Frasnian-Famennian Boundary and Extinction Event in Pelagic and Platform Carbonate Facies of South China

During the Late Devonian, the South China Block was located near the equator along the western margin of the Tethys Ocean, where carbonate sedimentation was highly developed. The Upper Kellwasser Event at the Frasnian–Famennian boundary represents the peak of the Late Devonian mass extinction and had a devastating impact on its ecosystems. However, studies of the F–F boundary and the associated extinction in South China remain relatively limited, partly because of controversies surrounding Frasnian conodont zonation and the relatively limited research on shallow-water platform facies. This study examines multiple sections from both pelagic and platform settings in South China, focusing on the characteristics of the F–F boundary, the evolution of conodont faunas, and the sedimentological impacts of the Upper Kellwasser Event. At the pelagic carbonate facies of the Lali and Duan sections in Guangxi Province, the Wuzhishan Formation is dominated by fossil-poor mudstones to wackestones. The conodont fauna is mainly composed of Palmatolepid species, whose stratigraphic ranges allow a correlation between the “Late Devonian Standard Conodont Zones” of Ziegler and Sandberg and the “Frasnian Conodont Zones” of Klapper and Kirchgasser. The Upper Kellwasser Event at the F-F boundary eliminated almost all Frasnian Palmatolepid species, and its event horizon is marked by black sediments and the onset of a positive carbon-isotope excursion. At the platform facies of the Quanzhou section, the late Frasnian Guilin Formation contains metazoan–microbial reefs with abundant skeletal builders, including corals and stromatoporoids. In contrast, the Famennian Dongcun Formation consists mainly of mottled wackestones with sparse fossils but abundant microbial aggregated grains (e.g., oncoids, cortoids, and lumps). The conodont fauna in this interval occurs mainly in the Famennian and is dominated by icriodid and polygnathid species. The Upper Kellwasser Event at the F-F boundary is constrained by the disappearance of Frasnian skeletal reef builders, the first appearance of Early Famennian conodonts (Palmatolepis subperlobata,Icriodus lateralis, and Icriodus deformatus), and the onset of a positive carbon-isotope excursion. The event horizon is further marked by a 1.5-m-thick interval of thrombolitic stromatolites which may be formed by a local microbe bloom. These sections improve the biostratigraphic correlation of the Frasnian–Famennian boundary between South China and other pelagic and platform carbonates successions, and help refine the conodont zonation framework for the region. They also demonstrate that the Upper Kellwasser Event had a profound impact on both platform and pelagic ecosystems in South China.