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Ancient Oil Deposits:
Many oil fields, both on land and in the water, are scattered throughout the state of California. California's large oil reserves are the result of large sedimentary rock basins and complex fault lines that create natural oil traps. The majority of oil in California formed during the late Tertiary period, which occured over 5 million years ago. During this time, these major rock basins laid beneath the surface of the ocean, where marine sediments accumulated as organims in the sea died. When the oceans receded, these areas became land and new layers of gravel and sand formed over the older marine sediments.
Click the link to view a map of oil, gas, and geothermal fields in California, 2001: California Energy Commission
La Brea Tar Pits:
The Los Angeles Basis has the most productive oil fields in the state. A well known field is the La Brea tar pits where the ocean receded about 100,000 years ago. While called tar pits, the liquid that seeps out of the ground is actually a form of oil called asphalt. The Rancho La Brea fossil depsoits formed about 40,000 years ago at the end of the Pleistocene Epoch and fossilized organims have been recovered from the pits.
Animals wandering through this area died when trapped in the sticky oil and their oil-covered bodies were prevented from decaying. Along with a variety of ancient plants, insects, and small organisms, the La Brea tar pits are known for their fossils of mammoths, mastodons, ground sloths, and sabertoothed cats. While the fossils within the tar pits are relatively young, the oil itself that preserves them is millions of years old.
Sources:
Ortega, Pat. 2002. "Return to the Ice Age: The La Brea Exploration Guide". The Natural History Museum of Los Angles County Foundation.
The Paleontological Research Institution. 2005. "Is There Oil in Your Backyard? The United States-West". Paleontological Research Institute: Petroleum Education.
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