Sedimentology encompasses the study of modernsediments such assand,[1]silt, andclay,[2] and the processes that result in their formation (erosion andweathering),transport,deposition anddiagenesis.[3] Sedimentologists apply their understanding of modern processes to interpret geologic history through observations ofsedimentary rocks andsedimentary structures.[4]
Sedimentary rocks cover up to 75% of theEarth's surface, record much of theEarth's history, and harbor thefossil record. Sedimentology is closely linked tostratigraphy, the study of the physical and temporal relationships between rock layers orstrata.
Thepremise that the processes affecting the earth today are the same as in the past is the basis for determining how sedimentary features in the rock record were formed. By comparing similar features today to features in the rock record—for example, by comparing modernsand dunes to dunes preserved in ancientaeolian sandstones—geologists reconstruct past environments.
There are four primary types ofsedimentary rocks: clastics, carbonates, evaporites, and chemical.
Sedimentary rocks provide a multitude of products which modern and ancient society has come to utilise.
The aim of sedimentology, studying sediments, is to derive information on the depositional conditions which acted to deposit the rock unit, and the relation of the individual rock units in a basin into a coherent understanding of the evolution of the sedimentary sequences and basins, and thus, the Earth's geological history as a whole.[citation needed]
The scientific basis of this is the principle ofuniformitarianism, which states that the sediments within ancient sedimentary rocks were deposited in the same way as sediments which are being deposited at the Earth's surface today.[6]
Sedimentological conditions are recorded within the sediments as they are laid down; the form of the sediments at present reflects the events of the past and all events which affect the sediments, from the source of the sedimentary material to the stresses enacted upon them afterdiagenesis are available for study.[citation needed]
Theprinciple of superposition is critical to the interpretation of sedimentary sequences, and in older metamorphic terrains or fold and thrust belts where sediments are often intenselyfolded or deformed, recognisingyounging indicators orgraded bedding is critical to interpretation of the sedimentary section and often the deformation and metamorphic structure of the region.[citation needed]
Folding in sediments is analysed with theprinciple of original horizontality, which states that sediments are deposited at their angle of repose which, for most types of sediment, is essentially horizontal. Thus, when the younging direction is known, the rocks can be "unfolded" and interpreted according to the contained sedimentary information.
Theprinciple of lateral continuity states that layers of sediment initially extend laterally in all directions unless obstructed by a physical object or topography.
Theprinciple of cross-cutting relationships states that whatever cuts across or intrudes into the layers of strata is younger than the layers of strata.
The methods employed by sedimentologists to gather data and evidence on the nature and depositional conditions of sedimentary rocks include;
The longstanding understanding of how somemudstones form has been challenged by geologists atIndiana University (Bloomington) and theMassachusetts Institute of Technology. The research, which appears in the December 14, 2007, edition ofScience, counters the prevailing view of geologists that mud only settles when water is slow-moving or still, instead showing that "muds will accumulate even when currents move swiftly." The research shows that some mudstones may have formed in fast-moving waters: "Mudstones can be deposited under more energetic conditions than widely assumed, requiring a reappraisal of many geologic records."[7]
Macquaker and Bohacs, in reviewing the research of Schieber et al., state that "these results call for critical reappraisal of all mudstones previously interpreted as having been continuously deposited under still waters. Such rocks are widely used to infer past climates, ocean conditions, and orbital variations."[8]
Considerable recent research intomudstones has been driven by the recent effort to commercially produce hydrocarbons from them asunconventional reservoirs, in both theshale gas andtight oil (or Light Tight Oil) plays.[9]
Recent research by an Australian sedimentologist,Dutkiewicz, has described how geocirculation is related to global temperatures and climate change. The research described carbon and water circulation, and impacts of heat on current and future capacity of carbon capture by the ocean.[10]
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