Sedimentology
Study of sediments and sedimentary rocks to interpret Earth's history.
Sedimentology is a subdiscipline of geology that studies modern sediments such as sand, silt, and clay, along with the processes of erosion, weathering, transport, deposition, and diagenesis that form them. By applying knowledge of modern processes to sedimentary rocks and structures, sedimentologists interpret geologic history. Sedimentary rocks cover up to 75% of the Earth's surface, record much of Earth's history, and contain the fossil record, making sedimentology closely linked to stratigraphy.
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Sedimentary rock types
Sedimentary rocks fall into four primary types: clastics, carbonates, evaporites, and chemical. Clastic rocks, composed of particles from weathering and erosion of precursor rocks, are classified by grain size and composition; the term siliciclastic sedimentary rocks is more appropriate than the older term clastic sedimentary rocks. Carbonates consist mainly of calcium carbonate precipitated by organic and inorganic processes. Evaporites form through surface water evaporation, commonly producing halite or gypsum. Chemical sedimentary rocks, including some carbonates, precipitate from aqueous solution, such as jaspilite and chert. Organic sedimentary rocks, like coal and oil shale, accumulate from biological detritus within clastic basins.
Importance of sedimentary rocks
Sedimentary rocks supply many products used by modern and ancient society. Marble, a metamorphosed limestone, serves artistic purposes. Dimension stone from sedimentary rocks appears in architecture, including slate for roofing and sandstone for load-bearing buttresses. Clay provides material for pottery, ceramics, and bricks; limestone yields cement and lime. Economically, sedimentary rocks host large SEDEX deposits of lead-zinc-silver, copper, gold, tungsten, uranium, and other minerals, gemstones, and industrial heavy mineral sands. Energy resources include petroleum, coal, oil shale, and much of the world's uranium. Groundwater aquifers are largely contained within sedimentary rocks, and understanding their extent depends on knowledge of the reservoir rocks.
Basic principles
Sedimentology aims to derive information on depositional conditions that formed a rock unit and to relate individual rock units within a basin to understand the evolution of sedimentary sequences and Earth's geological history. The principle of uniformitarianism holds that ancient sediments were deposited in the same way as sediments being deposited today. Sedimentological conditions are recorded in the sediments as they are laid down, and the present form of sediments reflects all past events from source material to post-diagenesis stresses. The principle of superposition is critical for interpreting sedimentary sequences; in deformed terrains, recognizing younging indicators or graded bedding is essential. The principle of original horizontality states that sediments are deposited at their angle of repose, essentially horizontal, allowing rocks to be unfolded when younging direction is known. The principle of lateral continuity holds that sediment layers initially extend laterally unless obstructed. The principle of cross-cutting relationships states that anything cutting across or intruding into strata is younger than the strata.
Recent developments
Research has challenged the longstanding view that mud only settles in slow-moving or still water, showing that muds accumulate even when currents move swiftly. This implies some mudstones may have formed under more energetic conditions, requiring reappraisal of many geologic records. These results call for critical reappraisal of all mudstones previously interpreted as continuously deposited under still waters, which are widely used to infer past climates, ocean conditions, and orbital variations. Recent research into mudstones has been driven by efforts to commercially produce hydrocarbons from them as unconventional reservoirs in shale gas and tight oil plays. Additionally, research has described how geocirculation relates to global temperatures and climate change, including carbon and water circulation and the impact of heat on the ocean's capacity for carbon capture.
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