Sandstone

Sandstone is a clastic sedimentary rock made up of sand-sized silicate grains, ranging from 0.0625 to 2 millimeters in diameter, that are held together by a mineral cement. It accounts for roughly 20 to 25 percent of all sedimentary rocks. The grains are mostly quartz or feldspar, as these minerals resist surface weathering best. While impurities can give sandstone any color, common shades include tan, brown, yellow, red, gray, pink, white, and black. Its beds often form prominent cliffs and landforms, and certain colors are strongly linked to specific places, like the red rock deserts of Arches National Park and other parts of the American Southwest. Because sandstone is porous and allows water and other fluids to percolate through, it can store large amounts, making it valuable for aquifers and petroleum reservoirs. Under metamorphism—usually from tectonic compression in mountain belts—quartz-rich sandstone can turn into quartzite.

Sandstone originates from clastic processes, not organic or chemical ones. Its sand grains come from physical and chemical weathering of bedrock, with erosion fastest in high-relief areas like volcanic arcs, continental rifts, and orogenic belts. Rivers or wind transport the eroded sand to depositional environments where tectonic activity has created space for sediment to accumulate. Forearc basins often collect sand rich in lithic grains and plagioclase, while intracontinental basins and grabens along continental margins are also common settings. As more sediment piles up, older sand gets buried and undergoes diagenesis—compaction and lithification. Early diagenesis, or eogenesis, occurs at shallow depths (a few tens of meters) and involves bioturbation and mineral changes, with slight compaction. The red hematite that colors red bed sandstones likely forms during this stage. Deeper burial leads to mesogenesis, where most compaction and lithification happen. Compaction increases as overlying sediment presses down, rearranging grains, deforming ductile ones like mica, and reducing pore space. Chemical compaction can also occur through pressure solution, where strained grain contact points dissolve, allowing closer packing. Lithification follows as higher temperatures speed up cement deposition, binding grains together. Pressure solution contributes by redepositing dissolved minerals in pore spaces. Mechanical compaction mainly occurs below 1,000 meters depth, chemical compaction continues to about 2,000 meters, and most cementation takes place between 2,000 and 5,000 meters. When erosion exposes buried sandstone again, telogenesis—the final diagenetic stage—occurs, where meteoric water can dissolve some cement, creating secondary porosity.

The bulk of sandstone consists of framework grains, which are sand-sized detrital fragments. Most are quartz or feldspar, the most weathering-resistant common minerals. Quartz grains dominate because of their hardness and chemical stability, allowing them to survive multiple recycling events and become rounded. They come from felsic plutonic rocks or older recycled sandstones. Feldspar is the second most abundant mineral, divided into alkali feldspars (ranging from KAlSi3O8 to NaAlSi3O8) and plagioclase feldspars (ranging from NaAlSi3O8 to CaAl2Si2O8). Lithic framework grains are pieces of source rock that haven't weathered into individual minerals; they can be fine- or coarse-grained igneous, metamorphic, or sedimentary rock, with volcanic clasts being most common. Accessory minerals make up a small percentage of grains and include micas (muscovite and biotite), olivine, pyroxene, and corundum. Many of these are denser than the main silicates and, being resistant to weathering, serve as indicators of source material.