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Colors of Sedimentary Rocks

Dividing Line

Red or reddish sandstones, mudstones, or shales may contain iron that has been oxidized, often indicating an environment in which the sediments were exposed to the atmosphere before or during burial. An example might be stream deposits or paleosols (fossil soils).

Greenish rocks may contain iron that has been reduced. A reducing environment may be one in which the sediments are buried in an oxygen-depleted environment (with inadequate oxygen) under water, such as a sea environment with limited circulation of nutrients or the quick burial of stream deposits before the iron has been oxidized. Good examples of reddish and greenish rocks in West Virginia formed in these manners include those of the Conemaugh Group exposed along I-79 north of Charleston.

Brown, black, or very dark gray rocks may contain large concentrations of organic matter (decayed plant and/or animal matter) and were generally deposited in an environment lacking oxygen. A prime example is coal, but other examples include black and very dark gray shales.

A distinction should be made between rocks colored all the way through like those above and rocks coated with a surface coloration. Reddish/yellowish coatings can often arise from acid mine drainage. Additionally, some iron and manganese minerals can also give a dark color to rocks, but most typically this coloration is in the form of a surface coating. Breaking open rocks with a surface crust of iron/manganese oxides and hydroxides often reveals that the coating is only a few millimeters thick and the "fresh" color of the rock is much lighter.

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