COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



467 



sediments ranging in thickness from 2000 to 5000 feet that were deposited in 

 embayments far more restricted in area than the seas of the Upper Jurassic and 

 Cretaceous time. The marine sedimentary formations of the Oligocene and 

 lower part of the Miocene series occupy still more restricted areas than those of 

 the Paleocene and Eocene, and near Carquinez Strait are more than 5000 feet 

 thick. The upper Miocene sandstones of the San Pablo group are far more 

 widely distributed and are nearly 2500 feet thick. They are characteristically 

 coarse-grained and were deposited in moderately shallow water which locally 

 was brackish or fresh. The Pliocene rocks crop out extensively in the north- 

 central part of the area and consist largely of alternating flows of andesite, 

 basalt, dacite, and rhyolite together with associated tuffs and agglomerates, 

 whose total thickness is 100 to 1200 feet. In Santa Rosa and Petaluma quad- 

 rangles marine sandstones contain invertebrate fossils closely allied to those of 

 the Merced formation in San Francisco. The beds in Marin and Sonoma coun- 

 ties are about 250 feet thick and rest unconformably upon the Franciscan 

 group. Near Petaluma Valley they interfinger with tuffs (Weaver, 1949). 



The Eel River embayment north of Cape Mendocino is the largest area 

 of Tertiary sediments, and the beds there are said to be 7000 to 11,000 

 feet thick and of Pliocene age. Another deposit extends along the coast at 

 Point Arena, where Miocene beds are several thousand feet thick. A third 

 deposit is near Clear Water Lake, where 4000 feet of lower Eocene beds 

 have been identified. 



Early Pliocene Phase 



In early Pliocene time before the Pliocene volcanics accumulated, the 

 entire area east of the San Andreas fault was folded and faulted, and then 

 deeply eroded. Particularly a great low-angle overthrust, the St. Johns 

 Mountain thrust fault, was formed at this time. 



Late Pliocene and Quaternary Phases 



The Pliocene volcanics were laid down on the beveled surface of the 

 older rocks, and later were moderately folded and broken by normal faults 

 and locally overturned and broken by thrust faults. 



Since the Pliocene beds in the Eel River embayment (Fig. 29.2) are 

 folded and faulted, the northern part of the northern Coast Ranges was 

 deformed in late Pliocene and Pleistocene time. This phase is similar to 

 that in the San Francisco Bay area on the south. The main middle area 



is undoubtedly structurally complex, but it seems reasonable to conclude 

 that it also was folded in late Pliocene and Pleistocene time, and perhaps 



during earlier phases. 



Late Pleistocene and Recent Movements 



As in the central Coast Ranges, there have been significant elevatory 

 movements since the compressional deformation. The movements seem to 

 be vertical and horizontal along faults, and also broader elevatory and 

 depressional warpings. 



Perhaps long before the compressional orogeny, the Klamath Moun- 

 tains area projected westward as a peninsula, with the flanking areas 

 below sea, especially on the north and west. A widespread erosion surface 

 is believed to have developed over the Klamaths during this time ( Fenne- 

 man, 1931). Then during the folding and thrusting on the north and west, 

 it was only elevated, the Klamaths standing like a buttress to the deform- 

 ing belts of Cretaceous and Tertiary strata. In relation to the trough sedi- 

 ments, the borders of the buttress were pushed westward up and over 

 them. 



Broad valleys were then cut in the high Klamath surface, according to 

 Fenneman (1931), but not in a single uplift because the valley walls are 

 terraced, and locally the floors of these broad valleys are themselves fairly 

 widespread erosion surfaces. The highest peaks in the Klamaths rise sev- 

 eral thousand feet above these broad valleys. In the Coast Ranges proper, 

 there are remnants of erosion surfaces, but they have probably been 

 jostled about in fault block movements. Their age, although most prob- 

 ably post-folding and post-thrusting, is not clearly demonstrable nor easy 

 to compare with the Klamath peneplain and the broad valleys cut in it. 



A great uplift affected the Klamaths and adjoining areas after the ero- 

 sion of the broad valleys. Deep inner valleys 1000 to 2000 feet deep were 

 cut and later glaciated. As the glaciation is generally recognized as Wis- 

 consin, it would follow that the uplift and high-erosion surface are pre- 

 Wisconsin in age. The uplift of the Klamaths may have been associated 

 with the adjacent compressional orogeny, or it may have followed closely. 

 At any rate, the uplift and dissection must have occurred in middle or 

 post-middle Pleistocene. The narrow continental shelf was added to the 



