268 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Nevadan intrusions there; and he also reviews the southern continuation 

 of the batholithic province into Baja California. A group of slates and 

 argilhtes, with some quartzites, lie west of the main batholith and form 

 most of the Santa Ana Mountains. Triassic fossils have been collected there 

 in several places. Somewhat more metamorphosed remnants of these 

 rocks occur within the batholith. The group is known as the Bedford Can- 

 yon formation, and about 20,000 feet of beds are exposed in the Santa Ana 

 Mountains. Parts of the formation may be older than Triassic and parts 

 may be Jurassic. The uniform argillaceous lithology is a dominant char- 

 acter. 



A group of volcanic beds, mostly mildly metamorphosed, andesitic 

 agglomerates, overlies the Bedford Canyon formation unconformably. 

 The extrusives have been called the Santiago Peak volcanics (Larsen, 

 1948), and they are probably many thousands of feet thick. They are 

 older than the batholithic intrusions and, therefore, are probably Jurassic 

 in age. 



Along the east side of the main batholithic region are coarsely crystal- 

 line schists, all of which contain much quartz. Interbeds of limestone 

 have yielded Mississippian fossils (Larsen, 1948). A quartzite sequence 

 with interbedded, coarse, mica schist is also thought to be Carboniferous. 

 It is some 12,000 feet thick. Larsen believes that the Paleozoic sediments 

 were metamorphosed and intruded by granite rocks before the deposition 

 of the Triassic rocks, and that this older metamorphism was more intense 

 than the later metamorphism of the Triassic rocks. 



NEVADAN OROGENY 



History of Concept 



The literature, up to the last few years, suggests that the Late Jurassic 

 folding and thrusting was followed very shortly by the great batholithic 

 intrusions, and that the two events occurred between the Kimmeridgian 

 and Portlandian. See Figs. 17.2 and 17.7. Recent isotope age determina- 

 tions have demonstrated fairly conclusively, however, that the intrusions 

 are mid- or early Late Cretaceous in age. Also new fossil finds have re- 

 sulted in a revision of concepts of the Upper Jurassic and Lower Creta- 



ceous stratigraphy which is not incompatible with a Mid-Cretaceous age 

 of the batholiths. 



Additional sampling and potassium-argon age determinations by Curtis 

 et al. (1958) indicate that granitic rocks along the northwest foothills of 

 the Sierra Nevada and in the southern Klamath Mountains are Tithonian 

 ( Portlandian ) in age, as the early geologists had concluded. Furthermore, 

 they found that several plutons in the Central Coast Ranges are early 

 Late Cretaceous (about Cenomanian to Senonian), the same age as the 

 plutons of Yosemite National Park. The various potassium-argon ages 

 to date in California are shown in Fig. 17.8. Curtis et al. conclude that the 

 bulk of the great batholiths of California are of the later date, but that 

 some are late Jurassic, and, as the earlier writers concluded, are closely 

 associated with the post-Kimmeridgian folding and thrusting. 



The term Nevadan orogeny has been used to denote those tectonic 

 events that occurred in the general region of the Sierra Nevada in a rather 

 limited interval of time between the Kimmeridgian and Portlandian. The 

 great batholiths are indelibly impressed in the literature as an outstand- 

 ing characteristic of the orogeny, so now with die recognition that the 

 main batholiths are much younger we are faced with a redefinition of the 

 term, Nevadan orogeny. It is here proposed to call those disturbances 

 and intrusions in Late Jurassic time (post-Bathonian ) the early Nevadan 

 orogeny, those of Early Cretaceous time the mid-Nevadan orogeny, and 

 those of Mid- and early Late Cretaceous time the late Nevadan orogeny 

 (see Fig. 17.7). 



General Characteristics 



The Jurassic and pre-Jurassic rocks thus far described were severely 

 folded and thrust-faulted in the Sierra Nevada, and then invaded by 

 granitic magma. The maximum deformation seems to have been con- 

 centrated along what is now the western slopes of the Sierra Nevada in 

 the zone of the western belt of Jurassic deposits. Overturned folds, some 

 of great amplitude, great thrusts, such as the Mother Lode zone, and mild 

 dynamic metamorphism were widespread. The eastern belt of Triassic and 

 Jurassic rocks, near the present crest of the Sierra Nevada, is strongly 

 folded, but less dynamically metamorphosed. The eastern belt of Triassic 



