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



63 



These Lower Miocene beds are exposed best along the La 

 Brea Creek where they form an acute anticline. This forma- 

 tion holds the oil of the region and the logs of the 

 oil wells, on La Brea Creek, kindly loaned by the Watsonville 

 Oil Company, indicate a thickness of at least 1500 feet. Most 

 of this thickness is of clay shale, sandstone, and siliceous shale. 

 Conglomerates are less frequent. The oil occurs in the sand- 

 stone, for the most part, though several of the wells penetrated 

 rich shale members. 



Another large but doubtfully mapped area of Lower 

 Miocene beds is exposed on the south slope of the Pescadero 

 canon, and occupies a faulted anticline. (See pi. 14, fig. 2.) 



Monterey Shale. — The predominant formation of the Miocene, 

 and the one which, structurally, has affected to a great extent the 

 present topography of the region, is the great thickness of 

 bituminous shale. A very considerable part of this formation 

 has been removed by erosion, and while its present maximum 

 thickness is about 3000 feet, it was doubtless originally much 

 thicker. Strata of subsequent age in the region have been, 

 in great part, derived from these shales and we are forced to 

 the conclusion that this formation has been much in evidence 

 in the areal geology since its first uplift above sea-level. 



The Monterey shale, as a whole, is a remarkably uniform 

 formation and the conditions under which the beds were 

 deposited must have been stationary and uniform through long 

 periods of time. The shales vary in color from an almost pure 

 white to a medium brown and the texture is very fine, varying 

 only within small limits. AYhere these beds rest upon the 

 Franciscan rocks in the northern part of the area, there is a 

 slight coarseness in texture and some of the strata here may 

 properly be called sandstone. Interspaced at fairly regular 

 intervals throughout the formation, but especially in its lower 

 part, are beds, varying in thickness from one to three feet, 

 holding a high percentage of lime. These beds always form 

 prominent outcrops, and north of La Brea Creek form the only 

 means of determining the structure of the formation as a whole 

 in that part of the area. Just north of Chittenden Lake there 

 are several beds of banded chert, some specimens of which are 

 translucent. 



