70 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Devonian rocks of the western assemblage appear to be widespread through- 

 out north-central Nevada, but are most abundant from the Shoshone Range 

 eastward. These lack the basic volcanic flows and pyroclastics characteristic of 

 Cambrian and Ordovician rocks of the western assemblage, but locally contain 

 silicic pyroclastics, much chert and shale, and a litde calcareous shale. 



In Slaven Canyon in the Shoshone Range and elsewhere in the Mt. Lewis 

 Quadrangle, there are at least 4,000 feet of strata composed dominantly of dark 

 gray to black chert with some dark shale, a little sandstone, and very small 

 amounts of limestone. These have yielded ostracods and conodonts of Middle 

 Devonian age. Similar rocks on and south of Bald Mountain, in the northern 

 Toyabe Range southwest of Cortez, are probably correlative. 



Tuffaceous shale and calcareous shale on the east side of Pine Valley about 

 8 miles south of Carlin have also yielded conodonts of Devonian age. These 

 rocks are associated with Silurian and Ordovician rocks in the upper plate of 

 the Roberts Mountains thrust (Roberts et al., 1958). 



In the southern Klamath Mountains siliceous black shales and slates 

 containing thin beds of sandstone and fossiliferous limestone, now largely 

 recrystalized, make up the Kennett formation of Devonian age. It crops 

 out in two restricted belts, and rests unconformably on the older rocks. 

 Devonian strata are not known in the Sierra Nevada or Coast Ranges 

 south of the Klamath Mountains in California. 



Late Devonian Orogeny 



Toward the end of the Devonian period, according to Nolan ( 1943 ) , a 

 geanticline began to rise in central Nevada, approximately along the 

 transition zone of eugeosynclinal and miogeosinclinal sediments. See Fig. 

 6.5. The uplift divided the geosyncline into a western and an eastern 

 trough, and the distribution of Devonian sediments is reflected in two 

 ways, viz., by the almost complete removal of the earlier Devonian 

 deposits along the axis of the arch, and by an eastward shift to the vicinity 

 of Eureka, Nevada, of the zone of maximum sedimentation. The geanti- 

 cline was later named the Manhattan (Eardley, 1947). Since then a large 

 amount of significant field work has been done and the geanticline has 

 come to be recognized as a belt of major orogeny, and has been called 

 the Antler orogenic belt ( Roberts et al., 1958 ) . 



At the close of the Devonian, fundamental changes took place along 

 the western part of the area of predominantly carbonate deposition 



(miogeosyncline). The carbonate rocks were folded and overridden by 

 the Roberts Mountains thrust plate that brought clastic and volcanic 

 rocks of equivalent age but different facies from the west or northwest. 

 Clastic rocks eroded from the rising upland in the west marked the end 

 of the broad geosyncline in north-central Nevada as it had existed earlier, 

 and introduced a change to narrow straits and embayments in the 

 orogenic belt during the remainder of the Paleozoic. The clastic rocks 

 do not resemble the assemblages laid down in the geosyncline during 

 early and middle Paleozoic, but overlap all of them. On the west, over- 

 lapping rocks rest with angular unconformity on rocks of the western 

 and transitional assemblages; in the Carlin area, west of Elko, the un- 

 conformity is much less marked; and on the east, the discordance fades 

 out and the overlapping rocks interfinger with the eastern assemblage 

 rocks and grade eastward into the carbonate section of late Paleozoic 

 age of eastern Nevada and western Utah. Examine Figs. 6.9, 6.14, and 

 6.15. 



In latest Devonian or earliest Mississippian time a sharp anticline rose 

 in the site of the Stansbury Range of west-central Utah. It was eroded 

 down to the Cambrian before early Mississippian seas covered it (see 

 Fig. 6.12). Coarse slide debris accumulated on its northwest flank, and 

 sand dunes were blown northward for several miles to build a sandstone 

 unit several hundred feet thick. The angular unconformity and the com- 

 pleteness of the anticline, about 30 miles long and 5 miles wide as mapped 

 by Rigby ( 1958 ) , are particularly impressive. 



No Devonian strata are known in the Raft River Mountains of north- 

 western Utah; only Pennsylvanian strata in fault contact with Precam- 

 brian rocks have been mapped, and the Devonian relations have not 

 been specifically deciphered (Felix, 1956). Small remnants of al- 

 lochthonous Paleozoic (?) strata occur on the Precambrian rocks, and 

 the possibility exists that this area may be a continuation of the Stans- 

 bury anticline and a belt where orogeny was more severe than to the 

 south. The belt may join the Antler orogenic belt to the northwest. See 

 Fig. 6.5. More details of the Antler orogenic belt will be given in the 

 discussion of the Mississippian, Pennsylvanian, and Permian strata. 



