Editing Carboniferous (section)

== Geology ==

=== Stratigraphy ===
Stages can be defined globally or regionally. For global stratigraphic correlation, the ICS ratify global stages based on a [[Global Boundary Stratotype Section and Point]] (GSSP) from a single [[Geological formation|formation]] (a [[stratotype]]) identifying the lower boundary of the stage. Only the boundaries of the Carboniferous System and three of the stage bases are defined by global stratotype sections and points because of the complexity of the geology.<ref name="Lucas-2022">{{Cite journal |last1=Lucas |first1=Spencer G. |last2=Schneider |first2=Joerg W. |last3=Nikolaeva |first3=Svetlana |last4=Wang |first4=Xiangdong |date=2022 |title=The Carboniferous timescale: an introduction |url=https://www.lyellcollection.org/doi/10.1144/SP512-2021-160 |journal=Geological Society, London, Special Publications |language=en |volume=512 |issue=1 |pages=1–17 |doi=10.1144/SP512-2021-160 |bibcode=2022GSLSP.512....1L |s2cid=245208581 |issn=0305-8719}}</ref><ref name="Davydov-2012" /> The ICS subdivisions from youngest to oldest are as follows:<ref name="Cohen-2013">Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) [https://stratigraphy.org/ICSchart/ChronostratChart2020-03.pdf The ICS International Chronostratigraphic Chart]. Episodes 36: 199–204.</ref>
{|
|Series/epoch
|
|Stage/age
|Lower boundary
|-
| colspan="2" style="background-color: {{period color|Permian}};" |[[Permian]]
| style="background-color: {{period color|Asselian}};" |[[Asselian]]
|298.9 ±0.15 Ma
|-
| rowspan="4" style="background-color: {{period color|Pennsylvanian}};" |[[Pennsylvanian (geology)|Pennsylvanian]]
| rowspan="2" style="background-color: {{period color|Upper Pennsylvanian}};" |Upper
| style="background-color: {{period color|Gzhelian}};" |[[Gzhelian]]
|303.7 ±0.1 Ma
|-
| style="background-color: {{period color|Kasimovian}};" |[[Kasimovian]]
|307.0 ±0.1 Ma
|-
| style="background-color: {{period color|Middle Pennsylvanian}};" |Middle
| style="background-color: {{period color|Moscovian}};" |[[Moscovian (Carboniferous)|Moscovian]]
|315.2 ±0.2 Ma
|-
| style="background-color: {{period color|Lower Pennsylvanian}};" |Lower
| style="background-color: {{period color|Bashkirian}};" |[[Bashkirian]]
|323.2 ±0.4 Ma
|-
| rowspan="3" style="background-color: {{period color|Mississippian}};" |[[Mississippian (geology)|Mississippian]]
| style="background-color: {{period color|Upper Mississippian}};" |Upper
| style="background-color: {{period color|Serpukhovian}};" |[[Serpukhovian]]
|330.9 ±0.2 Ma
|-
| style="background-color: {{period color|Middle Mississippian}};" |Middle
| style="background-color: {{period color|Visean}};" |[[Visean]]
|346.7 ±0.4 Ma
|-
| style="background-color: {{period color|Lower Mississippian}};" |Lower
| style="background-color: {{period color|Tournaisian}};" |[[Tournaisian]]
|358.9 ±0.4 Ma
|}

==== Mississippian ====
The Mississippian was proposed by [[Alexander Winchell]] in 1870 named after the extensive exposure of lower Carboniferous [[limestone]] in the upper [[Mississippi River]] valley.<ref name="Stanley-2015" /> During the Mississippian, there was a marine connection between the [[Paleo-Tethys Ocean|Paleo-Tethys]] and [[Panthalassa]] through the [[Rheic Ocean]] resulting in the near worldwide distribution of marine faunas and so allowing widespread correlations using marine [[biostratigraphy]].<ref name="Lucas-2022" /><ref name="Davydov-2012" /> However, there are few Mississippian [[volcanic rock]]s, and so obtaining [[Radiometric dating|radiometric dates]] is difficult.<ref name="Lucas-2022" />

The Tournaisian Stage is named after the Belgian city of [[Tournai]]. It was introduced in scientific literature by Belgian geologist [[André Dumont (geologist)|André Dumont]] in 1832. The GSSP for the base of the Carboniferous System, Mississippian Subsystem and Tournaisian Stage is located at the [[La Serre]] section in [[Montagne Noire]], southern France. It is defined by the first appearance of the [[conodont]] ''[[Siphonodella|Siphonodella sulcata]]'' within the evolutionary lineage from ''[[Siphonodella|Siphonodella praesulcata]]'' to ''Siphonodella sulcata''. This was ratified by the ICS in 1990. However, in 2006 further study revealed the presence of ''Siphonodella sulcata'' below the boundary, and the presence of ''Siphonodella'' ''praesulcata'' and ''Siphonodella sulcata'' together above a local [[unconformity]]. This means the evolution of one species to the other, the definition of the boundary, is not seen at the La Serre site making precise correlation difficult.<ref name="Davydov-2012" /><ref name="Stratigraphy.org">{{Cite web |title=International Commission on Stratigraphy |url=https://stratigraphy.org/gssps/visean |access-date=2023-11-12 |website=stratigraphy.org}}</ref>[[File:Carboniferous regional subdivisions.png|thumb|upright=2|Chart of regional subdivisions of the Carboniferous Period]]The Viséan Stage was introduced by André Dumont in 1832 and is named after the city of [[Visé]], [[Liège Province]], Belgium. In 1967, the base of the Visean was officially defined as the first black limestone in the Leffe [[facies]] at the Bastion Section in the [[Dinant|Dinant Basin]]. These changes are now thought to be ecologically driven rather than caused by evolutionary change, and so this has not been used as the location for the GSSP. Instead, the GSSP for the base of the Visean is located in Bed 83 of the sequence of dark grey [[limestone]]s and [[shale]]s at the [[Peng Chong|Pengchong]] section, [[Guangxi]], southern China. It is defined by the first appearance of the [[Fusulinida|fusulinid]] ''Eoparastaffella simplex'' in the evolutionary lineage ''Eoparastaffella ovalis – Eoparastaffella simplex'' and was ratified in 2009.<ref name="Davydov-2012" />

The Serpukhovian Stage was proposed in 1890 by Russian stratigrapher [[Sergei Nikitin (geologist)|Sergei Nikitin]]. It is named after the city of [[Serpukhov]], near Moscow and currently lacks a defined GSSP. The Visean-Serpukhovian boundary coincides with a major period of glaciation. The resulting sea level fall and climatic changes led to the loss of connections between marine basins and [[endemism]] of marine fauna across the Russian margin. This means changes in [[Biome|biota]] are environmental rather than evolutionary making wider correlation difficult.<ref name="Davydov-2012" /> Work is underway in the [[Ural Mountains|Urals]] and Nashui, [[Guizhou]] Province, southwestern China for a suitable site for the GSSP with the proposed definition for the base of the Serpukhovian as the first appearance of conodont ''[[Lochriea|Lochriea ziegleri]].''<ref name="Stratigraphy.org" />

==== Pennsylvanian ====
The Pennsylvanian was proposed by [[J. J. Stevenson (geologist)|J.J.Stevenson]] in 1888, named after the widespread coal-rich strata found across the state of Pennsylvania.<ref name="Stanley-2015" /> The closure of the Rheic Ocean and formation of Pangea during the Pennsylvanian, together with widespread glaciation across [[Gondwana]] led to major climate and sea level changes, which restricted marine fauna to particular geographic areas thereby reducing widespread biostratigraphic correlations.<ref name="Lucas-2022" /><ref name="Davydov-2012" /> Extensive volcanic events associated with the assembling of Pangea means more radiometric dating is possible relative to the Mississippian.<ref name="Lucas-2022" />

The Bashkirian Stage was proposed by Russian stratigrapher [[Sofia Semikhatova]] in 1934. It was named after [[Bashkiria (1917–1919)|Bashkiria]], the then Russian name of the republic of [[Bashkortostan]] in the southern Ural Mountains of Russia. The GSSP for the base of the Pennsylvanian Subsystem and Bashkirian Stage is located at [[Arrow Canyon Range|Arrow Canyon]] in [[Nevada]], US and was ratified in 1996. It is defined by the first appearance of the conodont ''[[Declinognathodus|Declinognathodus noduliferus]]''. Arrow Canyon lay in a shallow, tropical seaway which stretched from Southern California to Alaska. The boundary is within a [[Cyclothems|cyclothem]] sequence of [[Sequence stratigraphy|transgressive]] limestones and fine [[sandstone]]s, and [[Sequence stratigraphy|regressive]] [[mudstone]]s and [[breccia]]ted limestones.<ref name="Davydov-2012" />

The Moscovian Stage is named after shallow marine limestones and colourful [[clay]]s found around Moscow, Russia. It was first introduced by Sergei Nikitin in 1890. The Moscovian currently lacks a defined GSSP. The fusulinid ''Aljutovella aljutovica'' can be used to define the base of the Moscovian across the northern and eastern margins of Pangea, however, it is restricted in geographic area, which means it cannot be used for global correlations.<ref name="Davydov-2012" /> The first appearance of the conodonts ''Declinognathodus donetzianus'' or ''Idiognathoides postsulcatus'' have been proposed as a boundary marking species and potential sites in the Urals and Nashui, Guizhou Province, southwestern China are being considered.<ref name="Stratigraphy.org" />

The Kasimovian is the first stage in the Upper Pennsylvanian. It is named after the Russian city of [[Kasimov]], and was originally included as part of Nikitin's 1890 definition of the Moscovian. It was first recognised as a distinct unit by A.P. Ivanov in 1926, who named it the "''Tiguliferina''" Horizon after a type of [[brachiopod]]. The boundary of the Kasimovian covers a period of globally low sea level, which has resulted in [[Unconformity|disconformities]] within many sequences of this age. This has created difficulties in finding suitable marine fauna that can used to correlate boundaries worldwide.<ref name="Davydov-2012" /> The Kasimovian currently lacks a defined GSSP; potential sites in the southern Urals, southwest USA and Nashui, Guizhou Province, southwestern China are being considered.<ref name="Stratigraphy.org" />

The Gzhelian is named after the Russian village of [[Gzhel (selo), Moscow Oblast]], near [[Ramenskoye, Moscow Oblast|Ramenskoye]], not far from Moscow. The name and type locality were defined by Sergei Nikitin in 1890. The Gzhelian currently lacks a defined GSSP. The first appearance of the fusulinid ''Rauserites rossicus'' and ''Rauserites'' ''stuckenbergi'' can be used in the [[Boreal Sea]] and Paleo-Tethyan regions but not eastern Pangea or Panthalassa margins.<ref name="Davydov-2012" /> Potential sites in the Urals and Nashui, Guizhou Province, southwestern China for the GSSP are being considered.<ref name="Stratigraphy.org" />

The GSSP for the base of the Permian is located in the Aidaralash River valley near [[Aqtöbe]], Kazakhstan and was ratified in 1996. The beginning of the stage is defined by the first appearance of the conodont ''[[Streptognathodus|Streptognathodus postfusus]].''<ref>Davydov, V.I., Glenister, B.F., Spinosa, C., Ritter, S.M., Chernykh, V.V., Wardlaw, B.R. & Snyder, W.S. 1998. [https://www.researchgate.net/publication/237222028_Proposal_of_Aidaralash_as_Global_Stratotype_Section_and_Point_GSSP_for_base_of_the_Permian_System Proposal of Aidaralash as Global Stratotype Section and Point (GSSP) for base of the Permian System]. Episodes, 21, 11–17.</ref>

=== Cyclothems ===
A cyclothem is a succession of non-marine and marine [[sedimentary rock]]s, deposited during a single sedimentary cycle, with an [[Erosion surface|erosional surface]] at its base. Whilst individual cyclothems are often only metres to a few tens of metres thick, cyclothem sequences can be many hundreds to thousands of metres thick and contain tens to hundreds of individual cyclothems.<ref name="Montañez-2022" /> Cyclothems were deposited along [[Continental shelf|continental shelves]] where the very gentle gradient of the shelves meant even small changes in sea level led to large advances or retreats of the sea.<ref name="Stanley-2015" /> Cyclothem lithologies vary from [[mudrock]] and carbonate-dominated to coarse siliciclastic sediment-dominated sequences depending on the paleo-topography, climate and supply of sediments to the shelf.<ref name="Fielding-2021">{{Cite journal |last=Fielding |first=Christopher R. |date=2021-06-01 |title=Late Palaeozoic cyclothems – A review of their stratigraphy and sedimentology |url=https://www.sciencedirect.com/science/article/pii/S0012825221001124 |journal=Earth-Science Reviews |volume=217 |pages=103612 |bibcode=2021ESRv..21703612F |doi=10.1016/j.earscirev.2021.103612 |issn=0012-8252 |s2cid=233618931}}</ref>
[[File:Red Wharf Limestone Formation, Red Wharf Bay, Anglesey, North Wales, UK.jpg|alt=A cliff with pale grey beds of limestone overlain by orange sandstone, above which are more pale grey mudstones and limestones. A large fracture in the limestone is filled by a bulbous extension of the sandstone down into the limestone.|thumb|Cliff section through the Serpukhovian Red Wharf Limestone Formation, [[Wales]]. A marine limestone at the base of the cliff is overlain by an orange-coloured fluvial sandstone. Subaerial exposure of the limestone during a period of falling sea level resulted in the formation of a karstic surface, which has then been infilled by the river sands. A thin, estuarine silty mudstone overlays the sandstone, which in turn is overlain by a second marine limestone.]]
The main period of cyclothem deposition occurred during the [[Late Paleozoic icehouse|Late Paleozoic Ice Age]] from the Late Mississippian to early Permian, when the waxing and waning of [[ice sheet]]s led to rapid changes in [[eustatic sea level]].<ref name="Fielding-2021" /> The growth of ice sheets led global sea levels to fall as water was locked away in glaciers. Falling sea levels exposed large tracts of the continental shelves across which river systems eroded channels and valleys and vegetation broke down the surface to form [[soil]]s. The non-marine sediments deposited on this erosional surface form the base of the cyclothem.<ref name="Fielding-2021" /> As sea levels began to rise, the rivers flowed through increasingly water-logged landscapes of swamps and lakes. [[Peatland|Peat mires]] developed in these wet and oxygen-poor conditions, leading to coal formation.<ref name="Woodcock-2012" /> With continuing sea level rise, coastlines migrated landward and [[River delta|deltas]], [[lagoon]]s and [[Estuary|esturaries]] developed; their sediments deposited over the peat mires. As fully marine conditions were established, limestones succeeded these marginal marine deposits. The limestones were in turn overlain by deep water black shales as maximum sea levels were reached.<ref name="Stanley-2015" />

Ideally, this sequence would be reversed as sea levels began to fall again; however, sea level falls tend to be protracted, whilst sea level rises are rapid, ice sheets grow slowly but melt quickly. Therefore, the majority of a cyclothem sequence occurred during falling sea levels, when rates of [[erosion]] were high, meaning they were often periods of non-deposition. Erosion during sea level falls could also result in the full or partial removal of previous cyclothem sequences. Individual cyclothems are generally less than 10 m thick because the speed at which sea level rose gave only limited time for sediments to accumulate.<ref name="Fielding-2021" />

During the Pennsylvanian, cyclothems were deposited in shallow, [[Inland sea|epicontinental]] seas across the tropical regions of [[Laurasia|Laurussia]] (present day western and central US, Europe, Russia and central Asia) and the [[North China craton|North]] and [[South China cratons]].<ref name="Stanley-2015" /> The rapid sea levels fluctuations they represent correlate with the glacial cycles of the Late Paleozoic Ice Age. The advance and retreat of ice sheets across Gondwana followed a 100 kyr [[Milankovitch cycles|Milankovitch cycle]], and so each cyclothem represents a cycle of sea level fall and rise over a 100 kyr period.<ref name="Fielding-2021" />

=== Coal formation ===
[[File:Hyden Formation over Pikeville Formation (Middle Pennsylvanian; Jackson North roadcut, Breathitt County, Kentucky, USA) 1.jpg|alt=Photo of a road cutting through a thick and repeating sequence of pale grey to black rock strata.|thumb|Hyden Formation over Pikeville Formation in the Pennsylvanian of Kentucky, US. The exposure has Pennsylvanian-aged cyclothemic sedimentary rocks of the Breathitt Group. The upper part of the roadcut is Hyden Formation, consisting of mixed siliciclastics and coal. The lower part is Pikeville Formation, also having mixed siliciclastics and coal.]]
Coal forms when organic matter builds up in waterlogged, [[Anoxic waters|anoxic]] swamps, known as peat mires, and is then buried, compressing the peat into coal. The majority of Earth's coal deposits were formed during the late Carboniferous and early Permian. The plants from which they formed contributed to changes in the Carboniferous Earth's atmosphere.<ref name="NelsenDiMichelePetersBoyce2016PNAS">{{cite journal |last1=Nelsen |first1=Matthew C. |last2=DiMichele |first2=William A. |last3=Peters |first3=Shanan E. |last4=Boyce |first4=C. Kevin |date=19 January 2016 |title=Delayed fungal evolution did not cause the Paleozoic peak in coal production |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=113 |issue=9 |pages=2442–2447 |bibcode=2016PNAS..113.2442N |doi=10.1073/pnas.1517943113 |pmc=4780611 |pmid=26787881 |doi-access=free}}</ref>

During the Pennsylvanian, vast amounts of organic debris accumulated in the peat mires that formed across the low-lying, humid equatorial wetlands of the [[foreland basin]]s of the [[Central Pangean Mountains]] in Laurussia, and around the margins of the North and South China cratons.<ref name="NelsenDiMichelePetersBoyce2016PNAS" /> During glacial periods, low sea levels exposed large areas of the continental shelves. Major river channels, up to several kilometres wide, stretched across these shelves feeding a network of smaller channels, lakes and peat mires.<ref name="Woodcock-2012" /> These wetlands were then buried by sediment as sea levels rose during [[interglacial]]s. Continued crustal [[subsidence]] of the foreland basins and continental margins allowed this accumulation and burial of peat deposits to continue over millions of years resulting in the formation of thick and widespread coal formations.<ref name="NelsenDiMichelePetersBoyce2016PNAS" /> During the warm interglacials, smaller coal swamps with plants adapted to the temperate conditions formed on the [[Siberia (continent)|Siberian craton]] and the western Australian region of Gondwana.<ref name="Stanley-2015" />

There is ongoing debate as to why this peak in the formation of Earth's coal deposits occurred during the Carboniferous. The first theory, known as the delayed fungal evolution hypothesis, is that a delay between the development of trees with the wood fibre [[lignin]] and the subsequent evolution of lignin-degrading fungi gave a period of time where vast amounts of lignin-based organic material could accumulate. Genetic analysis of [[Basidiomycota|basidiomycete]] fungi, which have [[enzyme]]s capable of breaking down lignin, supports this theory by suggesting this fungi evolved in the Permian.<ref>{{Cite journal |last1=Floudas |first1=Dimitrios |last2=Binder |first2=Manfred |last3=Riley |first3=Robert |last4=Barry |first4=Kerrie |last5=Blanchette |first5=Robert A. |last6=Henrissat |first6=Bernard |last7=Martínez |first7=Angel T. |last8=Otillar |first8=Robert |last9=Spatafora |first9=Joseph W. |last10=Yadav |first10=Jagjit S. |last11=Aerts |first11=Andrea |last12=Benoit |first12=Isabelle |last13=Boyd |first13=Alex |last14=Carlson |first14=Alexis |last15=Copeland |first15=Alex |date=2012-06-01 |title=The Paleozoic Origin of Enzymatic Lignin Decomposition Reconstructed from 31 Fungal Genomes |url=https://ui.adsabs.harvard.edu/abs/2012Sci...336.1715F |journal=Science |volume=336 |issue=6089 |pages=1715–1719 |doi=10.1126/science.1221748 |pmid=22745431 |bibcode=2012Sci...336.1715F |issn=0036-8075|hdl=10261/60626 |osti=1165864 |hdl-access=free }}</ref><ref>{{Cite web |last=Biello |first=David |title=White Rot Fungi Slowed Coal Formation |url=https://www.scientificamerican.com/article/mushroom-evolution-breaks-down-lignin-slows-coal-formation/ |access-date=2024-01-06 |website=Scientific American |language=en}}</ref> However, significant Mesozoic and Cenozoic coal deposits formed after lignin-digesting fungi had become well established, and fungal degradation of lignin may have already evolved by the end of the Devonian, even if the specific enzymes used by basidiomycetes had not.<ref name="NelsenDiMichelePetersBoyce2016PNAS" /> The second theory is that the geographical setting and climate of the Carboniferous were unique in Earth's history: the co-occurrence of the position of the continents across the humid equatorial zone, high biological productivity, and the low-lying, water-logged and slowly subsiding sedimentary basins that allowed the thick accumulation of peat were sufficient to account for the peak in coal formation.<ref name="NelsenDiMichelePetersBoyce2016PNAS" />