Sandstone

Any sedimentary rock composed of stony grains between 1/16 mm and 2 mm in diameter that are cemented together is a sandstone.

Sandstone forms from beds of sand laid down under the sea or in low-lying areas on the continents. As a bed of sand subsides into the earth's crust , usually pressed down by over-lying sediments, it is heated and compressed. Hot water flows slowly through the spaces between the sand grains, importing dissolved minerals such as quartz , calcium carbonate, and iron oxide. These minerals crystallize around the sand grains and cement them together into a sandstone. Spaces remain between the grains, resulting in a porous, spongelike matrix through which liquids can flow.

Petroleum and natural gas are often found in sandstones. They do not form there, but seek to float to the surface by percolating through water-saturated sandstones. Sandstone layers shaped into domes by folding or other processes (and overlaid by non-porous rock) act as traps for migrating oil and gas, that ascend into them but then have no way out. Such traps are much sought after by oil companies; indeed, most sandstone sedimentologists work for the petroleum industry.

Another useful feature of sandstones is that they tend to record the surface conditions that prevailed when their sands were created and deposited. For example, the diagonal laminations often seen running across sandstone beds (cross-bedding) record the direction and speed of the water or wind that deposited their original sand. Furthermore, the ratio of feldspar to quartz in a sandstone reveals whether its sand was produced by rapid erosion , such as occurs in young, steep mountain ranges, or more slowly, such as occurs in flatter terrain. Since sand beds are often deposited rapidly by wind or water, tracks of reptiles—and even the pocks made by individual raindrops—may be preserved as fossils in sandstone.

A sandstone may be uplifted to the surface and broken down by weather into sand. This sand may be deposited in a bed that subsides, turns to sandstone, returns to the surface, breaks down into sand again, and so on. Some individual grains of sand have participated in more than 10 such cycles, each of which lasts on the order of 200 million years.

See also Bedding; Petroleum detection; Petroleum extraction; Sedimentary rocks; Sedimentation

sandstone (arenite) Sedimentary rock type, formed of a lithified sand, comprising grains between 63 μm and 1000 μm in size, bound together with a mud matrix and a mineral cement formed during burial diagenesis. The main constituents are quartz, feldspar, mica, and general rock particles, although the proportions of these may vary widely.

sandstone Sedimentary rock composed of sand grains cemented in such materials as silica or calcium carbonate. Most sand grains contain quartz; other minerals in sandstone include feldspars and micas. Iron also occurs, which gives sandstones a reddish or brownish colour. Most sandstones form through the accumulation of river sediments on the seabed. They are then compressed and uplifted to form new lands.

sand·stone / ˈsan(d)ˌstōn/ • n. sedimentary rock consisting of sand or quartz grains cemented together, typically red, yellow, or brown in color.

sandstone(arenite) A type of sedimentary rock, formed of a lithified sand, and comprising grains between 63 μm and 1000 μm in size, bound together with a mud matrix and a mineral cement.