The significance of the ammonoids Paratirolites and Otoceras in correlating the Permian–Triassic boundary beds of Iran and the People's Republic of China
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Abstract:
Chinese geologists have correctly interpreted the sequence in south China as including the youngest known marine Permian (Changxingian Stage), followed by earliest Triassic, strata with Otoceras (Griesbachian Stage, Gangetian Substage). Most of the Changxingian ammonoids are known only from China but one recently described species, Shizoloboceras fusuiense, is evidently congeneric with Paratirolites vediensis, which characterizes latest Permian (Dorashamian) beds of the south U.S.S.R. and Iran. This indicates that the youngest Permian beds of Iran and China are correlative. Alternative correlations which have been suggested, namely with Changxingian including beds younger than Dorashamian, and Gangetian correlative with Dorashamian, are rejected. Below the Changxingian is the Lopingian (or Wuchiapingan), characterized by a variety of early otocerataceans. Lopingian is more or less correlative with Dzhulfian.South China is the only known place where ammonoids of Dzhulfian (= Lopingian), Dorashamian (= Changxingian), and Gangetian (lowermost Triassic) ammonoids occur in a formational sequence. It does not necessarily follow that the Changxingian–Gangetian interval was one of faunal continuity and continuous deposition. Paleozoic-type brachiopods that locally occur in the basal metre of the Triassic formations do not establish that the relationship between the Permian and Triassic formations is transitional. The boundary between these formations is distinct. Probably, these brachiopods are derived from the subjacent Permian strata and are not natural members of the Triassic fauna.Keywords:
Early Triassic
Conodont
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Correlative
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Early Triassic
Ravine
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Conodont
Radiolaria
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Conodont
Early Triassic
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Permian–Triassic extinction event
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Permian–Triassic extinction event
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Early Triassic
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Conodont
Early Triassic
Extinction (optical mineralogy)
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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 PermianTriassic 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.
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Chaohu is located in a deep part of carbonate ramp on the Lower Yangtze Block, which belonged to the low-latitude eastern Tethyan archipelago during the Early Triassic. Fossils were very rich in the Lower Triassic of Chaohu. Bivalves, ammonoids, conodonts were very common throughout the Lower Triassic, while fish fossils were generally rich in some beds of the upper part. It is one of the most typical sections for the Early Triassic chronostratigraphy in the world. Although various fossils had been studied in the 1980s and 1990s, recent studies based upon new and more detailed collections from the Lower Triassic of Chaohu showed that the conodont zonation needs revision. We collected Lower Triassic conodont fossils from continuous sections of the West Pingdingshan, North Pingdingshan and South Majiashan, Chaohu, Anhui Province, and updated zonations were made for each section. Eight conodont zones have been distinguished. They are, in ascending order, Hindeodus typicalis zone, Neogondolella krystyni zone, Neospathodus kummeli zone, Neospathodus dieneri zone, Neospathodus waageni zone, Neospathodus pingdingshanensis zone, Neospathodus homeri zone, and Neospathodus anhuinensis zone. The first occurrence of Neospathodus waageni eowaageni of the N. w. eowaageni subzone (i.e. the base of the N. waageni zone) is suggested as the marker to define the Induan–Olenekian boundary.
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Early Triassic
Chronostratigraphy
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In 1979 I made a preliminary attempt to correlate three important sections of Permian and Triassic rock graphically (Sweet, 1979). In the sections considered in that study, data on the ranges of conodonts in strata thought to be Lower Triassic were adequate to suggest that further work might result in a high-resolution biostratigraphic scheme of worldwide significance. Consequently, considerable time was invested in assembling data on the ranges of conodonts in widely distributed sections of Lower Triassic strata and, in 1988 (Sweet, 1988b) a high-resolution biostratigraphic framework was established for the Lower Triassic. That scheme resulted from graphic correlation of sections in Kashmir, Pakistan, Japan, western United States, northern Italy, and far-eastern USSR. Correlation based on the measured ranges in those sections of 29 conodont species resulted in a framework that is divisible at the 95% confidence level into 21 chronozones (or Standard Time Units), each equivalent to a unit 5 m thick in the standard reference section at Guryul Ravine, Kashmir, and each representing approximately the same interval of time.
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