Sedimentology

Sedimentology encompasses the study of modern sediments and understanding the processes that deposit them. It also compares these observations to studies of ancient sedimentary rocks. Sedimentologists apply their understanding of modern processes to historically formed sedimentary rocks, allowing them to understand how they formed.

Sedimentary rocks cover most of the Earth's surface, record much of the Earth's history, and harbor the fossil record. Sedimentology is closely linked to stratigraphy, the study of the physical and temporal relationships between rock layers or strata.

Uniformitarian geology, the premise 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 - for example, sand dunes in the Sahara or the Great Sand Dunes National Park near Alamosa, Colorado - to the ancient sandstones such as the Wingate Sandstone of Utah and Arizona, of the southwest USA; since both have the same features, both can be shown to have formed from eolian (wind) deposition^{[citation needed]}.

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Sedimentary rock types

There are four primary types of sedimentary rocks: clastics, carbonates, evaporites, and chemical.

• Clastic rocks are composed of particles derived from the weathering and erosion of precursor rocks and consist primarily of fragmental material. Clastic rocks are classified according to their predominant grain size and their composition. In the past, the term "Clastic Sedimentary Rocks" were used to describe silica-rich clastic sedimentary rocks, however there have been cases of clastic carbonate rocks. The more appropriate term is siliciclastic sedimentary rocks.
  • Organic sedimentary rocks are important deposits formed from the accumulation of biological detritus, and form coal and oil shale deposits, and are typically found within basins of clastic sedimentary rocks
• Carbonates are composed of various carbonate minerals (most often calcium carbonate (CaCO₃)) precipitated by a variety of organic and inorganic processes. Typically, the majority of carbonate rocks are composed of reef material^{[citation needed]}.
• Evaporites are formed through the evaporation of water at the Earth's surface and are composed of one or more salt minerals, such as halite or gypsum^{[citation needed]}.
• Chemical sedimentary rocks, including some carbonates, are deposited by precipitation of minerals from aqueous solution. These include jaspilite and chert.

Importance of sedimentary rocks

Sedimentary rocks provide a multitude of products which modern and ancient society has come to utilise.

• Art: marble, although a metamorphosed limestone, is an example of the use of sedimentary rocks in the pursuit of aesthetics and art
• Architectural uses: stone derived from sedimentary rocks is used for dimension stone and in architecture, notably slate, a meta-shale, for roofing, sandstone for load-bearing buttresses
• Ceramics and industrial materials: clay for pottery and ceramics including bricks; cement and lime derived from limestone.
• Economic geology: sedimentary rocks host large deposits of SEDEX ore deposits of lead-zinc-silver, large deposits of copper, deposits of gold, tungsten and many other precious minerals, gemstones and industrial minerals including heavy mineral sands ore deposits
• Energy: petroleum geology relies on the capacity of sedimentary rocks to generate deposits of petroleum oils. Coal and oil shale are found in sedimentary rocks. A large proportion of the world's uranium energy resources are hosted within sedimentary successions.
• Groundwater: sedimentary rocks contain a large proportion of the Earth's groundwater aquifers. Our understanding of the extent of these aquifers and how much water can be withdrawn from them depends critically on our knowledge of the rocks that hold them (the reservoir).

Basic principles

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.

The scientific basis of this is the principle of uniformitarianism, 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.

Sedimentological conditions are recorded within the sediments as they are laid down; and in reference to Salvador Dalí's Persistence of memory, 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 after diagenesis are available for study.

However, sedimentological study produces interpretations of past depositional and environmental conditions and care must be taken in analysing sedimentary rocks in a scientific manner in order to gain a picture of the events which occurred within the past.

The principle of superposition is critical to the interpretation of sedimentary sequences, and in older metamorphic terrains or fold and thrust belts where sediments are often intensely folded or deformed, recognising younging indicators or fining up sequences is critical to interpretation of the sedimentary section and often the deformation and metamorphic structure of the region.

Folding in sediments is analysed with the principle 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.

The principle of lateral continuity states that layers of sediment initially extend laterally in all directions unless obstructed by a physical object or topography.

The principle of cross-cutting relationships states that whatever cuts across or intrudes into the layers of strata is younger then the layers of strata.

Methodology of sedimentology

The methods employed by sedimentologists to gather data and evidence on the nature and depositional conditions of sedimentary rocks include;

• Measuring and describing the outcrop and distribution of the rock unit;
  • Describing the rock formation, a formal process of documeenting thickness, lithology, outcrop, distribution, contact relationships to other formations
  • Mapping the distribution of the rock unit, or units
• Descriptions of rock core (drilled and extracted from wells during hydrocarbon exploration)
• Sequence stratigraphy
  • Describes the progression of rock units within a basin
• Describing the lithology of the rock;
  • Petrology and petrography; particularly measurement of texture, grain size, grain shape (sphericity, rounding, etc), sorting and composition of the sediment
• Analysing the geochemistry of the rock
  • Isotope geochemistry, including use of radiometric dating, to determine the age of the rock, and its affinity to source regions

Recent developments in sedimentology

The longstanding understanding of how the mudstones and other fine grained sediments form is being challenged as being in error, according to research by geologists at Indiana University (Bloomington) and the Massachusetts Institute of Technology. The research, which appears in the December 14th edition of Science, counters the prevailing view of geologists that mud only settles when water is placid, instead showing that "muds will accumulate even when currents move swiftly."

What is most interesting for students of the geological and fossil record is how this research potentially overturns the previous view on mudstone deposition, erosion, and re-deposition, as the press release outlines:

"The finding feels like something of a vindication, Schieber says. He and his colleagues have (genially) argued about whether muds could deposit from rapidly flowing water. Schieber had posited the possibility after noting an apparent oddity in the sedimentary rock record."^[1]

References

See also

• Important publications in sedimentology
• Sequence stratigraphy
• Rock formation
• Coal
• Oil shale
• Ore genesis
• Clastic rocks