Stratigraphic Significance of Microbialites Near the Frasnian-Famennian Boundary at the Liujing Section, Guangxi, South China

The Frasnian–Famennian (F–F) event marks one of the ‘Big Five’ mass extinction events of the Phanerozoic, which profoundly caused the devastation of metazoan reef ecosystems and proliferation of microbialites. The Liujing section (Hengzhou, Guangxi), a well-established stratotype marine section of Devonian in China, provides a continuous Upper Devonian succession. This study revisited Liujing section from high-resolution carbon isotope records, conodonts biostratigraphy and sedimentological analysis, with emphasis on two distinct microbialite development. The lower microbialite layer (~50 cm thick), occurring in the uppermost Frasnian linguiformis Zone, consists of Renalcis–Shuguria dendrolites and is interpreted as having been transported from upper slope settings, coinciding with the main excursion of the Upper Kellwasser Event. The upper microbialite layer (~4 m thick) is composed of Izhella dendrolites, which also occur in situ within the uppermost Frasnian linguiformis Zone, indicating the possible F–F extinction event horizons in Liujing section. Carbon isotope profiles exhibit several positive excursions (>4‰) within debris-flow deposits, conodonts analysis yield all Frasnian conodont within the rudstone, reflecting potential reworking of older Frasnian carbonates. Whereas the microbialite interval shows stable isotopic values consistent with enhanced organic carbon burial and elevated primary productivity during the crisis. The combined results highly demonstrates that the F–F boundary is positioned near above the two microbialite layers, with the Renalcis–Shuguria dendrolites and the Izhella dendrolites marking the microbial resurgence in a relatively stable, low-oxygen slope environment. However, different dominant microbes indicate distinct proliferation background, which coincide with the frequent changing environment during the extinction event. These results strongly suggest that the microbes blooming start before the extinction event and underscore the critical role of high-resolution stratigraphic framework in deciphering the tempo and mode of microbialite development in response to the F–F event, and provide a reference for correlating shallow- to deep-water microbial facies across the F–F transition in South China.