ALASKA AND THE YUKON 



615 



IDIDAROD 

 FAULT 



Kk 





KUSKOKWIM 

 RIVER 

 t 



KIOKLUK MOUNTAINS 



Kk 



Th 



■','///,'/,'- 



HOLITNA FAULT 



TAYLOR MOUNTAINS 



^:c^Mmmmmm?m^nn^ 



Kk 



«r:.Tqm;--:yN, 



: -:--sooo' 



K"pPCg 



MILES 



KT<PCg 



15 



Fig. 39.7. Cross section of central Kuskokwim region, Alaska. After Cady ef al., 1955. Th, Holkuk basalt; 

 Ki, Ididarod basalt; Kk, Kuskokwim group; K"fiPCj.., Gemuk group. 



probably in late Paleocene time. See structure of Fig. 39.7. According to 



Cady et al. (1955); 

 « 



. . . Late in Late Cretaceous time the deposits in the geosyncline were up- 

 lifted slightly above sea level, and the lava flows of the iditarod basalt spread 

 'out over the uppermost strata of the Kuskokwim group. 



The geanticlinal tracts moved closer together in earliest Tertiary time, 

 probably because the more rigid continental platform and Pacific Ocean floor 

 approached one another and decreased the width of the mobile belt. The 

 geosynclinal accumulations of the Kuskokwim group, which were structurally 

 less competent than the geanticlines, were as a result thrown into folds that 

 were draped around the margins of the geanticlines, and were also grouped 

 into rather extensive anticlinorial uplifts, such as the Gemuk anticlinorium, 

 ;\vhich includes an upbuckled portion of the floor of the Kuskokwim geo- 

 syncline. Biotite basalt sills and dikes and albite rhyolite sheets, sills, and dikes, 

 pardy concordant with the enclosing formations, were intruded in the geo- 

 synclinal rocks and underlying strata near the close of folding. 



Nonmarine Early Tertiary rocks, presumably of Eocene age, occur as 

 ierosional remnants on the Nevadan complex in several areas of central 

 JAlaska, particularly in the Healy basin (Fig. 39.2), north of the Alaska 

 JRange. They consist of claystone, sandstone, conglomerate, and lignite 

 'up to 5000 feet thick. Sedimentary rocks of this age are believed to have 

 'been deposited extensively in what are now the alluvium-floored low- 

 ! land basins. These Eocene sediments were gently, and locally strongly, 

 deformed in Oligocene or early Miocene time. 



Strong uplift occurred lastly at the close of the Tertiary and during the 

 Quaternary to produce the high mountain ranges and upland areas of 

 i central Alaska. 



Coast Range Orogenic Belt 



The Coastal Range orogenic belt as here defined is much like the 

 Central and Northern Coast Ranges of California, inasmuch as the bed- 



rock geology is the Nevadan complex with deformed Tertiary beds 

 superposed. 



The belt shown on Fig. 39.2 is widest in the Cook Inlet and Prince 

 William Sound region where it includes the Chugach and Kenai Moun- 

 tains. Three areas of Tertiary rocks are recognized, the Gulf of Alaska 

 Tertiary province, the Cook Inlet Tertiary province, and the Aniakchak 

 Tertiary province. 



Gulf of Alaska Tertiary Province 



Stratigraphy. The Yakataga basin of Tertiary deposits is an arcuate 

 lowland and foothills belt. The province borders the Gulf of Alaska 

 from the Copper River delta 300 miles southeastward to Icy Point, and 

 extends inland up to 40 miles to include the southern front of the Chugach 

 and St. Elias ranges. Although generally lowlands, the Gulf of Alaska 

 Tertiary province includes groups of hills and unnamed moun- 

 tains in the Katalla district up to 5000 feet above sea level, the Robinson 

 Mountains in the Yakataga district rising to 9000 feet, the Chaix and 

 Samovar Hills along die north margin of the Malaspina Glacier to 6000 

 feet, and a ridge in the Lituya district up to 3500 feet. Elevations above 

 1500 feet are covered by permanent snow fields and glaciers. 



Typical sections of the Tertiary rocks are given in Fig. 39. S. 



Three major subdivisions of Tertiary rocks are recognized on the basis of 

 gross lithologic characteristics and fossil evidence. These units arc believed 

 to correspond to major changes in the depositional environment oJ the Yakataga 

 geosyncline. 



The oldest unit, of Eocene and possibly early Oligocene age. consists pre- 

 dominandy of interbedded or intertonguing nonmarine coal-bearing strata 

 and shallow marine or brackish water strata. Fossil plants and marine in- 

 vertebrates in this unit are regarded as indicating subtropical to temperate 

 climate on land and tropical to warm-temperate marine environment. This 



