Sedimentary rocks, which form from compressed layers of accumulated material like sand, mud, and organic remains, often exhibit distinct bands of color (stratification). The different colors are a direct consequence of the chemical elements present in the sediment and the environmental conditions during deposition and later alteration. Understanding these variations requires looking closely at the mineral composition and subsequent chemical changes the rock undergoes over geologic time.
The Mineral Basis for Sedimentary Rock Color
The color of sedimentary rocks is primarily determined by the presence and chemical state of iron compounds. Iron is a common element in Earth’s crust, and its reactions with oxygen act as the main coloring agent. Reddish, yellow, and brown hues are caused by ferric iron, which is iron in an oxidized state, similar to rust.
The mineral hematite, an iron oxide, is responsible for deep red colors, while limonite and goethite produce yellows and browns. Conversely, when iron exists in a reduced state as ferrous iron, it imparts grayish or greenish colors to the rock layers. This difference reflects the presence or absence of free oxygen in the environment where the sediment formed.
Organic matter, the decayed remains of ancient plants and animals, is another major coloring agent. When sediment accumulates in environments with low or no oxygen, the organic material does not fully decompose. This preserved carbonaceous material results in the dark gray and black layers seen in rocks like black shale. Light-colored sedimentary rocks, such as pure quartz sandstones, are generally light because they lack these coloring agents.
Layering Due to Changes in Depositional Environment
The initial formation of distinct color layers occurs during deposition. This layering happens because geological conditions in a depositional basin, such as a lake bed or a marine shelf, shift over time. Each environmental change alters the type of sediment and the chemical state of the coloring minerals being laid down, resulting in a new color layer.
A significant factor controlling the color of a layer is the availability of oxygen at the time of sediment settling. For instance, deposition in a well-oxygenated environment, like a shallow river channel, allows iron to fully oxidize, resulting in a red layer. If the area later becomes a deep, stagnant swamp, conditions become anoxic (oxygen-poor).
These low-oxygen conditions prevent iron from oxidizing and allow organic matter to accumulate, resulting in the deposition of a dark gray or black layer directly on top of the red one. Subsequent shifts in sea level or climate can repeatedly change these conditions, causing an alternating sequence of red, green, or black layers to stack up. This environmental change during accumulation is the fundamental cause of widespread color banding.
Post-Formation Chemical Alteration and Color Bands
While primary layering is set during deposition, a rock’s color can be modified or created long after it has solidified, a process called diagenesis. This secondary coloration is driven by the movement of groundwater through the rock’s pore spaces. The groundwater acts as a chemical delivery system, carrying dissolved iron and other ions through the permeable layers.
As the water moves, it can encounter a chemical boundary known as a redox front, where oxidizing conditions meet reducing conditions. This boundary causes dissolved iron to precipitate or existing iron minerals to change their chemical state, creating sharp color contrasts that cut across the original sedimentary layers. For example, oxidizing groundwater can “bleach” a dark, reduced layer by dissolving the ferrous iron, or it can introduce new ferric iron to create a bright red band.
A striking form of secondary coloration is Liesegang banding, which appears as concentric, undulating, or rhythmic rings of color. This phenomenon is caused by the periodic precipitation of iron oxides from a diffusing solution within the rock. The bands are created when an ion solution achieves a critical saturation level, precipitates a ring of iron-rich cement, and then continues to diffuse until it precipitates the next ring, independent of the rock’s original layering.