Clastics

Clastics, in geology, refers to clastic sedimentary rocks or sediments formed from clasts—fragments of preexisting rocks or minerals that have been produced by weathering, transported, deposited, and lithified. They are contrasted with chemical sedimentary rocks, formed by precipitation from solution, and organic (biochemical) rocks, formed from accumulated biological matter.

The essential feature of clastics is the presence of discrete fragments. Clasts range in size from boulders and cobbles to pebbles, sand, silt, and clay. They may be angular or well-rounded, and their degree of sorting (uniformity of clast size) and maturity (transport history) reflects the energy and distance of transport. The matrix, when present, is finer material that fills the spaces between larger clasts and contributes to the rock’s texture.

Formation involves several stages: weathering to create clasts, erosion and transport to move them, deposition of the fragments upon available surfaces, and diagenesis that leads to lithification through compaction and cementation. Cementing minerals commonly include silica, calcite, and iron oxides, which bind clasts together to form solid rock.

Clastic rocks are commonly classified by clast size and rock texture. Gravel-sized clastics include conglomerate (rounded clasts) and breccia (angular clasts). Sand-sized clastics form sandstone, which can show varying degrees of sorting and cementation (examples include quartz arenite, arkose, and lithic sandstone). Finer-grained clastics include siltstone and mudstone, which includes shale fossils in many settings.

Depositional environments for clastics are diverse, ranging from high-energy rivers and alluvial fans to deserts (aeolian sands), lakes, deltas, and shallow marine settings, with deep-water turbidites representing more distal deposits. Clastics are economically important as groundwater and hydrocarbon reservoirs and are key records of past environments and geologic history.