Sediments are unconsolidated materials derived from the physical and chemical breakdown of pre-existing rock, or from the accumulation of biological or chemical matter. These particles are transported by water, wind, or ice before settling, compacting, and cementing to form sedimentary rock. Classification is foundational to geology, providing insight into the environmental conditions and processes during their formation. Sediments are categorized primarily based on two factors: the origin of the source material and the physical size of the individual particles.
Classification Based on Origin
The most fundamental way to classify sediments is by their source, which defines the material’s initial composition. This approach separates sediments into three main groups that reflect distinct geological processes.
Clastic, or detrital, sediments originate from the mechanical weathering and erosion of rock on land. These fragments are transported and deposited, making them the most abundant sediment type on Earth’s surface.
Chemical sediments form from materials that precipitate directly out of a water solution, such as seawater or lake water. This occurs when dissolved ions become oversaturated and bond together to form solid mineral crystals. Precipitation often happens due to changes in water temperature, pressure, or concentration.
Biogenic sediments are composed of the remains of living organisms, either microscopic or macroscopic. The hard parts of these organisms, such as shells and skeletons, accumulate after the organism dies. This links the sediment composition directly to the biological productivity of the environment where it formed.
Classification of Clastic Sediments
Clastic sediments are classified based on their texture, specifically the size of the individual grains. The standard for this classification is the Udden-Wentworth scale, which provides precise millimeter boundaries for each size category. Particle size is a direct indicator of the energy of the transporting medium, such as a river or wind current.
The coarsest category is gravel, which includes all particles larger than two millimeters in diameter. Sediment sizes within this range, such as pebbles, cobbles, and boulders, require high-energy environments like fast-flowing mountain streams or powerful ocean waves for transport. When lithified, this material forms conglomerate if the fragments are rounded, or breccia if the fragments remain sharp and angular.
Sand-sized particles range from two millimeters down to one-sixteenth of a millimeter, forming sandstone upon cementation. Sand is commonly transported by moderate-energy currents, accumulating in environments like beaches, deserts, and river channels. Sand is the smallest size generally visible to the unaided eye.
The finest categories are silt and clay, often grouped together as mud, and they are deposited only in low-energy conditions. Silt ranges from one-sixteenth of a millimeter down to 0.004 mm, forming siltstone when compressed. Clay particles are the smallest, measuring less than 0.004 millimeters, and their accumulation forms fine-grained rocks like mudstone or shale.
Classification of Chemical and Biogenic Sediments
Chemical and biogenic sediments are classified based on their mineralogy and composition, reflecting the specific chemical reactions or biological processes that created them.
Chemical Sediments
Chemical sediments, also known as hydrogenous sediments, include evaporites, which form when a body of water evaporates, leaving behind dissolved minerals. Examples include rock salt (halite) and gypsum, which are common evaporite minerals.
Other chemical sediments can precipitate from solution around hydrothermal vents or through the slow accumulation of minerals on the seafloor, such as manganese nodules. Classification is tied directly to the mineral that dominates the deposit, such as a silica-rich chert or an iron-rich formation. The formation of these deposits indicates specific chemical conditions, like high salinity or low oxygen, in the past environment.
Biogenic Sediments
Biogenic sediments are categorized by the composition of the remains of the organisms. Calcareous sediments are the most common, consisting largely of calcium carbonate (CaCO3) skeletons from organisms like foraminifera and coccolithophores. The deep-sea accumulation of these remains forms calcareous ooze, which eventually lithifies into limestone.
Siliceous biogenic sediments, such as siliceous ooze, are composed of opaline silica (SiO2) from organisms like diatoms and radiolarians. The presence of either calcareous or siliceous material provides clues about the temperature and depth of the ancient ocean, as the solubility of their skeletal materials varies with water chemistry. Organic sediments, such as peat and coal, are also classified biogenically based on their high content of carbon-rich, non-skeletal organic matter derived from plant life.