266 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



mian or older volcanic rocks over the two coarse clastic formations; and (5) 

 later folding, faulting, and erosion providing the present outline of the range. 



These relations record orogeny of Laramide age and undoubtedly mean 

 that the Laramide belt of the Central Rockies ( Chapter 22) spread west- 

 ward over most of Nevada. The folding of the King Lear elastics before 

 the deposition of the Pansy Lee conglomerate records deformation prob- 

 ably in Early Cretaceous time, and this has been labeled the mid- 

 Nevadan orogeny on Fig. 17.7. 



CENTRAL AND NORTHERN CALIFORNIA 



In the California Sierra Nevada region, Taliaferro (1942) summarizes 

 an eastern belt of Triassic and Jurassic rocks and a western belt of Juras- 

 sic rocks. The two were probably continuous, but due to the Nevadan 

 orogeny (to be described immediately) a dividing mass 25 to 50 miles 

 wide of Calaveras rocks and granite of the Sierra Nevada batholith exists. 

 The eastern belt consists of discontinuous areas of Upper Triassic and 

 Jurassic sediments and volcanics. Doubtless these formed a continuous 

 belt at one time, but as they lie in the region of maximum plu tonic in- 

 vasion and maximum Tertiary uplift, they have been obliterated or re- 

 moved by erosion in many places. Near the northern end of the Sierra 

 Nevada, the Milton formation represents the Triassic and Jurassic rocks, 

 and where not engulfed by the plutons or removed by erosion it lies in a 

 broad, steep-limbed syncline, practically free from minor crumbling, 

 thrusting, and overturning (Taliaferro, 1942). The conglomerates contain 

 abundant debris of the Paleozoic rocks (Calaveras) and thicken and 

 coarsen westward. It seems clear, therefore, that the Milton was derived 

 from the west. 



The best-exposed and most complete section of the east belt is on the 

 north fork of the American River in Placer County. On the west limb of 

 the syncline, basic and intermediate volcanics and radiolarian cherts, 200 

 feet thick, are followed by 12,800 feet of conglomerates, sandstones, hard 

 slaty shales, and fine-grained andesitic tuffs. The center of the syncline is 

 occupied by 9500 feet of intermediate and basic flows, agglomerates, and 



tuffs. Only about 900 feet of sediments and tuffs lie below the volcanics 

 on the east limb of the syncline, the lower part having been obliterated by 

 batholithic intrusions. Well-preserved Upper Triassic fossils are found 

 at and near the base of the sediments on the west limb of the syncline, 

 Lower Jurassic fossils 2500 feet above the base, and Middle Jurassic fos- 

 sils 9500 feet above the base; no fossils have been found in the upper 

 13,000 feet of the sediments and volcanics. The section is well exposed 

 and no unconformities or disconformities have been observed. Possibly 

 part of the upper 13,000 feet is equivalent to the Mariposa slate of the 

 western belt. The upper volcanics are possibly equivalent to the extensive 

 Logtown Ridge volcanics lying between Amador and Mariposa west 

 of the Mother Lode. No evidence supports the idea that the Milton of 

 the eastern belt was separated from the Mariposa and Logtown Ridge of 

 the western belt either by deposition in separate basins or by a period of 

 batholithic intrusion and orogeny (Taliaferro, 1942). See column 2, 

 Fig. 17.2. 



In comparing the sediments of the eastern belt of the Sierras with those 

 of the trough of western Nevada, it appears that Lower and Middle 

 Triassic sediments were deposited in the central part of the trough which 

 lay in western Nevada, and then Upper Triassic sediments overlapped on 

 highlands both westward and eastward. See Fig. 17.8. Great subsidence 

 occurred in early Middle, and early Late Jurassic time; the center of the 

 Jurassic trough migrated west of that of the Triassic trough; and over- 

 lap on the western volcanic orogenic belt was extensive. 



The western belt is made up of the Amador group and the Mariposa 

 slates in the Sierra Nevada and northwestward in Oregon, of the Dothan 

 and Galice. The Amador and Dothan are probably Middle Jurassic in 

 age, with their upper beds containing Late Jurassic fossils. The Mariposa 

 and Galice are early Late Jurassic. The great bulk of the Amador consists 

 of volcanics and elastics, but red and green radiolarian cherts and dense, 

 unfossiliferous limestones are common. On the Cosumnes River, 1200 

 feet of conglomerates and sandstones are at the base of the Amador. On 

 the Merced River, radiolarian cherts, tuffs, and shales are over 1500 feet 

 thick, and these overlie about 1400 feet of pillow basalts. The entire 

 Amador group ranges in thickness from 5000 to 15,000 feet, and usually 



