The Sedimentary Record of Marine–Continental Transitional Shales in the Upper Triassic of Xujiahe Formation, Southeast Sichuan Basin, China

Marine–continental transitional shale is a focus of global energy exploration, offering significant but underexplored hydrocarbon potential. Unlike well-studied marine shales, these deposits pose challenges due to complex interactions between marine and continental influences. The lower Xujiahe Formation in the southeastern Sichuan Basin exemplifies this uncertainty, with its depositional environment debated as either continental or transitional. Resolving this issue is critical for refining facies models and improving exploration strategies. This study aims to determine the depositional environment of the lower Xujiahe Formation by integrating sedimentological, paleontological, and geochemical evidence. Field observations identify tidal rhythmites, reverse cross-stratification, and double mud drapes, indicative of tidal influence. Fossil assemblages, including Sulcusicystis sp. and marine-influenced sporopollen sequences, further support marine influence and align with records from the Tanba and Qilixia sections in northeastern Sichuan. Geochemical analysis reveals Sr concentrations (24.47–194.43 ppm), Sr/Ba ratios (0.11–0.65), m-values (4.37–33.08), and CaO/(Fe + CaO) ratios (0.03–0.80), suggesting freshwater to brackish conditions. V/Cr (0.92–2.22) and U/Th (0.18–0.48) ratios indicate a weakly oxidizing environment. Kerogen analysis classifies the organic matter as type II2–III, suggesting periodic marine influence during deposition. These findings confirm that the lower Xujiahe Formation represents a marine–continental transitional facies, refining previous facies interpretations and providing a basis for more targeted shale gas exploration in the Sichuan Basin and comparable basins worldwide.