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Abstract

California is along the western borderland of North America where sedimentary deposits accumulated along the eastern Pacific margin. During that time, subduction diastrophism prevailed in the late Mesozoic -early Cenozoic era, and right-lateral strike-slip shear diastrophism prevailed in the middle and late Cenozoic era. The latter diastrophism generated an extremely unstable north-trending mobile belt that includes what is now the Coast Range Province west of the stable Great Valley Province. Cumulative right-slip on the northwest-trending San Andreas fault within this belt has displaced units within it as much as 500 km (300 mi).

The sedimentary series of late Mesozoic-Cenozoic age is composed of two major divisions. The lower division is a great thickness of marine clastic turbidites derived from the rising continental magmatic arc to the east and consisting of a primarily Cretaceous (Great Valley) sequence and lower Tertiary sequence. The upper division is composed of variable marine and terrestrial deposits. It consists of a middle Tertiary sequence of clastic and siliceous sediments and volcanic rocks; an upper Tertiary sequence of clastic sediments; and a Quaternary sequence of valley alluvial sediments. In the Coast Range province the sequences are generally separated by unconformities.

The upper division sequences were deposited on the forearc trough and in deeply subsiding marine basins forming within the evolving mobile belt. The marine sequences are petroliferous. The hydrocarbons accumulated on the basin flanks in updip stratigraphic traps that resulted from recurring movements during deposition within the mobile belt, especially near the San Andreas fault. They also accumulated in numerous structural traps formed in these sequences, such as closed anticlines and updip accumulations against minor faults.

Most of the hydrocarbon accumulations in and near the Coast Range province are in the extreme southeastern part, between the Taft-McKittrick area of the southern San Joaquin Valley and the Santa Maria-San Luis Obispo coastal area. In this segment the middle Tertiary sequence contains a petroliferous siliceous shale unit (Monterey Shale) of Miocene age that is probably the major source of hydrocarbons in California.

The Taft-McKittrick oil district is the most productive in California. This district contains numerous oil accumulations in both stratigraphic and structural closures in many sands of Pliocene to Eocene age, and includes several large surface deposits of asphalt and tar sand, and one oil shale deposit.

The coastal area includes the Santa Maria basin, which contains several major fields with heavy to medium oil, mostly in fractured Monterey shale in closed anticlines and stratigraphic traps. This area also contains several tar sand and oil shale deposits. The small Pismo basin to the north contains one heavy-oil field and many tar sands in upper Miocene strata.

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