466 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



In the Los Angeles basin, a number of unconformities within the upper 

 Miocene and the Pliocene section indicate a succession of uplifts along 

 the major structural trends during these times ( Wissler, 1941 ) . 



The present condition of the crust in southern California is one of 

 decided instability. Folding, thrusting, and high-angle faulting have not 

 only manifested themselves in earthquakes and buckled pipelines, cables, 

 and pavements; through precise surveys, the amount and rate of the move- 

 ments have been measured in places. Gilluly (1949) reviews these move- 

 ments and concludes that the present is a time of typical orogeny. The 

 seismicity of the western Cordillera will be considered in Chapter 31. 



It is evident that the division of the structural history of southern Cali- 

 fornia into phases is not altogether a satisfactory treatment, because the 

 deformation was prolonged and shifting in time and place. Rasin sub- 

 sidence, sediment accumulation, the tilting and erosion of marginal beds, 

 and the rise and truncation of anticlines in nearby and related areas all 

 went on together. Very little time is represented in some of the angular 

 unconformities, hardly enough for a change to occur in the faunas, yet 

 the angular unconformities have caught and fixed the rise of landmasses 

 in process of movement in the same manner almost as a photograph 

 stops an object in motion. Perhaps the unconformities should not be con- 

 sidered rigidly as indicators of separate widespread impulses. In the 

 analyses of Paleozoic and Mesozoic orogenies in the great system of west- 

 ern Cordilleran troughs, the theory seems repeatedly substantiated that 

 deformation was almost continuous in an oceanward volcanic archipelago, 

 and that from time to time the compressive movements spread into the 

 flanking trough and deformed the sediments in it to variable intensities 

 and distances. These deformational waves off the main belt of constant 

 unrest probably constitute our orogenic impulses or phases in the Coast 

 Ranges. 



NORTHERN COAST RANGES OF CALIFORNIA 



General Features 



The Northern Coast Ranges, as generally defined, extend from San Fran- 

 cisco Bay to Trinidad Head and perhaps beyond. They are bounded on 



the east by the Sacramento Valley and on the north by the Klamath Moun- 

 tains. They are composed mostly of Franciscan-Knoxville strata, but other 

 pre-Tertiary formations may be present; and in this respect, they contrast 

 with the Central and Southern Coast Ranges, which in good part are made 

 up of Tertiary deposits. 



The southern end of the Northern Coast Ranges is not greatly different 

 from the northern end of the Central Coast Ranges. In both the Tertiary 

 is prominent, but northward it is limited to a few small basins and to the 

 marginal areas. Most of the hills and valleys are probably underlain only 

 by Mesozoic rocks. The complex structure of the Mesozoic and Cenozoic 

 rocks, their poor outcrops in many places, and their slight economic im- 

 portance as yet, have contributed to a lack of detailed geologic work 

 except in a few areas. 



Weaver ( 1949 ) has published on seven quadrangles north of San 

 Francisco Ray, and reports that the hills there are arranged in three 

 blocks, one west of the San Andreas fault, the Montara block; one east 

 of it and west of the Tolay fault (a northwestward extension of the 

 Haywards fault system), the Francisco-Marin block; and one east of the 

 Tolay fault, the Berkeley Hills block. Refer to Figs. 29.1 and 19.2. Each 

 block is tilted toward the northeast. The Franciscan group constitutes the 

 surface exposures in most of the intermediate block, and the eastern block 

 is made up of more than 30,000 feet of Jurassic to Quaternary marine and 

 fresh-water sediments, together with about 1200 feet of Pliocene andesites, 

 rhyolites, and tuffs. 



These sediments probably accumulated in structural troughs whose areas and 

 physical environments changed greatly during the Cretaceous and Tertiary. The 

 lower portion consists of clay shales and subordinate amounts of sandstone and 

 conglomerate as much as 17,000 feet thick, containing a marine fauna of am- 

 monites, pelecypods, and gastropods. These rocks include the Jurassic and 

 Lower Cretaceous portions of the Knoxville formation and the Upper Creta- 

 ceous Chico. Several faunal zones may be distinguished in the Knoxville, but 

 the formation in the mapped area cannot be subdivided on a lithologic basis. 

 The chico formation consists of interbedded shales and sandstones about 7000 

 feet thick. 



The Paleocene is represented by the Martinez formation, and the Eocene in 

 ascending order by the Capay shale and the Domengine and Markley sand- 

 stones. The formations of the Paleocene and Eocene series consist of marine 



