Black Shale

(petrology) Very thinly bedded shale rich in sulfides such as pyrite and organic material deposited under barred basin conditions so that there was an anaerobic accumulation. Also known as biopelite.

A dark mud rock rich in organic carbon. Black shales are typically very fine-grained and contain pyrite, phosphate, and abnormally large amounts of heavy metals. They commonly display excellent fissility and well-preserved planktonic and nektonic faunas and plant debris. Benthic fossils are rare or absent. Some black shales are sources of hydrocarbons. See also Shale.

Black shales are enigmatic deposits. Although the large organic carbon content (3–15%) must have required reducing conditions of deposition, there are few unambiguous indicators of the specific environment, most especially of the depth of water. See also Marine sediments; Sedimentology.

Black shales are typically well laminated on a scale of millimeters. Laminae are produced by variations in the supply of sediment, such as seasonal alternations of clay and planktonic algae. Delicate laminae can be preserved only in the total absence of benthic life, for burrowing animals disrupt lamination, producing bioturbated texture. Hence it is possible to differentiate between totally anaerobic conditions of deposition and marginally oxygenated (dysaerobic) conditions by recognizing laminated or bioturbated fabrics in a shale.