Earliest Triassic Microbialite Micro- to Megastructures in the Huaying Area of Sichuan Province, South China: Implications for the Nature of Oceanic Conditions after the End-Permian Extinction

摘要

The end-Permian extinction reflects one of the greatest biotic crises in earth history. Evidence of the event and its aftermath is well preserved in Permian/Triassic transitional strata of Sichuan Province in South China, originally deposited in the western part of the Yangtze epicontinental sea. Lowest Triassic strata (the Hindeodus parvus conodont Zone) lie with apparent conformity on uppermost Permian skeletal packstone and wackestone, and are overlain by deeper subtidal thinly bedded argillaceous lime mudstone. These beds are characterized by thrombolitic mesostructures that exhibit macrostructures of highly variable columnar shapes, showing a distinctive stratigraphic succession in bed form from planar to domed and subspherical forms. Spheroidal and ellipsoidal micritic bodies (30 µm in average diameter) are preserved in clusters and are interpreted as calcified coccoidal microbes. The earliest Triassic thrombolitic columns and masses are considered to have been constructed by vertical and lateral accretion of mesoclots of microbial origin. Even after the end-Permian extinction, microbial formation of carbonates apparently predominated in Sichuan in localized subtidal to intertidal environments.