Shale Formation of shale
 Black shales are deposited in anoxic or low oxygen environments, such as in stagnant water columns. They are common in some Palaeozoic and Mesozoic strata, Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc. R. Zangerl and E.S. Richardson 1963. Fieldiana Memoirs'  '4', Field Museum of Natural History, Chicago. 352J.D. Vine and E.B. Tourtelot 1970. Geochemistry of black shale deposits - a summary report.