UPPER CRETACEOUS. 669 



formations in the district and seems to represent a separate stratigraphic unit, although no 

 unconformity has been made out at its base. 



The lower half of the upper division consists chiefly of massive sandstone beds, often weather- 

 ing cavernous. * * * The sandstone is usually drab, medium grained, and not very hard. 



Above the succession of concretionary sandstone beds, and forming the upper half of the 

 upper division of the Knoxville-Chico, comes a less prominent and less uniform succession of 

 beds in which shale is predominant. * * * rphis upper half of the upper division consists 

 of two main members predominantly of shale, each approximating 1,000 feet thick, separated by 

 a much thiimer but more prominent member of concretionary sandstone, in general similar to 

 that lower down. * * * ^he lower of the two shale members has few exposures but is 

 notable for the presence in it of different kinds of deposits, such as blackish, thinly bedded 

 clay shale and yellowish and whitish calcareous and arenaceous shale and sand, and for the 

 presence of ammonites, baculites, Inoceramus, and other fossils, which weather out in a frag- 

 mentary state on the surface of the clayey soU. * * * ^he sandstone separating the two 

 shale members varies from 200 to 300 or 400 feet in thickness and consists of yellowish-gray and 

 brown sandstone full of large brown concretions. It is of the same type as the characteristic 

 Chico concretionary sandstone farther dow'n in the section. * * * The uppermost member 

 is one of particular interest owing to its individuality among the known types of Cretaceous 

 deposits, its large content of organic material, its petroliferous nature, and its similarity to the 

 Eocene beds. It is at least 1,200 feet thick and consists principally of shale but has a consid- 

 erable admixture of sand and sandstone. For convenience it will be referred to as a whole as 

 the purple shale member. The most characteristic beds of this zone are of purplish-brown, fairly 

 hard, thinly bedded, both siliceous and calcareous clay shale, in which the tests of Foraminifera 

 are very abundant. This shale bears a resemblance to some of the less siliceous shale in the 

 upper part of the overlying Tejon (Eocene), especially to that shale as it occurs in the southern 

 part of this district, and is not unlike some argillaceons phases of the Monterey shale (middle 

 Miocene) of the outer Coast Ranges and the shale mapped as Santa Margarita (?) (middle 

 Miocene) along Reef Ridge and at other points in this district. * * * 



Above the purple shale of the uppermost Cretaceous lie dark clay and clay shale beds 

 aggregating several hundred feet in thickness, which appear to form an upward continuation of 

 the shale of the Chico but which are mapped with the Tejon (Eocene). Fossils recently found 

 near the northern edge of the Coalinga district show that the higher of these beds belong with the 

 sandstone of the Tejon above, but there is reason to beheve that 200 feet or so of beds at the 

 base belong in the Chico. * * * Whether the line of separation between the Cretaceous 

 and Tertiary occurs at the top of the purple shale or somewhat higher, within the darker clay 

 shale, it is noteworthy that it should occur within a zone of fine sediments and be marked by no 

 break of evident stratigraphic importance. However, in spite of the apparent transition between 

 the shale of the Cretaceous and that of the Eocene in the northern part of the Coalinga district, 

 it is believed that they are separated by an unconformity. The evidence of this is the progressive 

 <iisappearance of the upper members of the Knoxville-Chico southward in the district, whereas 

 the basal Tejon is believed to represent a fairly constant horizon. * * * This thinning 

 and disappearance of the members is probably the effect of erosion upon them before the 

 deposition of the Eocene. 



For a general correlation of the Mesozoic and Cenozoic formations of the Coast 

 Range, see Chapter XVII (p. 818). 



In the Santa Cruz quadrangle the Chico is very thick. Branner "'' states : 



The Chico strata along the coast are made up for the most part of hard siliceous shale, sand- 

 stone, and massive, coarse conglomerate. The general dip of the beds, for a distance of 3 miles 

 along the coast south of the mouth of Pescadero Creek, is toward the southwest and the section 

 apparently exposes about 9,400 feet of strata. These strata dip at high angles, however, and 

 may be repeated hy folds, or to some extent by faults, thereby making the thiclmess appear greater 

 than it is. This is the case 2 miles north of Pigeon Point, where massive conglomerate is faulted 

 into contact with sandstone and shale. 



