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STRUCTURAL GEOLOGY OF NORTH AMERICA 



older Tertiary volcanics which had been tilted and eroded. Some late 

 Tertiary and Quaternary volcanic fields are also prominent. The volcanic 

 rocks throughout are basalt and andesite lavas and related pyroclastics 

 (Lord et al, 1947). 



The major areas of Tertiary volcanic rocks are as follows. In Yukon 

 Territory and extending into Alaska irregular isolated areas occur which 

 lie mainly within two northwesterly trending belts. The easterly of the 

 two belts extends from near Carmacks, at the mouth of the Lewes River 

 to and into Alaska, and is here called the Yukon field. The westerly belt 

 lies along the northeast flank of the St. Elias Mountains and is here 

 called the St. Elias field (see map, Fig. 37.1). 



In northern Rritish Columbia one area, in part of Quaternary age, 

 extends north from Telegraph Creek on the Stikine River for about 80 

 miles (the Telegraph Creek field) and another floors the Rocky Moun- 

 tain Trench for perhaps 150 miles along Finlay and Fox rivers ( the Fin- 

 lay River field). The latter consists of sediments and volcanics of late 

 Oligocene and early Miocene age. 



The largest field is in the southern interior of Rritish Columbia where 

 major accumulations in places several thousand feet thick extend 350 

 miles in a northwesterly direction and 150 miles in a northeasterly direc- 

 tion. These are flat-lying and commonly referred to as the plateau basalts. 

 The accumulation is labeled "Plateau Volcanic Field" on the map. A 

 large and associated field immediately to the northwest is here called 

 the Fort Frazer. It consists in the Fort Frazer area of a lower series of 

 upper Oligocene and lower Miocene sediments and volcanics dipping at 

 angles up to 30 degrees and overlain unconformably by an upper series 

 of nearly horizontal basalt, andesites, and other volcanic rocks about 

 2000 feet thick. 



An informative Tertiary section is found in the Okanogan Valley close 

 to the International Boundary. It is described as follows: 



Here the Springhrook formation, perhaps of Paleocene age, and composed 

 of soils, alluvium, talus, stream and lake deposits, and tuff, rests on a pre- 

 Tertiary rock surface of steep relief. These strata accumulated in the valleys and 

 are overlain by and to some extent interlayered with the andesites, basalts, and 



pyroclastic rocks of the Marron formation, which buried the valleys and 

 reached a thickness of more than 4,000 feet. The White Lake formation, 

 consisting mainly of lake and stream deposits with coal, was deposited on the 

 Marron strata from which most of their materials were derived. They are locally 

 as steep as 65 degrees and 4,000 feet or more thick. Their age is probably late 

 Eocene, but they may be somewhat younger. The White Lake strata are 

 overlain unconformably by beds of more gendy dipping andesitic breccia and 

 agglomerate, which are succeeded upwards by agglomerate and conglomerate. 

 The youngest conglomerate beds are horizontal and of pre-Pleistocene age 

 (Lord et al, 1947). 



Volcanic activity occurred on a much reduced scale in the Quaternary 

 period. In Yukon very young lavas occur and a loose, white volcanic 

 ash is widespread which is at best only a few thousand years old. In the 

 Telegraph Creek field Hoodoo Mountain on Iskut River may still be an 

 active volcano. Recent lavas have been noted in several places along the 

 coast. 



RELATION OF VOLCANISM TO TECTONIC PROVINCES 



Most all post-batholithic volcanism in the Canadian Cordillera is 

 limited to the batholithic belt. Minor activity has occurred west of the 

 major Coast Range batholith in the island archipelago, but the major 

 activity was to the east of the island belt, and very approximately between 

 the western zone of batholiths and the eastern zone. The fields are dis- 

 continuous and the volume of extruded rock is apparently not large. 

 About one-tenth of the batholithic belt is covered. 



The three trenches shown on the map, Fig. 37.1, are believed to be 

 Tertiary grabens, but little is known about them. The Finlay River 

 volcanic field fills the Rocky Mountain trench for a distance of about 

 180 miles. This is the only occurrence of volcanics in association with 

 the trenches whose combined length in Canada is 3000 miles. It can be 

 thought, therefore, that the association is accidental and not genetical. In 

 South America, the trenches and volcanism seem more closely associated. 



The row of stratovolcanoes of the Cascades extends into southwestern 

 British Columbia. Pliocene, Pleistocene, and Recent activity is noted in 



