Isarcicella Isarcica Zone Research Articles

Integrated biochemostratigraphy of the Permian-Triassic boundary interval and Lower Triassic succession in the eastern Cimmerian continent (Baoshan block, West Yunnan, southwest China)

The records of extreme biotic and environmental crises have been well documented worldwide from Permian-Triassic successions. However, the Cimmerian continent, a Gondwana-derived continental terrane/block strip, has received insufficient attention in terms of its eastern part including the Baoshan block. Here, we report an integrated conodont biostratigraphic zonation and carbon isotope profile from the Permian-Triassic boundary (PTB) beds and Lower Triassic succession of the Baoshan block. The conodont faunas obtained from the dolomitized Hewanjie Formation at the Dajianshan section are dominated by Hindeodus and Isarcicella species. Representatives of Eurygnathodus and the Ellisoniidae are also present. Five conodont zones are recognized in ascending order: the Hindeodus parvus Zone, the Isarcicella staeschei Zone, the Isarcicella isarcica Zone, the Hindeodus postparvus Zone, and the Eurygnathodus costatus Zone. The carbon isotope curve exhibits two pulses of negative excursion around the PTB. In general, both the conodont zones and the carbon isotope profile correlate well with those from other well-documented sections in the world. The Permian-Triassic boundary, extinction horizons and Induan-Olenekian boundary are calibrated in terms of conodont and carbon isotopic data. In addition, conodont biofacies analyses show that the depositional water-depth was shallow in general, as indicated by the lack of gondolellids and neospathodids and the presence of ellisonids and Eurygnathodus.

Conodonts and Carbon Isotopes during the Permian-Triassic Transition on the Napo Platform, South China

Two Permian-Triassic boundary (PTB) sections (Pojue and Dala) are well exposed in an isolated carbonate platform (Napo Platform) on the southwestern margin of the Nanpanjiang Basin, South China. These sections provide an insight into the transition across the PTB and a detailed investigation of the conodont biostratigraphy and inorganic carbon isotopes is presented. The PTB at the Pojue Section is placed at the base of Bed 10B (a dolomitized mudstone found below a microbialite horizon), defined by the first occurrence of Hindeodus parvus. At the Dala Section, four conodont zones occur. They are, in ascending order, the Hindeodus parvus Zone, Isarcicella staeschei Zone, Isarcicella isarcica Zone and Clarkina planata Zone. Comparison with the Pojue Section suggests the PTB at Dala also occurs at the base of dolomitized mudstone below a microbialite horizon, although the first occurrence of Hindeodus parvus is near the top of a microbialite bed: an occurrence that is also seen in other platform sections. The succeeding microbialite beds developed during the ongoing PTB mass extinction phase. This time was characterized by low carbon isotope values, and a microbialite ecosystem that provided a refuge for selected groups (bivalves, ostracods and microgastropods) that were likely tolerant of extremely high temperatures.

Biostratigraphy and sedimentology of Upper Permian and Lower Triassic strata at Masore, Western Slovenia

Upper Permian and lowermost Triassic strata of the Masore section in western Slovenia have been restudied by means of bio- and lithostratigraphy. This section is mainly characterized by a carbonate succession of the Bellerophon Formation deposited in a shallow marine ramp environment that was located in the western part of the Paleotethys. The Permian-Triassic boundary (PTB) transition is marked by laminated microbialites – stromatolites interpreted to reflect a deeper ramp environment. The conodont elements recovered enabled the recognition of the Hindeodus praeparvus Zone of the uppermost Permian (Upper Changhsingian) in the Bellerophon Formation. Gondolellids are documented in the PTB transitional interval with microbial microfacies, whereas the Isarcicella isarcica Zone (Lower Griesbachian, Lower Triassic) is recognized in the tectonically separated Werfen Formation just above the microbial microfacies part of the section. The lowermost part of the microbialites is characterized by Late Permian species of foraminifers indicating that at least this part of the section is still Upper Permian.

Conodont biostratigraphy of a new Permo-Triassic boundary section at Wenbudangsang, north Tibet

The Wenbudangsang section is a newly discovered continuous succession of marine carbonates that straddles the Permian–Triassic boundary (PTB), extending from the late Permian to the earliest Triassic (Induan). The section is located in the Wenbudangsang area of Geji County, northern Tibet, and tectonically sits within the northern Gangdise Stratigraphic division. Conodonts are abundant throughout this section, except being comparatively rare and less diverse in the Permian strata below the distinctive PTB Clay Bed.The conodonts are mainly represented by Hindeodus–Isarcicella Pa elements, Neogondolella Pa elements, and ramiform elements, with few Neospathodus Pa elements present at the top of the section. Two conodont zonal successions have thus been determined based on Hindeodus–Isarcicella and Neogondolella species. The Hindeodus–Isarcicella succession includes the uppermost Permian Hindeodus julfensis Zone, Hindeodus praeparvus Zone, and the lowermost Triassic Hindeodus parvus Zone, Isarcicella staeschei Zone, Isarcicella isarcica Zone, and Hindeodus postparvus Zone. The Neogondolella succession consists of the upper Permian Neogondolella changxingensis–Neogondolella deflecta–Neogondolella postwangi Zone, Neogondolella yini–Neogondolella zhangi Zone, Neogondolella meishanensis Zone, the Neogondolella taylorae Zone that crosses the PTB, and the lowermost Triassic Neogondolella krystyni Zone and Neogondolella prediscreta n. sp. Zone. The top of the section is defined by the Neospathodus kummeli Zone and Neospathodus cristagalli Zone. Two new species, Hindeodus microdentatus n. sp. and N. prediscreta n. sp., are established and described in this paper.The PTB conodont successions recognised in the new Wenbudangsang section have been closely correlated herein with other well-known PTB sections: the GSSP Meishan section of South China, sections in Abadeh in Iran, and sections at Spiti in India. Importantly, the conodont faunas from Wenbudangsang now provide an opportunity to effectively correlate the PTB hindeodid and gondolellid zonal schemes given the high abundance of both assemblages, which is unique to this section.

Permian-Triassic conodonts from Dajiang (Guizhou, South China) and their implication for the age of microbialite deposition in the aftermath of the End-Permian mass extinction

The widespread microbialites deposition that followed the End-Permian mass extinction in the Tethyan realm have been intensively studied because of the evidence they provide on the nature of this crisis and its aftermath. However, the age of the microbialite event remains controversial. New conodont collection across the Permian-Triassic (P-T) transition from Dajiang (Guizhou Province, South China) in this study enable us to discriminate four conodont zones, in ascending order, they are: Hindeodus parvus zone, Isarcicella lobata zone, Isarcicella isarcica zone and Hindeodus sosioensis zone. The age of microbialite in the P-T transition at the Dajiang Section is considered to be within the Hindeodus parvus zone and thus to clearly post-date the main extinction crisis. Reviewing the age of onset of microbialites throughout the Tethyan regions reveals two different ages: a Hindeodus changxingensis zone age is dominant in south-western and westernmost Tethys, whilst most other regions show microbialite deposition began in the Hindeodus parvus zone. Our investigation also indicates that two conodont changes occur at this time: an increase of hindeodid species immediately following a sequence boundary and the mass extinction, and a phase of extinction losses in the earliest Triassic Isarcicella isarcica zone during highstand development.

Conodont biostratigraphy and evolution across Permian-Triassic boundary at Yangou Section, Leping, Jiangxi Province, South China

Series of large conodont samples with 20 species and 3 similar species in 3 genera were collected from the Permian-Triassic (P-T) boundary sequence in a shallow carbonate facies at Yangou (沿沟), Leping (乐平) County, Jiangxi (江西) Province, South China. On the basis of the distributions of the identified species, seven conodont zones have been recognized in ascending order as follows, Neogondolella changxingensis zone, Neogondolella yini zone, Hindeodus changxingensis zone, Neogon- dolella taylorae zone, Hindeodus parvus zone, Isarcicella staeschei zone, and Isarcicella isarcica zone. The successive occurrences of Hindeodus changxingensis, Neogondolella taylorae and Hindeodus parvus serve as proxies for defining the P-T biostratigraphy boundary at the base of Sub-bed 21-4 of Bed 21 in the Yangou Section. Correlations with the Meishan Section are also discussed in terms of conodont bio- stratigraphy. Three successive conodont faunal assemblages are grouped through the P-T transitional

A newly discovered earliest Triassic chert at Gaimao section, Guizhou, southwestern China

The Permian–Triassic (PTB) boundary section at Gaimao (Huaxi District, Guiyang, Guizhou Province) records a unique lithological transition between the Permian and Triassic in South China. Thus, bioclastic limestones of the Changxing Formation are overlain by radiolarian-bearing siliceous rocks of the Dalong Formation and ultimately limestone/mudstone alternations of the Shabaowan Formation. Conodont sampling of the section revealed well-preserved elements that enabled a conodont biostratigraphy to be established. The occurrences of Hindeodus julfensis and Hindeodus typicalis in Changxing Formation bioclastic limestone indicate a Late Permian age. Four conodont zones have been established in Dalong and Shabaowan Formations. In ascending order those are the Isarcicella isarcica Zone, Neogondolella (or Clarkina) planata Zone, Neogondolella krystyni Zone and Neospathodus dieneri Zone. The presence of typical Griesbachian conodonts I. isarcica and Isarcicella staeschei in upper Dalong Formation indicates that this formation's radiolarian-bearing siliceous mudrocks are of earliest Triassic age and not restricted to the Permian as previously thought. Thin section analysis reveals that the upper Dalong Formation contains spheroidal radiolarians but not a diverse assemblage of Permian holdover taxa. The new data proves that, following the end-Permian mass extinction of radiolarian, the abundance of these siliceous planktons was maintained at least locally enabling chert deposition to persist. Thus, the Gaimao section provides an exception to the ‘Early Triassic Chert Gap’, the first of its kind in Palaeo-Tethys, and indicates that the end of chert deposition does not always mark the end-Permian mass extinction.

Revised conodont zonation and conodont evolution across the Permian–Triassic boundary at the Shangsi section, Guangyuan, Sichuan, South China

Large conodont samples (20 kg at most) have been collected from the uppermost Permian and the lowermost Triassic (beds 26–33) at Shangsi, northern Sichuan. These have enabled us to establish parallel hindeodid and gondolellid conodont zones through the section similar to previously recognized in the Meishan section. In ascending order, three newly established gondolellid zones at the Shangsi section are: Neogondolella yini Zone (bed 26), Neogondolella meishanensis Zone (beds 27a–27c), Neogondolella taylorae Zone (beds 28a–29 d). Also in ascending order, four revised or newly established hindeodid zones are: Hindeodus changxingensis Zone (beds 28a–29b) Hindeodus parvus Zone (beds 29c–30a), Isarcicella lobata Zone (beds 30b–31a), Isarcicella isarcica Zone (bed 31b and above). According to the new data, the first occurrence of H. parvus is about 2 m lower than the previously reported horizon. On the basis of both hindeodid and gondolellid taxa, the PTB is placed 22 cm above the base of bed 28a. This conodont zonation of the Shangsi section will help improve correlations of global Permian–Triassic boundary sections.

Pyrite framboid study of marine Permian–Triassic boundary sections: A complex anoxic event and its relationship to contemporaneous mass extinction

Size analysis of pyrite framboids has been undertaken on epicontinental Permian- Triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia. Mid-paleolatitude sections from the margins of the Boreal (Spitsbergen, Greenland) and Neotethyan oceans (Western Australia) reveal intense anoxia throughout the Permian-Triassic boundary interval with euxinic conditions frequently developing, and dysoxia encountered even in relatively shallow-water settings above storm wave base. At equatorial paleolatitudes, weakly oxygenated (dysoxic) conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition, although euxinia was rare. Western and eastern Tethyan locations re- veal a complex and unstable redox history: anoxia in the Hindeodus praeparvus Zone was replaced by oxygenated facies in the Permian- Triassic boundary interval (H. changxingensis to H. parvus zones). Oxygen-poor deposition returned during the succeeding Isarcicella isarcica Zone. The more persistent and intense development of oxygen restriction in cooler water, mid-paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of Permian-Triassic bound- ary anoxia. In higher paleolatitudes the ben- thic invertebrate extinctions occurred during a prolonged phase of oxygen-poor deposition, while in equatorial Tethyan locations benthic losses occurred at the end of the fi rst anoxic phase (in the late H. praeparvus Zone).

Early Triassic conodont–palynological biostratigraphy of the Meishan D Section in Changxing, Zhejiang Province, South China

The Lower Triassic, consisting of the Yinkeng Fm., Helongshan Fm. and the lower part of the Nanlinghu Fm., is well developed at the Meishan D Section of Changxing County, Zhejiang Province, South China. Based on detailed study of the conodont biostratigraphy from the interval of the Permian–Triassic boundary and the Lower Triassic in the Meishan D Section, 8 conodont zones are recognized in ascending order as follows: the Neogondolella changxingensis yini– Hindeodus praeparvus Zone (beds 24a–24e, top of the Changxing Fm.), the Neogondolella meishanensis meishanensis– Hindeodus eurypyge Zone (beds 25–27b, base of the Yinkeng Fm.), the Hindeodus parvus Zone (beds 27c–27d, first conodont zone of Triassic, the global index for the base of Triassic), the Isarcicella staeschei Zone (beds 28–29a, lower part of the Yinkeng Fm), I. isarcica Zone (beds 29b–51, lower part of the Yinkeng Fm.), the Neogondolella tulongensis– N planata Zone (beds 52–72, from upper part of the Yinkeng Fm. to lower part of the Helongshan Fm.), the Neospathodus kummeli Zone (beds 73–92, middle part of the Helongshan Fm.), and the Neospathodus cristagalli– N dieneri Zone (beds 93–111, from upper part of the Helongshan Fm. to the base of the Nanlinghu Fm.). At the Meishan D Section, the palynological biostratigraphy is also studied herein. The palynological assemblage of Lundbladispora– Taeniaesporites– Equisetosporites is found in the intervals which contain the conodont Isarcicella isarcica Zone and the lower part of the Neogondolella tulongensis– N. planata Zone.

Restudy of conodont zonation and evolution across the P/T boundary at Meishan section, Changxing, Zhejiang, China

Large conodont samples have been collected from the uppermost Permian and lowermost Triassic (beds 24–29) at the Meishan section — the Global Stratotype Section and Point (GSSP) for the Permian–Triassic Boundary. This new information revises the ranges of some important conodont species in this section. For the first time, usable numbers of Neogondolella specimens have been found from the basal-most Triassic, enabling the establishment of parallel hindeodid and gondolellid zonations through the same section over this interval. In ascending order, the refined hindeodid zones are as follows: Hindeodus latidentatus Zone (bed 24a), Hindeodus praeparvus Zone (24b–26), Hindeodus changxingensis Zone (27a–b), Hindeodus parvus Zone (27c), Isarcicella staeschei Zone (27d–28), Isarcicella isarcica Zone (bed 29 and above). Three revised and newly established Neogondolella zones are proposed. In ascending order these are as follows: Neogondolella yini Zone (bed 24 and below), Neogondolella meishanensis Zone (25–26), Neogondolella taylorae Zone (27a and above). Neogondolella taylorae—a proposed earliest Triassic marker species, first appears in bed 27a, just below specimens of Hindeodus cf. parvus in bed 27b. Three successive conodont faunal assemblages are identified through the Permian–Triassic transitional interval.

Biostratigraphy and event stratigraphy in Iran around the Permian–Triassic Boundary (PTB): Implications for the causes of the PTB biotic crisis

The conodont succession and stratigraphic events around the Permian–Triassic boundary (PTB) have been investigated in detail in the open sea deposits of Iran (Abadeh and Shahreza in central Iran, and Jolfa and Zal in northwestern Iran). This investigation produced a very detailed conodont zonation from the Clarkina nodosa Zone up to the Isarcicella isarcica Zone. All significant events have been accurately located and dated within this zonation, and the duration of most of these conodont zones has been calculated by cross-correlation with continental lake deposits that display obvious Milankovitch cyclicity. The unusually short duration of all conodont zones in the interval from the C. nodosa up to the Hindeodus parvus Zone indicates that there was persistent high ecological stress during this time interval. Most of the conodont zones can be accurately correlated with South China. In the interval from the C. hauschkei Zone to the H. parvus Zone, even correlation with the Arctic is possible. Within three thin stratigraphic intervals, the Changhsingian (Dorashamian) warm water conodont fauna of the C. subcarinata lineage is replaced by a cool water fauna with small H. typicalis, rare Merrillina sp., and cool water Clarkina that have very widely spaced denticles. The uppermost cool water fauna horizon comprises the lower C. zhangi Zone and can be accurately correlated with continental beds by recognition of a short reversed magnetozone below the long uppermost Permian–lowermost Triassic normal magnetozone. In Iran and Transcaucasia, this short reversed zone comprises the upper C. changxingensis– C. deflecta Zone and most of the C. zhangi Zone. Its top lies 50 cm below the top of the Paratirolites Limestone (s.s.) in the Dorasham 2 section, which is at the beginning of the upper quarter of the C. zhangi Zone. In the Germanic Basin, this short palaeomagnetic interval comprises the lower and the basal part of the upper Fulda Formation. On the Russian Platform, the Nedubrovo Formation belongs to this short reversed magnetic interval. In its upper part (corresponding to the top of the lower C. zhangi Zone, see above) there is a fallout of mafic tuffs from the Siberian Trap event that originated about 3000 km away in eruption centres in the Siberian Tungusska Basin. In the Germanic Basin and in Iran, this horizon contains volcanic microsphaerules. Thus, a direct correlation can be made between the immigration of a cool water fauna into the tropical realm and an exceptionally strong interval of explosive activity during the Siberian Trap volcanic episode. These faunal changes are the same as those found at the base of the Boundary Clay, suggesting that a short cooling event at this horizon also was due to intense volcanism. Additional influence by a bolide impact cannot be excluded. Most of the events in the interval from the C. nodosa up to the I. isarcica Zone (upper Changhsingian to middle Gangetian) in the Iranian sections can be also observed in other marine sections (e.g., in Meishan) and even in continental sections of the Germanic Basin. Of particular significance is the fact that, in the investigated Iranian sections, the PTB lies either in red sediments or in light grey sediments (as in Abadeh) that contain an ostracod fauna indicative of highly oxygenated bottom waters. Therefore, anoxia cannot be the reason for the PTB extinction event in this region, even though anoxia does cause locally or regionally elsewhere an overprint on the extinction event.

Conodont zonation across the Permian–Triassic boundary at the Xiakou section, Yichang city, Hubei Province and its correlation with the Global Stratotype Section and Point of the PTB

Xiakou section is situated north of the central Yangtze platform with successive Late Permian to Early Triassic strata, thicker than the Global Stratotype Section and Point (GSSP) of the Permian–Triassic boundary (PTB). We recognized three conodont zones across the PTB in this section by high-precision biostratigraphy. They are the Hindeodus latidentatus – Clarkina meishanensis Zone, Hindeodus parvus Zone, and Isarcicella isarcica Zone in ascending order, which correspond well with the GSSP. The present biostratigraphic work on Xiakou section certainly confirmed the conodont zonation through the PTB at the GSSP and also testified to the correctness of the PTB demarcation in Xiakou section. Based on this precise demarcation of the PTB, the appearance of a clay layer, bed ZSJI266, just beneath the PTB should be an important advantage over the GSSP, because this clay layer approaches to the PTB much in contrast with those two clay layers, bed 26 and bed 28, in the GSSP. Therefore, the radiometric isotopic dating in bed ZSJI266 at Xiakou section should give a more accurate age for the PTB.