by Harold L. Levin
Chapter 5 - page 5
The Sedimentary Archives
Color of Sedimentary Rocks
Color of sedimentary rocks provides useful clues to the depositional environment.
- Black and dark gray coloration in sedimentary rocks generally indicates the presence of
organic carbon and/or iron.
Organic carbon in sedimentary requires anoxic environmental conditions
(lacking free oxygen), which might be found in some
quiet water marine environments, as well as deep lakes, and estuaries.
In these environments, iron combines with sulfur to form the mineral pyrite (FeS2),
which can also contribute to the black color.
Shales are commonly black to dark gray, suggesting deposition in deep, quiet water, anoxic environments.
Black, organic-rich sediments may also form in environments where the accumulation of organic matter exceeds the capacity of the environment to oxidize it.
- Red (often more of a maroon or a pink), and brown, purple,
or orange coloration in sedimentary rocks indicates the presence of iron oxides.
In well-oxygenated continental sedimentary environments,
the iron in the sediments is oxidized to form hematite or ferric iron oxide
colors the sediment red, brown, or purple.
These rocks are called red beds.
Red beds typically indicate deposition in continental (or transitional)
sedimentary environments such as flood plains, alluvial fans, and deltas.
Red beds can also form in marine environments (due to oxidation of the iron in the
sediment after deposition), or to erosion of red sediment from the land.
Red beds interlayered with evaporites indicate warm, arid conditions.
Red Silurian iron ore (hematite) from the Clinton Group near Birmingham, Alabama.
Trails in red siltstone from the Culpeper Basin, Virginia. Photo courtesy of Pamela Gore.
Banded Iron Formation. The red bands are hematite, and are interbedded with chert.
- Green and gray coloration in sedimentary rocks also indicates the presence of iron,
but in a reduced (rather than an oxidized) state.
Ferrous iron (Fe+2) generally occurs in oxygen-deficient environments.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 17, 2005