Black shales which form in anoxic conditions contain reduced free carbon along with ferrous iron (Fe 2+ ) and sulfur (S 2− ). Source: Internet
In many places around the world, dark anoxic shales were formed during this interval. Source: Internet
See Stanley (1999), pp. 481 482 These shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. Source: Internet
Clays are the major constituent of shales and other mudrocks. Source: Internet
Geologists can classify oil shales on the basis of their composition as carbonate -rich shales, siliceous shales, or cannel shales. Source: Internet
Geology Underpinning Houston's land surface are unconsolidated clays, clay shales, and poorly cemented sands up to several miles deep. Source: Internet