MESOZOIC SYSTEMS ALONG THE PACIFIC 



277 



conglomerate. The gritty beds are charged commonly with small angular 

 fragments of black argillite. All the coarser types are decidedly feld- 

 spathic. Some of these sediments could probably be called graywacke. 



In southeastern Alaska, Buddington and Chapin (1929) have noted 

 numerous outcrops of Triassic rocks and others that may be Triassic. All 

 the strata that carry fossils are Upper Triassic, and they seem to be 

 divisible into three units, one consisting of sediments and the other two 

 of volcanic rocks with a little intercalated sedimentary material. The 

 volcanic formations are differentiated from volcanic formations of other 

 periods on the basis of their faunas. Their character and structural rela- 

 tions over wide areas are insufficiently known, and their lithology is too 

 similar, to separate them otherwise. They comprise green andesitic flows, 

 breccia, and tuffs. The lava predominantly shows pillow structure but is 

 in part amygdaloidal and in part polygonally jointed. Much of the breccia 

 has a limestone matrix and is in considerable part the result of primary 

 disaggregation of the radial-jointed pillows. The basal part of the volcanic 

 rocks on Kuiu Island consists of interbedded limestone and green 

 andesitic tuff and lava with local conglomeratic beds. On Kupreanof 

 Island, the volcanic formation has a bed of conglomerate 150 to 200 feet 

 thick in local areas at its base. The basal Triassic conglomerates and the 

 unconformable relations to the Paleozoics have been discussed previously 

 in the section on the late Permian or early Triassic orogenic phase. 



On Kupreanof Island and the islands southeast of Kake, Upper Triassic 

 sediments overlie the upper limestone division of the Permian and are 

 overlain in apparent conformity by volcanic rocks of late Triassic age. 

 Locally there is a thick bed of coarse conglomerate of the Upper Triassic 

 volcanic rocks. On the northeast side of Kuiu Island, however, the Upper 

 Triassic volcanic rocks overlie the lower division of the Permian without 

 any apparent angular unconformity. The volcanic rocks of Kuiu Island 

 also carry a different fauna from those on Kupreanof Island. Uncon- 

 formities are indicated, therefore, not only at the base of the Upper 

 Triassic, but within it (Buddington and Chapin, 1929). 



The Triassic occurrences in British Columbia and Washington are so 

 little known that unconformities within the beds assigned to this period, 

 if they exist, are not known. It is of interest, however, to recall the un- 



conformities below and above the Upper Triassic beds of western Nevada, 

 and to note the same position of breaks in southeastern Alaska. 



Another series of beds that was intruded by the great Coast Range 

 batholith in southeastern Alaska has been assigned questionably to the 

 Jurassic. Some of these beds may be Lower Cretaceous and some Triassic 

 and Paleozoic. They have been divided into two groups for mapping pur- 

 poses, namely, a predominantly sedimentary facies consisting of gray- 

 wacke, black slate, and conglomerate; and a predominantly volcanic 

 facies consisting of schistose greenstone made up of breccia, flows and 

 tuffs, and black slate and graywacke. 



These questionable Jurassic and Lower Cretaceous rocks are believed 

 to overlie the Paleozoic and Triassic formations unconformablv. A pro- 

 nounced angular unconformity separates Jurassic from Devonian forma- 

 tions at the north end of Kupreanof Island, but where the Jurassic rests 

 on the Triassic the break is more in the nature of a disconformity ( Bud- 

 dington and Chapin, 1929). 



The Jurassic ( or Lower Cretaceous ) slate and graywacke appear to be 

 much less metamoq)hosed than the older Mesozoic formations, but this is 

 certainly in part due to their character. In all the formations, the 

 argillaceous beds are most resistant to recrystallization, and their abun- 

 dance in this series gives the Jurassic formations a misleading appearance 

 of minor metamorphism. The pebbles and cobbles of the intercalated 

 conglomerates are very markedly flattened as a result of very strong 

 pressure. 



To summarize, map, Fig. 17.14 may be referred to again. Triassic and 

 Jurassic strata were deposited east of the present Canadian Rockies, and 

 this basin of deposition was separated from a broad region of deposition 

 by a Mesozoic geanticline which now is displayed chiefly as the Beltian 

 terrane. The sediments of the eastern basin are miogeosynclinal and shelf 

 types, whereas the sediments west of the geanticline are eugeosynclinal. 

 and may have accumulated in several deep troughs. Due to the great 

 batholiths that occupy much of the region of southern British Columbia 

 it is apparently impossible to recognize the original extent of the basins or 

 their number. Triassic and Jurassic strata occur in places from Kootenay 

 lake westward to the Pacific Ocean. Since thev are laden with volcanic 



