The pattern of southern California Tertiary basins has been formed, changed, and controlled by the trends of many major lateral faults. Their dominant northwest-southeast trends are interrupted by an ancient belt of east-west lateral faults which also caused basins to form across the region.

The structural pattern and, therefore, the erosional and depositional patterns were changing almost continuously during the Tertiary. Broad Eocene land and sea features were broken up by regional emergence and block faulting during the Oligocene. However, the general structural pattern lasted into early Miocene, when regional submergence began. Regional transgression continued through Miocene time, with few interruptions, over an increasingly irregular terrane formed by a developing complex fault-block pattern of basins and ranges. Great reversals of vertical relations between blocks and great lateral offsets occurred through Miocene and Pliocene times. Many islands or high land masses, deep embayments, and basins were formed at different times only to founder or be broken up. The dep sitional areas and types varied greatly and constantly with most of the coarser clastic sediments being deposited as submarine slides and turbidites.

New general block deformation ended the Miocene, and the Pliocene began with a different pattern of emergences, although many existing basins were deepened. Marine sediments of the entire region, because of this rapidly changing geography, were mostly coarse clastics derived from land; lithofacies became increasingly divergent and restricted. However, thick organic deposits were formed over large areas during times of greatest submergence in middle and late Miocene times. Considerable non-marine deposition occurred in coastal as well as interior valleys through Tertiary time except during early Pliocene. Most of the Tertiary basins were similar to those of the present. Even the ecology of some Tertiary basins is similar to that of modern basins, including the Gulf of California and th Imperial Valley.

The comparatively meager subsurface data from the 15,000-square-mile area offshore indicate that the same structural, erosional, and depositional histories took place there in the Tertiary as in the Tertiary basin region now emerged onshore. Offshore structural features are much less eroded or obscured, and sediments there are generally finer, thinner, and more organic.

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