geologic history 717 



Geologic History of Alaska in Triassic Time 



The youngest known Paleozoic rocks of Alaska are the Permian ( ?) 

 limestones, which carry a fauna closely allied to that of the Artinskian of 

 Eussia. The wide distribution of this limestone indicates that there was 

 a period of profound submergence in late Paleozoic time. The Artinskian 

 sea probably extended over the larger part, if not all, of Alaska. 



Since Lower Triassic rocks have not been recognized in Alaska, it is 

 believed that the end of Paleozoic time was marked by a wide-spread 

 emergence which continued until the beginning of the Upper Triassic. 



The Permian or early Triassic lavas of the Pacific coastal belt, which 

 are represented by the Nikolai greenstone of the Chitina Valley, the basic 

 lavas and tuffs of the upper Susitna Valley, the ellipsoidal lavas of Kenai 

 Peninsula and Kodiak Island, the greenstones of the Uiamna-Clark Lake 

 district, and the ellipsoidal lavas of Hamilton Bay (and which may be 

 represented by the Orca greenstones of Prince William Sound and by 

 some of the greenstones of southeastern Alaska), indicate a period of in- 

 tense, wide-spread, and probably long-continued volcanic activity that 

 probably occurred in early Triassic time. These volcanic rocks, as far as 

 known, are not intercalated with marine sediments and presumably were 

 poured out on land during the Permian or early Triassic period of emer- 

 gence. The non-fossiliferous sedimentary rocks which are intercalated 

 with the Orca greenstones are, in the writer's opinion, not marine. The 

 volcanic activity of this period apparently extended throughout the entire 

 region south of the Alaska Range, from the Alaska Peninsula eastward 

 and southward into British Columbia. Volcanic rocks of this date are 

 not known north of the Alaska Eange and probably were never present 

 there, although their absence may be due to post-Triassic erosion. This 

 sharp limitation of the distribution of these rocks along the present line 

 of the major mountain axis of Alaska is very significant and, in the 

 writer's opinion, clearly indicates that this line marked the northern limit 

 of the supposed early Triassic volcanic activity. 



Middle Triassic time was probably a period of emergence in Alaska, 

 for Middle Triassic rocks are known only in Seward Peninsula. 



The Upper Triassic was a time of submergence, when limestone-form- 

 ing seas swept over large Alaskan areas. The initial Upper Triassic sub- 

 mergence was probably in Karnic time, when limestones were deposited 

 throughout the greater part, if not all, of the present Pacific Mountain 

 region, being known along the Pacific coast from southeastern Alaska to 

 Cook Inlet and in the Copper and the Susitna valleys. These limestones 

 locally attain a thickness of at least 3,000 feet and, in general, are not in- 

 Lll — Bull. Geol. Soc. Am., Vol. 27, 1915 



