Biostratigraphic Correlation of the Permian–Triassic Transition in the Weibei Area, Shaanxi Province, Based on Mixed Continental–Marine Biotas

The Permian–Triassic mass extinction caused severe biotic collapse in both marine and continental ecosystems. Although the marine Permian–Triassic boundary has been extensively studied and a unified marine stratigraphic framework has been established, direct correlation with continental successions remains problematic. Terrestrial–marine transitional facies provide an important bridge for linking marine and continental records, and their mixed biotas are particularly valuable for biostratigraphic correlation. In this study, we investigated three Permian–Triassic transitional sections in the Weibei area of Shaanxi Province, namely the Shichuanhe, Zishiya, and Houzhougongmiao sections. Based on systematic taxonomic study of mixed continental–marine biotas, we re-established the biotic composition and assemblage characteristics of the Permian–Triassic succession in this area. A total of 5 species in 5 genera, 19 species in 16 genera, and 17 species in 14 genera were identified from the Shichuanhe, Zishiya, and Houzhougongmiao sections, respectively. Four fossil assemblages were recognized. In the Shichuanhe section, the conchostracan Euestheria gutta–Palaeolimnadiopsis vilujensis assemblage from the middle Sunjiagou Formation indicates the Permian–Triassic transitional interval. In the Zishiya section, the Modiolus sp.–Wilkingia sp. bivalve assemblage from the Sunjiagou Formation is assigned to the Late Permian, whereas the Palaeoneilo elliptica–Pteria cf. murchisoni assemblage from the Qishan Formation indicates the Early Triassic (Olenekian). In the Houzhougongmiao section, the Eumorphotis multiformis–Claraia sp. bivalve assemblage and the Magniestheria mangaliensis–Magniestheria subcircularisconchostracan assemblage from the upper Qishan Formation indicate an Early Triassic (Induan) age. These assemblages clarify the biostratigraphic correlation of the three sections and enrich the record of Permian–Triassic transitional faunas in the Weibei area. They also provide important evidence for correlation between marine and continental strata. In addition, four transgressive beds were recognized across the Permian–Triassic transition, suggesting that at least four transgression events affected the Weibei area during this interval.