porcelanite - formed from the reprecipitation of silica during the conversion of siliceous sediments to sedimentary rock - Monterey Formation, CA - display specimen

Porcelanite from the Monterey Formation, Lake Lopez, San Luis Obispo County, California

The Monterey Formation is a diatomaceous accumulation of sediments in basins that were adjacent western North America between 15 and 5.5 million years ago. It is a part of a circum-Pacific ring of diatomaceous sediments that resulted from major changes in paleoclimate, coastal upwelling and areas of biosiliceous productivity that occurred at that time. Intensified coastal upwelling resulted in high plankton productivity and rapid deposition of siliceous ooze from the constant rain of dead diatoms and radiolarians falling to the sea floor. 

The sediments that accumulated farthest from the coast were primarily calcareous and siliceous. These sediments were rapidly and deeply buried. During diagenesis, biogenic silica dissolved and reprecipitated as more stable opal and eventually, with time and increased temperature, forming porcelanite, chalcedony and chert.  Diagenesis refers to the conversion of sediments into sedimentary rock. Where clay is a larger component of the sediments, diagenesis forms porcelanite, which has a silica content of 50 to 85%. With a silica content of 90% to 95%, the rock would be chert.

The Monterey Formation is the source rock for most of California’s petroleum production. The field photo shows tilted beds of porcelanite in the Monterey Formation near Lake Lopez.

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