Sedimentary rocks come in a wide variety of colors. The color of a sedimentary rock depends on its composition and the minerals present when it formed. Sedimentary rocks are formed from particles of other rocks, organic matter, or biological materials that accumulate over time. As layer upon layer builds up, the sediment is compressed into sedimentary rock. The composition and color of the original sediment determine the properties of the final rock.
Main Factors That Influence Sedimentary Rock Color
There are three primary factors that determine sedimentary rock color:
- The original composition and color of the sediment prior to lithification
- The presence of organic materials such as plant debris
- The presence of mineral impurities including metal oxides and hydroxides
The composition of the original sediment is the most significant factor. Sediment that accumulates in layers comes from weathered and eroded rocks upstream. The minerals present in the source rocks impart color. As mineral composition varies, so does color. Sedimentary rock also forms from accumulations of organic matter. The presence of dark plant debris and carbon results in darker grays and blacks. Finally, mineral impurities including iron oxides lend red and oranges, while hydroxides contribute brown, yellow, and green hues.
Composition of Common Colored Sedimentary Rocks
Let’s take a closer look at the composition of some common colored sedimentary rocks:
Tan/Beige
Tan or beige sedimentary rocks often form from weathered granite. Granite contains abundant quartz and feldspar. As these minerals break down, the resulting sediment is light tan. Sandstone is a prime example of a tan sedimentary rock. The most common mineral in sandstone is quartz. Quartz has no color, so accumulations of quartz sediment result in pale tan or white sandstones.
Red
Red sedimentary rocks get their color from iron oxide or hematite. Iron readily oxidizes and stains sediment red. Redbeds are red-colored sedimentary strata. They form when iron is present during deposition and oxidation occurs early after burial. Redbeds can also form when iron leaches into existing sedimentary beds and causes staining.
Green
Green sedimentary rocks contain minerals with greenish hues. Glauconite, a hydrated iron potassium silicate mineral, is the most common contributor of green colors. It forms authigenically during the transformation of sediment to rock. Other minerals that tint rocks green include chlorite, serpentine, and clay minerals. Green mudstones and siltstones often contain abundant glauconite.
Black
Black sedimentary rocks contain abundant carbon, typically in the form of charcoal, graphite, coal, or organic-rich shale. Coal forms from the accumulation of land plant debris. Its black color results from carbon. Black shale accumulates in low oxygen environments where organic material is preserved. This organic material is carbon-rich and gives the rock a dark gray or black color.
Gray
Gray is the most common sedimentary rock color. Pure quartz sediments are white, but impurities usually lend a gray tone. Clay minerals within shales produce soft grays. Carbonates like limestone are also typically medium to dark gray.
Orange/Yellow
Orange and yellow sedimentary rocks are colored by hydroxide and hydrate minerals. Common color contributors include goethite, limonite, jarosite and other iron hydroxides that lend yellows, oranges, browns and ochre hues. Uranium oxides can also stain rocks fluorescent yellow.
Environments That Produce Distinctively Colored Sedimentary Rock
Certain depositional environments produce sedimentary rocks with characteristic colors:
| Environment | Typical Rock Colors | Cause of Color |
|---|---|---|
| Fluvial (rivers) | Tan, gray, red | Quartz, shales, iron oxides |
| Shallow marine | Green, gray | Glauconite, clays |
| Lacustrine (lakes) | Gray, black | Carbonates, organic matter |
| Bogs/swamps | Black | Coal, carbon-rich shales |
| Deep marine | Black, gray | Chert, shales |
| Deltaic | Red, brown | Iron oxides |
| Arid | Red, orange | Iron oxides |
Rivers produce tan sandstones from quartz and feldspar sediment. Shallow marine environments are conducive to glauconite and clay formation, resulting in greenish sedimentary rocks. Lakes generate gray and black organic-rich mudstones. Swamps and bogs accumulate plant debris that transforms into black coal and shale. Anoxic deep marine settings preserve organic carbon as black shale. Deltaic regions produce red beds through iron oxide formation. Arid climates also oxidize iron and generate red rocks.
Post-Depositional Processes That Alter Sedimentary Rock Color
After initial deposition, a number of processes can alter sedimentary rock color:
- Oxidation: Exposure to oxygen causes oxidation of iron-bearing minerals, changing color to red, orange, or yellow.
- Reduction: Water saturation in the absence of oxygen promotes reducing conditions. This removes reds and oranges and produces gray or greenish tones.
- Weathering: Physical and chemical weathering break down mineral constituents, modifying color.
- Hydrocarbon intrusion: The forced intrusion of hydrocarbons into sedimentary strata generates black bituminous staining.
- Volcanism: Heat from intrusive volcanic bodies bakes and alters the color of surrounding sedimentary rocks.
- Metamorphism: With increasing heat and pressure, sedimentary rocks undergo metamorphism. This recrystallizes minerals, modifying original colors.
Oxidation and reduction are two of the most common processes impacting color after deposition. Exposure during uplift typically oxidizes iron and generates red hues. Burial and water saturation drives reducing conditions and removes reds. Sedimentary rocks provide clues about the depositional conditions under which they formed as well as post-depositional processes that have modified their original characteristics.
Clues Provided by Sedimentary Rock Color
The color of sedimentary rocks reveals details about their composition and post-depositional history. Here are some key insights that can be gleaned from color:
- Tan/white – Formed from quartz-rich sediment like granite wash or pure quartz sand.
- Red – Iron oxides are present from arid, oxidizing environments.
- Green – Contains glauconite from shallow marine settings.
- Black – Rich in organic material like coals or hydrocarbon source rocks.
- Gray – Deposited under reducing conditions; shales.
- Orange/yellow – Iron hydroxides formed after initial deposition.
- Mixed/variegated colors – Subjected to multiple episodes of oxidation and reduction.
Additionally, uniform vs. irregular color banding provides information on the depositional environment. Evenly banded strata indicate regular cycles of sedimentation in environments like deep marine. Irregular color patterns formed in settings with greater variability like deltas.
Conclusion
Sedimentary rocks exhibit a wide spectrum of colors. These colors provide valuable clues about original composition and depositional environments. Tan quartzites formed from sandstone deposited in rivers and oceans. Black organic-rich shales accumulated in low oxygen conditions. Redbeds with iron oxides originated in arid continental areas. Green glauconite-rich sediments formed in shallow marine settings. After deposition, oxidation, reduction, weathering, and metamorphism continue to modify initial rock color.
Examining sedimentary rock color and patterns sheds light on provenance, depositional processes, and post-formation alterations. The next time you see vividly colored bands on a roadside outcrop, think about the fascinating geologic history encoded in these sedimentary strata.
