458 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Typical Formations 



Age 



Assignments 







in California 



Current Usage 



Grouping 



by 



R. D. Reed 



Upper San Pedro 



Upper 

 Pleistocene 



Pleistocene 







Lower San Pedro, Saugus, 



Lower 



Upper 



' 





Tulare 



Pleistocene 



Neogene 







Etchegoin, Pico, Repetto 



Pliocene 







Neogene 



Santa Margarita, Monterey, 



Upper and 



Lower 







Modelo, Topango, Temblor 



Middle Miocene 



Neogene 



i 





Vaqueros, Temblor, Pleito, 



Lower Miocene 



Upper 



> 





San Lorenzo, San Ramon 

 Kreyenhagen, Tejon, Capay, 



and Oligocene 

 Eocene and 



Paleogene 

 Lower 



l 



Paleogene 



Domengine, Meganos, Martinez, 



Paleocene 



Paleogene 



J 





lone, Poway 











The upper Eocene (Tejon, Markley, Kreyenhagen, Gaviota, and 

 Wheatland) has a more limited distribution than the Middle Eocene. 

 Very slight folding and faulting may have intervened, but no mountains 

 were built, and the same seaways as before persisted, though somewhat 

 restricted. The Kreyenhagen has some bentonite and vitric tuff beds, and 

 the Wheatland has some andesitic debris, both indicating continued small- 

 scale volcanic activity. 



The Oligocene strata have even a more restricted distribution than the 

 upper Eocene, but occupy the same basins. They generally rest uncon- 

 formably on Eocene sediments and, in turn, are generally unconformably 

 overlain by the Miocene. The sediments regarded as Oligocene at present 

 are those of the San Lorenzo group. Volcanism occurred during the Oligo- 

 cene in the Mount Diablo and San Francisco Ray regions, where more 

 than 100 feet of rhyolite tuff occurs in the Kirker formation. 



The disconformities and slight angular unconformities that are known 

 in the Eocene and Oligocene might indicate comparative quiet in strong 

 contrast to the preceding and succeeding periods. This seeming lack of 

 important diastrophism, however, may be more apparent than real be- 

 cause of lack of evidence. The Upper Jurassic and Cretaceous unconform- 

 ities show that the various crustal movements were strongest in the western 



coastal region, the volcanic archipelago, and died out eastward. The 

 same may be true of the Eocene and Oligocene (Taliaferro, 1943b). 



The structures formed probably represent the general effect of several 

 episodes of movement. Although both folding and faulting occurred, nor- 

 mal faulting in the Diablo Range of great magnitude predominated. It 

 was during the Early Tertiary phase that the uplift and westward tilting of 

 the Gabilan Mesa (Diablo uplift) occurred, approximately along a line 

 corresponding to the present position of the San Andreas fault. This north- 

 eastern boundary fault may be thought of as ancestral to the San Andreas 

 fault in the central Coast Ranges, where the two coincide. The southwest- 

 ern side of the uplift is irregular, with several smaller faults. See Fig. 

 29.4. 



Late Miocene Phase 



Over most of the central Coast Ranges, the Miocene began with gentle 

 sinking, and basins of the early Tertiary were first uniformly flooded and 

 then overlapped. Early in middle Miocene, the uniform and gentle 

 sinking gave way to sharper downwarping, and great thicknesses of sedi- 

 ments accumulated locally. It is believed that the movement was caused | 

 by compression and that the interbasin areas rose at the same time as the 

 basins sank. The heterogeneous pre-Tertiary basement is believed to have 

 precluded uniform folding throughout the Coast Ranges. An important 

 and rather long-enduring trough developed along the western downtilted 

 side of the Gabilan Mesa, west of the Santa Lucia Range. The trough east 

 of the range continued to sink and expand both southward and northward, 

 until a connection was made with the sea in the site of the present Mon- 

 terey Ray. 



The crest of the Coalinga anticline, now composed of Franciscan, stood 

 above sea level throughout the Miocene. 



The effect of movements during the later upper Miocene cannot be 

 clearly evaluated in all places, because erosion incident to later severe 

 deformation has removed much of the evidence. This is especially true in 

 the Santa Lucia Range. However, in the northern part of the Castle 

 Mountain Range, the nature of upper Miocene deformation is well shown. 

 Figure 29.7 has been prepared to illustrate the structural evolution. Santa 



