Cambrian arc-continent collision in the Paleozoides of Kazakhstan

A model of the Cambrian evolution of the Paleozoides in Kazakhstan is presented on the basis of consideration of Cambrian rock complexes. The Middle Cambrian collision of an island arc and a passive continental margin was the main event of this evolution. Three stages of the Paleozoides evolution are recognized in the Cambrian. The first, precollision stage embraces a time span from the Early Cambrian to the first half of the Amgian Age of the Middle Cambrian in which the following elements of the continent-ocean transitional zone may be reconstructed: passive continental margin-backarc basin with oceanic crust-island arc-oceanic basin. At the second, collision stage (the second half of the Amgian Age), the backarc basin was closed and the continental margin and island arc collided, forming a suture zone. At the third, postcollision stage (the Mayan Age of the Middle Cambrian-Early Ordovician), the structure of the convergent margin was complicated. The volcanic belt started to develop on the accreted continental margin. A system of ensimatic island arcs originated at the same time and migrated toward the outer basin. The active rifting was typical of the inner part of the continental margin.     

A New Ectasian Event of Basitic Magmatism in the Southern Siberian Craton

Doklady Earth Sciences - On the basis of U–Pb dating of zircon and baddeleyite from gabbro–dolerite of the Goloustnaya dyke swarm (southern margin of the Siberian Craton), the age of...     

Sedimentology of early Pliocene sandstones in the south-western Taiwan foreland: Implications for basin physiography in the early stages of collision

This work presents sedimentological observations and interpretations on three detailed sections of the Pliocene Yutengping/Ailiaochiao formations, deposited in the early stages of collision in Taiwan. Seven facies associations record paleoenvironments of deposition ranging from nearshore to lower offshore with a strong influence of tidal reworking, even in shelfal sub-tidal environments, and a pro-delta setting characterized by mass-flows. The association of shallow facies of the upper offshore to lower shoreface with pro-delta turbidite facies sourced in the orogen to the east suggests a peculiar setting in which turbidite deposition occurred below wave base but on the shelf, in water depths of probably less than 100 m. This adds to the examples of “shallow turbidites” increasingly commonly found in foreland basins and challenges the classical view of a “deep” early underfilled foreland basin. Time series analysis on tidal rhythmites allow us to identify a yearly signal in the form of periodic changes of sand-supply, energy and bioturbation that suggests a marked seasonality possibly affecting precipitation and sediment delivery as well as temperature. The Taiwan foreland basin may also present a potentially high-resolution record in shallow sediments of the early installation of monsoonal circulation patterns in east Asia. We confirm partly the paleogeography during the early stages of collision in Taiwan: the Chinese margin displayed a pronounced non-cylindrical geometry with a large basement promontory to the west in place of the modern Taiwan mountain range. Collision in Taiwan may have happened at once along the whole length of the modern mountain range, instead of progressively from north to south as classically considered.     

Dike Magmatism in the Evolution of the Transform Active Continental Margin of the Siberian Craton in the Ediacaran

Doklady Earth Sciences - An Ediacaran complex of dike rocks has been identified for the first time in the Yenisei Ridge orogen. These igneous rocks are represented by basic, intermediate and acidic...     

Magmatism evolution and carbonatite-granite association in the neoproterozoic active continental margin of the Siberian craton: Thermochronological reconstructions

Neoproterozoic carbonatites and related igneous rocks, including A-type granites in the Tatarka-Ishimba suture zone of the Yenisey Ridge are confined to a horst-anticlinal structure that was formed in a transpression setting during the oblique collision between the Central Angara terrane and the Siberian craton. The carbonatites, associating mafic (including alkaline) dikes as well as the Srednetatarka nepheline syenites are the oldest igneous formations of the Tatarka active continental margin complex. Geochronological data indicate that magmatic evolution continued in the studied anticline for nearly 100 m.y. On the earliest stage carbonatites were formed and on the last stage — the emplacement of mantle-crustal A-type Tatarka granites took place. According to new U/Pb zircon studies, the earliest rocks in the Tatarka pluton are A-type leucogranites aged 646 ± 8 Ma. The younger ⁴⁰Ar/³⁹Ar ages of carbonatites obtained for phlogopites (647 ± 7 and 629 ± 6 Ma) are related to the last tectonic events in the studied region of the Tatarka-Ishimba suture zone, which are coeval with the formation of the A-type granitoids (646–629 Ma).     

中亚巴尔喀什成矿带晚古生代岩浆活动与斑岩铜矿成矿时代

巴尔喀什成矿带是中亚成矿域重要的晚古生代斑岩铜钼成矿带。通过该成矿带科翁腊德、博尔雷和阿克斗卡地区与斑岩铜成矿作用密切相关的花岗斑岩类岩体锆石SHRIMP U-Pb定年,主量、稀土和微量元素地球化学,Sr、Nd同位素示踪分析,进一步厘定了斑岩铜成矿作用的时代,并推测了板块构造环境。斑岩铜成矿时代分为两期：早期约为327 Ma,形成科翁腊德和阿克斗卡超大型斑岩铜矿床;晚期约为316 Ma,形成博尔雷大型斑岩铜矿床。与成矿有关的斑岩类主要为高钾钙碱性系列花岗岩,可能为火山岛弧环境,部分具有埃达克岩特征和经典岛弧花岗岩类特征。斑岩类ε_Sr(t)和ε_Nd(t)的变化范围分别为-6.35~34.03和-0.46~5.53。其中,科翁腊德-博尔雷地区斑岩类来源于亏损地幔与大陆地壳表层物质（老地壳物质）的显著混染作用,而阿克斗卡地区斑岩类直接来自于亏损地幔。将巴尔喀什成矿带与我国西准噶尔成矿带进行了对比,认为可能属于同一个晚古生代斑岩铜钼成矿带。     

Relationship between Mesozoic Magmatism and Subduction in the Da Hinggan-Yanshan Area

High-resolution tomographic images of the belt crossing the Japan Trench-Changbai Mountains-Dong Ujimqin Qi are represented in this paper, revealing the shape of a subducted slab in the western Pacific region and characteristics of the lithospheric structures under the Changbai Mountains and the Da Hinggan Mountains. Studies of the spatial distribution, subduction time and the time-lag between the subduction and magmatism, combined with petrology and isotope geochemistry of the Late Mesozoic volcano-plutonic rocks from the Da Hinggan Mountains-Yanshan Mountains have further proved the independence of magmatic activities from the subduction of the Pacific plate. The Mesozoic tectono-thermal evolutionary history and structural characteristics of the lithosphere in the Da Hinggan Mountains and North China suggest that the formation and evolution of magma have probably a close relationship with the delamination and thinning of the continental lithosphere and the underplating resulting from the consequent up     

Interplay of magmatism,sedimentation, and collision processes in the Siberian craton and the flanking orogens

The interplay of geodynamic and sedimentation processes in the Central Asian orogen and the Siberian craton is discussed in several aspects: (i) general tectonics of the Central Asian orogen, (ii) correlation of deposition and collision events, (iii) deposition history and sediment sources on the northern and eastern margins of the Siberian craton, compared, and (iv) history of the Central Asian orogen (Altaids) and formation of Early Mesozoic sedimentary basins.Chemical and isotope compositions and geochronology of Neoproterozoic–Paleozoic sedimentary sequences indicate deposition synchronicity in basins of different types, within both the craton and the orogen. Thus geodynamic models of deposition in separate basins provide reliable evidence of the history of orogens flanking the Siberian craton.The study has confirmed the existence of the Vendian–Early Paleozoic Charysh–Terekta–Ulagan–Sayan–Olkhon strike-slip suture between the continental-margin complexes of Siberia and Kazakhstan, with the crust of juvenile and mixed types, respectively. Late Paleozoic large-scale strike-slip faulting deformed the previous tectonic framework and caused tectonic mixing of the older structures on different margins. This superposed deformation makes it difficult to decipher the paleogeography, paleotectonics, and paleogeodynamics of the Central Asian orogen.     

Paleoproterozoic accretion in the Northeast Siberian craton: Isotopic dating of the Anabar collision system

Geochronological database considered in the work and characterizing the Anabar collision system in the Northeast Siberian craton includes coordinated results of Sm-Nd and Rb-Sr dating of samples from crustal xenoliths in kimberlites, deep drill holes, and bedrock outcrops. As is inferred, collision developed in three stages dated at 2200–2100, 1940–1760, and 1710–1630 Ma. The age of 2000–1960 Ma is established for substratum of mafic rocks, which probably originated during the lower crust interaction with asthenosphere due to the local collapse of the collision prism. Comparison of Sm-Nd and Rb-Sr isochron dates shows that the system cooling from ≈700 to ≈300°C lasted approximately 300 m.y. with a substantial lag relative to collision metamorphism and granite formation. It is assumed that accretion of the Siberian craton resulted in formation of a giant collision mountainous structure of the Himalayan type that was eroded by 1.65 Ga ago, when accumulation of gently dipping Meso-to Neoproterozoic (Riphean) platform cover commenced.     

Fragments of oceanic islands in accretion–collision areas of Gorny Altai and Salair, southern Siberia, Russia: early stages of continental crustal growth of the Siberian continent in Vendian–Early Cambrian time

The Altai-Salair area in southern Siberia is a Caledonian folded area containing fragments of Vendian–Early Cambrian island arcs. In the Vendian–Early Cambrian, an extended system of island arcs existed near the Paleo-Asian Ocean/Siberian continent boundary and was located in an open ocean realm. In the present-day structural pattern of southern Siberia, the fragments of Vendian–Early Cambrian ophiolites, island arcs and paleo-oceanic islands occur in the accretion–collision zones. We recognized that the accretion–collision zones were mainly composed of the rock units, which were formed within an island-arc system or were incorporated in it during the subduction of the Paleo-Asian Ocean under the island arc or the Siberian continent. This system consists of accretionary wedge, fore-arc basin, primitive island arc and normal island arc. The accretionary wedges contain the oceanic island fragments which consist of OIB basalts and siliceous—carbonate cover including top and slope facies sediments. Oceanic islands submerged into the subduction zone and, later were incorporated into an accretionary wedge. Collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high-pressure rocks. Our studies of the Gorny Altai and Salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites. Therefore, it is important to recognize paleo-islands in folded areas. The study of paleo- islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high-pressure rocks.     

内蒙古东部双井子中三叠世同碰撞壳源花岗岩的确定及其对西伯利亚与中朝古板块碰撞时限的约束

内蒙古东部西拉木伦河以北双井子花岗岩侵入古生代晚期增生-碰撞杂岩,遭受了中生代强烈韧性构造变形的改造,在岩石学方面以含有白云母为特征,在岩石化学方面表现为富SiO2、Al2O3和碱质,Na2O/K2O比值小于1,A/CNK比值为1.02～1.08,A/NK比值为1.20～1.35,轻稀土元素富集,Eu异常不明显,微量元素中Ti和Sc亏损、特别是Nb和Ta亏损、Th相对富集,^143Nd/^144Nd同位素比值介于0.512229～0.512286,143Nd/144Nd初始比值为0.512068～0.512199,其εNd（t）均为负值,介于-2.71～-5.28,Nd模式年龄介于1000～1300Ma;^87Sr/^86Sr同位素比值介于0.713516～0.722096,^87Sr/^86Sr初始比值为0.705756～0.709187.这些特征揭示出该岩体是壳源的,可能主要来源于古生代增生-碰撞杂岩和相对古老的大陆边缘的重熔.从该岩体采集的两个样品选出的锆石SHRIMP U-Pb年龄分别为229.2±4.1Ma和237.5±2.7Ma,表明该岩体是在三叠纪中期侵位的.结合区域地质资料和前人相关研究成果,推测西伯利亚与中朝古板块之间沿西拉木伦缝合带的碰撞始于二叠纪中期（约270Ma）,于三叠纪中期结束;从三叠纪岩浆岩的露头推测,这一碰撞事件形成了从北山向东通过内蒙古南部到吉林中部的近东西走向的巨型山脉;区域上晚三叠世岩浆活动形成于该山脉演化晚期的伸展构造背景,标志着该区地壳演化新阶段的开始.     

大兴安岭—燕山晚中生代岩浆活动与俯冲作用关系

本文通过对华北的大兴安岭－燕山地区晚中生代火山－深成岩岩石化学、同位素地质学的分析，认为该地区岩浆的形成和演化与板内伸展环境下的底侵作用有关，而与太平洋板块俯冲无直接的成因联系。本文展示了较高分辨率的日本海沟－长白山－蒙古额尔德尼查干的地震层析图像，揭示了太平洋俯冲板片的形态特征；再结合对俯冲时间及俯冲作用与岩浆作用时差的研究，进一步证明板块俯冲与研究区的岩浆活动无直接关系。最后通过底侵作用年代学的研究，将大兴安岭－燕山中生代岩浆活动与层析图象显示的大陆板内软流圈上涌体联系起来。     

Proterozoic basic magmatism of the Siberian Craton: Main stages and their geodynamic interpretation

Geological data on the Precambrian basic complexes of the Siberian Craton and their isotopic age are considered. The three main episodes of Precambrian basic magmatism of Siberia correspond to certain stages of the geodynamic evolution of the craton and the Earth as a whole. In the Late Paleoproterozoic, largely in the south and the north of the craton, the basic rocks were emplaced against the background of post-collision extension, which followed the preceding collision-accretion stage responsible for the formation of the craton. In the Mesoproterozoic, primarily in the north of the craton, basic magmatism was controlled by dispersed within-plate extension apparently caused by the impact of a mantle plume. Neoproterozoic basic magmatism widespread in the southern and southeastern parts of the craton was caused by rifting, which accompanied breakdown of the Rodinia supercontinent and opening of the Paleoasian ocean along the southern margin of the Siberian Craton.     

Biogeographic relations between the northern and southern continents during the Mesozoic and Cenozoic

Analysis of the affinities of terrestrial vertebrates and plants in the northern and southern hemispheres from early Mesozoic times onwards ought to throw light on the changing positions of the continents and varying role of Tethys as a marine barrier. In the Triassic the cosmopolitan distribution of reptiles and ferns indicate a persistent land communication between Laurasia and Gondwana. Jurassic dinosaurs and plants also exhibit strong affinities between the two hemispheres, implying free land communication until quite late in the period, and at least intermittent land connections must be inferred for the Cretaceous. Standard plate tectonic reconstructions of a Pangaean supercontinent and a low stand of sea level in the Triassic and early Jurassic are consistent with the biogeographic inference of free north-south land communication but later continental breakup and eustatic rise of sea level pose a problem.Mammal faunas indicate that by mid Eocene times India was united with Asia, an interpretation supported by local geological and palaeomagnetic data. Africa was effectively isolated by sea from Eurasia until the late Oligocene — early Miocene, when free intermigration of land vertebrates commenced in the Middle East. At the same time an essentially uniform Tethyan invertebrate fauna was broken up into eastern and western components on either side of the Middle East land corridor, as a consequence of convergence of Africa-Arabia on Eurasia and a significant fall of sea level. North and South America were separated by a significant marine barrier until the Pliocene, when the Panama Isthmus was uplifted. This allowed free intermigration of land mammals while promoting divergence of formerly homogeneous marine faunas on either side of the isthmus.Disparities with plate tectonic reconstructions are noted, and it is suggested that the width of the Tethyan barrier may have been overestimated. Possible geotectonic implications are briefly discussed.

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Zusammenfassung Die Analyse der Beziehungen terrestrischer Vertebraten und Pflanzen der nördlichen und südlichen Hemisphäre vom frühen Mesozoikum an vermag die sich verändernden Positionen der Kontinente und die unterschiedliche Bedeutung der Tethys als eine marine Barre zu erhellen. Während der Trias weist die weltweite Verbreitung von Reptilien und Farnen auf eine durchgehende Landverbindung zwischen Laurasia und Gondwana hin. Jurassische Dinosaurier und Pflanzen belegen ebenfalls deutliche Beziehungen zwischen den beiden Hemisphären, woraus sich eine freie Landverbindung bis in den späten Bereich dieser Periode ergibt und schließlich müssen intermittierende Landverbindungen für die Kreide angenommen werden. Die Rekonstruktionen des Pangaea-Superkontinents und der niedrige Meeresspiegelstand der Trias und frühen Jurazeit stimmen überein mit den biogeographischen Deutungen einer freien Nord-Süd-Landverbindung, jedoch bereiten das spätere Auseinanderbrechen des Kontinents und der eustatische Meeresspiegelanstieg Probleme.Säugerfaunen zeigen an, daß noch im Mitteleozän Indien mit Asien vereinigt war, eine Deutung, die durch lokale geologische und paläomagnetische Daten erhärtet wird. Afrika war vollständig von Eurasien durch das Meer getrennt bis in das späte Oligozän—frühe Miozän hinein, als der freie Austausch von Landvertebraten im mittleren Osten begann. Zur gleichen Zeit teilte sich eine einheitliche Tethys-Invertebratenfauna in östliche und westliche Komponenten auf entlang beider Seiten des Landkorridors des mittleren Ostens. Dieses war die Folge des Zusammenschlusses von Afrika-Arabien mit Eurasien und einem signifikanten Absinken des Meeresspiegels. Nord- und Südamerika waren bis in das Pliozän hinein durch eine deutliche marine Barre voneinander getrennt, nämlich bis sich der Panama-Isthmus heraushob. Hierdurch wurde ein freier Austausch der Landsäugetiere möglich, während sich die vorher einheitlichen marinen Faunen auf beiden Seiten des Isthmus auseinander entwickelten.Es gibt Unterschiede zu plattentektonischen Rekonstruktionen und es wird angenommen, daß die Breite der Tethis als Barre eventuell überschätzt wurde. Möglichkeiten geotektonischer Zusammenhänge werden kurz diskutiert.

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Résumé L'analyse des affinités entre vertébrés et plantes terrestres dans les hémisphères nord et sud à partir du début du Mésozoïque devrait jeter le jour sur le changement de position des continents et sur le rôle varié de la Téthys en tant que barrière marine. La distribution cosmopolite des reptiles et fougères du Triasique indique une communication par voie de terre persistente entre la Laurasie et le Gondwana. Les dinosaures et plantes du Jurassique montrent également de grandes affinités entre les deux hémisphères, impliquant une libre communication sur terre presque jusqu'à la fin de la période, et on peut supposer des connections au moins intermittentes pendant le Crétacé. Les reconstructions classiques de la plaque tectonique d'un supercontinent pangéen et un niveau marin bas pendant le Triassique et le début du Jurassique sont en accord avec la théorie biogéographique d'une libre communication nord—sud par voie de terre; mais la séparation ultérieure des continents et la remontée eustatique du niveau des mers pose un problème.La faune mammifère indique que, vers le milieu de l'Eocène, l'Inde était unie à l'Asie. Cette interprétation est confirmée par les données géologiques et paléomagnétiques locales. L'Afrique était en fait isolée de l'Eurasie par la mer jusque vers le fin de l'Oligocène — début du Miocène, quand la libre migration des vertébrés terrestres commença dans le Moyen-Orient. Au même moment, par suite de la convergence Africo-arabique sur l'Eurasie et de la baisse considérable du niveau des mers, une faune invertébrée téthysienne essentiellement uniforme, se divisa en un groupe occidental et un groupe oriental de chaque côté du corridor continental du Moyen-Orient. Le nord et le sud de l'Amérique étaient séparés par une barrière marine considerable jusqu'au Pliocène quand l'isthme re Panama se fût soulevé. Ceci permit l'intermigration des mammifères terrestres et, en même temps, provoqua de chaque côté de l'isthme la divergence d'une faune marine auparavant homogène.Des disparités avec les reconstructions de la plaque tectonique sont notées, et on suggère que la largeur de la barrière téthysienne aurait pu être exagérée. Des implications géotectoniques probables sont brièvement exposées.

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Precambrian sedimentation in the Urik-Iya Graben,southern Siberian Craton: Main stages and tectonic settings

The Paleoproterozoic sedimentary and volcanic-sedimentary sequences of the Urik-Iya Graben at southern flank of the Siberian Craton have been studied. Based on the isotopic U-Pb LA-ICP-MS dating of detrital zircons contained in the clastic fraction of the studied rocks, three main extension stages accompanied by sedimentation are recognized; each stage is characterized by certain types of sediments and conditions of their accumulation. The oldest rocks (Ingashi Formation) mark early extension events (～1.91?1.87 Ga), which were caused by collapse of the orogen that arose due to collision of the Biryusa and Sharyzhalgai blocks. The basin formed as a result of extension is regarded as an aulacogen. Granitoids of the Sayan Complex were emplaced in the cratonic lithosphere at the final stage of the first extension stage. The second stage of extension started ～1.75 Ga ago as a response to the effect of the inferred mantle plume on the lithosphere of the Siberian Craton. It was accompanied by deposition of the Daldarma Formation. Stress inversion took place at the final stage (～1.70 Ga), and an intracratonic fold zone arose at the place of the paleoaulacogen. The third extension stage (1.65?1.60 Ga) corresponds to the time of molasse accumulation in pull-apart basins (Yermosokha Formation). The final stage of rifting was marked by emplacement of granitoids (Chernaya Zima Complex, 1.53 Ga), which completed the active tectonic events in the region. Afterward, the Urik-Iya Graben transformed into a stable intracratonic domain. The available data allow us to revise the tectonic history of the Urik-Iya Graben. In light of new evidence, this structural unit may be interpreted as a long-evolving paleoaulacogen. The series of revealed sedimentation settings reflects the formation of a consolidated continental lithosphere at the southern flank of the Siberian Craton.     

Certain problems of the structure and evolution of transition zones between continents and oceans

A type of continental-oceanic transition zone, referred to as the Columbian transition zone, is distinguished from two other commonly known types of these zones. The subsidence of the Earth's crust, typical of all transition zones, is shown to be connected (by geophysical properties) to the transformation of continental crust into intermediate crust and later into oceanic. The most likely mechanisms of such changes are the basification of continental crust, its foundering, block by block, into the heated upper mantle, and its substitution by new oceanic crust. The evolution of transition zones of the Pacific type is largely influenced by deep faults, which reach down to the level of undepleted mantle. From this level, the volatile products rise to the surface which results in the formation of calc-alkali magmas on island arcs. The Benioff zones are deep faults, whose inclinations are dependent on the density contrasts in the upper mantle on either side of the Benioff zones. The denser mantle flows beneath the mantle of lower density. This phenomenon is depicted by plate tectonics as subduction.On the whole, the evolution of transition zones gives rise to the growth of the oceans at the expense of the continents, though oceanic crust becomes thicker by addition of volcanogenic layers composed of andesite, in the transition zones (type two) of the Pacific type at island arcs.     

Special Issue Devoted to Petroleum Geology in Marginal Sea Deepwater Settings—Take the South China Sea as an Example Preface

High‐resolution tomographic images of the belt crossing the Japan Trench‐Changbai Mountains‐Dong Ujimqin Qi are represented in this paper, revealing the shape of a subducted slab in the western Pacific region and characteristics of the lithospheric structures under the Changbai Mountains and the Da Hinggan Mountains. Studies of the spatial distribution, subduction time and the time‐lag between the subduction and magmatism, combined with petrology and isotope geochemistry of the Late Mesozoic volcano‐plutonic rocks from the Da Hinggan Mountains‐Yanshan Mountains have further proved the independence of magmatic activities from the subduction of the Pacific plate. The Mesozoic tectono‐thermal evolutionary history and structural characteristics of the lithosphere in the Da Hinggan Mountains and North China suggest that the formation and evolution of magma have probably a close relationship with the delamination and thinning of the continental lithosphere and the underplating resulting from the consequent upwelling of the asthenosphere. On the other hand, the large‐scale strike‐slip fault system, resulting from sinistral shearing of the Pacific plate relative to the Asian continent in the Mesozoic, is responsible for the formation and emplacement of magma on the continental margin. It was the intense crust‐mantle interaction, together with structural deformation at the shallower levels that led to the large tectono‐magmatic belt in the East Asian continental margin.     

中国早石炭世早期阶和亚阶的划分与对比

简要回顾了中国早石炭世早期地层“阶”和“亚阶”划分的历史沿革,阐述了划分的基本原则,即在全球性或大区域性关键地质事件和生物进化事件研究的基础上,将“阶”和“亚阶”的底界应置于地质事件之后某一或某些主导生物门类谱系演化的分支点上,且以某一主导生物门类的新分子在谱系中的首次出现来定义。根据地层的分布和发育特征,拟将中国早石炭世地层分为岩关阶和大塘阶2个阶,其中岩关阶可分为3个亚阶：塘林冲村亚阶、大圩镇亚阶和小圩镇亚阶。岩关阶的底界以牙形刺Siphonodella sulcata的首次出现来确定,与现行的国际泥盆—石炭系界线相一致;岩关阶的顶界以牙形刺Gnathodus homopunctatus的首次出现来确定,略高于现行的由有孔虫定义的国际杜内—维宪阶界线位置。认为现行的国际杜内—维宪阶界线没能规避岩关阶晚期事件（海平面下降事件）的影响,大量以Eoparastaffella为代表的底栖有孔虫的出现正是此次事件的产物。