Abstract

New biostratigraphic (ammonoids and conodonts) and geochemical data (carbonate carbon and oxygen stable isotopes, 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'antang Formation lies on top of the Tianjingshan Formation, which is identified by the occurrence of peritidal cycles with stromatolite layers, and includes a distinct karst surface in its upper part. Below the karst surface, negative trends in carbonate carbon and oxygen isotopes can be interpreted as the result of meteoric diagenesis. Above, the Ma'antang Formation is subdivided into five units. The lower part (Units 1–3) mostly consists of bioclastic limestones with thin clayey siltstone intercalations, but at least three intervals of terrigenous deposits are present. A lithological change from mainly carbonates to terrigenous clastic rocks occurs between Units 3 and 4. This interval is biostratigraphically well constrained by the occurrence of ammonoids from the clayey 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 1 and 2, which should be dated to the early Carnian and could be correlated to similar perturbations in western Tethys and in the Nanpanjiang Basin, South China. These correlated carbon isotopic oscillations were attributed to the onset of the Carnian Pluvial Episode (CPE). The two known Chinese records of the CPE both display a broad negative carbon isotopic excursion that encompasses a significant part of the Carnian, whereas the western Tethys records feature possibly multiple, short-lived isotopic excursions. The present isotopic data show that the CPE began roughly contemporaneously with the first terrigenous input at Ma'antang. It is unclear whether the minor siliciclastic intervals within Units 1–3 at Ma'antang could be correlated to similar terrigenous episodes in the western Tethys region. So far, Ma'antang is the only locality of the Sichuan Basin where the carbon isotopic perturbation related to the Carnian Pluvial Episode has been documented.