as the Lipalian series by Gussow ( L957). In northern 

 Utah, it may find representation in the Mineral Fork tillite and Mutual 

 formation (Crittenden et al., 1952). 



Angular unconformities have been recognized in a number of places up 

 and down the trough between the Beltian and Metaline sequences and 

 between them and the overlying Cambrian. In central Arizona Mazat z a l 

 Mountains) the Apache (Beltian) group is tilted, beveled, and overlain 

 by the Cambrian. In the Grand Canyon of northern Arizona, the Grand 

 Canyon series (Beltian) group is tilted, faulted, beveled, and overlain by 

 the Cambrian. In north-central Utah 12,000 to 15,000 feet of the Big 

 Cottonwood series (Beltian) and the Mutual formation are cut out 

 beneath the basal Cambrian angular unconformity. 



In western Montana and southeastern British Columbia Deiss (1935) 

 believes the Beltian strata were strongly uplifted, tilted, mildly folded, 

 and eroded before the Cambrian beds were laid down. In the Purcell 

 Range Cambrian beds lie across various Purcell formations (Beltian) 

 through a stratigraphic interval of 8000 feet, and although the dis- 

 cordance is generally slight, in one place it is 90 degrees (White, 1959). 

 Large sills and dikes are present in this region and probably accompanied 

 the orogeny. Campbell (1959) recognizes an unconfromity between 

 Middle Cambrian and Beltian strata in northwestern Montana and north- 

 ern Idaho in which up to 18,000 feet of Beltian is missing. 



Stimulated by a paper by Weiss (1959) the writer has prepared a 

 cross section from northeastern Washington across southern British- 

 Columbia to Waterton, Alberta, showing postulated conditions at the 

 beginning of Middle Cambrian time (Fig. 4.6). The Beltian correlatives 

 would be the Deer Trail, Priest River, and Lower Purcell groups. The 

 L T pper Purcell group would include the Monk, Three Sisters, and Horse- 

 thief Creek association, the Huckleberry, Leola, Irene, and Purcell vol- 

 canics, and the basal Huckleberry, Shedroof, and Toby conglomerates. It 

 may be seen that the Upper Purcell group rests unconformly on the 

 Beltian and is introduced by a thick and widespread conglomerate. This 

 unconformity is taken specifically to mark the orogeny of the Beltian 



