High Resolution Biostratigraphy Constrains Diachronous Depositional Events: Examples of the Frasnian-Famennian Transition at the Changtang Section, South China

The biostratigraphic and lithostratigraphic frameworks are frequently interrupted by event depositions, such as storm or slump induced turbidites. Thus, it is a prerequisite to identify these depositional events and to warrant the normal marine depositional sequence before establishing the reliable stratigraphic frameworks. In this study, we conduct conodont biostratigraphy and petrography analyses across the Frasnian-Famennian Boundary (FFB) at the Changtang section in South China. Eight conodont zones are recognized in stratigraphic order across the FFB at the Changtang section, extending from the upper Frasnian Palmatolepis bogartensis Zone to the lower Famennian Pa. crepida Zone. However, conodonts from the Pa. subperlobata Zone through the lower Pa. delicatula platys Zone exhibit a mixed faunal signature, incorporating both Frasnian and Famennian taxa. Petrographic analysis reveals that the Frasnian components are allochthonous, having been reworked from underlying strata and incorporated into a carbonate breccia deposited during the Pa. delicatula platys Zone. Consequently, the FFB at Changtang is situated within an event bed and does not preserve a primary, undisturbed marine depositional sequence. High-resolution biostratigraphic correlation with other FFB sections in South China that record similar event deposits indicates marked diachroneity in the timing of brecciation and turbidite emplacement. This spatiotemporal heterogeneity implies that these sedimentary events were genetically independent at the inter-basinal scale, precluding a synchronous or causal linkage to the Frasnian-Famennian mass extinction. Moreover, quantitative conodont abundance data demonstrate that the pronounced decline in conodont density across the FFB at Changtang is best explained by a collapse in primary productivity rather than by dilution effects associated with elevated sedimentation rates.