64 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 6.1. Major Paleozoic tectonic elements of western United States. The eugeosynclinal 

 bondary was farther east than shown in Permian time. The Wasatch line through southern 

 Nevada has been called the Las Vegas line (Welch, 1959). 



sandy and few are calcareous. The quartzites are generally nearly pure, but 

 the sandstones are either graywackes or feldspathic sandstones. The chert units, 

 partly of volcanic derivation, range from a few inches to several hundred feet 

 thick; individual chert layers are lenticular and range from a fraction of an 

 inch to 3 feet. They are separated by shaly partings which are also lenticular; 

 laterally, chert units grade into siliceous shale units with subordinate chert. 

 The volcanic rocks are largely andesitic or basaltic pillow lavas and pyroclastics 

 that accumulated mainly in a marine environment; most are highly albitic. 

 Siliceous pyroclastic rocks locally form thick sections. The volcanic rocks are 

 highly lenticular, and probably formed around many source centers (Roberts 

 etal, 1958). 



Another characteristic of the sediments of the eugeosyncline is their 

 metamorphism. The thick sequences, especially in the Sierra Nevada, 

 Klamath Mountains, western British Columbia, and southeastern Alaska, 

 are made up of phyllites; slates; argillites; quartz, chlorite, hornblende, 

 and calcareous schists; hornblende gneiss; recrystallized chert; marble; 

 meta-conglomerate; meta-andesite; and various metamorphosed pyro- 

 clastics. Still another characteristic is the presence of great intrusive 

 bodies of later age, and the metamorphism of the sediments about the 

 intrusions. 



The sediments of the miogeosyncline, on the other hand, are not 

 metamorphosed. Many of the sands are cemented with silica and termed 

 quartzite, but little dynamic metamorphism incident to Paleozoic, 

 Mesozoic, or Tertiary orogeny has occurred. 



The medial belt in central Nevada contains transitional types of the 

 two environments, and became not only a geanticline but a belt of 

 orogeny in late Devonian time. The western eugeosynclinal strata were 

 thrust many miles eastward to rest on miogeosynclinal strata of strikingly 

 different lithology. 



BASINS AND UPLIFTS OF THE WESTERN UNITED STATES 

 AND SOUTHERN BRITISH COLUMBIA 



Cambrian Basins 



The miogeosyncline is noted for its Cambrian sections (Fig. 6.2). At 

 one locality in southern Nevada and California 17,000 feet of Lower, 

 Middle, and Upper Cambrian beds have been measured. 



