462 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



App S Miles 



Fig. 29.9. Ventura basin showing conditions before Middle Pleistocene folding. Section P-P', 

 Fig. 29.1. 



there is little definite correspondence of the various terrace levels over 

 wide areas. Over limited areas, there may be very definite intervals be- 

 tween terrace; a few miles away, the terraces may be equally well 

 developed; but the intervals between terraces in the two areas differ. 

 Furthermore, the marine terraces along the coast cannot be correlated 

 definitely with the terraces of the interior valleys, but there is strong evi- 

 dence that the coastal area has very recently been uplifted more than the 

 interior (Taliaferro, 1943b). 



The San Francisco Bay area was probably depressed rather than up- 

 lifted, but it is not possible to say that the entire lowland and bay was 

 depressed subsequent to the folding and thrusting because it may have 

 been left that way as orogeny progressed ( Taliaferro, 1943b ) . 



The terraces have been cited as evidence of widespread epeirogeny, 

 but Taliaferro thinks they may be due to gentle folding or upbowing of 

 the ranges. 



A few but indisputable examples of tilted beaches are known, but the 

 structural meaning is yet obscure. 



SOUTHERN COAST OR TRANSVERSE RANGES OF CALIFORNIA 



Principal Structural Features 



The Southern Coast Ranges trend in an east-west direction which is 

 transverse to that of the Central and Peninsular Ranges. See Fig. 29.2. 

 The relief features as well as the faults and folds are generally so oriented. 



The formations and structure of the southern part of the San Joaquin 

 basin are shown in cross section in Fig. 29.5. A cross section of the Ven- 

 tura basin, restored to the time preceding the major deformation, is pre- 

 sented in Fig. 29.9. The cross sections L-L', M-M', and N-N', Fig. 29.10, 

 and O-O', Fig. 29.11, are representative of the present structure and major 

 groups of beds in various parts of the southern Coast Ranges. 



Early Tertiary Phase 



Paleocene, Eocene, Oligocene, and early Miocene times were generally 

 characterized by subsidence of the basins previously mentioned, but at 

 times during these epochs slight surges of crustal unrest are attested by 

 conglomerates and local small-angle unconformities. During the Eocene, 

 the greatest subsidence occurred, and it centered in the Ventura basin. 

 The Paleocene beds are generally coarse, variable in lithology, and of 

 restricted distribution. In most places, the contact with Cretaceous beds 

 is difficult to locate, and the two systems seem conformable. Aside from 

 the coarser aspect, the Paleocene beds are not much different from the 

 Cretaceous. In one locality, an angular unconformity of 30 degrees has 

 been noted (Reed, 1933), and it has been taken to mean gentle folding 

 in places at the beginning of the Tertiary. 



The Eocene sediments were generally finer, and consisted of arkosic 

 sandstones and silty and sandy shales. They accumulated to a depth of 

 11,900 feet in the Ventura basin. Perhaps the total thickness there of 

 Paleocene and Eocene beds, the Martinez and Tejon formations, was 

 20,000 feet. See thickness contours of Fig. 29.6. The Eocene deposits 

 spread over much larger areas than the Paleocene, but the subsidence 

 followed the earlier troughs or defined them better. Toward the end of the 

 Eocene or during the Oligocene, the areas of deposition remained large, 

 but the facies represented became highly varied. They included the Poway 



