Sediment: deposition, composition and geological significance

Composition and classification

Sediments are commonly classified by grain size and origin. Grain-size terms in general use include clay, silt, sand and gravel; these sizes largely control transport behavior and permeability. By origin, sediments are described as:

• Clastic: composed of fragments of preexisting rocks (for example, sand and mud).
• Chemical: formed by precipitation of minerals from solution (for example, evaporites and some carbonate deposits).
• Biogenic: made of accumulated remains of organisms (for example, shell beds, peat).

Transport and depositional environments

Transport mechanisms determine where sediment is deposited. Moving water (rivers, tides, waves), wind (aeolian systems), glaciers and gravity all carry particles of different sizes and deposit them when the transporting energy drops. Common depositional settings include river channels and floodplains, deltas, beaches and continental shelves, lakes, deserts and glacial tills. For discussions of wind and water transport and typical environments, consult resources on wind and water transport and depositional environments.

Sedimentary structures and records

Sediments often display structures formed during or shortly after deposition. Examples are bedding and lamination, cross-bedding in dunes, ripple marks in shallow water, and graded bedding from settling in turbidity currents. These features, together with fossil content and geochemical signals, make sediments important archives of past climates, sea-level change, and biological activity. The scientific study of these processes and records is called sedimentology.

Burial, diagenesis and rock formation

Once buried beneath younger layers, sediments are subject to compaction and chemical alteration. Mineral growth and cementation can bind grains together in a process called lithification, turning loose sediment into sedimentary rock. The combined physical and chemical changes occurring after deposition are described as diagenesis; more information on transformation to rock appears in many guides to sedimentary rock formation.

Economic, environmental and practical importance

Sediments are economically significant as aquifer host materials and as reservoirs for oil and gas. They supply construction aggregates (sand, gravel), fertile soils, and raw materials such as industrial minerals. Environmentally, sediment transport influences water quality, coastal stability and habitat distribution. Because sediments preserve records of past environments, they are key to archaeology, paleoclimatology and environmental reconstruction.

Study methods and everyday meaning

Fields methods include mapping, coring, and sampling; laboratory techniques include grain-size analysis, petrography, geochemistry and dating methods. In everyday usage, the word "sediment" can mean any material that settles to the bottom of a liquid. Simple demonstrations and explanations of particulate settling are common in teaching materials on sediment in liquids, and familiar examples include deposits seen in bottled drinks or natural waters and the particulate residue sometimes present in beer (see common examples). For further reading, introductory textbooks and field guides in sedimentology are recommended.