Classification based on origin
Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts,
volcanism, and other minor processes.
Clastic sedimentary rocks
Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic rocks are composed largely of
feldspar, rock (lithic) fragments,
clay minerals, and
mica; any type of mineral may be present, but they in general represent the minerals that exist locally.
Clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the
Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions:
gravel (>2 mm diameter),
sand (1/16 to 2 mm diameter), and
clay is <1/256 mm and
silt is between 1/16 and 1/256 mm). The classification of clastic sedimentary rocks parallels this scheme;
breccias are made mostly of
sandstones are made mostly of
mudrocks are made mostly of the finest material. This tripartite subdivision is mirrored by the broad categories of
lutites, respectively, in older literature.
The subdivision of these three broad categories is based on differences in clast shape (
breccias), composition (
sandstones), grain size or texture (
Conglomerates and breccias
Conglomerates are dominantly composed of rounded
breccias are composed of dominantly angular
Sedimentary rock with sandstone in
Sandstone classification schemes vary widely, but most geologists have adopted the Dott scheme,
 which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains.
- Composition of framework grains
- The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components
feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance.
- Quartz sandstones have >90% quartz grains
- Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains
- Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grains
- Abundance of muddy matrix material between sand grains
- When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud (silt and/or clay sized particle).
- "Clean" sandstones with open pore space (that may later be filled with matrix material) are called arenites.
- Muddy sandstones with abundant (>10%) muddy matrix are called wackes.
sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme
 is widely used by sedimentologists, common names like
arkose, and quartz sandstone are still widely used by non-specialists and in popular literature.
Mudrocks are sedimentary rocks composed of at least 50%
clay-sized particles. These relatively fine-grained particles are commonly transported by
turbulent flow in water or air, and deposited as the flow calms and the particles settle out of
Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud.
 Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles.
 Most authors use "
shale" as a term for a
fissile mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock.
Biochemical sedimentary rocks
Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include:
- Most types of
limestone are formed from the calcareous skeletons of organisms such as
Coal, formed from plants that have removed
carbon from the atmosphere and combined it with other elements to build their tissue.
- Deposits of
chert formed from the accumulation of siliceous skeletons of microscopic organisms such as
Chemical sedimentary rocks
Chemical sedimentary rock forms when mineral constituents in
supersaturated and inorganically
precipitate. Common chemical sedimentary rocks include
oolitic limestone and rocks composed of
evaporite minerals, such as
halite (rock salt),
"Other" sedimentary rocks
This fourth miscellaneous category includes rocks formed by
volcanic breccias, and other relatively uncommon processes.