Depositional environment

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A diagram of various depositional environments Main depositional environments.svg
A diagram of various depositional environments

In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases, the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available (e.g. banded iron formations).

Contents

Types of depositional environments

Diagram to show the different depositional environments in which tsunami deposits are formed - partly after Shanmugam 2006 Tsunami deposit environments.png
Diagram to show the different depositional environments in which tsunami deposits are formed – partly after Shanmugam 2006
Depositional environmental model of the Araripe Basin formations, NE Brazil Araripe Basin - depositional environments and sequence stratigraphy.jpg
Depositional environmental model of the Araripe Basin formations, NE Brazil

Continental [2] [3]

Transitional [4]

Marine [7]

Others

Recognition of depositional environments in ancient sediments

Depositional environments in ancient sediments are recognised using a combination of sedimentary facies, facies associations, sedimentary structures and fossils, particularly trace fossil assemblages, as they indicate the environment in which they lived.

Related Research Articles

Shale 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 and tiny fragments 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.

Sedimentary rock 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 the 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.

Sediment Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.

Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.

Deposition (geology) Geological process in which sediments, soil and rocks are added to a landform or landmass

Deposition is the geological process in which sediments, soil and rocks are added to a landform or landmass. Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.

Mudflat Coastal wetlands where sediments have been deposited by tides or rivers

Mudflats or mud flats, also known as tidal flats, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal flat ecosystems are as extensive globally as mangroves, covering at least 127,921km² of the Earth's surface. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries; they are also seen in freshwater lakes and salty lakes alike, wherein many rivers and creeks end. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of estuarine silts, clays and aquatic animal detritus. Most of the sediment within a mudflat is within the intertidal zone, and thus the flat is submerged and exposed approximately twice daily.

Conglomerate (geology) Coarse-grained clastic sedimentary rock with mainly rounded to subangular clasts

Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contain 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.

Turbidite Geologic deposit of a turbidity current

A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.

Phosphorite

Phosphorite,phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxide (P2O5). Marketed phosphate rock is enriched ("beneficiated") to at least 28%, often more than 30% P2O5. This occurs through washing, screening, de-liming, magnetic separation or flotation. By comparison, the average phosphorus content of sedimentary rocks is less than 0.2%. The phosphate is present as fluorapatite Ca5(PO4)3F typically in cryptocrystalline masses (grain sizes < 1 μm) referred to as collophane-sedimentary apatite deposits of uncertain origin. It is also present as hydroxyapatite Ca5(PO4)3OH or Ca10(PO4)6(OH)2, which is often dissolved from vertebrate bones and teeth, whereas fluorapatite can originate from hydrothermal veins. Other sources also include chemically dissolved phosphate minerals from igneous and metamorphic rocks. Phosphorite deposits often occur in extensive layers, which cumulatively cover tens of thousands of square kilometres of the Earth's crust.

Mudrock 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.

Clastic rock 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 with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha or extinct cnidaria were important reef builders.

Cross-bedding Sedimentary rock strata at differing angles.

In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original depositional layering is tilted, such tilting not being the result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers, which are known as cross-strata.

Ripple marks Wave structures created in sediments by bottom current

In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.

Cyclic sediments are sequences of sedimentary rocks that are characterised by repetitive patterns of different rock types (strata) or facies within the sequence. Processes that generate sedimentary cyclicity can be either autocyclic or allocyclic, and can result in piles of sedimentary cycles hundreds or even thousands of metres thick. The study of sequence stratigraphy was developed from controversies over the causes of cyclic sedimentation.

Ecca Group The second of the main subdivisions of the Karoo Supergroup of geological strata in southern Africa

The Ecca Group is the second of the main subdivisions of the Karoo Supergroup of geological strata in southern Africa. It mainly follows conformably after the Dwyka Group in some sections, but in some localities overlying unconformably over much older basement rocks. It underlies the Beaufort Group in all known outcrops and exposures. Based on stratigraphic position, lithostratigraphic correlation, palynological analyses, and other means of geological dating, the Ecca Group ranges between Early to earliest Middle Permian in age.

Bioclast

Bioclasts are skeletal fossil fragments of once living marine or land organisms that are found in sedimentary rocks laid down in a marine environment—especially limestone varieties around the globe. some of which take on distinct textures and coloration from their predominate bioclasts—that geologists, archaeologists and paleontologists use to date a rock strata to a particular geological era.

Sedimentary structures Geologic structures formed during sediment deposition

Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.

Shallow water marine environment

Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils.

Sorthat Formation

The Sorthat Formation is a geologic formation on the island on Bornholm, Denmark. It is of latest Pliensbachian-Lower Toarcian age. Plant fossils have been recovered from the formation, along with several traces of invertebrate animals. The Sorthat Formation is overlain by fluvial to lacustrine gravels, along with sands and clay along with parts covered by coal beds, that are part of the Aalenian-Bathonian Bagå Formation. In fact the Sorthat Formation was included on most of late SXX studies as the lowermost part of the Bagå Formation, recovering the latest Pliensbachian to lower Aalenian boundary, that was dismissed after comparing the geology of the two. The Sorthat strata recover a mostly marginal deltaic to marine unit, where large streams fluctuated to the east where a large river system was established at the start of the Toarcian. At the northwest the local Vulcanism, that started on the lower Pliensbachian, extended along the North Sea and mostly from southern Sweden. At this time, the Central Skåne Volcanic Province and the Egersund Basin expulsed most of his strata, with influences on the local tectonics. The Egersund Basin has abundant fresh prophyritic Nephelinite lavas and dykes of lower Jurassic Age, with a composition nearly equal to those found on the clay pits. That reveals the translation of strata from the Continental margin by large fluvial channels, that ended on the sea deposits of the Ciechocinek Formation Green series.

References

  1. Shanmugam G. (2006). "The Tsunamite Problem". Journal of Sedimentary Research. 6 (5): 718–730. Bibcode:2006JSedR..76..718S. doi:10.2110/jsr.2006.073.
  2. "Basics—Table of Depositional Environments". commons.wvc.edu. Retrieved 2020-01-13.
  3. "6.3 Depositional Environments and Sedimentary Basins – Physical Geology". opentextbc.ca. Retrieved 2020-01-13.
  4. "6.3 Depositional Environments and Sedimentary Basins – Physical Geology". opentextbc.ca. Retrieved 2020-01-13.
  5. "Basics—Table of Depositional Environments". commons.wvc.edu. Retrieved 2020-01-13.
  6. "Basics—Table of Depositional Environments". commons.wvc.edu. Retrieved 2020-01-13.
  7. "6.3 Depositional Environments and Sedimentary Basins – Physical Geology". opentextbc.ca. Retrieved 2020-01-13.
  8. "Basics—Table of Depositional Environments". commons.wvc.edu. Retrieved 2020-01-13.