Stratigraphic Correlation and Sedimentary Environment Analysis of the Cambrian Qiongzhusi Formation, Southern Sichuan Basin: Insights from Trace Fossils

The Qiongzhusi Formation of the Cambrian in southern Sichuan Basin is a key interval for shale gas exploration. The analysis of trace fossils and body fossils is critical for regional stratigraphic correlation and sedimentary environment analysis. Based on field outcrop sections and drilling cores of the Qiongzhusi Formation, this study presents an integrated analysis of trace fossils and body fossils. Fossil types and their vertical distribution characteristics were statistically analyzed, and well-outcrop correlation was conducted by integrating lithological and logging data to evaluate the significance of biological evolution in indicating changes in sedimentary environments and stratigraphic boundaries. A total of 8 genera and 8 species of trace fossils, and 14 genera and 19 species of body fossils, were identified in the study area. Representative trace fossils include Planolites, Palaeophycus,Helminthopsis, and Neonereites. Body fossils mainly consist of trilobites, palaeocopids, brachiopods, sponges, and zhijinites. The results show that vertical variations in trace fossils and body fossils record a transition of sedimentary environments from deep-water basin and deep-water shelf to shallow-water shelf. The occurrence of the deep-water trace fossil Neonereites in Member 3 indicates the deepest water conditions and may correspond to the maximum flooding surface. Between Members 5 and 6, a shallowing-upward trend in water depth indicates a regressive event. In Member 7, trace fossil diversity and bioturbation intensity reach their peak, representing the most flourishing stage of the ecosystem. The coordinated variations in trace fossils and body fossils effectively record changes in sedimentary environments and sea level, and provide reliable biostratigraphic markers for identifying regressive surfaces and maximum flooding surfaces. This study provides biostratigraphic evidence for the fine-scale stratigraphic division and reconstruction of the regional sedimentary system of the Qiongzhusi Formation in southern Sichuan Basin.