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STRUCTURAL GEOLOGY OF NORTH AMERICA 



part of the Yakataga formation on early Tertiary and pre-Tertiary rocks. 

 Near Lituya Bay in the Lituya district the narrow belt of Tertiary rocks 

 is folded into a shallow syncline and a strongly asymmetric anticline. These folds 

 pass to the southeast into a seaward-facing homocline which, at Icy Point, is 

 overturned. Upper Tertiary rocks in the outlier in the northern part of the 

 Lituya district form a broad syncline trending northwest. 



Cook Inlet Tertiary Province 



The Cook Inlet Tertiary province includes the Cook Inlet lowland 

 and the lower part of the Susitna River valley. About 75 miles to the 

 east of the lower Susitna River basin and separated from it by the 

 Talkeetna Mountains, is the Copper River Tertiary and Quaternary basin. 

 The basins are floored extensively with Quaternary deposits, and these 

 are believed to cover Tertiary beds which crop out mostly in marginal 

 areas. See map, Fig. 39.1. 



Stratigraphy. The chief display is a coal-bearing series of nonmarine 

 elastics. In the Kenai lowland the strata have been named the Kenai 

 formation. They consist of partly indurated sand, silt, clay with thin 

 conglomerate lenses and many thin beds of sub-bituminous coal or lignite, 

 and have a thickness of at least 4700 feet. The formation is presumed 

 to be Eocene and to rest unconformably on the deformed Mesozoic 

 rocks. 



At a locality on the northwest margin of the Cook Inlet province, 900 

 feet of clay, sand, and gravel, presumed to be Eocene, rests with angular 

 unconformity on highly deformed slate and graywacke of Mesozoic age. 

 The unconsolidated beds are overlain, apparently conformably, by 

 500-1100 feet of coarse gravel, possibly of Oligocene or younger age. 



Structure. The Tertiary beds of the Cook Inlet province are not as 

 much deformed as those of the Gulf of Alaska province. For the most 

 part they are nearly flat or only gently tilted or folded. In some marginal 

 areas dips up to 60 degrees have been observed. 



Aniakchak Tertiary Province 



A Tertiary area, here called the Aniakchak Tertiary province, com- 

 poses the southwestern half of the Alaska Peninsula. The Upper Jurassic 

 and Cretaceous rocks are overlain with minor unconformity by Early 



Tertiary nonmarine, coal-bearing arkosic sandstones and shales and 

 much fragmental volcanic material interbedded with flows. These rocks 

 are presumed to underlie much of the Shelikof Strait depression. See Fig. 

 39.1. 



Marine strata of Eocene, Miocene, and Pliocene (?) age are exposed 

 in the Herendeen Bay area and Shumagin Islands. 



The Early Tertiary strata of the Alaska Peninsula are in general 

 gently tilted or folded. Several well-defined anticlines with flank dips of 

 5 to 45 degrees have been mapped. One is 30 miles long. 



Laramide Oogenic Belt 



The Laramide orogenic belt lies north of and adjacent to the Nevadan. 

 It is made up of two parts, the Foothills or gently deformed belt, and 

 the main or strongly deformed. In contrast to the Nevadan belt, the 

 Laramide involves the Paleozoic platform-type sediments, as well as 

 Mesozoic sediments mostly of miogeosynclinal nature. Also, intrusive 

 masses are few and not so large as in the Nevadan. The line drawn on 

 Fig. 39.2 separating the Nevadan from the Laramide was determined 

 mostly from the distribution of late Mesozoic intrusions, viz., most of 

 the intrusions lie south of the line. Included in the Laramide belt, ac- 

 cordingly, are the Yukon basin, Seward uplift, Hogatza arch, Kobuk 

 basin, Brooks Range geanticline, and the Arctic Foothills belt. 



Brooks Range Geanticline. The northern limb of the Brooks Range 

 geanticline consists of slightly metamorphosed Devonian and Carboni- 

 ferous rocks. Dark clastic rocks of the Sadlerochit formation (Permian 

 and Early Triassic) generally overlie the lighter carbonate rocks of 

 the Lisburne group (Mississippian) and form conspicuous hogbacks 

 along the northern edge of the range. The structure of the northern 

 half of the geanticline is one of folds and thrusts. 



The southern limb consists of early Paleozoic metamorphic rocks and 

 Silurian limestone. Tight folds and thrust faults toward the north repeat 

 the formations in numerous subparallel belts (D. J. Miller, 1959). 



At least 10,000 feet, and perhaps 15,000 feet, of Devonian and Carboni- 

 ferous sedimentary rocks including much limestone were deposited in a 

 Paleozoic basin in the area of the present Brooks Range. Most of the 



