LATER TERTIARY (MIOCENE AND PLIOCENE). 817 



may be definitely made out, which prove the intervention of periods of time during which land 

 conditions existed over wide areas or locally. The important post-Eocene formations that 

 represent the epochs of submergence of the land in the area now occupied by the Coalinga 

 district are the Vaqueros (lower Miocene) , the Santa Margarita ( ?) (upper middle Miocene) , 

 the JacaHtos (early upper Miocene) , the Etchegoin (late upper Miocene) , and the Tulare (Plio- 

 cene and lower Pleistocene) formations. The Tulare formation is probably in large part of 

 different origin from the others but is similar to them in the general features of its appearance. 

 These formations are all united into one series in the monocline dipping down the east flank of 

 Joaquin Ridge in the northern part of the district and again in the monocline dipping away 

 from Reef Ridge in the southern portion, but the character of the series is not entirely the same 

 in the two regions. 



In the northern part of the district the base of the Miocene-Pliocene is formed of coarse and 

 fine oil-impregnated sands unconformably overlying the whitish and purphsh petroliferous 

 Eocene (Tejon) shales, these sands being overlain by prominent sandstone beds (the "reef beds"), 

 a prominent zone of white siliceous shale (the "indicator"), and soft sand up to the base of a 

 zone of bluish-gray and variegated clay, sand, gravel, and serpentine, detritus locally known as 

 the Big Blue. Up to this point the beds are fossUiferous, have a thickness of about 650 feet, and 

 are mapped as Vaqueros (lower Miocene). The Big Blue has a thickness of about 300 feet and 

 is nonfossUiferous. It corresponds in stratigraphic position to the Monterey shale (middle 

 Miocene) of regions nearer the coast, but nothing has been discovered to indicate that it may 

 belong to that formation. It is overlain by a thickness of about 175 feet of sand, sandstone, and 

 conglomerate beds fuU of immense oysters, barnacles (Tamiosoma), scallop shells (Pecten), and 

 other fossils. These fossUiferous beds will be referred to as the Tamiosoma zone. They are 

 overlain by 400 to 500 feet of sand and gravel beds up to the base of a very prominent gravel 

 zone full of petrified wood. The beds from the base of the Big Blue up to this point are mapped 

 as the Santa Margarita ( ?) formation (upper middle Miocene). The gravel zone with fossil wood 

 forms the base of a succession of sand, sandstone, gravel, and clay beds extending up to the base 

 of a prominent zone of bluish-gray sand beds having near their base a rich fossil bed, the Glycym- 

 eris zone. The succession of beds up from the base of the fossil-wood gravel zone to this point 

 has a thickness of about 1,600 feet and is mapped as the JacaHtos formation (early upper Miocene). 

 The fossil bed (Glycymeris zone) and bluish sands immediately overlying grade upward into 

 sand and clay beds, the whole forming a thickness of about 1,700 feet, which is mapped as the 

 Etchegoin formation (uppermost Miocene), and this finally is overlain by poorly exposed coarse 

 gravel deposits, which are mapped as the Tulare formation (Phocene-lower Pleistocene). The 

 total tliickness of the succession in the Coalinga field thus outlined is about 4,600 feet, exclusive 

 of the Tulare formation, which can not be measured in tliis portion of the district. 



In the southern part of the district the basal portion of the Miocene-Phocene series consists 

 of about 700 to 900 feet of steeply dipping hard sandstone and conglomerate beds forming the 

 face of Reef Ridge. These overlie, with an important though usually not apparent unconformity, 

 the shale of the Tejon (Eocene) and are locally petroliferous. At the summit they grade into 

 softer beds which are overlain by hard sihceous shale. Up to this shale the beds are fossihferous 

 and are mapped as the Vaqueros formation. The overlying shales are hard and whitish, form a 

 prominent zone varying up to 1,200 feet in tlaickness, and are mapped as Santa Margarita (?), 

 although only tentatively referred to that formation. These shales are overlain by a great suc- 

 cession of beds of sandstone, shale, sand, clay, gravel, and conglomerate of many varieties, hav- 

 ing a thickness as measured in a section south of Big Tar Canyon of about 9,500 feet. This 

 succession is divided on the map, on the basis of criteria to be discussed later, into three approxi- 

 mately equal divisions corresponding to the formations in the north, namely, the JacaHtos (early 

 upper Miocene), Etchegoin (uppermost Miocene), and the Tulare (Pliocene-lower Pleistocene). 

 The total thickness of the Miocene, Pliocene, and lower Pleistocene series measurable in the 

 above-mentioned single section is over 11,000 feet. 



The following table, from Arnold and Anderson's report,^* gives a correlation of 

 the Mesozoic and Cenozoic formations of the California Coast Range : 



4801] °— 12 52 



