608 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



19,850 feet. Mt. Sanford in the Wrangell Mountains is 16,000 feet high. 

 The Aleutian Range has a general summit level of 3000 to 6000 feet and 

 includes a number of active and dormant volcanoes. The famous Valley 

 of Ten Thousand Smokes and the great crater of Aniakchak are situated 

 at the southwest end of the Aleutian Range. 



An oceanward arc of ranges, more truly called coast ranges, extends 

 through the Cook Inlet and Prince William Sound region. The Kenai 

 Mountains forming the Kenai peninsula east of Cook Inlet connect with 

 the Chugack Range, bordering the coast, and this merges with the St. 

 Elias Range. Some writers have grouped the St. Elias Range entirely with 

 the Coast Ranges, but its geology is too little known to permit a definite 

 conclusion. At the west end, the Kenai Range probably is continued by 

 the low mountains of Kodiak Island, and thence by beveled or buried ele- 

 ments in the shelf between the Aleutian Islands and Aleutian trench to 

 Unimak Island. 



The Shelikof Strait-Cook Inlet and Susitna River depression effectively 

 separates portions of the inner ranges from the outer. The Copper River 

 Valley and its tributary, the Chitina River, form another separating de- 

 pression. The Talkeetna Mountains break the continuity of the two 

 depressions and, anomalously, seem to bridge the two great mountain 

 systems. 



PALEOZOIC GEOSYNCLINE AND RELATED OROGENY 



Most of the Paleozoic rocks of Alaska are exposed in the Rrooks Range, 

 Seward peninsula, Central Plateau, and Alexander peninsula. The latter 

 has already been considered in a previous chapter. The Alaska, Nutzotin, 

 and Wrangell Ranges also contain Paleozoic rocks, and a nearby belt ex- 

 tends along part of Copper River and Chitina River valleys. The towering 

 mass of Mt. McKinley in the Alaska Range is eroded mostly from de- 

 formed Paleozoic strata. 



For a detailed study of the Paleozoic rocks of Alaska, Smith's 17. S. Geo- 

 logical Survey Professional Paper 192 should be consulted, particularly 

 the large correlation chart in the pocket. The formations are well devel- 

 oped in the Tanana-Yukon region of the Central Plateau, and a resume of 

 them is given on page 609. 



The igneous rocks of the Tanana-Yukon region have been summarized 

 by Mertie ( 1935 ) . Basic lavas of basaltic and diabasic character have 

 been extruded during at least five geologic epochs in the Paleozoic. The 

 first was in the Middle Ordovician, the second in the Middle Devonian, 

 and the last three during three epochs of the Carboniferous. Granular 

 intrusives of the same general character accompanied the extrusion of the 

 lavas, but the volume of such rocks is relatively small. Some rhyolite and 

 dacitic lavas and tuffs are found among the Carboniferous lavas, but gen- 

 erally speaking, lavas of acidic or intermediate character are rare. Ultra- 

 basic rocks were intruded during the late Devonian epoch. 



The volcanism that occurred during the Carboniferous period in 

 Alaska, according to Mertie ( 1935 ) was greater than in any other period 

 and most intense in the Alaska Range. The eruption of the basic lavas was 

 accompanied by epeirogenic movements that persisted into the Triassic. 



It is immediately clear that the above rocks represent the eugeosyn- 

 clinal assemblage previously recognized and described in the western 

 Cordillera of southeastern Alaska, British Columbia, Washington, Oregon, 

 California, and Nevada. The presence of the basic intrusives in the vol- 

 canic assemblage suggests that the belt was more the site of the archi- 

 pelago than an adjacent trough. 



In northern Alaska, the Paleozoic rocks are mainly sandstones, shales, 

 and limestones, and are typical of the miogeosyncline or shelf, also previ- 

 ously described in the Cordillera of Canada and the United States. No 

 volcanic rocks have been found in the sediments. A resume, as listed in 

 the correlation chart of Professional Paper 192, is given on page 610. 



The Upper Devonian and Mississippian rocks of a southeastern area 

 of the Brooks Range have recently been measured, and the section is 

 given in Fig. 39.3. Although about 8000 feet of strata of the two systems 

 are present, they are regarded as platform-type deposits and not miogeo- 

 synclinal by Bowsher and Dutro (1957). The massive lower and middle 

 members of the Kanayut conglomerate help to define a region of uplift 

 in the Late Devonian. See Fig. 39.12. The above strata are overlain by 

 variegated shale and siltstone, the Siksikpuk formation, about 350 feet 

 thick, which is probably Permian in age, and then the Shublik formation 

 of Triassic age. The Pennsylvanian is missing over all Alaska except the 

 northeast corner. 



