Monterey Shale

The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with massive outcroppings of the formation in areas of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islands. The formation, as a major oil-producing geological formation, has had its strata extensively investigated and mapped for petroleum exploration and is of major importance to understanding the complex geological history of the state of California. Its rocks are mostly highly siliceous strata that vary greatly in composition, stratigraphy, and tectono-stratigraphic history.

The Monterey Formation strata vary distinctly between its lower Miocene members showing indications of poor coastal upwelling with fossil assemblages and calcareous-siliceous rocks formed from diatoms and coccolithophorids, to its middle and upper Miocence upwelling-rich assemblages and its unique highly siliceous rocks from diatom-rich planktons forming diatomites, porcelainites, and banded cherts.