A Medley of Potpourri

Sunday, January 5, 2020

Ring species

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Ring_species

In a ring species, gene flow occurs between neighbouring populations of a species, but at the ends of the "ring" , the populations cannot interbreed.

The coloured bars show natural populations (colours), varying along a cline. Such variation may occur in a line (e.g. up a mountain slope) as in A, or may wrap around as in B. Where the cline bends around, populations next to each other on the cline can interbreed, but at the point that the beginning meets the end again, as at C, the differences along the cline prevent interbreeding (gap between pink and green). The interbreeding populations are then called a ring species.

In biology, a ring species is a connected series of neighbouring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region (sympatry) thus closing a "ring". The German term Rassenkreis, meaning a ring of populations, is also used.

Ring species represent speciation and have been cited as evidence of evolution. They illustrate what happens over time as populations genetically diverge, specifically because they represent, in living populations, what normally happens over time between long-deceased ancestor populations and living populations, in which the intermediates have become extinct. The evolutionary biologist Richard Dawkins remarks that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension".

Formally, the issue is that interfertility (ability to interbreed) is not a transitive relation; if A can breed with B, and B can breed with C, it does not mean that A can breed with C, and therefore does not define an equivalence relation. A ring species is a species with a counterexample to the transitivity of interbreeding. However, it is unclear whether any of the examples of ring species cited by scientists actually permit gene flow from end to end, with many being debated and contested.

History

The Larus gulls interbreed in a ring around the arctic. 1: L. fuscus, 2: Siberian population of L. fuscus, 3: L. heuglini, 4: L. vegae birulai, 5: L. vegae, 6: L. smithsonianus, 7: L. argentatus
Herring gull (Larus argentatus) (front) and lesser black-backed gull (Larus fuscus) (behind) in Norway: two phenotypes with clear differences

The classic ring species is the Larus gull. In 1925 Jonathan Dwight found the genus to form a chain of varieties around the Arctic Circle. However, doubts have arisen as to whether this represents an actual ring species. In 1938, Claud Buchanan Ticehurst argued that the greenish warbler had spread from Nepal around the Tibetan Plateau, while adapting to each new environment, meeting again in Siberia where the ends no longer interbreed. These and other discoveries led Mayr to first formulate a theory on ring species in his 1942 study Systematics and the Origin of Species. Also in the 1940s, Robert C. Stebbins described the Ensatina salamanders around the Californian Central Valley as a ring species; but again, some authors such as Jerry Coyne consider this classification incorrect. Finally in 2012, the first example of a ring species in plants was found in a spurge, forming a ring around the Caribbean Sea.

Speciation

The biologist Ernst Mayr championed the concept of ring species, claiming that it unequivocally demonstrated the process of speciation. A ring species is an alternative model to allopatric speciation, "illustrating how new species can arise through 'circular overlap', without interruption of gene flow through intervening populations…" However, Jerry Coyne and H. Allen Orr point out that rings species more closely model parapatric speciation.

Ring species often attract the interests of evolutionary biologists, systematists, and researchers of speciation leading to both thought provoking ideas and confusion concerning their definition. Contemporary scholars recognize that examples in nature have proved rare due to various factors such as limitations in taxonomic delineation or, "taxonomic zeal"—explained by the fact that taxonomists classify organisms into "species", while ring species often cannot fit this definition. Other reasons such as gene flow interruption from "vicariate divergence" and fragmented populations due to climate instability have also been cited.

Ring species also present an interesting case of the species problem for those seeking to divide the living world into discrete species. All that distinguishes a ring species from two separate species is the existence of the connecting populations; if enough of the connecting populations within the ring perish to sever the breeding connection then the ring species' distal populations will be recognized as two distinct species. The problem is whether to quantify the whole ring as a single species (despite the fact that not all individuals can interbreed) or to classify each population as a distinct species (despite the fact that it can interbreed with its near neighbours). Ring species illustrate that species boundaries arise gradually and often exist on a continuum.

Examples

Ensatina salamanders example of ring species

Speculated evolution and spread of the greenish warbler.

P. t. trochiloides

P. t. obscuratus

P. t. plumbeitarsus

P. t. "ludlowi"

P. t. viridanus

Note: The P. t. nitidus in the Caucasus Mountains is not shown

Many examples have been documented in nature. Debate exists concerning much of the research, with some authors citing evidence against their existence entirely. The following examples provide evidence that—despite the limited number of concrete, idealized examples in nature—continuums of species do exist and can be found in biological systems. This is often characterized by sub-species level classifications such as clines, ecotypes, complexes, and varieties. Many examples have been disputed by researchers, and equally "many of the [proposed] cases have received very little attention from researchers, making it difficult to assess whether they display the characteristics of ideal ring species."

The following list gives examples of ring species found in nature. Some of the examples such as the Larus gull complex, the greenish warbler of Asia, and the Ensatina salamanders of America, have been disputed.

Acanthiza pusilla and A. ewingii
Acacia karroo
Alauda skylarks (Alauda arvensis, A. japonica and A. gulgula)
Alophoixus
Aulostomus (Trumpetfish)
Camarhynchus psittacula and C. pauper
Chaerephon pumilus species complex
Ensatina salamanders
Euphorbia tithymaloides is a group within the spurge family that has reproduced and evolved in a ring through Central America and the Caribbean, meeting in the Virgin Islands where they appear to be morphologically and ecologically distinct.
Great tit (however, some studies dispute this example)
The greenish warbler (Phylloscopus trochiloides) forms a species ring, around the Himalayas. It is thought to have spread from Nepal around the inhospitable Tibetan Plateau, to rejoin in Siberia, where the plumbeitarsus and the viridanus appeared to no longer mutually reproduce.
Hoplitis producta
House mouse
Junonia coenia and J. genoveva/J. evarete
Lalage leucopygialis, L. nigra, and L. sueurii
Larus gulls form a circumpolar "ring" around the North Pole. The European herring gull (L. argentatus argenteus), which lives primarily in Great Britain and Ireland, can hybridize with the American herring gull (L. smithsonianus), (living in North America), which can also hybridize with the Vega or East Siberian herring gull (L. vegae), the western subspecies of which, Birula's gull (L. vegae birulai), can hybridize with Heuglin's gull (L. heuglini), which in turn can hybridize with the Siberian lesser black-backed gull (L. fuscus). All four of these live across the north of Siberia. The last is the eastern representative of the lesser black-backed gulls back in north-western Europe, including Great Britain. The lesser black-backed gulls and herring gulls are sufficiently different that they do not normally hybridize; thus the group of gulls forms a continuum except where the two lineages meet in Europe. However, a 2004 genetic study entitled "The herring gull complex is not a ring species" has shown that this example is far more complicated than presented here (Liebers et al., 2004): this example only speaks to the complex of species from the classical herring gull through lesser black-backed gull. There are several other taxonomically unclear examples that belong in the same species complex, such as yellow-legged gull (L. michahellis), glaucous gull (L. hyperboreus), and Caspian gull (L. cachinnans).
Pelophylax nigromaculatus and P. porosus/P. porosus brevipodus (the names and classification of these species have changed since the publication suggesting a ring species)
Pernis ptilorhynchus and P. celebensis
Perognathus amplus and P. longimembris
Peromyscus maniculatus
Phellinus
Platycercus elegans (Crimson rosella) complex
Drosophila paulistorum
Phylloscopus collybita and P. sindianus
Phylloscopus (Willow warblers)
Powelliphanta
Rhymogona silvatica and R. cervina (the names and classification of these species have changed since the publication suggesting a ring species)
Melospiza melodia, a song sparrow, forms a ring around the Sierra Nevada of California with the subspecies heermanni and fallax meeting in the vicinity of the San Gorgonio Pass.
Todiramphus chloris and T. cinnamominus

Cline (biology)

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cline_(biology)

In biology, a cline (from the Greek “klinein”, meaning “to lean”) is a measurable gradient in a single character (or biological trait) of a species across its geographical range. First coined by Julian Huxley in 1938, the “character” of the cline referred to is usually genetic (e.g. allele frequency, blood type), or phenotypic (e.g. body size, skin pigmentation). Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next.

A cline refers to a spatial gradient in a specific, singular trait, rather than a gradient in a population as a whole. A single population can therefore theoretically have as many clines as it has traits. Additionally, Huxley recognised that these multiple independent clines may not act in concordance with each other. For example, it has been observed that in Australia, birds generally become smaller the further towards the north of the country they are found. In contrast, the intensity of their plumage colouration follows a different geographical trajectory, being most vibrant where humidity is highest and becoming less vibrant further into the arid centre of the country. Because of this, clines were defined by Huxley as being an “auxiliary taxonomic principle”; that is, clinal variation in a species is not awarded taxonomic recognition in the way subspecies or species are.

While the terms “ecotype” and “cline” are sometimes used interchangeably, they do in fact differ in that “ecotype” refers to a population which differs from other populations in a number of characters, rather than the single character that varies amongst populations in a cline.

Drivers and the evolution of clines

Two populations with individuals moving between the populations to demonstrate gene flow.

Clines are often cited to be the result of two opposing drivers: selection and gene flow (also known as migration). Selection causes adaptation to the local environment, resulting in different genotypes or phenotypes being favoured in different environments. This diversifying force is countered by gene flow, which has a homogenising effect on populations and prevents speciation through causing genetic admixture and blurring any distinct genetic boundaries.

Development of clines

Clines are generally thought to arise under one of two conditions: “primary differentiation” (also known as "primary contact" or "primary intergradation" ), or “secondary contact” (also known as "secondary introgression", or "secondary intergradation").

Primary differentiation

Primary differentiation is demonstrated using the peppered moth as an example, with a change in an environmental variable such as sooty coverage of trees imposing a selective pressure on a previously uniformly coloured moth population. This causes the frequency of melanic morphs to increase the more soot there is on vegetation.

Clines produced through this way are generated by spatial heterogeneity in environmental conditions. The mechanism of selection acting upon organisms is therefore external. Species ranges frequently span environmental gradients (e.g. humidity, rainfall, temperature, or day length) and, according to natural selection, different environments will favour different genotypes or phenotypes. In this way, when previously genetically or phenotypically uniform populations spread into novel environments, they will evolve to be uniquely adapted to the local environment, in the process potentially creating a gradient in a genotypic or phenotypic trait.

Such clines in characters can not be maintained through selection alone if lots of gene flow occurred between populations, as this would tend to swamp out the effects of local adaptation. However, because species usually tend to have a limited dispersal range (e.g. in an isolation by distance model), restricted gene flow can serve as a type of barrier which encourages geographic differentiation. However, some degree of migration is often required to maintain a cline; without it, speciation is likely to eventually occur, as local adaptation can cause reproductive isolation between populations.

A classic example of the role of environmental gradients in creating clines is that of the peppered moth, Biston betularia, in the UK. During the 19th century, when the industrial sector gained traction, coal emissions blackened vegetation across northwest England and parts of northern Wales. As a result of this, lighter morphs of the moth were more visible to predators against the blackened tree trunks and were therefore more heavily predated relative to the darker morphs. Consequently, the frequency of the more cryptic melanic morph of the peppered moth increased drastically in northern England. This cline in morph colour, from a dominance of lighter morphs in the west of England (which did not suffer as heavily from pollution), to the higher frequency of melanic forms in the north, has slowly been degrading since limitations to sooty emissions were introduced in the 1960s.

Secondary contact

Secondary contact between two previously isolated populations. Two previously isolated populations establish contact and therefore gene flow, creating an intermediate zone in the phenotypic or genotypic character between the two populations.

Clines generated through this mechanism have arisen through the joining of two formerly isolated populations which differentiated in allopatry, creating an intermediate zone. This secondary contact scenario may occur, for example, when climatic conditions change, allowing the ranges of populations to expand and meet. Because over time the effect of gene flow will tend to eventually swamp out any regional differences and cause one large homogenous population, for a stable cline to be maintained when two populations join there must usually be a selective pressure maintaining a degree of differentiation between the two populations.

The mechanism of selection maintaining the clines in this scenario is often intrinsic. This means that the fitness of individuals is independent of the external environment, and selection is instead dependent on the genome of the individual. Intrinsic, or endogenous, selection can give rise to clines in characters though a variety of mechanisms. One way it may act is through heterozygote disadvantage, in which intermediate genotypes have a lower relative fitness than either homozygote genotypes. Because of this disadvantage, one allele will tend to become fixed in a given population, such that populations will consist largely of either AA (homozygous dominant) or aa (homozygous recessive) individuals. The cline of heterozygotes that is created when these respective populations come into contact is then shaped by the opposing forces of selection and gene flow; even if selection against heterozygotes is great, if there is some degree of gene flow between the two populations, then a steep cline may be able to be maintained. Because instrinsic selection is independent of the external environment, clines generated by selection against hybrids are not fixed to any given geographical area and can move around the geographic landscape. Such hybrid zones where hybrids are a disadvantage relative to their parental lines (but which are nonetheless maintained through selection being counteracted by gene flow) are known as “tension zones”.

Another way in which selection can generate clines is through frequency-dependent selection. Characters that could be maintained by such frequency-dependent selective pressures include warning signals (aposematism). For example, aposematic signals in Heliconius butterflies sometimes display steep clines between populations, which are maintained through positive frequency dependence. This is because heterozygosity, mutations and recombination can all produce patterns that deviate from those well-established signals which mark prey as being unpalatable. These individuals are then predated more heavily relative to their counterparts with "normal" markings (i.e. selected against), creating populations dominated by a particular pattern of warning signal. As with heterozygote disadvantage, when these populations join, a narrow cline of intermediate individuals could be produced, maintained by gene flow counteracting selection.

Secondary contact could lead to a cline with a steep gradient if heterozygote disadvantage or frequency-dependent selection exists, as intermediates are heavily selected against. Alternatively, steep clines could exist because the populations have only recently established secondary contact, and the character in the original allopatric populations had a large degree of differentiation. As genetic admixture between the population increases with time however, the steepness of the cline is likely to decrease as the difference in character is eroded. However, if the character in the original allopatric populations was not very differentiated to begin with, the cline between the populations need not display a very steep gradient. Because both primary differentiation and secondary contact can therefore give rise to similar or identical clinal patterns (e.g. gently sloping clines), distinguishing which of these two processes is responsible for generating a cline is difficult and often impossible. However, in some circumstances a cline and a geographic variable (such as humidity) may be very tightly linked, with a change in one corresponding closely to a change in the other. In such cases it may be tentatively concluded that the cline is generated by primary differentiation and therefore moulded by environmental selective pressures.

No selection (drift/migration balance)

While selection can therefore clearly play a key role in creating clines, it is theoretically feasible that they might be generated by genetic drift alone. It is unlikely that large-scale clines in genotype or phenotype frequency will be produced solely by drift. However, across smaller geographical scales and in smaller populations, drift could produce temporary clines. The fact that drift is a weak force upholding the cline however means that clines produced this way are often random (i.e. uncorrelated with environmental variables) and subject to breakdown or reversal over time. Such clines are therefore unstable and sometimes called “transient clines”.

Clinal structure and terminology

Clinal characters change from one end of the geographic range to another. The extent of this change is reflected in the slope of the cline.

The steepness, or gradient, of a cline reflects the extent of the differentiation in the character across a geographic range. For example, a steep cline could indicate large variation in the colour of plumage between adjacent bird populations. It has been previously outlined that such steep clines may be the result of two previously allopatric populations with a large degree of difference in the trait having only recently established gene flow, or where there is strong selection against hybrids. However, it may also reflect a sudden environmental change or boundary. Examples of rapidly changing environmental boundaries like this include abrupt changes in the heavy metal content of soils, and the consequent narrow clines produced between populations of Agrostis that are either adapted to these soils with high metal content, or adapted to "normal" soil. Conversely, a shallow cline indicates little geographical variation in the character or trait across a given geographical distance. This may have arisen through weak differential environmental selective pressure, or where two populations established secondary contact a long time ago and gene flow has eroded the large character differentiation between the populations.

The gradient of a cline is related to another commonly referred to property, clinal width. A cline with a steep slope is said to have a small, or narrow, width, while shallower clines have larger widths.

Types of clines

Categories and subcategories of clines, as defined by Huxley.

According to Huxley, clines can be classified into two categories; continuous clines and discontinuous stepped clines. These types of clines characterise the way that a genetic or phenotypic trait transforms from one end of its geographical range of the species to the other.

Continuous clines

In continuous clines, all populations of the species are able to interbreed and there is gene flow throughout the entire range of the species. In this way, these clines are both biologically (no clear subgroups) and geographically (contiguous distribution) continuous. Continuous clines can be further sub-divided into smooth and stepped clines.

Continuous smooth clines are characterised by the lack of any abrupt changes or delineation in the genetic or phenotypic trait across the cline, instead displaying a smooth gradation throughout. Huxley recognised that this type of cline, with its uniform slope throughout, was unlikely to be common.
Continuous stepped clines consist of an overall shallow cline, interspersed by sections of much steeper slope. The shallow slope represents the populations, and the shorter, steeper sections the larger change in character between populations. Stepped clines can be further subdivided into horizontally stepped clines, and obliquely stepped clines.
- Horizontally stepped clines show no intra-population variation or gradation in the character, therefore displaying a horizontal gradient. These uniform populations are connected by steeper sections of the cline, characterised by larger changes in the form of the character. However, because in continuous clines all populations exchange genetic material, the intergradation zone between the groups can never have a vertical slope.
- In obliquely stepped clines, conversely, each population also demonstrates a cline in the character, albeit of a shallower slope than the clines connecting the populations together. Huxley compared obliquely stepped clines to looking like a “stepped ramp”, rather than taking on the formation of a staircase as in the case of horizontally stepped clines.

Discontinuous stepped clines

Unlike in continuous clines, in discontinuous clines the populations of species are allopatric, meaning there is very little or no gene flow amongst populations. The genetic or phenotypic trait in question always shows a steeper gradient between groups than within groups, as in continuous clines. Discontinuous clines follow the same principles as continuous clines by displaying either

Horizontally stepped clines, where intra-group variation is very small or non-existent and the geographic space separating groups shows a sharp change in character
Obliquely stepped clines, where there is some intra-group gradation, but this is less than the gradation in the character between populations

Clines and speciation

Parapatric speciation

It was originally assumed that geographic isolation was a necessary precursor to speciation (allopatric speciation). The possibility that clines may be a precursor to speciation was therefore ignored, as they were assumed to be evidence of the fact that in contiguous populations gene flow was too strong a force of homogenisation, and selection too weak a force of differentiation, for speciation to take place. However, the existence of particular types of clines, such as ring species, in which populations did not differentiate in allopatry but the terminal ends of the cline nonetheless do not interbreed, cast into doubt whether complete geographical isolation of populations is an absolute requirement for speciation.

Because clines can exist in populations connected by some degree of gene flow, the generation of new species from a previously clinal population is termed parapatric speciation. Both extrinsic and intrinsic selection can serve to generate varying degrees of reproductive isolation and thereby instigate the process of speciation. For example, through environmental selection acting on populations and favouring particular allele frequencies, large genetic differences between populations may accumulate (this would be reflected in clinal structure by the presence of numerous very steep clines). If the local genetic differences are great enough, it may lead unfavourable combinations of genotypes and therefore to hybrids being at a decreased fitness relative to the parental lines. When this hybrid disadvantage is great enough, natural selection will select for pre-zygotic traits in the homozygous parental lines that reduce the likelihood of disadvantageous hybridisation - in other words, natural selection will favour traits that promote assortative mating in the parental lines. This is known as reinforcement and plays an important role in parapatric and sympatric speciation.

Clinal maps

Clines can be portrayed graphically on maps using lines that show the transition in character state from one end of the geographic range to the other. Character states can however additionally be represented using isophenes, defined by Ernst Mayr as “lines of equal expression of a clinally varying character”. In other words, areas on maps that demonstrate the same biological phenomenon or character will be connected by something that resembles a contour line. When mapping clines therefore, which follow a character gradation from one extreme to the other, isophenes will transect clinal lines at a right angle.

Examples of clines

Bergmann's Rule demonstrated showing the difference in size between a larger northern fox, whose range spans colder regions, and a smaller desert fox, whose range is primarily in hot regions.

Although the term “cline” was first officially coined by Huxley in 1938, gradients and geographic variations in the character states of species have been observed for centuries. Indeed, some gradations have been considered so ubiquitous that they have been labelled ecological “rules”. One commonly cited example of a gradient in morphology is Gloger's Rule, named after Constantin Gloger, who observed in 1833 that environmental factors and the pigmentation of avian plumage tend to covary with each other, such that birds found in arid areas near the Equator tend to be much darker than those in less arid areas closer to the Poles. Since then, this rule has been extended to include many other animals, including flies, butterflies, and wolves.

Other ecogeographical rules include Bergmann's Rule, coined by Carl Bergmann in 1857, which states that homeotherms closer to the Equator tend to be smaller than their more northerly or southerly conspecifics. One of the proposed reasons for this cline is that larger animals have a relatively smaller surface area to volume ratio and therefore improved heat conservancy – an important advantage in cold climates. The role of the environment in imposing a selective pressure and producing this cline has been heavily implicated due to the fact that Bergmann’s Rule has been observed across many independent lineages of species and continents. For example, the house sparrow, which was introduced in the early 1850s to the eastern United States, evolved a north-south gradient in size soon after its introduction. This gradient reflects the gradient that already existed in the house sparrow’s native range in Europe.

Interbreeding populations represented by a gradient of coloured circles around a geographic barrier

The Larus gulls interbreed in a ring around the arctic.
1: Larus argentatus argentatus, 2: Larus fuscus (sensu stricto), 3: Larus fuscus heuglini, 4: Larus argentatus birulai, 5: Larus argentatus vegae, 6: Larus argentatus smithsonianus, 7: Larus argentatus argenteus

Ring species are a distinct type of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline. In the case of Larus gulls, the habitats of the end populations even overlap, which introduces questions as to what constitutes a species: nowhere along the cline can a line be drawn between the populations, but they are unable to interbreed.

In humans, clines in the frequency of blood types has allowed scientists to infer past population migrations. For example, the Type B blood group reaches its highest frequency in Asia, but become less frequent further west. From this, it has been possible to infer that some Asian populations migrated towards Europe around 2,000 years ago, causing genetic admixture in an isolation by distance model. In contrast to this cline, blood Type A shows the reverse pattern, reaching its highest frequency in Europe and declining in frequency towards Asia.

Saturday, January 4, 2020

Geology of the southern North Sea

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Geology_of_the_southern_North_Sea

Location of the Southern North Sea provided by GeoMapApp

Aerial image of the North Sea provided by the United States Geological Survey

The North Sea basin is located in northern Europe and lies between the United Kingdom, and Norway just north of The Netherlands and can be divided into many sub-basins. The Southern North Sea basin is the largest gas producing basin in the UK continental shelf, with production coming from the lower Permian sandstones which are sealed by the upper Zechstein salt. The evolution of the North Sea basin occurred through multiple stages throughout the geologic timeline. First the creation of the Sub-Cambrian peneplain, followed by the Caledonian Orogeny in the late Silurian and early Devonian. Rift phases occurred in the late Paleozoic and early Mesozoic which allowed the opening of the northeastern Atlantic. Differential uplift occurred in the late Paleogene and Neogene. The geology of the Southern North Sea basin has a complex history of basinal subsidence that had occurred in the Paleozoic, Mesozoic, and Cenozoic. Uplift events occurred which were then followed by crustal extension which allowed rocks to become folded and faulted late in the Paleozoic. Tectonic movements allowed for halokinesis to occur with more uplift in the Mesozoic followed by a major phase of inversion occurred in the Cenozoic affecting many basins in northwestern Europe. The overall saucer-shaped geometry of the southern North Sea Basin indicates that the major faults have not been actively controlling sediment distribution.

Geological history

Paleozoic era

Two major orogenic events occurred in this era, the Caledonian Orogeny and the Variscan Orogeny, allowing a complex geologic history to begin. During the late Silurian and early Devonian the Caledonian Orogeny occurred with episodes of uplift and erosion leaving unconformities. The Caledonian event occurred due to the collision of three land masses – Laurentia, Baltica, and Avalonia – which would eventually lead to the creation of Pangea. This collision allowed for a mountain belt to form NW–SE in the northern portion of the current basin, and in the south extending SW–NE. Following the Caledonian Orogeny approximately 380 Ma the Variscan Orogeny started and ended near the Permian. During this time period the orogeny caused Carboniferous rocks to become folded and faulted. The last collision occurred in the late Carboniferous where two super continents collided leading to the Varsican mountain range, Laurasia and Gondwanaland. Late Permian deposition of evaporites created the Zechstein supergroup which act as a salt cap for the fine grained sediment.

Mesozoic era

During this era the end of extensional tectonics had been well constrained in the southern North Sea basin; the extension occurred from the late Carboniferous to the Triassic. There had been some reactivation of Varsican basement faults due to the subsidence of the Sole Pit Basin and allowing basin tilts creating a peripheral graben system around the basin. Due to the reactivation of the basement faults it led to the beginning of halokinesis in the basin. The halokinesis allowed major uplift during the Mesozoic because of the presence of salt and the reactivation of basement faults; the thrusting permitted the sediment to thrust over the diapers and float on top of the Zechstein salt. Due to the Kimmerian phase uplift in the northern portion of the North Sea, it allowed subsidence and deposition to fill the basin, creating sandstone. Due to differential loading along the faults, salt diapers developed and played a huge role in the southern North Sea basin and all salt tectonic structures. Reverse faulting associated with late Carboniferous basin inversion is recorded by a wide range of Carboniferous stratigraphy subcropping the Permian sediments.The subcrop pattern indicates a strong influence of NW–SE tectonic trends during this inversion. This inversion event was followed by deposition of upper Carboniferous red beds, which pass up into sands of the Permian Rotliegend Group; these are overlain by evaporites of the Zechstein Supergroup. A major phase of basin inversion during or at the end of the Late Cretaceous affected many basins in northwestern Europe, including the Sole Pit Basin and the Cleveland Basin, and has been attributed to strike-slip reactivation of basement faults.

Cenozoic era

During the end of the Mesozoic and into the Cenozoic era the Alpine orogeny occurred which led to reactivation of faults and structures. In the beginning of the Tertiary, inversion involving basin tilt and reactivation of basement faults transpired. The center part of the southern North Sea basin comprises the Silver Pit and Sole Pit trough and the Cleaver Bank High, which are all distinguished by a series of salt swells and walls which occurred in the Tertiary. A reversal of basin tilt during the Tertiary uplifted the thick sedimentary wedge in the Sole Pit Trough to form the Sole Pit High. Since the orogeny reactivated the Mesozoic rifts it permitted the Zechstein salts to act as a buffer or detachment layer separating two structural regimes, which can lead to traps for natural resources.

Tectonic phases

Caledonian phase

During the Paleozoic there were three major landmasses that collided, Laurentia, Baltica, and Avalonia closing the Iapetus ocean. The event created a mountain chain trending North to South in the northern portion and an East to West trend in the South. The reason being that there is a North to South trend in the North is because Laurentia coming from the West and Baltica coming from the east meeting at the center to create a compressional regime. Through time eventually Avalonia coming from the south closing the Iapetus ocean, collided with the two landmasses to create a T-junction giving an East to West trend in the southern portion. This event is the first major event that would lead to the creation of Pangea. The tectonic event comprised the entire Ordovician and into the early Devonian, the Caledonian rocks are the basement of the current North Sea.

Variscan phase

From the late Devonian to the end of the Permian ending in the Paleozoic era the Variscan Orogeny occurred. The super continents of Gondwanaland and Laurussia collided creating an extensive mountain range just east of the pre-existing Caledonian mountains and creating Pangea the super continent at the end of the Variscan phase. The collision of these plates plays an important role in the potential of hydrocarbons in the Southern North Sea basin. The start of this phase is the collapsing of the Caledonian orogeny and a general extensional regime which would cause a depression to fill with sediment. There are four major phases in this orogenic event. First phase known as the Bretonian reflected in changes in the sediment input and the reactivation of a south plunging subduction zone. The second phase, the Sudetian, was of volcanic event and extrusive metamorphic and igneous rocks with uplift and mild folding of grabens in the vicinity which lead to inversion. The Asturian tectonic phase created fragmentation of the Variscans and its foreland due to the complex fault system of conjugate shear faults and secondary extensional faults. The last major phase, the Staphanian, caused the majority of faulting and deformation expressed in wrench faults. The accumulation of hydrocarbons in the south was permitted due to the basin that was formed, the foreland basin was barely disturbed by tectonic events in the northern region and eventually sealed up by the salt caps of the Zechstein formation. Since the Caledonian and Variscan orogeny are closely related in time both events helped create Pangea and the Caledonians slowly phase into the Variscan orogeny.

Kimmerian phase

Pangea animation

The break up of Pangea occurs during the Kimmerian tectonic phase for most of the Mesozoic, until the early-mid Cretaceous, this marks the start of creating the present position of our continents today. During the Jurassic, rifting activity reaches its maximum and North America starts to move apart from Eurasia following that event in the Cretaceous the southern part of North America starts to open up the Atlantic Ocean with the separation of South America and Africa. At the end of the Mesozoic the North Sea reached its final position where it lies in present day. Throughout the Cretaceous rifting eventually slowed down and came to a halt which later created the North Sea failed rift system because the regional stresses had shifted on to North America. The Jurassic is probably the most important geological time for hydrocarbon exploration in the North Sea. Many accumulations are in Jurassic reservoir, the Kimmeridge clay is considered the most important source rock and structures formed during rifting form excellent traps. In the first place rifting was responsible for the deposition of organic rich source rock due to anoxic conditions in the deep isolated rift basins. Possibly the most important phase to create structures and traps for the natural resources we try to collect today.

Alpine phase

This phase is currently active today and started in the Cretaceous.In the late Cretaceous and in the Tertiary inversion phases in the Southern North Sea region occurred due to the Alpine orogeny and its compressional stresses. Since there had been inversion the Zechstein salt played a huge role by acting as a buffer between two structural regimes. Although the phase reactivated pre-existing faults it allowed the salt tectonics to remain active during the Tertiary as the sediments were deposited, and later became penetrated by the salt diapers. The Alpine phase did add more structural confusion to the geologic history, but it also help create more traps with the Zechstein salt.

Sedimentary formations

Main Formations

Rotliegend group

The sandstones of the Southern North Sea region form gas reservoirs. Deposition started in the early Permian, and near the end of the early Permian finer sediment was deposited in an environment of lacustrine and saline/sabkha.

Zechstein group

The Zechstein group consists of evaporites which sealed the Rotliegend group for reservoir formation. Sedimentation was dominated by the development of mixed carbonate-evaporite depositional system throughout the southern Permian basin. Climatic conditions allowed the deposition of five major sedimentary cycles of progressive progradation and desiccation of the basin after an initial recharge through basin flooding.

Cromer Knoll group

The Cromer Knoll is deposited on top of an unconformity at the base of the Cretaceous period. Regional uplift and erosion allowed the unconformity to appear in the late Triassic and depositing the Cromer Knoll and chalk groups.

Salt tectonics

Zechstein salt cap

Salt tectonics is the movement of a significant amount of evaporites encompassing salt rock within a stratigraphic sequence of rocks. Within the southern North Sea basin this plays a huge role in the oil and gas industry because the tectonic events throughout the geologic timescale allowed these halokinesis structures to trap areas of natural resources. The major salt basins were clearly deposited by gravity driven measurements with three basinal areas: the German, English and Norwegian basins. The southern North Sea basin concerns the English and German Zechstein salt basins. The German basin can be categorized as a salt wall which is a linear diapiric structure possibly related either to basement faulting or to the controlling effect of regional dip, and the English basin is categorized as a salt pillow type of structure, developed in association with thinning of overlying beds but without diapiric effects. The major types of salt structures in this basin are salt pillows or swells which lie in the cores of buckle fold structures.

Petroleum geology

Location of oilfields (green dots) and gasfields (red dots) in the Southern North Sea

In general the reservoir potential is restricted to aeolian sandstone, although poorer quality potential reservoirs are found in fluvial sediment. About 85% of the gas production in the southern North Sea basin comes from the pre-Zechstein Permian sandstones and 13% from the Triassic fluvial sandstones. The sandstone deposited prior to the Zechstein evaporites are essentially the area in which the oil industry is pulling the natural resources from due to high quality seal from the salt diapers and pillows which acted as a buffer between structural segments. Triassic sequence fluvial sandstones are of lesser quality of a reservoir because it was not sealed in a trap such as the Rotliegend.

North German basin

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/North_German_basin

The North German Basin located in western Europe, represented as the green region defined by USGS

The North German Basin is a passive-active rift basin located in central and west Europe, lying within the southeastern most portions of the North Sea and the southwestern Baltic Sea and across terrestrial portions of northern Germany, Netherlands, and Poland. The North German Basin is a sub-basin of the Southern Permian Basin, that accounts for a composite of intra-continental basins composed of Permian to Cenozoic sediments, which have accumulated to thicknesses around 10–12 kilometres (6–7.5 mi). The complex evolution of the basin takes place from the Permian to the Cenozoic, and is largely influenced by multiple stages of rifting, subsidence, and salt tectonic events. The North German Basin also accounts for a significant amount of Western Europe's natural gas resources, including one of the world's largest natural gas reservoir, the Groningen gas field.

Regional tectonic evolution

The regional tectonic evolution of the North German Basin coincides with of the evolution of the Southern Permian Basin, the basin across central and western Europe. From the late Neoproterozoic Era to Carboniferous Period, Europe underwent the Caledonian Orogeny and Variscan Orogeny. These crustal accretion events produced the present day regional lithosphere, and by the time of the post-orogenic collapse of the Variscan Orogeny the supercontinent Pangea had completely formed. After the formation of Pangea, much of the region underwent crustal instability and thus developing the extensive Permo-Carboniferous magmatic province. This magmatism led to the extrusion of abundant volcanic successions such as the Northeast German Basin, Northwest Polish Basin, and Oslo Rift, while also causing the formation of 70 rift basins throughout the Permian Basin. The regions most evolved and voluminous magmatism occurred within the North German Basin dating back to 297-302 Ma.

Basin evolution

Initial rifting

The initiation of the Northern German Basin took place in the Late Carboniferous approximately 295-285 Ma (Million Years Ago) in association collapse of the Variscan Orogeny due to wrenching tectonics in the over-thickened crust in the northern foreland of the Variscan Orogeny. The initiation formed by crustal rifting and wrenching in addition to huge amounts of volcanism(>40,000 km³ ) and magmatism, can only be approximately dated due to the extensive (>250 Ma) poly-phased subsidence of the region. The most evident dating method has been done using SHRIMP (Sensitive High-Resolution Ion Microprobe) Zircon ages, allowing for dating of sediments produced during the magmatic flare-up during the Permian. The wrench tectonics, magmatic inflation, and mantle lithosphere erosion took place gave a regional uplift allowing for an increase in crustal erosion.

Main phase of subsidence

20 million years post-rifting, the North German Basin experienced a rapid accumulation of sediments, >2,700 m (8,900 ft) of strata from the Upper Rotliegend Unit to the Bunter Unit, thus experiencing maximum thermal subsidence from the Late Permian to the Middle Triassic. This rapid burial of sediments lead to subsidence rates of 220 m per million years due to the drastic increase in crustal load. Another important influence of this subsidence is due to the thermal relaxation of the lithospheric magmatic inflation, thus allowing the basin to deepen with the accumulation of the sediment.

Secondary rifting

During the Triassic-Early Jurassic, 252 to 200 Ma, there was a phase of new north to south rifting events due to the break up of the super-continent Pangea caused W-E extension across the Northern German Basin. These extensions in the crust created the Triassic grabens such as the local the Gluckstadt Graben, while also initiating the salt tectonics seen in the region. This rifting event was then followed by another phase of subsidence due to sedimentary loading and lithospheric thermal relaxation.

Doming

During the Middle-Late Jurassic, the center of the North Sea underwent a doming acknowledged by the Middle Jurassic erosional unconformity, the erosion of >1,000 m (3,300 ft) of Upper Triassic and Lower Jurassic strata. The dome raised above sea level during the Middle Jurassic and began to deflate due to rifting in the Late Jurassic. Though the mechanism forming the North Sea Rift Dome is not particularly well understood, the development of the dome seems to be consistent with an active rift model having a broad-based (1,250 km or 780 mi diameter) plume head influencing the Late Jurassic rifting.

Tertiary rifting

In the Late Jurassic, the third rifting event took place in response to the North Sea doming event. Major extensional faulting and rifting began approximately 157-155 Ma allowing for the Zechstein evaporites to form a detachment between basement rocks and upper stratigraphy largely influencing the natural gas and oil formation seen across the North German Basin. Organic-rich mudstones from the Kimmeridge Clay Formation is the source of the majority of the North German Basin's hydrocarbons which was restricted from migrating upward by the Zechstein salt.

Inversion

In the Late Cretaceous, a significant phase of inversion took place due to the reactivation of strike-slip basement faults. Inversion of the region responded significantly to the orientation of compression, such that faults like the E-W Elbe Fault System was inverted 3–4 km (1.9–2.5 mi) while the N-S Grabens did not experience significant uplift.

Final subsidence

During the Cenozoic, the last phase of subsidence occurred. During the Oligocene to Miocene, many of the basement faults were reactivated by the strike-slip faults during the Late Cretaceous inversion. The reactivation of these basement faults triggered more halokinesis. Slight inversion due to the salt tectonics allowed for minor amounts of Miocene and Pliocene deposits, which were later buried by widespread delta and glacial deposits during the Quaternary, resulting in rapid subsidence.

Stratigraphy

This figure breaks down the stratigraphic units of the North German Basin through time.

The depositional history of the North German Basin is recorded within the stratigraphy sequence of sediments, which make up the basin. The poly-phase deposition of the basin can be broken down into strati-graphic units, each with their own distinct characteristics. The sedimentary basin was assembled above the Lower Paleozoic crystalline basement formed during the Caledonian Orogeny about 420-400 Ma.