Vol. (3] Jones: The Geology of the Sargent Oil Field. 



59 



Igneous. — Extending across the southwest part of the area 

 and forming a part of the same plutonic mass which enters 

 into the composition of the Gavilan range to the south, and 

 probably also the Santa Cruz range to the north, is an extensive 

 mass of rather basic quartz-diorite. This rock, together with 

 the more acidic facies to the north and to the south, forms the 

 base of the Coast Ranges. 



The geologic relations of the quartz-diorite, its petrographic 

 and chemical characters, were discussed by Reid :! in his paper 

 on the igneous rocks near Pajaro. Reid's views have been 

 in the main, corroborated by the writer. Rather extensive 

 quarrying at Logan near the Pajaro River Canon has exposed 

 the rock-mass so that its relations to the overlying sedimentaries 

 can be well studied. Southeast from this point the diorite is 

 exposed in the transverse valleys for a mile or more, and exten- 

 sive areas of it are found in the vicinity of Fremont Peak to the 

 south. North of the Pajaro River the diorite quickly disappears 

 beneath the overlying sedimentaries, and does not reappear on 

 the surface again for several miles. 



Generally speaking, the plutonic mass may be said to lie in 

 the anticline of a somewhat complicated fold of Miocene sedi- 

 ments. The contact, of course, is depositional and not intrusive. 

 The lowest sedimentary rock exposed as resting on the diorite 

 is a coarse, non-fossiliferous, brown sandstone. This is well 

 exposed near the quarry on the railroad track just south of 

 Logan. Above this, conformably, lie great thicknesses of 

 Miocene shale. Resting across the truncated edges of both 

 sandstone and shale beds and across the eroded surface of the 

 diorite, is a series of friable sands and gravels which are 

 probably closely allied to the Merced formation. The upper 

 surface of the diorite mass is water-worn and these sands, with 

 an abundant marine fauna, fill the crevices. These relations are 

 best seen on the southwestern side of the range or hills formed 

 by the diorite mass. On the northeastern side these relations are 

 obscured by faulting along the San Andreas rift. 



The San Andreas fault-zone is quite broad in this vicinity 

 and the fracturing of the diorite has been extensive, so that 



3 Univ. Calif. Publ. Bull. Dept. Geol., 3, 1902, pp. 173-190. 



