High resolution bio- and chemostratigraphic framework at the Frasnian-Famennian boundary: Implications for regional stratigraphic correlation between different sedimentary facies in South China
Abstract As one of the “Big Five” mass extinctions in Phanerozoic, the Frasnian-Famennian (F-F) boundary event records the extinction of 40 % of marine invertebrate genera. The F-F boundary is defined by the first appearance of conodont Palmatolepis subperlobata as well as the abundant occurrences of Pa. ultima at the base of the Lower Pa. triangularis Zone. The F-F boundary is also broadly coincident with a positive excursion in carbon isotopes, suggesting the possible linkage between the perturbation of marine carbon cycle and the biotic crisis. However, the correlation between conodont biostratigraphy and carbon isotope chemostratigraphy has not been established yet, preventing the high-resolution regional stratigraphic correlation among different sedimentary facies. In this study, we studied the conodont biostratigraphy and analyzed carbonate carbon (δ 13 C carb ) and organic carbon (δ 13 C org ) isotopes of the Baisha section in the Guangxi Province, South China. The first appearance of index conodont fossil Pa. subperlobata for the Lower Pa. triangularis Zone, i.e. the F-F boundary, is coincident with the onset of a 3‰ positive excursion in δ 13 C carb , while Pa. triangularis and Pa. subperlobata occur nearly simultaneously, but predate the disappearances of Pa. linguiformis and other typical Frasnian elements. In addition, a positive excursion in δ 13 C org is associated with the decline of δ 13 C carb near the base of the Middle Pa. triangularis Zone, which is marked by the first appearance of Pa. delicatula . Another important Famenian element Pa. minuta , the index fossil for the Upper Pa. triangularis Zone, first appears when δ 13 C org starts to decline. The offset between δ 13 C org and δ 13 C carb excursions could be attributed to the oceanic anoxia (the Kellwasser Event) near the F-F boundary, followed by the mixing of surface and deep water in earliest Famenian. The high resolution bio- and chemostratigraphic framework can be used as the standard for regional stratigraphic correlation and identification of the F-F boundary in different depositional environments in South China. We propose that the F-F boundary can be identified from a conodont-poor carbonate section by the onset of the positive excursion in δ 13 C carb , whereas the F-F boundary in a non-fossiliferous siliciclastic section might be placed slightly below the positive excursion in δ 13 C org .