MESOZOIC SYSTEMS ALONG THE PACIFIC 



285 



intrusive rocks and a younger group of more silicic-alkalic intrusive rocks. 



Structure of Southeastern Alaska. Most pervading of any structural 

 feature is the isoclinal folding. In a belt 15 to 30 miles wide adjacent to 

 the west side of the Coast Range batholith, the isoclinal folds are slightly 

 or markedly overturned toward the southwest. See Fig. 17.20. In general 

 the axes or axial planes of the isoclinal folds escape detection, and uni- 

 form dips occur through wide intervals. In the Wrangell-Revillagigedo 

 belt of metamorphic rocks near the batholith, the dip of the schist and 

 gneiss is 60 to 90 degrees northeast. In the outer part of the belt, the dip 

 is 30 to 50 degrees northeast. On the east side and at the northeast end 

 of Prince of Wales Island, the isoclinal folds are overturned toward the 

 northeast (Buddington and Chapin, 1929). 



Out of the numerous isoclinal and close folds, a number of anticlinoria 

 and synclinoria may be recognized. Two dominate the structure of the 

 Alexander peninsula, namely: the Junean synclinorium next west to the 

 great batholith, and west of the synclinorium the Prince of Wales-Kuiu 

 anticlinorium. Both synclinorium and anticlinorium seem to be divided 

 into branching or parallel synclinoria. The axes of all the major fold com- 

 plexes are shown on the map of Fig. 17.19. 



The formations exposed in the Prince of Wales-Kuiu anticlinorium are 

 almost exclusively Paleozoic, and make up a belt 40 to 50 miles wide. In 

 the trough of the synclinorium the Upper Jurassic and Lower Cretaceous 

 formations are exposed. The Keku-Gravina synclinorium is dominantly a 

 shallow downwarp of Tertiary formations. They rest unconformably on 

 Mesozoic formations which in general are folded into the great Junean 

 synclinorium. 



On the west flank of the Prince of Wales-Kuiu anticlinorium in the 

 Sitka district is the Sitka Mesozoic belt. A western belt of Tertiary sedi- 

 ments and volcanics is exposed along the shore north of Cross Sound, and 

 the southern half of Kruzof Island on the north side of Sitka Sound is 

 composed of Quaternary volcanics (Mt. Edgecombe). 



Through Frederick Sound is an axis of cross folding. Jurassic beds are 

 exposed at intervals along the south side of Admiralty Island and form a 

 considerable part of the coastline. They appear to have been folded along 

 two axes — the usual north-northwest axis and another about parallel to 

 Frederick Sound — that is, east-west. The gently dipping Tertiary lava 

 beds about Frederick Sound seem to represent a broadly folded anticline 

 from the center of which they have been eroded away. The axis of the 

 anticline strikes northeast, approximately in the direction of the Mesozoic 

 cross fold, and Buddington suggests that the forces in Tertiary time were 

 oriented almost at right angles to those that effected the folding in pre- 

 Tertiary time. 



Structure of the Island Ranges of British Columbia. For a clear dis- 

 cussion of the topographic elements of British Columbia refer to Peacock 

 (1935). Under the present heading, the area west of the Coast Range 

 batholith in British Columbia is signified. No summary treatment of the 

 folds and faults of this great island region has been written such as Bud- 

 dington and Chapin's account of southeastern Alaska, although numerous 

 reports of specific areas are available. They are chiefly Suminanj Reports 

 of the Geological Survey of Canada. Even if possible, it does not seem 

 feasible for the present writer to attempt a synthesis, but in general it 

 appears that the same type of structure as in southeastern Alaska con- 



Fig. 17.20. Structure section in southeastern Alaska, after Buddington and Chapin, 1929. A, 

 across Gravina and part of Revillagigedo Islands, showing Triassic beds thrust over Devonian. 



I B and B', continuous section from Iphigenia Bay to the mainland. C, across Kuin and Kupreanof 

 Islands to the mainland. D and D', continuous section along the south side of Frederick sound 

 to the mainland. Upper Jurassic of Lower Cretaceous intrusives: dt, diorite; md, monzodiorite; 

 qd, quartz diorite. Metamorphic rocks, probably Ordovician to Jurassic or later, Wrangell- 

 Revillagigedo belt: sgp, schistose greenstone and green phyllite; ph, phyllite; sph, md, crystalline 



| schist and phyllite with beds of marble; gn, layered gneisses. Lower and Middle Ordovician: 

 Ogs, indurated graywacke with slate, andesitic volcanics, chert, conglomerate, and limestone. 

 Silurian: Sar, andesitic volcanics and conglomerate; SI, limestone, with thick conglomerate, 

 sandy beds or argillaceous beds (Sc); Sgr, predominant graywacke. Middle Devonian: Dsa, 

 slate, limestone and chert with interbedded andesitic volcanics; Da, andesitic lava, breccia and 



conglomerate with limestone cobbles; Dgt, predominantly graywacke and tuffaceous beds; Dsr, 

 sediments, including graywacke, conglomerate, slate, limestone and chert, with associated 

 volcanics. Mississippian: Cmc, chert, quartzite and limestone. Permian: Cpc, conglomerate, lime- 

 stone, sandstone, andesitic and basaltic volcanics; Cpl, limestone with white chert layers. 

 Triassic: Icl, conglomerate, sandstone, and limestone; lis, slate with sandstone in upper part; 

 Trav, andesitic volcanics, including breccias and lava flows locally interbedded with sediments. 

 Jurassic or Cretaceous: Kgs, graywacke, slate, and conglomerate with tuff and limestone; Kgr, 

 greenstone volcanics. Lower Cretaceous: Ksg, slate and graywacke with chert nodules, impure 

 limestone, and conglomerate. Eocene: Tra, rhyolite and andesite volcanics, conglomerate, and 

 dacite porphyry sills; Tba, basaltic and andesitic lava with some breccia and conglomerate. 

 Quaternary: Qb, basalt and tuff. 



