The floors of San Pedro and Santa Monica basins off southern California lie at a depth of nearly 1000 meters. Yet they contain layers of clean sand interbedded with normal deep-water green muds. The sands and their associated beds of silt and of fine gravel are believed to have been rapidly deposited by turbidity currents which started as mass movements at the shallow heads of submarine canyons and at the tops of basin slopes. A similar sequence is present in Pliocene sediments of completely filled basins now on land, for which the shales serve as source beds of petroleum and the sandstones as reservoir beds.

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