12 
OIL DISTRICTS OF SOUTHERN CALIFORNIA. 
several localities. Tentatively, however, the line between the Sespe 
(Eocene) and the Vaqueros (lower Miocene) formations has been 
drawn at an indefinite horizon in the rusty beds described, at a point 
where the sandstone no longer predominates but is largely replaced 
by shale, yet below the lowermost lower Miocene fossils found. 
OIL, IN THE UPPER SESPE BEDS. 
It is probable that a portion of the oil yielded by the Fourfork and 
Tar Creek wells has been drawn from the rusty series at the summit 
of the red beds or else from similar thinner beds in the lower part of 
the Vaqueros formation. It will be observed that the entire series 
of beds designated the Sespe shows a remarkable distribution of 
petroleum, in both its vertical and horizontal extent. 
VAQUEROS FORMATION. 
AGE AND GENERAL CHARACTER. 
The formation designated Vaqueros, appears to be largely lower 
Miocene. The position of 3,000 feet of shale, such as make up the 
Vaqueros, below beds which are certainly well down in the Miocene, 
suggests the possibility that the lower part of the shale extends into 
the Oligocene. By a rough estimate the maximum thickness of the 
Vaqueros formation is between 2,000 and 3,000 feet, but the amount 
of sediment deposited evidently varied greatly from point to point. 
From the base upward the following more or less distinct zones are 
recognizable: Shales, purplish, rusty, and gray in color, purplish pre¬ 
vailing, perhaps 500 feet; a conspicuous zone of gray shale, 500 feet; 
a series of deep maroon, brown, and gray shales, approximately 
700 feet; and an upper body of siliceous shale and limestone, gray, 
but weathering a pronounced yellow, 500 or 600 feet. The formation 
is therefore distinctly one of shale from top to bottom. Through¬ 
out its extent lenticular limestones occur, with a tendency to form 
continuous bands a foot or two thick. Much of the shale also is 
calcareous. 
Through the lower half of the basal purplish zone are threaded 
thin layers of calcareous grits or gritty limestones. Where the quartz 
ingredient is sufficient, they resemble certain beds in the underlying 
rusty zone at the top of the Sespe formation. These beds are fossilif- 
erous, and from them were gathered the following representative 
forms, determined by Ralph Arnold: 
Fossils from Vaqueros formation. 
TAR CREEK, BY TRAIL SIDE. 
Modiolus sp. 
Ostrea cf. idriaensis Gabb. 
Pecten sespeensis Arnold (PI. XXXIII, figs. 1, la, 2). 
Turritella ineziana Conrad var. sespeensis Arnold (PI. XLI, fig. 6). 
