614 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



ceous strata in the immense thickness of 40,000 to 65,000 feet. The rocks 

 are dark, interbedded shale and fine-grained graywacke. Breccia and 

 conglomerate facies are present in a few localities. The record of crustal 

 deformation and sedimentation is described as follows: 



. . . Sediments were eroded from emerged areas of the geanticlines and were 

 carried by streams to the trough of the intervening Kuskokwim geosyncline, 

 where scores of thousands of feet of sediments were deposited while subsidence 

 continued, during latest Early Cretaceous and early and possibly middle 

 Late Cretaceous time. The sediments were drawn from older rocks exposed 

 in the geanticlines — phyllite, slate, quartzite, limestone, siltstone, chert, basalt, 

 and andesite. 



The geanticlines, particularly the Aniak-Ruby geanticline continued to be 

 uplifted rapidly during at least the early part of the Late Cretaceous time, 

 and areas of sharp relief evidently appeared from which the older rocks were 

 violently eroded and subjected to disintegration almost entirely mechanical. 

 The disintegration products, chiefly angular silt and sand-size fragments, were 

 transported fairly short distances to the Kuskokwim geosyncline. The submarine 

 relief of the belt of the Kuskokwim geosyncline, like the subaerial relief of 

 the geanticlines, was continually steepened in the early Late Cretaceous epoch, 

 particularly along the borders of the trough. Sediments left by the streams in 

 this marginal area formed loose, unconsolidated deposits that were continually 

 and repeatedly upset by the steepening of the trough borders, and slid down 

 the submarine slopes of the trough. Part of the silt and sand involved in the 

 slides became incorporated in turbidity currents of high density and were 

 distributed in the otherwise unagitated water below wavebase. The sediments 

 of the slides and of the turbidity currents came to rest to form the interbedded 

 graywacke and shale of the Kuskokwim group. The graywacke beds formed at 

 the time of sliding, and are possibly related to turbidity currents capable of 

 transporting the sand-size particles. The latter settled at depths at which the 

 currents were checked by seawater of equal density. Shale beds were laid 

 down in more quiet intervals of setding. Beds of graywacke, many of which 

 are as much as two feet thick, were probably formed in a very short time by 

 this process, an instant of time in the geologic sense (Cady et al. 1955). 



In the Alaskan peninsula 800 feet of Cretaceous arenaceous limestone 

 occur in the Herenden Bay area, and 1175 feet of marine limestone, 

 sandstone, shale, and conglomerate are noted on the southeast flank 

 of the Talkeetna Mountains (Miller, 1959). These are part of the 

 Matanuska basin. See Fig. 39.2. 



Earliest Tertiary deposits occur in the Matanuska Valley and consist 

 of shale, sandstone, conglomerate, and coal. They have a maximum 



thickness of 7000 feet and are tentatively assigned to the Paleocene. They 

 are considered by Payne (1955) to represent the closing phase of sedi- 

 mentation in the Matanuska basin. 



MESOZOIC AND CENOZOIC OROGENIES 



Belts of Orogeny 



The eugeosyncline of southern Alaska during the Paleozoic and 

 Mesozoic eras bespeaks almost constant orogeny. The platform region 

 to the north was involved principally in Mid-Cretaceous and Early Terti- 

 ary orogeny. The southern margin of the eugeosyncline was involved 

 in deformation during the Cenozoic, and thus a parallel with the 

 western United States is at hand, for we have to deal with the Laramide 

 belt on the north, the Nevadan belt in the southern half, and the Coast 

 Range belt along the southern margin. 



Nevadan Orogenic Belt 



The Nevadan belt (Fig. 39.2) is characterized by Paleozoic and 

 Mesozoic eugeosynclinal strata, by their intense deformation and low- 

 grade metamorphism over large areas, by voluminous and numerous 

 batholithic intrusions, and by the fact that the climatic orogenic events 

 took place in the latter half of the Mesozoic. Three phases of Mid- and 

 Late Jurassic orogeny are noted by unconformities in the Jurassic 

 sequence in the Matanuska basin, and a fourth phase in the earliest 

 Cretaceous (Miller, 1959). Intrusive activity began in Mid-Jurassic 

 and continued through Late Jurassic in the Talkeetna geanticline. The 

 major and intense deformation of the Alaska and Seymore basins oc- 

 curred in late Early Cretaceous time (late Neocomian and Aptian). It 

 was accompanied by the intrusion of the major batholiths there. 



Jurassic orogeny is obscure in central Alaska but strata of pre-Albian 

 age (late Early Cretaceous) are strongly deformed and considerably 

 intruded. The batholiths may be Jurassic as well as Early Cretaceous. 



The Kuskokwim group of Late Cretaceous age in the Kushokwim basin 

 and similar strata in the Yukon basin (Fig. 39.2) were strongly folded, 



