17. 



MESOZOIC SYSTEMS 

 ALONG THE PACIFIC 



WESTERN NEVADA 



Central and western Nevada and all California were involved in 

 orogeny during the Mesozoic era, and the index map, Fig. 17.1, shows 

 the chief areas and features with which the following discussion is con- 

 cerned. The map also extends eastward to central Utah where late 

 Mesozoic disturbances occurred. These will be discussed in Chapter 18. 



A trough of geosynclinal proportions centered in western Nevada in 

 Triassic and early Jurassic time. It has already been referred to in con- 

 nection with the Permian and Mesozoic geanticline in central Nevada. 

 See tectonic maps, Plates 8, 9, and 10. In general, its axis probably lay 

 slightly east of the axis of the Permian trough. In the Hawthorne and 



Tonapah quadrangles, Nevada, it sank and received a total thickness of 

 sediments of about 30,000 feet (Muller and Ferguson, 1936). The sedi- 

 ments are predominantly marine elastics, cherts, and limestones with a 

 considerable proportion of more or less altered pyroclastic rocks and 

 lavas in the lower and upper parts of the section. 



The table, Fig. 17.2, shows the sequence of Mesozoic formation there 

 and elsewhere in western Nevada, California, and southern Oregon. 

 The Lower Triassic Candelaria formation rests with marked erosional 

 unconformity on the thin Permian sandstones and grits and in places on 

 the beveled Ordovician strata. A slight disturbance, therefore, affected 

 the area in late Permian time and probably reflects greater orogeny in the 

 westward-lying orogenic belt. During the deposition of the Candelaria 

 formation, the area of sedimentation as well as the western highland were 

 comparatively quiet, and shales, sandy shales, sandstones, some of tuf- 

 faceous aspect, and scattered, thin layers of limestone were deposited. 

 Then marked volcanism and orogeny occurred to the west in middle 

 Triassic time, and over 12,000 feet of strata, chiefly pyroclastics and lavas, 

 accumulated. This group of rocks is known as the Excelsior formation. 

 The lavas range in composition from andesite through quartz latite to 

 rhyolite. Alteration, principally epidotization and chloritization, has af- 

 fected the formation over wide areas. Volcanic breccias, especially those 

 containing altered andesite fragments, are abundant; and in some sections 

 they exceed the effusive rocks in amount. In the Pilot and Excelsior 

 ranges, a considerable thickness, estimated to exceed 8000 feet, consists 

 of massively bedded chert. Examination under the microscope shows this 

 rock to be an extremely fine-grained water-laid tuff, cemented and largely 

 replaced by cryptocrystalline quartz. Interbedded with the chert are dark 

 tuffaceous slate, a little impure sandstone, and some lava and breccia. 



The volcanics were then subjected to erosion for a time but not much 

 disturbed before the thick Upper Triassic sequence accumulated. Dark 

 limestone and dolomite predominate, but siliceous argillite, argillite, 

 calcareous shale, shale, and chert pebble conglomerates are not uncom- 

 mon. These beds are known as the Luning formation. Above the limestone 

 and dolomite sequence are 420 feet of purple to black shale and dark 

 brown limestone, known as the Gabbs formation. The Gabbs is conforma- 



260 



