Salt tectonics is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This is due both to the low density of salt, which does not increase with burial, its low strength. Salt structures have been found in more than 120 sedimentary basins across the world. Structures may form during continued sedimentary loading, without any external tectonic influence, due to gravitational instability. Pure halite has a density of 2160 kg/m3; when deposited, sediments have a lower density of 2000 kg/m³, but with loading and compaction their density increases to 2500 kg/m³, greater than that of salt. Once the overlying layers have become denser, the weak salt layer will tend to deform into a characteristic series of ridges and depressions, due to a form of Rayleigh–Taylor instability. Further sedimentation will be concentrated in the depressions and the salt will continue to move away from them into the ridges.
At a late stage, diapirs tend to initiate at the junctions between ridges, their growth fed by movement of salt along the ridge system, continuing until the salt supply is exhausted. During the stages of this process the top of the diapir remains at or near the surface, with further burial being matched by diapir rise, is sometimes referred to as downbuilding; the Schacht Asse II and Gorleben salt domes in Germany are an example of a purely passive salt structure. Such structures do not always form; this can be due to a high strength overburden or to the presence of sedimentary layers interbedded within the salt unit that increase both its density and strength. Active tectonics will increase the likelihood of salt structures developing. In the case of extensional tectonics, faulting will both reduce the strength of the overburden and thin it. In an area affected by thrust tectonics, buckling of the overburden layer will allow the salt to rise into the cores of anticlines, as seen in salt domes in the Zagros Mountains.
If the pressure within the salt body becomes sufficiently high it may be able to push through its overburden, this is known as forceful diapirism. Many salt diapirs may contain elements of both passive salt movement. An active salt structure may pierce its overburden and from on continue to develop as a purely passive salt diapir. In those cases where salt layers do not have the conditions necessary to develop passive salt structures, the salt may still move into low pressure areas around developing folds and faults; such structures are described as reactive. When one or more salt layers are present during extensional tectonics, a characteristic set of structures are formed. Extensional faults propagate up from the middle part of the crust until they encounter the salt layer; the weakness of the salt prevents the fault from propagating through. However, continuing displacement on the fault offsets the base of the salt and causes bending of the overburden layer; the stresses caused by this bending will be sufficient to fault the overburden.
The types of structures developed depend on the initial salt thickness. In the case of a thick salt layer there is no direct spatial relationship between the faulting beneath the salt and that in the overburden, such a system is said to be unlinked. For intermediate salt thicknesses, the overburden faults are spatially related to the deeper faults, but offset from them into the footwall; when the salt layer becomes thin enough, the fault that develops in the overburden is aligned with that beneath the salt, forms a continuous fault surface after only a small displacement, forming a hard-linked fault. In areas of thrust tectonics salt layers act as preferred detachment planes. In the Zagros fold and thrust belt, variations in the thickness and therefore effectiveness of the late Neoproterozoic to Early Cambrian Hormuz salt are thought to have had a fundamental control on the overall topography; when a salt layer becomes too thin to be an effective detachment layer, due to salt movement, dissolution or removal by faulting, the overburden and the underlying sub-salt basement become welded together.
This may cause the development of new faults in the cover sequence and is an important consideration when modeling the migration of hydrocarbons. Salt welds may develop in the vertical direction by putting the sides of a former diapir in contact. Salt that pierces to the surface, either on land or beneath the sea, tends to spread laterally away and such salt is said to be "allochthonous". Salt glaciers are formed on land where this happens in an arid environment, such as in the Zagros Mountains. Offshore tongues of salt are generated that may join together with others from neighbouring piercements to form canopies. On passive margins where salt is present, such as the Gulf of Mexico, salt tectonics control the evolution of deep-water sedimentary systems. A significant proportion of the world’s hydrocarbon reserves are found in structures related to salt tectonics, including many in the Middle East, the South Atlantic passive margins and the Gulf of Mexico. Plasticity – The deformation of a solid material undergoing non-reversible changes of shape in response to applied forces Gorleben salt dome NOAA site on brine pools Salt Tectonics Publications
The Cenozoic Era meaning "new life", is the current and most recent of the three Phanerozoic geological eras, following the Mesozoic Era and extending from 66 million years ago to the present day. The Cenozoic is known as the Age of Mammals, because the extinction of many groups allowed mammals to diversify so that large mammals dominated it; the continents moved into their current positions during this era. Early in the Cenozoic, following the K-Pg extinction event, most of the fauna was small, included small mammals, birds and amphibians. From a geological perspective, it did not take long for mammals and birds to diversify in the absence of the large reptiles that had dominated during the Mesozoic. A group of avians known as the "terror birds" grew larger than the average human and were formidable predators. Mammals came to occupy every available niche, some grew large, attaining sizes not seen in most of today's mammals; the Earth's climate had begun a drying and cooling trend, culminating in the glaciations of the Pleistocene Epoch, offset by the Paleocene-Eocene Thermal Maximum.
Cenozoic, meaning "new life," is derived from Greek καινός kainós "new," and ζωή zōḗ "life." The era is known as the Cænozoic, Caenozoic, or Cainozoic. The name "Cenozoic" was proposed in 1840 by the British geologist John Phillips; the Cenozoic is divided into three periods: the Paleogene and Quaternary. The Quaternary Period was recognized by the International Commission on Stratigraphy in June 2009, the former term, Tertiary Period, became disused in 2004 due to the need to divide the Cenozoic into periods more like those of the earlier Paleozoic and Mesozoic eras; the common use of epochs during the Cenozoic helps paleontologists better organize and group the many significant events that occurred during this comparatively short interval of time. Knowledge of this era is more detailed than any other era because of the young, well-preserved rocks associated with it; the Paleogene spans from the extinction of non-avian dinosaurs, 66 million years ago, to the dawn of the Neogene, 23.03 million years ago.
It features three epochs: the Paleocene and Oligocene. The Paleocene epoch lasted from 66 million to 56 million years ago. Modern placental mammals originated during this time; the Paleocene is a transitional point between the devastation, the K-T extinction, to the rich jungle environment, the Early Eocene. The Early Paleocene saw the recovery of the earth; the continents began to take their modern shape, but all the continents and the subcontinent of India were separated from each other. Afro-Eurasia was separated by the Tethys Sea, the Americas were separated by the strait of Panama, as the isthmus had not yet formed; this epoch featured a general warming trend, with jungles reaching the poles. The oceans were dominated by sharks. Archaic mammals filled the world such as creodonts; the Eocene Epoch ranged from 56 million years to 33.9 million years ago. In the Early-Eocene, species living in dense forest were unable to evolve into larger forms, as in the Paleocene. There was nothing over the weight of 10 kilograms.
Among them were early primates and horses along with many other early forms of mammals. At the top of the food chains were huge birds, such as Paracrax; the temperature was 30 degrees Celsius with little temperature gradient from pole to pole. In the Mid-Eocene, the Circumpolar-Antarctic current between Australia and Antarctica formed; this disrupted ocean currents worldwide and as a result caused a global cooling effect, shrinking the jungles. This allowed mammals to grow to mammoth proportions, such as whales which, by that time, had become fully aquatic. Mammals like Andrewsarchus were at the top of the food-chain; the Late Eocene saw the rebirth of seasons, which caused the expansion of savanna-like areas, along with the evolution of grass. The end of the Eocene was marked by the Eocene-Oligocene extinction event, the European face of, known as the Grande Coupure; the Oligocene Epoch spans from 33.9 million to 23.03 million years ago. The Oligocene featured the expansion of grass which had led to many new species to evolve, including the first elephants, dogs and many other species still prevalent today.
Many other species of plants evolved in this period too. A cooling period featuring seasonal rains was still in effect. Mammals still continued to grow larger; the Neogene spans from 23.03 million to 2.58 million years ago. It features 2 epochs: the Miocene, the Pliocene; the Miocene epoch spans from 23.03 to 5.333 million years ago and is a period in which grass spread further, dominating a large portion of the world, at the expense of forests. Kelp forests evolved, encouraging the evolution such as sea otters. During this time, perissodactyla thrived, evolved into many different varieties. Apes evolved into 30 species; the Tethys Sea closed with the creation of the Arabian Peninsula, leaving only remnants as the Black, Red and Caspian Seas. This increased aridity. Many new plants evolved: 95% of modern seed plants evolved in the mid-Miocene; the Pliocene epoch lasted from 5.333 to 2.58 million years ago. The Pliocene featured dramatic climactic changes, which led to modern species and plants; the Mediterranean Sea dried up for several million years (because the ice ages reduced sea levels, disconnecting the Atlantic from
Cross-cutting relationships is a principle of geology that states that the geologic feature which cuts another is the younger of the two features. It is a relative dating technique in geology, it was first developed by Danish geological pioneer Nicholas Steno in Dissertationis prodromus and formulated by James Hutton in Theory of the Earth and embellished upon by Charles Lyell in Principles of Geology. There are several basic types of cross cutting relationships: Structural relationships may be faults or fractures cutting through an older rock. Intrusional relationships occur when an igneous dike is intruded into pre-existing rocks. Stratigraphic relationships may be an erosional surface cuts across older rock layers, geological structures, or other geological features. Sedimentological relationships occur where currents have eroded or scoured older sediment in a local area to produce, for example, a channel filled with sand. Paleontological relationships occur where plant growth produces truncation.
This happens, for example. Geomorphological relationships may occur where a surficial feature, such as a river, flows through a gap in a ridge of rock. In a similar example, an impact crater excavates into a subsurface layer of rock. Cross-cutting relationships may be compound in nature. For example, if a fault were truncated by an unconformity, that unconformity cut by a dike. Based upon such compound cross-cutting relationships it can be seen that the fault is older than the unconformity which in turn is older than the dike. Using such rationale, the sequence of geological events can be better understood. Cross-cutting relationships may be seen cartographically and microscopically. In other words, these relationships have various scales. A cartographic crosscutting relationship might look like, for example, a large fault dissecting the landscape on a large map. Megascopic cross-cutting relationships are features like igneous dikes, as mentioned above, which would be seen on an outcrop or in a limited geographic area.
Microscopic cross-cutting relationships are those that require study by magnification or other close scrutiny. For example, penetration of a fossil shell by the drilling action of a boring organism is an example of such a relationship. Cross-cutting relationships can be used in conjunction with radiometric age dating to effect an age bracket for geological materials that cannot be directly dated by radiometric techniques. For example, if a layer of sediment containing a fossil of interest is bounded on the top and bottom by unconformities, where the lower unconformity truncates dike A and the upper unconformity truncates dike B, this method can be used. A radiometric age date from crystals in dike A will give the maximum age date for the layer in question and crystals from dike B will give us the minimum age date; this provides range of possible ages, for the layer in question. Principle of faunal succession Principle of lateral continuity Principle of original horizontality Cross Cutting. World of Earth Science.
Ed. K. Lee Lerner and Brenda Wilmoth Lerner. Gale Cengage, 2003. Nicolai Stenonis solido intra solidum naturaliter contento dissertationis prodromus... Florentiae: ex typographia sub signo Stellae Hutton, James. Theory of the Earth, 1795 Lyell, Charles. Principles of Geology, 1830
Intrusive rock is formed when magma crystallizes and solidifies underground to form intrusions, for example plutons, dikes, sills and volcanic necks. Intrusive rock forms within Earth's crust from the crystallization of magma. Many mountain ranges, such as the Sierra Nevada in California, are formed from large granite intrusions. Intrusions are one of the two ways igneous rock. Technically an intrusion is any formation of intrusive igneous rock. In contrast, an extrusion consists of extrusive rock. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons. Plutonic rocks form 7% of the Earth's current land surface. Coarse-grained intrusive igneous rocks that form at depth within the earth are called abyssal while those that form near the surface are called subvolcanic or hypabyssal. Intrusive structures are classified according to whether or not they are parallel to the bedding planes or foliation of the country rock: if the intrusion is parallel the body is concordant, otherwise it is discordant.
An intrusive suite is a group of plutons related in time and space.. Intrusions vary from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite. Intrusions can be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes: Batholith: a large irregular discordant intrusion Chonolith: an irregularly-shaped intrusion with a demonstrable base Cupola: a dome-shaped projection from the top of a large subterranean intrusion Dike: a narrow tabular discordant body nearly vertical Laccolith: concordant body with flat base and convex top with a feeder pipe below Lopolith: concordant body with flat top and a shallow convex base, may have a feeder dike or pipe below Phacolith: a concordant lens-shaped pluton that occupies the crest of an anticline or trough of a syncline Volcanic pipe or volcanic neck: tubular vertical body that may have been a feeder vent for a volcano Sill: a thin tabular concordant body intruded along bedding planes Stock: a smaller irregular discordant intrusive Boss: a small stock A body of intrusive igneous rock which crystallizes from magma cooling underneath the surface of the Earth is called a pluton.
If the pluton is large, it may be called a stock. Intrusive rocks are characterized by large crystal sizes, as the individual crystals are visible, the rock is called phaneritic; this is as the magma cools underground, while cooling may be fast or slow, cooling is slower than on the surface, so larger crystals grow. If it runs parallel to rock layers, it is called a sill. If an intrusion makes rocks above rise to form a dome, it is called a laccolith. How deep-seated intrusions burst through the overlying strata causes intrusive rock to be recognized: Veins spread out into branches, or branchlike parts result from filled cracks, the high temperature is evident in how they alter country rock; as heat dissipation is slow, as the rock is under pressure, crystals form, no vitreous chilled matter is present. The intrusions did not flow. Contained gases could not escape through the thick strata, thus form cavities, which can be observed; because their crystals are of the rough equal size, these rocks are said to be equigranular.
There is no distinction between a first generation of large well-shaped crystals and a fine-grained ground-mass. The minerals of each have formed in a definite order, each has had a period of crystallization that may be distinct or may have coincided with or overlapped the period of formation of some of the other ingredients. Earlier crystals originated at a time when most of the rock was still liquid and are more or less perfect. Crystals are less regular in shape because they were compelled to occupy the spaces left between the already-formed crystals; the former case is said to be idiomorphic. There are many other characteristics that serve to distinguish the members of these two groups. For example, orthoclase is feldspar from granite, while its modifications occur in lavas of similar composition; the same distinction holds for nepheline varieties. Leucite is common in lavas but rare in plutonic rocks. Muscovite is confined to intrusions; these differences show the influence of the physical conditions under which consolidation takes place.
Intrusive rocks formed at greater depths are called abyssal. Some intrusive rocks solidified in fissures as dikes and intrusive sills at shallow depth and are called subvolcanic or hypabyssal, they show structures intermediate between those of plutonic rocks. They are commonly porphyritic and sometimes vesicular. In fact, many of them are petrologically indistinguishable from lavas of similar composition. Ellicott City Granodiorite Guilford Quartz Monzonite Methods of pluton emplacement Norbeck Intrusive Suite Volcanic rock Woodstock Quartz Monzonite
Diagenesis is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation, at temperatures and pressures less than that required for the formation of metamorphic rocks. It does not include changes from weathering, it is any chemical, physical, or biological change undergone by a sediment after its initial deposition, after its lithification. This process excludes surface metamorphism; these changes happen at low temperatures and pressures and result in changes to the rock's original mineralogy and texture. There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures. Hydrothermal solutions, meteoric groundwater, permeability and time are all influential factors. After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis.
Porosity decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization. The study of diagenesis in rocks is used to understand the geologic history they have undergone and the nature and type of fluids that have circulated through them. From a commercial standpoint, such studies aid in assessing the likelihood of finding various economically viable mineral and hydrocarbon deposits; the process of diagenesis is important in the decomposition of bone tissue. The term diagenesis meaning "across generation", is extensively used in geology. However, this term has filtered into the field of anthropology and paleontology to describe the changes and alterations that take place on skeletal material. Diagenesis "is the cumulative physical and biological environment. In order to assess the potential impact of diagenesis on archaeological or fossil bones, many factors need to be assessed, beginning with elemental and mineralogical composition of bone and enveloping soil, as well as the local burial environment.
The composite nature of bone, comprising one-third organic and two thirds mineral renders its diagenesis more complex. Alteration occurs at all scales from molecular loss and substitution, through crystallite reorganization and microstructural changes, in many cases, to disintegration of the complete unit. Three general pathways of the diagenesis of bone have been identified: chemical deterioration of the organic phase. Chemical deterioration of the mineral phase. Biological attack of the composite, they are as follows: The dissolution of collagen depends on time and environmental pH. At high temperatures, the rate of collagen loss will be accelerated and extreme pH can cause collagen swelling and accelerated hydrolysis. Due to the increase in porosity of bones through collagen loss, the bone becomes susceptible to hydrolytic infiltration where the hydroxyapatite, with its affinity for amino acids, permits charged species of endogenous and exogenous origin to take up residence; the hydrolytic activity plays a key role in the mineral phase transformations that exposes the collagen to accelerated chemical- and bio-degradation.
Chemical changes affect crystallinity. Mechanisms of chemical change, such as the uptake of F− or CO3− may cause recrystallization where hydroxyapatite is dissolved and re-precipitated allowing for the incorporation of substitution of exogenous material. Once an individual has been interred, microbial attack, the most common mechanism of bone deterioration, occurs rapidly. During this phase, most bone collagen is lost and porosity is increased; the dissolution of the mineral phase caused by low pH permits access to the collagen by extracellular microbial enzymes thus microbial attack. When animal or plant matter is buried during sedimentation, the constituent organic molecules break down due to the increase in temperature and pressure; this transformation occurs in the first few hundred meters of burial and results in the creation of two primary products: kerogens and bitumens. It is accepted that hydrocarbons are formed by the thermal alteration of these kerogens. In this way, given certain conditions kerogens will break down to form hydrocarbons through a chemical process known as cracking, or catagenesis.
A kinetic model based on experimental data can capture most of the essential transformation in diagenesis, a mathematical model in a compacting porous medium to model the dissolution-precipitation mechanism. These models have been intensively applied in real geological applications. Diagenesis has been divided, based on hydrocarbon and coal genesis into: eodiagenesis and telodiagenesis. During the early or eodiagenesis stage shales lose pore water, little to no hydrocarbons are formed and coal varies between lignite and sub-bituminous. During mesodiagenesis, dehydration of clay minerals occurs, the main development of oil genesis occurs and high to low volatile bituminous coals are formed. During telodiagenesis, organic matter undergoes cracking and dry gas is produced. Early diagenesis in newly formed aquatic sediments is mediated by microorganisms using different electron acceptors as part of their metabolism. O
Exfoliation joints or sheet joints are surface-parallel fracture systems in rock, leading to erosion of concentric slabs. (See Joint. Follow topography. Divide the rock into sub-planar slabs. Joint spacing increases with depth from a few centimeters near the surface to a few meters Maximum depth of observed occurrence is around 100 meters. Deeper joints have a larger radius of curvature, which tends to round the corners of the landscape as material is eroded Fracture mode is tensile Occur in many different lithologies and climate zones, not unique to glaciated landscapes. Host rock is sparsely jointed isotropic, has high compressive strength. Can have concave and convex upwards curvatures. Associated with secondary compressive forms such as arching, A-tents Despite their common occurrence in many different landscapes, geologists have yet to reach an agreement on a general theory of exfoliation joint formation. Many different theories have been suggested, below is a short overview of the most common.
This theory was proposed by the pioneering geomorphologist Grove Karl Gilbert in 1904. The basis of this theory is that erosion of overburden and exhumation of buried rock to the ground surface allows compressed rock to expand radially, creating tensile stress and fracturing the rock in layers parallel to the ground surface; the description of this mechanism has led to alternate terms for exfoliation joints, including pressure release or offloading joints. Though the logic of this theory is appealing, there are many inconsistencies with field and laboratory observations suggesting that it may be incomplete, such as: Exfoliation joints can be found in rocks that have never been buried. Laboratory studies show that simple compression and relaxation of rock samples under realistic conditions does not cause fracturing. Exfoliation joints are most found in regions of surface-parallel compressive stress, whereas this theory calls for them to occur in zones of extension. One possible extension of this theory to match with the compressive stress theory is as follows: The exhumation of buried rocks relieves vertical stress, but horizontal stresses can remain in a competent rock mass since the medium is laterally confined.
Horizontal stresses become aligned with the current ground surface as the vertical stress drops to zero at this boundary. Thus large surface-parallel compressive stresses can be generated through exhumation that may lead to tensile rock fracture as described below. Rock expands upon heating and contracts upon cooling and different rock-forming minerals have variable rates of thermal expansion / contraction. Daily rock surface temperature variations can be quite large, many have suggested that stresses created during heating cause the near-surface zone of rock to expand and detach in thin slabs. Large diurnal or fire-induced temperature fluctuations have been observed to create thin lamination and flaking at the surface of rocks, sometimes labeled exfoliation. However, since diurnal temperature fluctuations only reach a few centimeters depth in rock, this theory cannot account for the observed depth of exfoliation jointing that may reach 100 meters. Mineral weathering by penetrating water can cause flaking of thin shells of rock since the volume of some minerals increases upon hydration.
However, not all mineral hydration results in increased volume, while field observations of exfoliation joints show that the joint surfaces have not experienced significant chemical alteration, so this theory can be rejected as an explanation for the origin of large-scale, deeper exfoliation joints. Large compressive tectonic stresses parallel to the land surface can create tensile mode fractures in rock, where the direction of fracture propagation is parallel to the greatest principle compressive stress and the direction of fracture opening is perpendicular to the free surface; this type of fracturing has been observed in the laboratory since at least 1900. Tensile cracks can form in a compressive stress field due to the influence of pervasive microcracks in the rock lattice and extension of so-called wing cracks from near the tips of preferentially oriented microcracks, which curve and align with the direction of the principle compressive stress. Fractures formed in this way are sometimes called axial cleavage, longitudinal splitting, or extensional fractures, are observed in the laboratory during uniaxial compression tests.
High horizontal or surface-parallel compressive stress can result from regional tectonic or topographic stresses, or by erosion or excavation of overburden. With consideration of the field evidence and observations of occurrence, fracture mode, secondary forms, high surface-parallel compressive stresses and extensional fracturing seems to be the most plausible theory explaining the formation of exfoliation joints. Recognizing the presence of exfoliation joints can have important implications in geological engineering. Most notable may be their influence on slope stability. Exfoliation joints following the topography of inclined valley walls, bedrock hill slopes, cliffs can create rock blocks that are prone to sliding; when the toe of the slope is undercut, sliding along exfoliation joint planes is if the joint dip exceeds the joint’s frictional angle. Foundation work may be affected by the presence of exfoliation joints, for example in the case of dams. Exfoliation joints underlying a dam foundation can create a significant l
Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is flora. Flora and other forms of life such as fungi are collectively referred to as biota. Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess Shale fauna". Paleontologists sometimes refer to a sequence of faunal stages, a series of rocks all containing similar fossils; the study of animals of a particular region is called faunistics. Fauna comes from the name Fauna, a Roman goddess of earth and fertility, the Roman god Faunus, the related forest spirits called Fauns. All three words are cognates of the name of the Greek god Pan, panis is the Greek equivalent of fauna. Fauna is the word for a book that catalogues the animals in such a manner; the term was first used by Carl Linnaeus from Sweden in the title of his 1745 work Fauna Suecica. Cryofauna refers to the animals that live in, or close to, cold areas.
Cryptofauna are the fauna. Infauna are benthic organisms that live within the bottom substratum of a water body within the bottom-most oceanic sediments, rather than on its surface. Bacteria and microalgae may live in the interstices of bottom sediments. In general, infaunal animals become progressively smaller and less abundant with increasing water depth and distance from shore, whereas bacteria show more constancy in abundance, tending toward one million cells per milliliter of interstitial seawater. Epifauna called epibenthos, are aquatic animals that live on the bottom substratum as opposed to within it, that is, the benthic fauna that live on top of the sediment surface at the seafloor. Macrofauna are soil organisms which are retained on a 0.5 mm sieve. Studies in the deep sea define macrofauna as animals retained on a 0.3 mm sieve to account for the small size of many of the taxa. Megafauna are large animals of any particular time. For example, Australian megafauna. Meiofauna are small benthic invertebrates that live in both freshwater environments.
The term meiofauna loosely defines a group of organisms by their size, larger than microfauna but smaller than macrofauna, rather than a taxonomic grouping. One environment for meiofauna is between grains of damp sand. In practice these are metazoan animals that can pass unharmed through a 0.5 – 1 mm mesh but will be retained by a 30–45 μm mesh, but the exact dimensions will vary from researcher to researcher. Whether an organism passes through a 1 mm mesh depends upon whether it is alive or dead at the time of sorting. Mesofauna are macroscopic soil animals such as nematodes. Mesofauna are diverse. Microfauna are microscopic or small animals. Other terms include avifauna, which means "bird fauna" and piscifauna, which means "fish fauna". Linnaeus, Carolus. Fauna Suecica. 1746 "Biodiversity of Collembola and their functional role in the ecosystem"