PALEOZOIC CORDILLERAN GEOSYNCLINE 



67 



No Cambrian or Ordovician fossils have been found in northern Cali- 

 fornia, Oregon, and all Washington except the northeast corner. The 

 lack of information about the western margin of the continent in Cam- 

 brian time, and in Ordovician as well, is disappointing. The oldest fossils 

 yet discovered along the Pacific margin in the United States and British 

 Columbia are Silurian. These have been found in the Klamath Mountains 

 by Wells (1956). Three metamorphic series underly the fossiliferous 

 Devonian strata there, according to Hinds (1939), and one or more of 

 these might be Ordovician and Cambrian. See Fig. 6.3. In southeastern 

 Alaska Buddington reports Ordovician fossils, but no Cambrian. In con- 

 clusion it may be assumed that the entire region west of central Nevada 

 was eugeosynclinal from Ordovician time to the close of the Paleozoic. 



Ordovician Basins 



A broad Ordovician basin exists in western Utah and Nevada with 

 miogeosynclinal type sediments in the eastern and eugeosynclinal type in 

 the western part ( see Fig. 6.3 ) . The formations and their lithologies are 

 shown in Fig. 6.9, which is a section across central Nevada and marks the 

 change from the eugeosyncline to the miogeosyncline. The miogeo- 

 synclinal sediments of western Utah are reviewed by Hintze ( 1951 ) and 

 summarized in the table of Fig. 6.12. 



Another basin, which was narrower and completely miogeosynclinal in 

 character (Fig. 6.12, Logan area), existed in southeastern Idaho and 

 northern Utah. For a review of the stratigraphy see Ross ( 1953 ) . In 

 both basins the rocks are dominantly limestones and dolomites, but con- 

 spicuous quartzite formations exist in each. The Swan Peak quartzite 

 of southeastern Idaho and northern Utah is about 500 feet thick, and the 

 Eureka quartzite and the Swan Peak quartzite of western Utah and 

 eastern Nevada are nearly 800 feet thick together. The Eureka quartzite, 

 537 feet thick at Ibex, Utah, overlies an 85-foot dolomite member, and 

 this overlies the Swan Peak quartzite, 249 feet thick. The dolomite mem- 

 ber wedges out east of Ibex, and there the upper quartzite rests directly 

 on the lower. The absence or near absence of these sandstones together 

 with a thinner Ordovician section in Utah southwest of Great Salt Lake 

 indicates an uplift there which Webb (1958) has defined and named the 



Fig. 6.4. Thickness and paleogeographic map of the Silurian. 



