COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



465 



part. The Oakridge uplift started to evolve when, at the end of Sespe time, 

 local subsidence ceased to be so rapid. The lower Miocene strata include 

 the Vaqueros sandstone and the Rincon (Temblor) clay shale. The mid- 

 dle Miocene has a basal limestone. Both middle and upper Miocene 

 contain a predominance of siliceous organic shale, including diatomite, 

 chert, and various other siliceous varieties. 



Middle and Late Miocene Phase 



The late Miocene phase (Fig. 29.6) is not well known in the southern 

 Coast Ranges, and the evidence that is available suggests only local, gen- 

 tle folding and volcanism. Near Santa Barbara, a coarse breccia of middle 

 late Miocene age occurs at the plunging ends of cross folds in the 

 east-west trending structures. Reed believes this breccia was formed dur- 

 ing the cross folding. In the cross section O-O', Fig. 29.11, the folding 

 seems to be mostly pre-middle Miocene, and the anticline grew by several 

 movements from Eocene to Miocene. It is probable that the Santa Barbara 

 district of the Santa Barbara embayment in middle Miocene time was 

 one of southward regional dips, fluted by a few low folds of northerly 

 trend. The most important of the cross folds was along the axis of the San 

 Rafael uplift and extended to Ventura (Reed and Hollister, 1936). East 

 and west of Los Angeles, in the Los Angeles basin, gentle folding oc- 

 curred in late Miocene time. In spite of the folding and the change to 

 heterogeneous facies in the late Miocene from homogeneous facies in 

 the middle Miocene, the boundaries of the two basins were much the 

 same ( Reed and Hollister, 1936 ) . 



Volcanic rocks form an important constituent of the middle and upper 

 JMiocene along the axis of the San Rafael uplift but are not conspicuous 

 jelsewhere. 



Pleistocene Phase 



Pliocene and lower Pleistocene deposits of considerable thickness occur 

 only in restricted parts of the Miocene basins. See Fig. 29.6. The three 

 thickest deposits of Pliocene beds in southern California are found in the 

 Maricopa, Ventura, and Los Angeles basins. The thicknesses are very 



great, possibly 10,000 to 20,000 feet in the first, 18,000 feet in the second, 

 and 10,000 in the third. The foraminifera in the lower Pliocene beds of 

 the Ventura and Los Angeles basins suggest that the sea was one to two 

 miles deep at the time of deposition. 



Lower Pleistocene was deposited in all the Pliocene basins, but only in 

 the western depressions did marine beds accumulate. Eastward, the beds 

 are continental and are 1000 to 5000 feet thick. 



The late Pliocene orogenic phase of the central Coast Ranges as de- 

 scribed by Taliaferro is not a "notable disturbance" in the southern Coast 

 Ranges, according to Reed and Hollister (1936). A disconformity is pres- 

 ent in the Repetto Hills, the Ventura basin, and the San Joaquin Valley 

 between the lower and upper Pico, but the break has not been observed 

 as an angular unconformity anywhere. 



Along the seaward margin of the Los Angeles basin, there is a pro- 

 nounced angular unconformity between lower San Pedro beds of early 

 and middle Pleistocene age and upper San Pedro beds of late Pleisto- 

 cene age (Reed and Hollister, 1936). The evident folds and thrusts of 

 southern California can best be explained, according to Reed and Hol- 

 lister, as having formed approximately at this time. Examine cross sections 

 L-L', M-M', N-N', Fig. 29.10, and O-O', Fig. 29.11. 



In the Ventura basin, about 5000 feet of lower Pleistocene beds have 

 been turned up so as to have dips of 30 to 90 degrees beveled by erosion 

 and covered by about 300 feet of upper Pleistocene fanglomerates. The 

 fossil Equus, cf. occidentalis, occurs both above and below the angular 

 conformity, apparently without change, and indicates that the structure 

 was formed during a very short period of time (Bailey. 1943). 



The structural history of the Kettleman Hills anticline is instructive. In 

 it, the Tulare formation, which is lower Pleistocene and not older than 

 latest Miocene, is folded apparently as strongly as the underlying forma- 

 tions. The anticline, therefore, was formed almost entirely in post-Tulare 

 time. After its rise, it was eroded until several thousand feet of rock were 

 removed from its axial part. Toward the south end, it was reduced to a 

 plain which then became buried in alluvium. After this, the alluvium 

 was arched into a new, though gentle, fold. 



