Sedimentary rocks are classified by their genesis: how they formed. All sedimentary rocks are made from the components of other, pre-existing rocks, that have been broken down, transported, and reassembled into new rocks. The classification of the rock gives information about this history, telling others about its characteristics by its very name.
Rocks can be disintegrated into their chemical components, dissolved by runoff water, and eventually deposited into a still water body. If the chemicals are precipitated by biological means (either directly by the organism, or indirectly by the organism modifying the environment), the resulting crystalline materials are biogenic sedimentary rocks. If the chemicals are precipitated by physicochemical processes (not biologically driven processes), the resulting crystalline materials are chemical sedimentary rocks.
Alternately, rocks can be broken down into fragments (clasts), then transported as individual pieces by wind, water, ice, or gravity-driven processes (landslides!) until eventual deposition. Some combination of burial, compaction, cementation, or crystallization lithifies these fragments into new sedimentary rocks. If the source rock is decomposed into clasts by weathering, the result is a siliciclastic rock. If the source rock is disintegrated into clasts by other processes, the resulting rock is classified by the process of clast production. For example, fragments produced by explosive igneous activity result in pyroclastic rocks, while those created by collapse or tectonism (cavern collapse!) are cataclastic rocks.
Each of these classifications is then subdivided into more categories related to the source material, transportation history, and composition.
Biogenic rocks are subdivided by the primary chemical components dissolved and precipitated. The primary type are chert (a silicate rock), phosphate rocks, and carbonates (formed from calcium carbonate). Carbonates are usually classified by either the Folk or Dunham classification schemes based on allochem and orthochem composition and texture. Allochem are pre-existing components of any origin that are held together by the orthochem cement (sparite) or crystallized matrix (micrite).
Chemical rocks are subdivided by the means of precipitation. Layered evaporites are minerals precipitated from solution during evaporation, forming a distinct mineralogical sequence dependent on the precipitation order.
Classification is imperfect. For example, banded iron formations are distinctly layered rocks formed when iron was precipitating out of the oceans. This precipitation was triggered by changes in ocean oxidation, as oxygen was a waste product of stromatolites. In this sequence of events, is a banded iron formation a biogenic rock (as iron was precipitated due to environmental changes caused by a living organism), or is the causation so indirectly tied to biological processes that the final precipitate is better classified as a chemical rock?
Siliciclastic rocks are initially subdivided by clast size. As larger fragments require more energy to transport than smaller fragments, this instantaneously gives information about rocks that were deposited in high or low energy environments, or were transported short or long distances from the source material. Subdivisions within these classifications often touch on the composition of the rocks, giving hints to the source material and depositional environments that modified the original components.
Rocks dominated by dominated by fragments greater than 2 millimeters (>2mm) are immediately subdivided by clast geometry. If the fragments are primarily angular, it is a breccia. If the fragments are any other geometry (subangular, subrounded, or rounded), it is a conglomerate. Within each of these categories, rocks are then classified by composition, transportation distance, and inferred origin. Looking at the clasts that compose the rocks, oligomictic rocks are composed of fragments from only a few varieties of rocks while polymictic rocks are composed of fragments from a great variety of rocks. If a rock is composed of clasts mostly from outside the depositional basin, it is extraformational, while if the clasts originate from inside the depositional basin, it is intraformational. Finally, context and textures within the rock may give clues as to the potential origin of the rock. If so, it can be classified as a tillite, the produce of melting glaciers, as a flow deposit, where fragments were transported by landslide, or any of a variety of other special names.
Rocks composed of sand-sized grains, less than 2 millimeters but greater than 1/16th of a millimeter (1/16-2mm), are unsurprisingly sandstones. These are subdivided by the percentage of the rock that is fine-grained matrix versus larger-grained framework, and by the mineral composition of the larger grains. If the matrix is less than or equal to 15% of the total rock (≤15%), it is an arenite. If the matrix is greater than 15% (>15%), it is a wacke.1 The mineral composition is broken down by percentage of quartz, feldspar, or lithic (intact rock fragment) materials, where the boundaries between rock names varies dependent on if it is an arenite or a wacke.
In the realm of very tiny grains, less than 1/16th of a millimeter (<1/16mm), are mudrocks. The percentage of clay-sized components determine the base name. On this scale, it’s more practically feasible to categorize by effective texture: if the rock feels gritty (up to a third clay, <33%), it is siltstone; if it feels loamy (one to two-thirds clay, 33-66%), it is mudstone; and if it feels slick or slimy (more than two-thirds clay, >66%), it is claystone. These classifications are enhanced by naming true colour (not surface staining), which can be indicative of both mineral composition and formation environment. Additional name classifications require laboratory analysis, adding on chemical composition, detrital mineralogy, or lithology of associated sedimentary rocks.
- This 15% matrix cut-off can also be applied to breccias and conglomerates. An intact, grain-supported framework with ≤15% matrix, where the grains are in direct contact, are orthoconglomerates. Unstable, non-intact matrix-support frameworks with >15% matrix (usually >50% matrix), effectively sandstones or mudstones with floating grains, are paraconglomerates. [↩]