Geochemical characteristics and paleo-environmental implications of Middle Permian Qixia Formation in Wangcang, northern Sichuan

The sedimentary paleoenvironment of the Middle Permian Qixia Formation in northeastern Sichuan Basin was studied based on outcrop observations and laboratory analyses. The data correlations of the δ¹³C and δ¹⁸O of 19 samples from the Qixia Formation showed that there was no linear relationship between them. The samples are reliable targets for paleoenvironmental analyses since δ¹⁸O > -6‰, Mn/Sr ≤ 0.6, Fe/Sr < 3, and MgO/CaO < 0.125. The contents of terrigenous elements (Al₂O₃ and TiO₂) are low, and are strongly and positively correlated. The ΣREE and Σ(Al₂O₃+TiO₂) also showed a positive relationship. The characteristics of other main and trace elements indicated that the Qixia Formation was formed in a hypoxic or oxygen-depleted environment. The Sr/Ba value ranges from 21.9-261.9, indicating that the sea level was relatively high, so the terrigenous material content is low, and the terri-genous detrital sediments were controlled by paleo-water depth during sedimentation. The carbon and oxygen isotopes revealed that the ancient oceanic temperature ranged from 17.62-28.43℃, the paleosalinity ranged from 129.07-135.21, and the ratio of MgO/CaO ranged from 0.007-0.029, revealing that the paleoenvironmental climate was relatively warm and humid. Water depth fluctuated within a small range and experienced two transgressive and regressive cycles.