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Sedimentology encompasses the study of modern sediments such as sand ^[1], mud (silt),^[2] and clay,^[3] and understanding the processes that deposit them.^[4] It also compares these observations to studies of ancient sedimentary rocks^[5] and sedimentary structures. 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 aeolian (wind) deposition^{[citation needed]}.

Sedimentary rock types

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

Importance of sedimentary rocks

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

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; 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.

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.

Methodology of sedimentology

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

Recent developments in sedimentology

The longstanding understanding of how some mudstones form has been challenged by geologists at Indiana University (Bloomington) and the Massachusetts Institute of Technology. The research, which appears in the December 14th, 2007, edition of Science, 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."^[6]

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."^[7]

References

^ Raymond Siever, Sand, Scientific American Library, New York (1988), ISBN 0 7167 5021 X.
^ P.E. Potter, J.B. Maynard, and P.J. Depetris, Mud and Mudstones: Introduction and Overview Springer, Berlin (2005) ISBN 3 5402 2157 3.
^ Georges Millot, translated [from the French] by W.R. Farrand, Helene Paquet, Geology Of Clays - Weathering, Sedimentology, Geochemistry Springer Verlag, Berlin (1970), ISBN 0 4121 0050 9.
^ Gary Nichols, Sedimentology & Stratigraphy, Wiley-Blackwell, Malden, MA (1999), ISBN 0 6320 3578 1.
^ Donald R. Prothero and Fred Schwab, Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy, W. H. Freeman (1996), ISBN 0 7167 2726 9.
^ Juergen Schieber, John Southard, and Kevin Thaisen, "Accretion of Mudstone Beds from Migrating Floccule Ripples," Science, 14 December 2007: 1760-1763.
See also "As waters clear, scientists seek to end a muddy debate," at PhysOrg.com (accessed 27 December 2007).
^ Joe H. S. Macquaker and Kevin M. Bohacs, "Geology: On the Accumulation of Mud," Science, 14 December 2007: 1734-1735.

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