The Ordovician Earth System
Black shales: An Ordovician perspective
Published:April 01, 2010
Ordovician black shales have been investigated extensively for at least three basic reasons: (1) they bear one of the system's most useful biostratigraphic tools, graptolites; (2) many of them are richly petroliferous; and (3) they are relatively widespread. Review and analysis of environments in which modern analogues of black shales may form indicate that they accumulate under hypoxic to anoxic conditions. Investigations of these environments in modern oceans indicate that they form in settings where an abundance of organic matter is present, and oxygen supply and possible resupply is shut off, or those in which oxygen resupply is at a slower rate than oxygen consumption. Ordovician black shales likely accumulated in environments relatively similar to modern hypoxic-anoxic environments under oceanic oxygen minimum zones as well as in shelf sea basins of varying depths and shallow shelf seas in which density stratification shuts off oxygen supplies.