DEFINITION OF SANDSTONE

Any medium-grained clastic sedimentary rock

composed predominantly of rounded or angular sand-sized

particles, set in a finer-grained matrix composed of silt or claysized

particles, and indurated or cemented by an agent such as

carbonate, silica, or iron. It is the consolidated equivalent of

sand, and in most cases it is composed of dominantly quartz

particles. The names of sandstones that are not composed of

80–90 percent quartz are typically preceded by a prefix

describing the main constituents, such as olivine-sandstone,

garnet-sandstone, lithic-sandstone, and so on. There is a wide

variety of textural and compositional types of sandstones

deposited in an equally diverse set of environments ranging

from fluvial and aeolian settings to deep marine settings. Various

types of sandstones comprise about 10–15 percent of the

total amount of sedimentary rocks in the Earth’s crust.

Grain size may vary in sandstones between very fine,

fine, medium, and coarse-grained sands, and the amount of

variation in grain sizes in the sediment is described by the

term sorting, where well-sorted refers to a fairly uniform

grain size, and poorly sorted refers to a wide variation in

grain size. The angularity of the edges of individual grains is

described by the term rounding, and more rounded grains

tend to be the ones that have traveled the greatest distance

from the source rock. Additionally, windblown grains tend to

be more highly rounded and even frosted than grains transported

by water.

Numerous studies over several decades have shown that

the composition of the major, minor, and heavy-mineral constituents

of sandstones may reflect the composition of the

source terrane, the weathering processes, the processes that

transported the sediment, and the diagenetic and metamorphic

effects. Compositionally sandstones are divided into two main

groups including the arenites and the wackes. Arenites have

less than 15 percent fine-grained matrix material between the

sand particles, whereas wackes have more than 15 percent clay

and fine-grained material in the matrix. Arkoses are feldsparrich

sandstones that are typically found close to their source in

continental settings, since feldspar is unstable and alters to clay

minerals with weathering and transportation.

See also CLASTIC ROCKS; SEDIMENTARY ROCKS.

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