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In general, the Los Angeles and Ventura areas have similar ecologic and tectonic histories. Foraminifera and other fossils indicate that when Tertiary time began there were widespread marine conditions in both areas, varying from shallow to bathyal depths. The average surface temperature of the Eocene sea is indicated to have been above 20° C., which is somewhat warmer than now. Emergence followed the Eocene, and up to 8000 feet of lower Oligocene continental beds (Refugian stage?) was deposited. Sedimentary and other features suggest that these beds accumulated on a broad flood plain of rather low gradient. These continental beds interfinger with marine sediments in the vicinity of Gaviota Pass. Marine conditions returned during middle and late Oligocene time (Zemorrian and Saucesian stages), and the fossiliferous Vaqueros sand was deposited over most of the area. From this time to late Miocene, the area was gradually lowered into the sea, to depths ranging from 2000 to 4000 feet. At the close of Miocene time, the large southern California marine province was divided—either by emergent or submarine barriers—into separate basins of deposition such as the Los Angeles, Ventura, Santa Maria, and San Joaquin basins. The Los Angeles and Ventura basins sagged to depths below 4000 feet during early Pliocene time. From that time to the present, these basins have gradually filled but with varying amounts of subsidence in some areas.

The Santa Maria Basin, in a general way, had the same history, but, in the area discussed here, the upper bathyal Miocene sediments overlap Mesozoic rocks. The fauna indicates a gradual filling of the basin to its present level. Here, subsidence was also a large factor. The Pliocene of the basin was deposited in waters not more than 1000 feet in depth, which is much shallower than depths reached in the Los Angeles and Ventura basins.

During Eocene time the southern San Joaquin Valley was covered by shallow subtropical to tropical marine waters. The lower Oligocene contains sediments of continental and shallow neritic origin.

This basin was covered deeply by marine waters during the Zemorrian stage, whereas the coastal area during this time was emergent or covered by shallow water. About the same conditions prevailed in all the basin areas during Miocene time. However, the San Joaquin Valley Basin, during Pliocene time, was under the influence of shallow seas and emergent conditions, whereas the coastal basins—especially during the Pliocene—were covered by deep marine waters.

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