Shale

Shale is a fine-grained sedimentary rock formed from mud. The mud is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called "fissility" (= ~splitting). Mudstones, on the other hand, are similar in composition but do not show the fissility.
Shales and mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales form in anoxic (without oxygen) conditions. They contain free carbon and iron sulfides such as pyrite. This produces the black colouration.
Formation of shale.
Shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands. They can also be deposited on the continental shelf, in relatively deep, quiet water. This process could have taken millions of years to complete.
'Black shales' are dark, as a result of being especially rich in unoxidized carbon. 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. The heavy metals may have got there by slow accumulation.