The Monterey Formation is a widespread, thick body of silica-rich rock laid down in Miocene time, approximately 16 to 6 million years ago. It's easily recognized throughout the Coast Ranges by its rhythmic bedding and cherty nature, although parts of it are shaly and full of fossils.
The Monterey Formation has interested California geologists for more than a century. The first generations of researchers found its highly siliceous composition a mystery, so unlike the sandstones and mudstones that typically form along continental margins.
Soon afterward a host of industry geologists mapped it in detail, because the Monterey Formation is also very rich in kerogen, the source material of petroleum. Today the Monterey Formation is also studied for the light it sheds on the tectonic evolution of western North America.
The thin, even beds of cherty sediment were laid down during a long interval during which vigorous growth and death of diatoms took place in richly nutritious surface waters. Upon their deaths, these one-celled organisms built up thick deposits in wide seafloor basins that were deep enough and far enough away from land to escape being filled with land sediment. The rhythmic beds are thought to be turbidites, formed by gentle sediment landslides into the heart of the basins. Because California was in the process of being pulled apart along the San Andreas fault at this time, earthquakes may account for a large share of these turbidites. Continued tectonic activity compressed the siliceous rocks, folded and fractured them, and piled them up into the growing Coast Ranges where they began to release their oil into overlying rocks.
Montaña de Oro State Park is the first place south of the Monterey Peninsula where the Monterey Formation crops out on the shore. Large exposures occur along the Southern California coast, corresponding to highly productive oilfields.