620 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Southern Foothills. The southern foothills are characterized by iso- 

 lated, irregular hills and ridges of sandstone, limestone, and chert 

 which rise above low shale areas of little relief. This section has the 

 structural complexity of the Brooks Range but differs in being composed 

 of less resistant rocks, including a great thickness of shale. Ridges and 

 hilltops are at altitudes of 2500 to 3500 feet and rise 1000 to 2000 feet 

 above the surrounding plains. The southern foothills are readily travers- 

 able by such vehicles as the weasel but not so easily by boat, plane, or 

 foot. Lakes suitable for landings by small float planes ( 1 to 2 passengers ) 

 are not abundant, and only a few lakes such as Noluk and Liberator are 

 suitable for larger float planes (3 to 6 passengers). The flat areas be- 

 tween the hills or along ridgetops are ideally suited to the use of 

 tracked vehicles. 



Northern Foothills Section. The northern foothills section differs 

 from the southern section in having more regular topography, including 

 persistent ridges and elongate mesas that reflect a simpler structure of 

 Appalachian-type folds, with minor cross faults, and a few major 

 overthrusts. Anticlines are commonly asymmetric with steeper limbs on 

 the north. 



TERTIARY VOLCANIC ROCKS 



Volcanics of the Coast Ranges 



The Wrangell Mountains (Figs. 37.1 and 39.1) consist of a major 

 Quaternary stratovolcanic accumulation. At least four major centers of 

 eruption form a cluster of majestic peaks, namely, Mt. Wrangell ( 14,000 

 feet), Mt. Drum (12,000 feet), Mt. Sanford (16,210 feet), and Mt. 

 Blackburn (16,140 feet). 



Of these only the first-named has been seen "smoking." Apparently volcanism 

 in this region did not begin until some time after an early Tertiary plain of 

 erosion had been formed, uplifted, and somewhat dissected. Since that time 

 there has been almost unceasing volcanic activity in different parts of the 

 area, during which the present huge agglomeration of flows, breccias, and 

 tuffs has accumulated. Most of these rocks are porphyries of medium coarseness 

 and light or dark-gray color. In composition the usual type is a hypersthene 

 or hornblende andesite, but more basic or more acidic phases range from 



basalt to dacite. The color of these rocks also shows a considerable variation 

 from the type, as brick-red, pink, lavender, brown, and greenish tones are bv 

 no means rare. The eastern limit of the lavas in the Copper River region that 

 may be correlated with the Wrangell lava is in the mountains adjacent to 

 Skolai Pass, where they cap many of the highland areas and unconformablv 

 overlie Paleozoic and younger sedimentary rocks. That the lavas in this area 

 are correlative with the older members of this volcanic series seems clearly 

 indicated by the extensive dissection they have undergone, whereby the 

 deep valleys of Skolai Creek and the Nizina River and Nizina Glacier have 

 been deeply trenched through them. None of these Tertiary-Recent lavas 

 shows evidence of marked deformation after they were poured out. The 

 thickness of the lava series differs considerably in different places, and no 

 measurements are available that disclose the total thickness of these beds in 

 the heart of the range. Partial sections have shown more than 4000 feet of 

 these volcanic rocks near Regal Glacier, in the Nizina Valley (Smith, 1939) 



Cook Inlet-Susitna Field 



Overlying the sedimentary coal-bearing and associated rocks in the 

 Matanuska area and extending both eastward into the Nelchina area 

 and northward into the Talkeetna Mountains is a series of andesitic 

 basalt flows with intercalated tuffs. They are nearly horizontal and at 

 least 1000 feet diick. They are deeply dissected and form cappings of 

 the highlands. In the Nelchina area certain rhyolites appear. The series 

 is thought to be late Eocene to Miocene in age (Smidi, 1939). 



Tertiary volcanic rocks are widespread in the Nevadan belt and only 

 a few examples will be mentioned. 



Volcanics of the Nevadan Orogenic Belt 



In the Kuskokwim region the Lower (?) and Upper Cretaceous 

 Kuskokwim group, is overlain disconformably by the Ididarod basalt, 

 also of Late Cretaceous age. It is regarded as the first of a succession of 

 volcanic rocks deposited in a continental environment. The Getmuna 

 rhyolite group and the Holokuk basalt are early to mid-Cenozoic in 

 age, and are separated from the older rocks by an angular unconformity. 

 In late Cenozoic time die Waterboot basalt was erupted. 



Intruded into the Kuskokwim sediments are a number of stocks of, 

 quartz monzonite, believed to be post-Holokuk basalt. 



In the Yukon-Tanana region an older unit consists of rhyolite, dacite, 



