To most field geologists 'black shales' are dark, fissile sedimentary rocks which are rich in both carbon and sulphur. The property of fissility probably develops only in sediments deposited from anoxic bottom waters. Carbon and sulphur-rich sediments are formed wherever relatively organic-rich material accumulates at such a rate that intensive diagenetic alteration takes place within the uppermost few metres of the sediment column, but relatively little happens below. Depositional waters must be marine (or at least sulphate-rich) but not necessarily anoxic.

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