624 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



uplifts to be part of a much larger uplift embracing the eastern end of 

 the Chukotski peninsula. 



Now, if the Geologic Map of the U.S.S.R. (1955) is consulted, the 

 Chukotski peninsula and adjacent areas to the west are found to be made 

 up of three geologic provinces, namely, ( 1 ) a deformed and considerably 

 intruded Cretaceous basin on the north; (2) a Tertiary Coast Range 

 province on the south; and (3) an intermediate Tertiary volcanic belt 

 in which it appears that the volcanics rest mostly on the Cretaceous com- 

 plex. See map, Fig. 39.2. The Cretaceous basin with its abundant Creta- 

 ceous volcanics and many batholiths and stocks seems similar to the 

 central geanticline and adjacent basins of southwestern Alaska, and if 

 tectonic connections on this basis are attempted several lines of evidence 

 support the postulate. 



The Coast Range orogenic belt is adjacent on the south in Siberia as 

 in Alaska. St. Lawrence Island with its major intrusions appears to be 

 Nevadan and falls within the projected Nevadan belt. See Fig. 39.2. The 

 shallow water shelf of the Bering Sea will contain both belts of orogeny, 

 and the outer margin of the shelf lies in the line of projection. By this 

 theory an erosion surface of the orogenic belts would have been buried 

 by the deltaic deposits of the Yukon and Kuskokwim rivers. The last 

 evidence suggestive of the northwest bend of the Nevadan and Coast 

 Range belts is the bathymetry of the shelf off the southeast side of the 

 Alaska peninsula. If the map of Fig. 39.11 is referred to, it will be seen 

 that the shelf is broad off Kodiak Island and westward to Unimak Island 

 (Fig. 39.1), and then narrows so that hardly any shelf exists along the 

 volcanic islands of the archipelago. The narrowing shelf margin projects 

 almost exactly to the Bering Sea shelf margin, as if this is a major tectonic 

 line. It may thus be imagined that this line marks the swing of the Coast 

 Bange belt toward the northwest and Anadyr Bay. 



The Seward uplift then becomes a coigne around which the Nevadan 

 belt wraps rather sharply. 



The Aleutian Archipelago is here considered a welt or geanticline that 

 has developed with customary curvature, volcanism, and trench from 

 ocean basin crust, whereas the Nevadan and Coast Range belts are 

 marginal to continental crust. The archipelago and the Coast Range belt 



have evolved probably simultaneously, although the archipelago is now 

 very active while the Coast Range belt under the Bering Sea is quiescent 



Bering Land Bridge 



With Nevadan, Laramide, and Coast Range belts extending from 

 Alaska to the Anadyr-Chukotski region of Siberia there can be little doubt 

 that land was continuous from one continent to the other many times 

 from the beginning of the Cretaceous to the present. 



Hopkins ( 1959 ) reports that if sea level were lowered 120 feet, only 

 a channel 20 miles wide would remain. If lowered 150 to 180 feet an 

 intercontinental land connection would be established via St. Lawrence 

 Island and the Diomede Islands. If lowered 300 feet, presumably to the 

 level during the maximum glaciation of the Wisconsin, Alaska and Siberia 

 would be joined by an almost featureless plain nearly 1000 miles wide 

 from the shrunken Bering Sea to the shore of the Arctic Ocean. 



YUKON TERRITORY AND THE DISTRICT OF MACKENZIE 



Geography 



The principal mountains and rivers of Yukon Territory and the ad- 

 jacent district of Mackenzie are shown on Fig. 39.1. The Selwyn Moun- 

 tains form the major drainage divide, with the several tributaries of the 

 Yukon River flowing to the west, and tributaries of the Mackenzie flowing 

 to the east and north. The long arcuate Mackenzie and Franklin Moun- 

 tains stand off to the northeast of the main Cordillera, with the Mackenzie 

 River flowing between the two ranges. Several plains and plateaus in 

 addition to those shown are recognized by various writers (Bostock, 

 1948; Martin, 1959), but the geographic nomenclature is not com- 

 pletely standardized. 



Stratigraphy 



Strata of every Paleozoic and Mesozoic system are present in the region 

 as well as rocks of Precambrian and Tertiary age. Dominant rock types 

 are as follows: Precambrian and Lower Cambrian, elastics; Middle and 

 Upper Cambrian, Ordovician, and Silurian, carbonates, black shales, and 



