Black shale

A shale with at least 1% carbonaceous material indicating a reducing environment.

A dark, thinly laminated carbonaceous shale, exceptionally rich in organic matter (5% or more carbon content) and sulfide (esp. iron sulfide, usually pyrite), and often containing unusual concentrations of certain trace elements (U, V, Cu, Ni). It is formed by partial anaerobic decay of buried organic matter in a quiet-water, reducing environment (such as in a stagnant marine basin) characterized by restricted circulation and very slow deposition of clastic material. Fossil organisms are preserved as a graphitic or carbonaceous film or as pyrite replacements.

Sometimes resembling cannel coal, often found as a roof to a coal, or in place of a coal, resting on a fire clay.

Ref: AGI