burned shale - teaching hand specimen of burned shale from the Monterey Formation, Ventura County, California

The Miocene Monterey Formation is the source of much of the petroleum produced in California. In the area of what is now Grimes Canyon in Ventura County, California, the shale spontaneously combusted with petroleum as the likely fuel. In some areas temperatures reached 1650˚C, melting phosphate-rich layers of the siliceous shale into a black glass. Nearby shale was baked or “burned” to form various shades of red, orange and pink.

The burned shale is currently being mined as a decorative rock to replace plantings on road embankments and medians, and to replace lawns where water use is restricted.

The Monterey shale is diatomaceous. The exoskeletons of diatoms are made of silica, which the one-celled organisms remove from seawater. Diatoms are the source of the silica that formed the black glass and the source of the petroleum that ignited. It’s estimated that the burning occurred at some point during the past 1,000 years.

Burned shales exist in the Fort Union Formation in the Powder River Basin in Wyoming. There the fuel was underlying low grade coal, which also spontaneously combusted to bake the overlying shale to similar shades of pink, red and orange. In places that shale contains fossil leaves, turned a dark maroon by the baking. Shale and glass similar to the material in Grimes Canyon also occurs in the Fort Union Formation in eastern Montana.

The field photo shows our field assistant, T Cat, standing on blocks of this shale that have fallen to the base of a cliff. Untrimmed, it's a mess. With the weathered surfaces trimmed off, it's a pretty rock.

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