6. 



PALEOZOIC 



CORDILLERAN GEOSYNCLINE 



DIVISIONS AND THEIR CHARACTERISTICS 



Schuchert is probably more responsible than anyone else for the use 

 of the expression Cordilleran geosyncline in describing the basins of 

 accumulation of sediments along the western margin of the continent. 

 He also used the term Rocky Mountain geosyncline. During the Mesozoic, 

 his "Cordilleran intermontane geanticline" split the overall broad and 

 irregular basins into two longitudinal divisions, but before the geanticline 

 became pronounced, the divisions were already evident by the nature of 

 their sediments, the western being an eugeosynclinal assemblage and the 

 eastern a miogeosynclinal. The eugeosyncline extended from mid-Nevada 



to the Pacific Coast, and the miogeosyncline from mid-Nevada to central 

 Utah (Fig. 6.1). The miogeosyncline is much better known than the 

 eugeosyncline. The basins of sedimentation and geography shifted some- 

 what from one period to another, but the broad overall relations remained 

 fairly constant. The change from the thick sedimentary sequence of the 

 miogeosyncline to the thin sediments of the shelf has been called the 

 Wasatch line (Kay, 1951), and for all Paleozoic periods except Silurian 

 the change is fairly abrupt and in much the same position. The broad 

 divisions as outlined were probably first recognized by Stille (1941) and 

 later elaborated on by Kay (1942, 1951, 1960) and Eardley (1947). 



The eugeosyncline probably sank more and received a greater thickness 

 of sediments than the miogeosyncline, but the extent of sediments in 

 both was great. The major difference lies in the character of the sedi- 

 ments. The eugeosyncline received a dominant amount of volcanic 

 material and graywacke, whereas the miogeosyncline was filled with 

 sandstones, quartzites, shales, limestones, and dolomites. The volcanic 

 material in the eugeosyncline is in several forms: flows, volcanic con- 

 glomerates, and various pyroclastics. The volcanics and graywackes occur 

 in every stratigraphic system from Upper Cambrian to Cretaceous. The 

 Permian especially was a time of excessive volcanism, and the volcanics 

 of that period have been traced from California and western Nevada to 

 Alaska (Wheeler, 1939; White, 1959). In the Humboldt Range of north- 

 western Nevada, over 10,000 feet of Permian strata, largely volcanic, have 

 been identified. 



Roberts et al. (1958) estimate that the miogeosynclinal strata in east- 

 ern Nevada and western Utah above the thick basal quartzite of the 

 Cambrian consist of 60 per cent limestone, 30 percent dolomite, 8 percent 

 shale, and 2 percent quartzite. They estimate that the eugeosvnclinal 

 strata, on the other hand, in the Sonoma Range and vicinity consist of 

 20-40 percent shale, 10-30 percent sandstone, graywacke, and quartzite, 

 up to 30 percent of chert, with shale partings, and up to 30 percent of 

 volcanic and pyroclastic rocks. 



The units are characteristically lenticular, and thin or thicken abrupdy 

 parallel with and normal to the geosynclinal trend. Limestone, generally shah' 

 or sandy, locally forms thin, discontinuous layers. The shale units are commonlv 



63 



