Sedimentary rocks exposed along the periphery of the San Fernando Valley indicate the area was a separate marine basin during Miocene and Pliocene time. Although the surrounding geology is well known, the basin is virtually unrecognized as a major Tertiary basin of Southern California.

Structure, stratigraphy, and sedimentology indicate the basin's history is similar to that of other Continental Borderland basins. Localized subsidence in the Late Middle Miocene formed the basin as a discrete unit. Basin filling took place during Pliocene and Pleistocene time. Benthonic Foraminifera indicate that the San Fernando basin was separate from the adjacent and deeper Ventura basin but remained an integral part of the east-west Ventura embayment.

Over 900 meters of Late Miocene and Pliocene sediments are well exposed on the south side of the basin. Shales and diatomites typify the Miocene sequence whereas silts and sands are characteristic of the Pliocene. Laminated diatomite was probably deposited in a subsill oxygen-deficient environment analogous to the existing Santa Barbara basin. Coarse, arkosic continental sands interfinger with Pliocene marine sediments at the eastern end of the basin.

Benthonic Foraminifera and Radiolaria show that

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subsidence to middle bathyal depths occurred during Late Miocene (Delmontian) and Early Pliocene (Repettian) time. Rapid shoaling during the Pliocene is evidenced by the systematic appearance of slope, shelf, and neritic foraminiferal faunas. Abundant Pliocene macrofossils also characterize shelf-depth deposits. Shallow-water micro and macrofaunas within deep water sediments are interpreted as displaced by turbidity currents or slumping. Planktonic Foraminifera and an increase in radiolarian (Spumellina) diameter suggest cool surface temperatures during the Late Miocene and increasingly warmer temperatures in the Early Pliocene.

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