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STRUCTURAL GEOLOGY OF NORTH AMERICA 



The belt of Vancouver Island may be projected south-southeastward to a 

 deposit of very thick Upper Cretaceous strata in Washington (see maps, 

 Figs. 17.13 and 17.22). If the two were connected, as seems possible, then 

 a narrow but deep trough formed in this area after the orogeny and 

 batholithic intrusions of the late Jurassic and early Cretaceous phase. The 

 trough was continuous to Graham Island and, if farther, then it must 

 now be west of any land in southeastern Alaska and, therefore, in the 

 continental shelf. 



Near the base of the series on Vancouver Island, coarse conglomerate 

 is found. It contains angular to subangular pebbles and boulders, on the 

 average 2 inches in diameter but varying greatly in dimensions, of vol- 

 canic rocks, granodiorite, argillite, and quartzite. This conglomerate prob- 

 ably indicates the existence of a closely adjacent highland being actively 

 elevated while the trough sank. The Upper Cretaceous strata of Van- 

 couver Island occur in an open basin and dip about 15 degrees toward 

 the center. The gentle folding occurred before the intrusion of dikes and 

 stocks which are believed to be Eocene or Oligocene in age (Gunning, 

 1930). 



Farther north on Graham Island, the Upper Cretaceous strata have 

 been folded somewhat more intensely. See cross sections of Fig. 17.23. It 



is evident that a very late Cretaceous or early Eocene episode of folding 

 affected the thick sediments of the narrow Upper Cretaceous trough. 

 Since the history of the Eocene in nearby Washington and Oregon is 

 chiefly one of trough subsidence, it seems best to assign the disturbance 

 to the later Upper Cretaceous and to relate it provisionally to the Santa 

 Lucian orogeny of the central Coast Ranges of California. 



Peacock (1935) imagines that the Upper Cretaceous of Vancouver and 

 Graham islands was once more widespread than now, and that by the 

 close of the Cretaceous wide arms of the sea washed the margins of 

 remnants of the once great mountain system reduced to insignificant 

 relief. Because no Upper Cretaceous rocks are known in southeastern 

 Alaska, it is concluded that the region there was land for the rest of 

 the Cretaceous. Not until Eocene time did any significant subsidence oc- 

 cur. 



It will be recalled that the Coast Ranges of California are composed 

 mostly of the trough sediments, and the Island Ranges of southern British 

 Columbia are only in small part latest Jurassic and Cretaceous; they are 

 mostly the Nevadan complex. In southeastern Alaska, an offshore belt of 

 post-batholithic Cretaceous strata may exist, however, but submerged 

 beneath the continental shelf. See Fig. 17.21. 



