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 se