66 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



ORDOVICIAN 



Fig. 6.3. Thickness and paleographic map of the Ordivician. 



In Chapter 4 the Precambrian Mazatzal and Reltian orogenic belts have 

 been described. Although the Beltian trough of sedimentation and later 

 belt of orogeny marked the first tectonic development parallel with the 

 present Pacific margin of the continent, the older Mazatzal belt seems to 

 have made an impress on the Paleozoic geosynclinal basins. The Trans- 

 continental Arch, which reflects the Mazatzal orogenic belt, borders 

 directly on the arch in southeastern Utah, Arizona, and Colorado, and 

 the two have the same trend to the southwest. See maps, Figs. 6.1 to 

 6.5 



An uplift here called the Raft River geanticline, is identified in north- 

 western Utah (Stokes, 1952; Felix, 1956) and southwestern Montana 

 ( Scholten, 1957) on the south and north sides of the Snake River volcanic 

 field respectively (see Fig. 6.11). Its extent northwestward cannot be told 

 because of the cover of Tertiary volcanic rocks and the intrusion of the 

 great Idaho batholiths, but in the interpretation rendered on Fig. 6.2 it 

 appears as a geanticlinal uplift between the eugeosyncline basin in north- 

 ern Nevada and the miogeosyncline of Utah. An unconformity in the 

 Upper Cambrian detected in the South Stansbury Mountains (Rigby, 

 (1958) with 700 feet of beds removed may be a lateral affect of the Raft 

 River geanticline (see Fig. 6.10). The erosion surface lies beneath the 

 Cole Canyon dolomite. 



Still farther north in northwestern Montana, northern Idaho, and 

 British Columbia is an extensive region of Precambrian strata, the Belt 

 series, and this is here interpreted to have been a fairly persistent struc- 

 tural feature from Cambrian time on. Evidence cannot be sited for 

 shoreline deposits and overlapping relations, but this is mostly due to 

 the extensive batholithic intrusions and metamorphism. Early geologists 

 considered the Beltian terrane the shore of an extensive, west-lying land 

 which they called Cascadia, but later ones have considered the Paleozoic 

 strata, beginning with Middle Cambrian, to have been deposited across 

 and then eroded away incident to the emergence of the modern gean- 

 ticline in Cretaceous and Tertiary times. Sloss ( 1950 ) however, suggests 

 a small uplift there, and his interpretation is reflected on the maps of the 

 Williston basin, Figs. 5.17, 5.18, and 5.19. The writer takes the view that 

 it has been a significant feature from Cambrian time on ( see Chapter 33 ) . 



