Monterey Formation
by Richard J. Behl, CSU Long Beach Geology

The Miocene Monterey Formation of California is part of a wide swath of unique sediments deposited around the Pacific Rim between about 17.5 and 6 million years ago. Although these sediments vary from place to place as local conditions differed, the sediments are distinctly diatomaceous, phosphatic, dolomitic, and rich in organic matter. They are therefore very important because they provide a detailed record of past oceanographic change and are the source rock for much of the oil and gas in California, Japan, and elsewhere. The diatoms, which form the bulk of the sediments when originally deposited, are unusually unstable and dissolve and precipitate as cement several times after burial to deeper and hotter depths below the surface of the Earth. In this way, soft diatomaceous sediments transform to much harder porcelanite and chert over time. The brittle cherts can deform and shatter under tectonic deformation and form excellent petroleum reservoirs. As you wander around coastal California, you can often see outcrops of the Monterey Formation. When old and weathered, the Monterey is usually a distinctly light color with well developed bedding, but at the beach the rocks may be varied and colorful. The ongoing research of Dr. Rick Behl and his students centers on the deposition and diagenesis of siliceous sediments and the origin of chert in both the Monterey Formation and the deep west Pacific Ocean.

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