0¥ THE WEST COAST 299 



Oligocene in California 

 8 an lorenzo formation 



Until the year 1914 the only marine beds in California that had been 

 definitely referred to the Oligocene were a limited area of outcrops in the 

 Santa Cruz Mountains. The San Lorenzo formation of this area was 

 described and named by Ealpli Arnold- in 1900. At the time these de- 

 posits were first studied it was believed that they graded upward into the 

 Vaqueros (Lower Miocene). Eecent work by Bailey Willis^ apparently 

 shows that there is an unconformity between the San Lorenzo beds of the 

 type section and those of the Lower Miocene. 



When Arnold described the San Lorenzo formation, he stated that the 

 reason for his belief in the Oligocene age of these beds was that the fauna 

 appeared to have both Eocene and Miocene affinities; this, together with 

 their stratigraphic position, would, he believed, be sufficient to place them 

 in the Oligocene. Later work has only emphasized the Eocene and Mio- 

 cene relationships of the San Lorenzo fauna, and thereby strengthened 

 the belief in their Oligocene age. 



Kecent investigations* have shown that deposits of San Lorenzo age are 



- For the original definition of the San Lorenzo formation and its fauna, see Profes- 

 sional Taper U. S. Geological Survey, no. 47, 1906, pp. 16-17. For description of stra- 

 tigraphy and lithology, see Folio U. S. Geological Survey, nos. 1-163, 1900, pp. 3-4. For 

 description of San Lorenzo fauna, see Troc. U. S. National Museum, vol. 34. 1908, no. 

 1617, pp. 345-388, pi. 33. 



3 An unpublished paper read before the Le Conte Geological Society in October, 1917. 



* B. L. Clark, in 1914, discovered an unconformity in the beds mapped as the Monterey 

 Group. See "The occurrence of Oligocene in the Contra Costa hills of middle Califor- 

 nia," University of California Publ. Bull. Dept. Geol., vol. 9, no. 2, 1915, pp. 9-21. The 

 fauna found below this unconformity proved to be that of the San Lorenzo, that from 

 the beds above being typical Monterey. 



Beds of San Lorenzo age are now known to extend from Mount Diablo to as far south 

 as Ventura County. In the Coast Ranges, bordering the west side of the San .loaquin 

 Valley, these deposits are composed chiefly of organic shales, locally known as the 

 Kreyenhagen shales. The work of the U. S. Geological Survey has shown that the 

 Kreyenhagen shales are separated by a marked unconformity from Monterey deposits 

 (Lower Miocene) above. See Ralph Arnold and Robert Anderson: Geology and oil re- 

 sources of the Coalinga district. Bull. U. S, Geol. Survey, no. 398, 1908, p. 80; Robert 

 Anderson and Robert W. Pack : Geology and oil resources of the west border of the San 

 .loaquin Valley, north of Coalinga, California, IT. S. Geol. Survey, Bull. no. 60, 1915, pp. 

 74-80. Also in this field there is an unconformity between these Oligocene shales and 

 the Tejon (Upper Eocene) deposits below. A typical San Lorenzo fauna has been ob- 

 tained from the Kreyenhagen shales. 



Beds of San Lorenzo age are found in the San Emigdio Mountains at the south end 

 of the San Joaquin Valley. Here these deposits have a maximum thickness of close to 

 3,000 feet, consisting chiefly of dark clay-shales and sandstones. These deposits were 

 studied by a party from the Paleontology Department of the University of California 

 during the summer of 1917. The results of this work show that these San Lorenzo beds 

 are here separated from those of typical Monterey by an unconformity ; also that in this 

 section there is an unconformity between the San Lorenzo beds and those of the Tejon 



