The existence of asphalt seeps (oil) and dense fossil accumulations at Rancho La Brea is dependent on the unique tectonic history of Southern California. The evolution of the San Andreas Fault zone is complicated and dependent on a number of past events.
During the Miocene (between 23 - 5 million years ago) the tectonic plates in the Southern California region changed from subduction to transform (strike-slip) faults. The crustal plates rotated which in turn stretched the L.A. region and formed a deep structural basin. Large amounts of marine sediments were deposited forming up to 6 miles deep! The L.A. Basin is one of the richest oil basins in the world in proportion to its size and volume. Later the Santa Monica Mountains uplifted and rivers flowing down brought large amounts of sands and gravels that subsequently buried the remains of plants and animals trapped on the surface of the asphalt seeps at Rancho La Brea.
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