Clark: The San Lorenzo Series of Middle California 73 



San Lorenzo Series to West and South of Mount Diablo 

 GENERAL STATEMENT 



The beds of the San Lorenzo series of the Concord Quadrangle to 

 the west of Mount Diablo were originally mapped and described in 

 the San Francisco folio as a part of the Monterey group. 58 Below 

 is given a table taken from page 10 of the Folio, showing the lithologic 

 and faunal zones of the Monterey as seen in the section to the south- 

 east of the town of Pinole. 



Palaeontologic 



Stratigraphic 



Petrographic 



Thickness 



subdivisions 



subdivisions 



character 



feet 





' Briones sandstone 



Sandstone 



800 



Upper Faunal Zone « 



Hercules shale member 



Bituminous shale 



500 





Briones sandstone 



Sandstone 



1000 





' Eodeo shale 



Bituminous shale 



670 





Hambre sandstone 



Sandstone 



1200 



Middle Faunal Zone 



Tice shale 



Bituminous shale 



460 



< 



Cursan sandstone 



Sandstone 



600 





Claremont shale 



Bituminous shale 









and chert 



250 



Lower Faunal Zone 



Sobrante sandstone 



Sandstone 



400 







Total thickness 



5880 



The lower faunal 



zone of the above 



section is known 



as the 



Agasoma gravidum zone, the middle as the Area montereyana zone, 

 and the upper as the Scutella breweriana zone. Mention has already 

 been made of the fact that there is a stratigraphic break between the 

 beds containing the Area montereyana fauna and those containing 

 the Agasoma gravidum fauna, which latter beds are, in this paper, 

 referred to the San Lorenzo series, also that this break occurs in the 

 Sobrante sandstone. The name Sobrante sandstone is retained for the 

 basal sands and conglomerates of the Monterey Group above this 

 break. 50 



To the west of Mount Diablo, beds containing the Agasoma gravi- 

 dum fauna are known in three general sections in the Concord Quad- 

 rangle, Contra Costa County : ( 1 ) In the northwest quarter of the 

 Concord Quadrangle near Bear Creek; (2) in the vicinity of the town 

 of Walnut Creek on both sides of a tightly folded syncline which will 

 be referred to as the San Ramon syncline; (3) to the southeast of the 

 town of Martinez and east of Muir station on both sides of the Pacheco 

 syncline. 



ss Lawson, A. C, San Francisco Folio, no. 193, U. S. Geol. Surv., pp. 1-21, 1914. 



59 The faunal list from the Agasoma gravidum beds, given in the Folio, needs 

 revision, as it contains a number of species which are characteristic of the Area 

 montereyana zone. 



