Geochemical characterization and paleoenvironmental analysis of the mudstone of the Lower Cretaceous Naijiahe Formation in the Liupanshan Basin

This research delves into the Lower Cretaceous Naijiahe Formation in the Liupanshan Basin, which is a typical terrestrial basin. By conducting a detailed examination of sedimentary features and geochemical signatures, the study aims to decipher the paleoenvironmental and paleoclimatic evolution preserved in lacustrine systems. Geochemical analysis uncovers unique elemental distributions in the mudstones of the Naijiahe Formation. When compared with the average values of the upper crust, major elements such as Na₂O and TFe₂O₃ are enriched, while the concentrations of MgO, K₂O, and TiO₂ are lower. Among trace elements, Ba, Sr, and Rb are enriched, whereas Th, Zr, and Hf show depletion. Weathering indices, such as the Chemical Index of Alteration（CIA）and the Index of Compositional Variability（ICV）, suggest that chemical weathering was relatively weak during sediment transport. Provenance discrimination diagrams, consistently imply that the source rocks are felsic, originating from acidic magmatic rocks in the Qinling-Qilian Orogenic Belt. Multiple geochemical proxies jointly indicate that the deposition occurred under arid, anoxic, and hypersaline conditions. As a representative terrestrial saline lake in North China, the salinization of water bodies and the formation of black shales in the Madongshan-Naijiahe Formation may well represent the regional responses to oceanic anoxic events during the greenhouse climate conditions of the Early Cretaceous.