68 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Tooele arch. The arch and erosion is pre-Fish Haven (see Fig. 6.12). 



A deep and evidently narrow trough of Ordovician sediments exists in 

 the Canadian Rockies of western Alberta and eastern Rritish Columbia. 

 It is interpreted to lie east of the Reltian geanticline and to be separated 

 by it from the basin of northeastern Washington containing the Ordo- 

 vician Ledbetter slate, also of miogeosynclinal type. The Ordovician strata 

 of the Canadian Rockies consist of 3000 to 7000 feet of limestone, shale, 

 and slate beds with fossils representing a range from Lower to Upper 

 in different places ( Lord et al., 1947 ) . 



According to Roberts et al. (1958): 



Rocks of Ordovician age that belong to the western assemblage (eugeosyn- 

 cline) are widely exposed throughout north-central Nevada. They underlie 

 large areas in the Sulphur Spring Range, Roberts Mountains, Tuscarora Moun- 

 tains, Cortez Mountains, northern Shoshone Range, Toyabe Range, Batde Moun- 

 tain, and the Sonoma Range. So far as known they are allochthonous. 



Merriam and Anderson (1942, p. 1694) used the name Vinini formation 

 for rocks of Ordovician age of the western assemblage in the Roberts Moun- 

 tains. They divided the formation into two units. The lower part of the Vinini, 

 Early Ordovician in age, consists of quartzite, limestone, and calcareous sand- 

 stone, and silty and shaly sediments with minor amounts of andesitic lava flows 

 and tuffs; perhaps the relatively abundant limestone here suggests an approach 

 to the transitional assemblage. The upper part of the Vinini, of Middle 

 Ordovician age, is composed of bedded chert and black organic shale, clearly 

 of normal western lithologic type. 



The most complete stratigraphic section of the Vinini formation thus far 

 seen is in the Tuscarora Mountains, northern Eureka County, about 5 miles north 

 of U.S. Highway 40. Strata of Early, Middle, and probably late Ordovician 

 age are present; no detailed measurements were made, but it is estimated that 

 the section is at least 7,000 feet thick. 



In the Shoshone Range, Battle Mountain, and Sonoma Range the proportion 

 of massive quartzite, chert, and volcanic material in the Ordovician rocks of 

 the western assemblage is larger than in the Vinini formation. These rocks were 

 named the Valmy formation in Battle Mountain (Roberts, 1949, 1951) where 

 they have been subdivided into two members. The lower part of the Valmy 

 consists mainly of rather pure, generally light-colored quartzite, dark gray and 

 greenish chert, some gray to black siliceous shale, and a significant amount 

 of greenstone. The upper member consists principally of dark thin-bedded 

 chert interbedded with dark shale and a little greenstone. The base of the 

 Valmy is concealed but at least 4,000 feet is present. The upper beds of the 

 Valmy are highly contorted, but are estimated to be 3,000 or more feet thick. 

 [Refer also to Ross (1961).] 



In the shelf region the Transcontinental Arch was nearly completely 

 emergent, or at least no Ordovician strata occur on it under a Devonian 

 and Mississippian cover, except for the Colorado sag. This embayment 

 probably did not extend all the way through to the western geosyncline 

 or the Williston basin because in the eastern Uinta Mountains of Utah 

 the Mississippian beds (possibly Devonian) rest directly on the Cam- 

 brian. 



The ancestral Sweetgrass arch was broadly emergent and well-defined. 



Silurian Basins 



The Silurian seas were more restricted than any others in Paleozoic 

 time. The Laketown dolomite of northern Utah and southeastern Utah 

 has been traced widely over western Utah and is the sole representative 

 of the Silurian thus far recognized there. In eastern and central Nevada 

 the Roberts Mountain formation and overlying Lone Mountain dolomite 

 correlate with the Laketown. The entire section is carbonate rock, and 

 over half of it is dolomite (see Figs. 6.9 and 6.12). 



Silurian rocks of eugeosynclinal aspect appear to be widespread in 

 north-central Nevada, but because they resemble the Ordovician units 

 they may not have been recognized in mapping. 



On the east side of Pine Valley about 8 miles south of Carlin, unnamed 

 black shale and tawny to buff tuffaceous shale and calcareous shale have 

 yielded Monograptus determined by R. J. Ross, Jr., to be of Silurian age. 

 The thickness of these beds is not known. 



Black shale containing Monograptus is reported by C. W. Merriam (oral 

 communication) from the vicinity of McClusky Pass in the northern part of 

 the Simpson Park Mountains. C. A. Nelson (oral communication) also reports 

 Monograptus in shale on the east side of Pine Valley near Mineral Hill. On 

 the west side of the Tuscarora Mountains in the valley of Mary's Creek, 

 graptolites that according to R. J. Ross, Jr., have affinities with Silurian forms 

 were collected by Roberts in 1954. Silurian strata (R. J. Ross, Jr.), including 

 about 4000 feet of sandstone, arkose, shale, and a little chert, from part of 

 the overriding plate of the Roberts Mountains thrust in the northern Shoshone 

 Range and in the Cortez Mountains. 



The beds containing graptolites of Silurian age are on the whole less cherty, 

 and contain more calcareous shale and limestone layers than the Vinini and 

 Valmy formations. On the other hand, the Silurian beds of the western 

 assemblage appear much less calcareous than the Silurian of the transitional 



