272 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



The granitic rocks were exposed by erosion at the time the Turonian 

 sediments were deposited (see Figs. 17.7 and 17.8) and hence, only a 

 short time separated the last intrusion from its exposing. It may thus be 

 assumed that uplift and erosion kept close pace with granitic emplace- 

 ment. From this Evernden et al. deduce that the space for the batholiths 

 was produced by the elevation of the roof slowly and by small increments, 

 and that the overlying sedimentary rocks were stripped by erosion as 

 rapidly as they rose. 



ANCESTRAL COAST RANGE SYSTEM 



Franciscan Basin 



Following the post-Kimmeridgian folding and thrusting (Fig. 17.9) 

 a trough or basin sank on the west in California, and in it an exceedingly 

 thick sequence of sediments accumulated. These are those of the Fran- 

 ciscan group and equivalents (Fig. 17.2). West of this trough lay a 

 sourceland of sediments, viewed as a narrow volcanic archipelago by 

 Taliaferro ( 1942 ) . The strata known as Franciscan crop out in the Coast 

 Ranges and the Shasta and Upper Cretaceous strata occur in the Sacra- 

 mento Valley. The thickness of the Franciscan is about 35,000 feet. That 

 of the Shasta series is about 10,000 feet and of the Upper Cretaceous on 

 the west side of Sacramento Valley is 15,000 feet. 



According to Irwin ( 1957 ) : 



The Franciscan group consists dominandy of detrital sedimentary rocks 

 with interbedded chemical sedimentary and volcanic rocks. The detrital 

 rocks are chiefly sandstones of the graywacke type, with interbedded shale 

 and conglomerate. Reliable criteria have not yet been described for dis- 

 tinguishing, either in hand specimen or under the microscope, between 

 detrital rocks of the Franciscan group and those of the Sacramento Valley 

 sequence. The most obvious and significant difference between the lithologic 

 character of the Franciscan group and that of the Sacramento Valley sequence 

 is the presence and local abundance of interbedded volcanic rocks and as- 

 sociated chemical sedimentary rocks in the Franciscan. The chemical sedimen- 

 tary rocks include rhythmically thin-bedded chert, and, much less abundandy, 

 a distinctive foraminiferal limestone. In addition, the Franciscan group includes 

 small areas of glaucophane schists. In some areas, strata of the Franciscan group 

 have been metamorphosed to slates and phyllites. 



The Franciscan group has been intruded by mafic and serpentinized ultra- 



mafic rocks, and has been highly faulted and pervasively sheared. The general 

 appearance of the Franciscan terrane, because of the net effect of the lithologic 

 heterogeneity and complex structural deformity, is in striking contrast to areas 

 underlain by strata of the Sacramento Valley sequence. 



The Knoxville formation as exposed along the west side of Sacramento Valley 

 between Wilbur Springs and Paskenta is perhaps 10,000 feet in average thick- 

 ness. The base is unknown, as along most of the valley the lowest exposed beds 

 are in fault contact with the belt of ultramafic rock. The Knoxville formation is 

 generally considered to consist typically of a thick section of thin-bedded shales 

 with small lenses of limestone, but interbedded sandstones and conglomerates 

 are locally abundant. Fossils indicate that it is Late Jurassic (Tithonian) in 

 age. One of its most characteristic and abundant fossils is Aucella piochii Gabb. 



The contact between the Knoxville formation and overlying Shasta series is 

 marked by a fairly abrupt and complete change in fauna, and at many places 

 by beds of conglomerate. Here, as well as at other places, the concept of a 

 "basal conglomerate" has much influenced the subdivision of the Sacramento 

 Valley sequence. Along much of Sacramento Valley the transition from one 

 unit to the other is one of nearly continuous deposition and, judged from broad 

 structural conformity, was accomplished with litde disturbance. 



The strata referred to the Shasta series have a higher ratio of sandstone to 

 shale than has the Knoxville formation. 



Upper Cretaceous strata along the west side of Sacramento Valley consist 

 of sandstones and shales and are about 15,000 feet in average thickness. They 

 represent only the lower part of the Upper Cretaceous section of northern 

 California. 



Mid-Cretaceous Phase (Mid-Cretaceous Orogeny) 



In many places in the Coast Ranges there is either a definite discon- 

 formity or a strong unconformity or overlap between the Shasta series 

 and the Upper Cretaceous strata. Especially in the Santa Lucian Range, 

 there is evidence of deep erosion and overlap. Along the crests of some 

 of the folds produced during this disturbance, the Lower Cretaceous 

 and Upper Jurassic beds were removed, so that the Shasta trough was 

 lifted in subparallel fragments. Other parts of the Shasta beds were 

 little affected. The orogeny represented by the unconformity has been 

 called the Mid-Cretaceous by Taliaferro (1943b). 



Mid-Upper Cretaceous Phase (Santa Lucian Orogeny) 



The Upper Cretaceous strata in the Coast Ranges are divisible into 

 two groups, the Pacheco and the Asuncion, which together make up 

 the Chico (Taliaferro, 1943b). See Fig. 17.2. The Pacheco consists in 



