Sedimentology

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Sedimentology encompasses the study of modern sediments such as sand, [1] silt, and clay, [2] and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. [3] Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures. [4]

Contents

Sedimentary rocks cover up to 75% 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.

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 to features in the rock record—for example, by comparing modern sand dunes to dunes preserved in ancient aeolian sandstones—geologists reconstruct past environments.

Sedimentary rock types

Middle Triassic marginal marine sequence of siltstones and sandstones, southwestern Utah. Triassic Utah.JPG
Middle Triassic marginal marine sequence of siltstones and sandstones, southwestern Utah.

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

Importance of sedimentary rocks

Mi Vida uranium mine in redox mudstones near Moab, Utah UraniumMineUtah.JPG
Mi Vida uranium mine in redox mudstones near Moab, Utah

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

Basic principles

Heavy minerals (dark) deposited in a quartz beach sand (Chennai, India). HeavyMineralsBeachSand.jpg
Heavy minerals (dark) deposited in a quartz beach sand (Chennai, India).

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 graded bedding 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 than the layers of strata.

Methodology

Centripetal desiccation cracks (with a dinosaur footprint in the center) in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah. MudcracksCentripetal.JPG
Centripetal desiccation cracks (with a dinosaur footprint in the center) in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah.

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

Recent developments

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 14, 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]

Considerable recent research into mudstones has been driven by the recent effort to commercially produce hydrocarbons from them as unconventional reservoirs, in both the shale gas and tight oil (or Light Tight Oil) plays. [8]

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. [9]

See also

Related Research Articles

<span class="mw-page-title-main">Shale</span> Fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2Si2O5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Petroleum geology is the study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for hydrocarbons.

<span class="mw-page-title-main">Stratigraphy</span> Study of rock layers and their formation

Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostratigraphy, biostratigraphy, and chronostratigraphy.

<span class="mw-page-title-main">Concretion</span> Compact mass formed by precipitation of mineral cement between particles

A concretion is a hard, compact mass formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin concretio "(act of) compacting, condensing, congealing, uniting", itself from con meaning 'together' and crescere meaning "to grow". Concretions form within layers of sedimentary strata that have already been deposited. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.

<span class="mw-page-title-main">Lithology</span> Description of its physical characteristics of a rock unit

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.

<span class="mw-page-title-main">Conglomerate (geology)</span> Coarse-grained sedimentary rock composed mostly of rounded to sub-angular fragments

Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.

Sequence stratigraphy is a branch of geology, specifically a branch of stratigraphy, that attempts to discern and understand historic geology through time by subdividing and linking sedimentary deposits into unconformity bounded units on a variety of scales. The essence of the method is mapping of strata based on identification of surfaces which are assumed to represent time lines, thereby placing stratigraphy in chronostratigraphic framework allowing understanding of the evolution of the earth's surface in a particular region through time. Sequence stratigraphy is a useful alternative to a purely lithostratigraphic approach, which emphasizes solely based on the compositional similarity of the lithology of rock units rather than time significance. Unconformities are particularly important in understanding geologic history because they represent erosional surfaces where there is a clear gap in the record. Conversely within a sequence the geologic record should be relatively continuous and complete record that is genetically related.

<span class="mw-page-title-main">Mudstone</span> Fine grained sedimentary rock whose original constituents were clays or muds

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from shale by its lack of fissility.

<span class="mw-page-title-main">Organic-rich sedimentary rocks</span>

Organic-rich sedimentary rocks are a specific type of sedimentary rock that contains significant amounts (>3%) of organic carbon. The most common types include coal, lignite, oil shale, or black shale. The organic material may be disseminated throughout the rock giving it a uniform dark color, and/or it may be present as discrete occurrences of tar, bitumen, asphalt, petroleum, coal or carbonaceous material. Organic-rich sedimentary rocks may act as source rocks which generate hydrocarbons that accumulate in other sedimentary "reservoir" rocks. Potential source rocks are any type of sedimentary rock that the ability to dispel available carbon from within it. Good reservoir rocks are any sedimentary rock that has high pore-space availability. This allows the hydrocarbons to accumulate within the rock and be stored for long periods of time. Highly permeable reservoir rocks are also of interest to industry professionals, as they allow for the easy extraction of the hydrocarbons within. The hydrocarbon reservoir system is not complete however without a "cap rock". Cap rocks are rock units which have very low porosity and permeability, which trap the hydrocarbons within the units below as they try to migrate upwards.

<span class="mw-page-title-main">Mudrock</span> Class of fine grained siliciclastic sedimentary rocks

Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 116 mm and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature.

<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.

<span class="mw-page-title-main">Graded bedding</span> Type of layering in sediment or sedimentary rock

In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Such a bed is also described as fining upward. Normally graded beds generally represent depositional environments which decrease in transport energy as time passes, but these beds can also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits.

<span class="mw-page-title-main">Bed (geology)</span> Layer of sediment, sedimentary rock, or pyroclastic material

In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces". Specifically in sedimentology, a bed can be defined in one of two major ways. First, Campbell and Reineck and Singh use the term bed to refer to a thickness-independent layer comprising a coherent layer of sedimentary rock, sediment, or pyroclastic material bounded above and below by surfaces known as bedding planes. By this definition of bed, laminae are small beds that constitute the smallest (visible) layers of a hierarchical succession and often, but not always, internally comprise a bed.

<span class="mw-page-title-main">Belt Supergroup</span>

The Belt Supergroup is an assemblage of primarily fine-grained sedimentary rocks and mafic intrusive rocks of late Precambrian (Mesoproterozoic) age. It is more than 15 kilometres (10 mi) thick, covers an area of some 200,000 km2, and is considered to be one of the world's best-exposed and most accessible sequences of Mesoproterozoic rocks. It was named after the Big Belt Mountains in west-central Montana. It is present in western Montana and northern Idaho, with minor occurrences in northeastern Washington and western Wyoming. It extends into Canada where the equivalent rocks, which are called the Purcell Supergroup, are exposed in southeastern British Columbia and southwestern Alberta. The rocks of the Belt Supergroup contain economically significant deposits of lead, zinc, silver, copper, gold, and other metals in a number of areas, and some of the Belt rocks contain fossil stromatolites.

<span class="mw-page-title-main">Iron-rich sedimentary rocks</span> Sedimentary rocks containing 15 wt.% or more iron

Iron-rich sedimentary rocks are sedimentary rocks which contain 15 wt.% or more iron. However, most sedimentary rocks contain iron in varying degrees. The majority of these rocks were deposited during specific geologic time periods: The Precambrian, the early Paleozoic, and the middle to late Mesozoic. Overall, they make up a very small portion of the total sedimentary record.

The geology of Nunavut began to form nearly three billion years ago in the Archean and the territory preserves some of the world's oldest rock units.

The geology of Utah includes rocks formed at the edge of the proto-North American continent during the Precambrian. A shallow marine sedimentary environment covered the region for much of the Paleozoic and Mesozoic, followed by dryland conditions, volcanism and the formation of the basin and range terrain in the Cenozoic. Utah is a state in the western United States.

The geology of Thailand includes deep crystalline metamorphic basement rocks, overlain by extensive sandstone, limestone, turbidites and some volcanic rocks. The region experienced complicated tectonics during the Paleozoic, long-running shallow water conditions and then renewed uplift and erosion in the past several million years ago.

The geology of the Northwest Territories has been mapped in different quadrangles by the Canadian government. The region has some of the oldest rocks in the world and among the oldest in North America, formed from several sections of stable craton continental crust, including the Slave Craton, Rae Craton and Hearne Craton. These rocks form the Archean and Proterozoic Precambrian basement rock of the region and are the subject of extensive research to understand continental crust and tectonic conditions on the early Earth.

References

  1. Raymond Siever, Sand, Scientific American Library, New York (1988), ISBN   0-7167-5021-X.
  2. Georges Millot, translated [from the French] by W.R. Farrand, Helene Paquet, Geology Of Clays - Weathering, Sedimentology, Geochemistry Springer Verlag, Berlin (1970), ISBN   0-412-10050-9.
  3. Gary Nichols, Sedimentology & Stratigraphy, Wiley-Blackwell, Malden, MA (1999), ISBN   0-632-03578-1.
  4. Donald R. Prothero and Fred Schwab, Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy, W. H. Freeman (1996), ISBN   0-7167-2726-9.
  5. Edward J. Tarbuck, Frederick K. Lutgens, Cameron J. Tsujita, Earth, An Introduction to Physical Geology, National Library of Canada Cataloguing in Publication, 2005, ISBN   0-13-121724-0
  6. 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).
  7. Joe H. S. Macquaker and Kevin M. Bohacs, "Geology: On the Accumulation of Mud," Science, 14 December 2007: 1734-1735.
  8. Robert G. Loucks, Robert M. Reed, Stephen C. Ruppel, and Daniel M. Jarvie "Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale", Journal of Sedimentary Research, 2009, v. 79, 848-861.
  9. "Global warming speeds up currents in the ocean's abyss". Samachar Central. 2022-03-25. Retrieved 2022-04-16.