New biostratigraphic (ammonoids and conodonts) and geochemical data (carbonate carbon and oxygen stableisotopes, and organic carbon stable isotopes) are presented for the Carnian (Upper Triassic) marine succession of Ma’antang,Jiangyou County, in the Upper Yangtze Block (NW margin of the Sichuan Basin, China). In this locality, the Ma’antangFormation lies on top of the Tianjingshan Formation, which is identified by the occurrence of peritidal cycles with stromatolitelayers, and includes a distinct karst surface in its upper part. Below the karst surface, negative trends in carbonate carbon andoxygen isotopes can be interpreted as the result of meteoric diagenesis. Above, the Ma’antang Formation is subdivided into fiveunits. The lower part (Units 1–3) mostly consists of bioclastic limestones with thin clayey siltstone intercalations, but at leastthree intervals of terrigenous deposits are present. A lithological change from mainly carbonates to terrigenous clastic rocksoccurs between Units 3 and 4. This interval is biostratigraphically well constrained by the occurrence of ammonoids from theclayey siltstone of Unit 4, suggesting a Julian 2 age (Early Carnian), and conodont associations allow us to place the Julian–Tuvalian boundary at 195 m, the upper part of Unit 4. A wide negative carbon isotope oscillation of about 2‰occurs in Units 1and 2, which should be dated to the early Carnian and could be correlated to similar perturbations in western Tethys and in theNanpanjiang Basin, South China. These correlated carbon isotopic oscillations were attributed to the onset of the CarnianPluvial Episode (CPE). The two known Chinese records of the CPE both display a broad negative carbon isotopic excursionthat encompasses a significant part of the Carnian, whereas the western Tethys records feature possibly multiple, short-livedisotopic excursions. The present isotopic data show that the CPE began roughly contemporaneously with the first terrigenousinput at Ma’antang. It is unclear whether the minor siliciclastic intervals within Units 1–3atMa’antang could be correlated tosimilar terrigenous episodes in the western Tethys region. So far, Ma’antang is the only locality of the Sichuan Basin where thecarbon isotopic perturbation related to the Carnian Pluvial Episode has been documented.

The carnian pluvial episode at ma’antang, jiangyou in upper yangtze block, southwestern China

Jin, Xin;Preto, Nereo;Rigo, Manuel;
2019

Abstract

New biostratigraphic (ammonoids and conodonts) and geochemical data (carbonate carbon and oxygen stableisotopes, and organic carbon stable isotopes) are presented for the Carnian (Upper Triassic) marine succession of Ma’antang,Jiangyou County, in the Upper Yangtze Block (NW margin of the Sichuan Basin, China). In this locality, the Ma’antangFormation lies on top of the Tianjingshan Formation, which is identified by the occurrence of peritidal cycles with stromatolitelayers, and includes a distinct karst surface in its upper part. Below the karst surface, negative trends in carbonate carbon andoxygen isotopes can be interpreted as the result of meteoric diagenesis. Above, the Ma’antang Formation is subdivided into fiveunits. The lower part (Units 1–3) mostly consists of bioclastic limestones with thin clayey siltstone intercalations, but at leastthree intervals of terrigenous deposits are present. A lithological change from mainly carbonates to terrigenous clastic rocksoccurs between Units 3 and 4. This interval is biostratigraphically well constrained by the occurrence of ammonoids from theclayey siltstone of Unit 4, suggesting a Julian 2 age (Early Carnian), and conodont associations allow us to place the Julian–Tuvalian boundary at 195 m, the upper part of Unit 4. A wide negative carbon isotope oscillation of about 2‰occurs in Units 1and 2, which should be dated to the early Carnian and could be correlated to similar perturbations in western Tethys and in theNanpanjiang Basin, South China. These correlated carbon isotopic oscillations were attributed to the onset of the CarnianPluvial Episode (CPE). The two known Chinese records of the CPE both display a broad negative carbon isotopic excursionthat encompasses a significant part of the Carnian, whereas the western Tethys records feature possibly multiple, short-livedisotopic excursions. The present isotopic data show that the CPE began roughly contemporaneously with the first terrigenousinput at Ma’antang. It is unclear whether the minor siliciclastic intervals within Units 1–3atMa’antang could be correlated tosimilar terrigenous episodes in the western Tethys region. So far, Ma’antang is the only locality of the Sichuan Basin where thecarbon isotopic perturbation related to the Carnian Pluvial Episode has been documented.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/3291104
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