188 C. SCHUCHERT THE NORTH AMERICAN GEOSYNCLINES 



PACIFIC SEQUEXT GEOSYNCLIXE 

 (See Maps, Figures 12 to IT) 



Let US first consider the shorter and narrower Pacific geosyncline, bnt 

 one with a far longer history than the eastern or Eocky Mountain trough. 

 It extended from the Alexandrian archipelago of southeastern Alaska 

 to the southern end of Lower California, and while its main history is of 

 Mesozoic time, yet during the whole of the Cenozoic the Calif ornian 

 sea was a decidedly subsiding geos}Ticline. Actually there were, however^ 

 two more or less distinct Pacific geosynclines with different histories. 

 To the northern one, restricted almost wholly to Canada and essentially 

 of Mesozoic time, it is proposed to give the name of British Columbia 

 geosyncline^ while the southern one, of much longer endurance, has 

 long been known as the California sea. 



All through the Paleozoic the Alexandrian embay ment of south- 

 eastern Alaska was in evidence, and its seas, beginning with the Silurian 

 and continuing with interruptions up to the close of the Permian, laid 

 down apparently not less than 12,000 feet of sediments, though the depth 

 may be a great deal more (see maps, figures 7 to 17). With the late 

 Pennsylvanian, however, this embayment became but the northern part 

 of the British Columbia geosyncline, and the eastern shoreline of the 

 northern part of the Cordilleran trough was moved far to the west. 

 This northern geosyncHne of late Paleozoic time continued unbroken 

 into the Calif ornian sea, which at this time still retained its former 

 wide eastern spread. The British Columbia geosyncline, however, was 

 not in typical development until Upper Triassic time, and was blotted 

 out during the earlier Upper Cretaceous, long before the Eocky Mountain 

 trough was folded into mountains. 



The Triassic deposits of the British Columbia geosyncline attain a 

 maximum depth, on Vancouver Island, of 13,000 feet, but nine-tenths 

 of this is volcanic extrusives. The Jurassic is well represented by from 

 3,000 to 8,000 feet of deposits. Of earliest Cretaceous strata there are 

 none, and the sea returned in late Lower Cretaceous times and continued 

 unbroken into that of the Pierre, during which interval there were de- 

 posited from about 4,000 to 15,000 feet of coarse elastics and lava flows. 

 In other words, the whole of the trough subsided, from the Middle Penn- 

 sylvanian to the close of the Cretaceous, something like 25,000 feet. 



Xow let us consider the southern half of the Pacific sequent geosyn- 

 cline, or, better, the Californian sea. The Shastan channel was clearly 

 in evidence with the Middle Devonian, and the Cordilleran seas did not 

 become narrowed until Middle Triassic time. Therefore it was in the 

 Upper Triassic that the Californian geosyncline made its appearance. 



