A detailed study was made of the foraminiferal paleoecology of the Middle Tertiary of the San Joaquin Valley, California. General trends of use in paleoecology include: (1) increase of authigenes away from shorelines, (2) progressive diversification of foraminiferal species and increase in foraminiferal abundance away from shore, and (3) concentration of planktonic species in the upper bathyal and outer shelf zones of marine basins. Bathymetry of modern homeomorphs of Tertiary species serves as the basis for establishing seven major biofacies for the California Tertiary, representing depths ranging from estuarine to deep bathyal conditions.

From Zemorrian to Luisian time, water depths in the San Joaquin marine basin were about 6,000 feet; in later Miocene time there was gradual shoaling, resulting in widespread shallow marine and paralic conditions in the Pliocene. Displaced faunas were most abundant near the base of steeper slopes of the reconstructed marine environments of the Middle Tertiary. Abyssal and shoal faunas appear to have longer geologic ranges, generally, than biofacies representing intermediate depths.

Paleotectonism was assessed in terms of vertical changes; changes amounted to many thousands of feet for each stage, especially in perimeter areas of the marine basin. Volumetric analyses suggest that about 900 cubic miles of subsidence occurred in the Zemorrian, with a progressive decrease to a minimum value of about 300 cubic miles in the Relizian, and increasing values for the remainder of the Miocene.

More than 5,000 cubic miles of rock represent the marine sediment deposited in the San Joaquin basin during the middle Tertiary stages; more than 4,000 cubic miles of this were deposited in bathyal marine conditions, and most of the oil produced has come from these sediments. Oil fields occur in areas that have been active tectonically, near steeper bottom slopes of the reconstructed environment, and where there are rapid changes in biofacies.

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