Vol. 6] Jones: The Geology of the Sargent Oil Field. 



73 



GEOLOGIC HISTORY. 



Pre-Franciscan. — Little is known of the Pre-Franciscan 

 geologic history of the central Coast Ranges and no additional 

 data was procured from the study of the Pajaro Valley region. 

 The intrusion of the quartz-diorite into a much older series of 

 sedimentaries whose age is not known, uplift, and the erosion of 

 all but remnants of the sedimentaries, are all the records we have 

 of that long period of time. 



Franciscan Time. — Little also can we learn from a study of 

 the Franciscan rocks of the conditions of deposition and the 

 physical geography of Franciscan time. That the period of 

 deposition was a long one and that conditions changed materially 

 during that time we know. 



Post-Franciscan and Pre-Tertiary. — The chief record in the 

 Pajaro Valley region of the time between the uplift of the Fran- 

 ciscan rocks and the Tertiary period is that of a long period of 

 erosion of the Franciscan. Some time during that interval these 

 rocks were invaded by peridotitic magma, which formed lacco- 

 lithic bodies. The Shasta-Chico is not represented in the section. 



Tertiary. Pre-Miocene. — Up to the beginning of the Miocene 

 there is no sedimentary record in this region. 



Miocene. — The lowest Tertiary rocks exposed are considered 

 to be of Lower Miocene age. Their lithologic characters indicate 

 a period of oscillation. Sandstones, conglomerates, and shales 

 follow one another without any definite sequence and so we 

 may consider that the depth of water over the area varied con- 

 siderably during this time. The Franciscan rocks evidently 

 formed land bodies at this time, as we find the Monterey shales 

 overlapping the Lower Miocene and resting directly upon the 

 Franciscan. 



At the beginning of Monterey time, without interruption of 

 deposition, there was a widespread submergence and deep water 

 prevailed all through the Monterey time, the depositional con- 

 ditions remaining constant and uniform. The Franciscan rocks 

 were doubtless largely covered by a thick mantle of sediments. 

 Subsequent to Monterey time and before the San Pablo there was 

 a profound uplift, accompanied by folding and faulting and 

 there was inaugurated a long period of erosion. 



