The Los Angeles basin contains a thick sedimentary sequence, mainly of late Miocene and Pliocene age. Sedimentary rocks from Cretaceous through Oligocene age crop out in places along the fringe of the basin, but these strata have not been encountered in the central portion of the basin.

A rich foraminiferal assemblage makes it possible to subdivide the Miocene and Pliocene into a number of stages and zones.

Luisian fauna, generally the oldest encountered within the basin, is indicated by Valvulineria californica and Anomalina salinasensis.

Strata of the overlying late Miocene, lower Mohnian stage, contain the marker fossils Bulimina uvigerinaformis, Bolivina modeloensis, and Epistominella gyroidinaformis.

The upper Mohnian fauna is recognized by the presence of Cassidulinella renulinaformis and Bolivina hughesi.

The upper Miocene Delmontian stage has fauna similar to the early Pliocene Repettian stage, but contains the marker Rotalia garveyensis and abundant Radiolaria.

The Pliocene is divided into three stages, these being--from oldest to youngest--Repettian, Venturian, and Wheelerian.

The Repettian stage is divided into 18 zones. Typical Repettian forms are: Ellipsonodosaria verneuili, Karreriella milleri, Bulimina rostrata, and Nonion pompilioides.

The Venturian stage has the markers Bulimina subacuminata and Bolivina sinuata.

Fauna of the Wheelerian stage includes Uvigerina peregrina, Epistominella pacifica and Bolivina interjuncta.

The early Pleistocene guide fossils include Cassidulina limbata and C. tortuosa.

The late Pleistocene fauna has the forms Elphidium poeyanum and Elphidiella hannai.

Water depth during Luisian time was about ±1,500 feet. Water depth increased at a fairly steady rate, through the Miocene and early Pliocene, until a maximum depth of about ±5,000 feet was reached during mid-Repettian time. The water depth then slowly decreased through late Repettian and sharply decreased during the Venturian. Wheelerian water depths commenced at ±2,000 feet and ended at ±1,000 feet. The basin was filled by the close of Pleistocene time.

The main production in the basin is from strata of Repettian age. This is followed by production from the late Miocene. Rocks of other ages produce relatively minor amounts of oil.

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