Sedimentary Rock

Sediments are loose, unconsolidated rock particles and ions (i.e., charged atoms) dissolved in aqueous solutions. Sedimentary rocks, which constitute about 75% of all rocks exposed at the surface of the Earth, form when the particles are cemented together or when crystals precipitate from the solutions. Sedimentary rocks (particularly sandstones) are important because they host petroleum deposits and most of the uranium used for generating nuclear energy. There are three types of sedimentary rocks: (1) clastic (or detrital), (2) biochemical, and (3) chemical.

Clastic sedimentary rocks constitute more than 75% of the total sedimentary rocks. These rocks are formed from cemented grains and particles derived from the breakdown of preexisting rocks of any type (e.g., igneous, sedimentary, or metamorphic). The classification of clastic sedimentary rocks is based mainly on the size of the grains. A clastic sedimentary rock formed by gravel (i.e., grains bigger than 2 mm [millimeters] in diameter: granules, pebbles, or boulders) is called conglomerate (if the grains are rounded) or breccia (if the grains are angular). A clastic sedimentary rock formed by grains between 1/16 and 2 mm in diameter is called sandstone. Sandstones can be further classified according to the most abundant minerals they contain. Sandstones formed mostly [Page 2521]by grains of the mineral quartz are called quartz sandstones (or quartz arenites), whereas sandstones formed mostly by grains of the mineral feldspar are called arkoses. Sandstones that include high amounts of fine-grained clay are called graywackes. Conglomerates, breccias, and sandstones are formed by grains that are coarse enough to be seen with the naked eye. In contrast, grains of silt (between 1/256 and 1/16 mm in diameter) and clay (smaller than 1/256 mm in diameter) are too small to be seen with the naked eye. Silt and clay grains, when cemented together, form the clastic rocks siltstones and claystones, respectively. Siltstones and claystones are sometimes collectively called mudstones. A mudstone that easily splits in different layers is a shale.

Colorful sedimentary layers in a road cut at Artist's Point in the Colorado National Monument near Grand Junction

Source: Linda Armstrong/iStockphoto

Biochemical sedimentary rocks are formed from the remains of organisms. Peat and bituminous coal are formed from plant fragments. Peat is commonly brown and porous, with plant fragments that are still visible. In contrast, bituminous coal is black without distinguishable plant remains. Fossiliferous limestone is formed from fossil shells and coral and is the most common biochemical sedimentary rock.

Sedimentary rocks belonging to the last type (chemical) form by precipitation from aqueous solutions. These rocks are classified according to their chemical composition. Some common chemical sedimentary rocks are limestone (formed by CaCO3), dolostone (CaMg(CO3)2), chert (SiO2), rock salt (NaCl), and rock gypsum (CaSO4·2H2O). Limestone is the rock that most commonly is responsible for the development of karst topography.