Sedimentary rocks form from the accumulation and consolidation of minerals, fragments of other rocks, or organic matter. There are three main types of sedimentary rocks: clastic, chemical, and organic.
Clastic Sedimentary Rocks
Clastic sedimentary rocks are composed of fragments of pre-existing rocks. The fragments can be from igneous, metamorphic, or other sedimentary rocks. The size of the fragments determines the name of the sedimentary rock.
Shale is made of clay and silt sized particles. The small size allows the particles to compact together to form dense, fine-grained shale. Shale layers can contain fossils since the fine particles allow good preservation of bones and shells.
Sandstone is composed of sand sized grains between 1/16 to 2 mm in diameter. Quartz is the dominant mineral. The size and composition of the grains and the type of cementing material classifies the type of sandstone such as quartz arenite (quartz grains) or arkose (feldspar grains). Pores between the sand grains allow for good permeability of fluids.
Conglomerate rocks contain large pebbles and sometimes boulders greater than 2 mm in diameter in a matrix of finer grained sediments. The sediment clasts are round from being transported by water. Conglomerate beds form in high energy river environments.
Chemical Sedimentary Rocks
Chemical sedimentary rocks form by precipitation of minerals from aqueous solution. Evaporation concentrates dissolved minerals to the point they crystallize out.
Rock salt forms from evaporation of seawater. It consists primarily of the mineral halite or NaCl. Large thick salt beds indicate past hot arid environments.
Iron oxides can accumulate in layers when iron rich waters evaporate and deposit hematite or magnetite. Red beds get their color from hematite coated grains. Or iron can be transported as colloids and precipitate out forming ironstone concretions.
Organic Sedimentary Rocks
Organic sedimentary rocks form from the accumulation of plant or animal debris.
Coal beds start as accumulations of plant matter, usually in swamp environments. With burial and time, the plant matter transforms into peat and eventually into coal. Coal grades into the harder anthracite as the coalification process proceeds.
Black shales contain abundant organic matter mixed with clay and silt sized particles. The organic matter gives them a dark color. The fine grains allow good preservation of the organic matter. Black shales often develop in deep marine basins where organic particles accumulate.
Fossils shells and skeletons can accumulate as carbonate rocks called coquina. The limestone rock made of shell fragments can form beach deposits or bioherms around reefs. Chalk beds are soft and fine-grained limestones composed of planktonic microfossils.
The diversity of sedimentary rocks provides evidence for past depositional environments and climates. Careful observation of the rock types, textures, and fossils provide clues to the geologic history.
