300 CLARK AND ARNOLD MARINE OLIGOCENE 



fairl}^ widespread throughout the State of California, and that in general 

 these deposits are separated from those of the Montere}^ by a stratigraphic 

 break. A stud}- of the known fauna of the San Lorenzo emphasizes the 

 importance of this break. 



PALEOGEOGRA FH Y 



While much remains to be done before all the details are known about 

 the distribution of the San Lorenzo deposits in California, yet enough is 

 now known to enable us to outline in a general way the original basin of 

 deposition, for apparently in California there was only one general basin. 

 This was a long geosynclinal trough extending from at least as far south 

 as the San Emigdio Mountains, at the south end of the San Joaquin 

 Valley, to the region of Mount Diablo and possibly for 200 miles or more 

 farther north. The deepest part of this trough was along where is now 

 the eastern edge of the Coast Ranges or along the western side of the 

 present San Joaquin Valley. This is indicated by the organic shales 

 found in this region. Over a large part of the western Coast Ranges the 

 San Lorenzo deposits are apparently absent, and where present they are 

 represented by shallow-water deposits, carbonaceous clay-shales, and sand- 

 stones. 



This big mediterranean sea probably connected with the main ocean to 

 the north in the region of the present Santa Cruz Mountains, in which 

 area is the type locality of the San Lorenzo. Very probably there was a 

 connection to the south; however, not enough detail is known about the 

 distribution of these deposits to enable one to say definitely where such 

 a connection might be. The outline map of California (figure 1) gives 

 the probable outlines of the San Lorenzo sea. This will undoubtedly be 

 considerably modified with future work. 



STRATIGRAPHIC RELATIONSHIP TO UPPER EOCENE 



The San Lorenzo deposits in California appear to show a closer rela- 

 tionship to those of the Tejon (Upper Eocene) than to deposits of the 

 Monterey (Middle and Lower Miocene). The comparison of the distri- 

 bution of the Tejon strata with those of the San Lorenzo in California 

 shows that the areas covered by these two seas were very much the same, 



(Upper Eocene). It is probable that these deposits will be found to extend a consider- 

 able distance to the south of the San Emigdio Mountains into Ventura Covinty. 



San Lorenzo deposits are now known to exist in the western Toast Ranges of Califor- 

 nia. They are believed to be present in the Santa Luoia Mountains bordering the coast 

 and along the Salinas Valley to the east of here. Very little study has been made of 

 the San Lorenzo deposits in this western area. In all the mapping that has been done, 

 these beds have been included with those of the liower Miocene (Monterey). 



