klastisk

Klastisk, in geology, is the Danish term for rocks and processes characterized by clasts—fragments of pre-existing rocks or minerals formed by weathering and erosion. The English equivalent is clastic. Clastic material is transported by water, wind, or ice, deposited, and lithified through diagenesis, which includes compaction and cementation.

Clasts vary in size from clay and silt to sand grains, pebbles, cobbles, and boulders. The texture—grain size, sorting, and rounding—records transport history and depositional environment. Classification commonly distinguishes siliciclastic rocks, made mostly of silicate minerals (notably quartz and feldspar), from lithic clastic rocks that contain substantial rock fragments.

Common clastic rocks include shale (finely grained and fissile), siltstone, sandstone (sand-sized grains), conglomerate (rounded clasts), and breccia (angular clasts). Subtypes such as arkose and quartz arenite reflect clast composition and diagenetic cementation.

Diagenesis cements these particles into rock and can alter mineralogy. Clastic rocks preserve clues to provenance and palaeoenvironments, such as rivers, deltas, beaches, deserts, and submarine fans. The study of clastic textures and compositions helps interpret source areas and sediment transport patterns.

Clastic rocks are contrasted with crystalline, chemical, and biogenic sedimentary rocks, which form by precipitation of minerals from solution or accumulation of biological material.