Element Geochemical Characteristics and Geological Significance of Mudstones from the Middle Jurassic Shaximiao Formation in Sichuan Basin, Southwest China

Thick sequences of terrestrial multicolored mudstonesof the MiddleJurassic Shaximiao Formation in the Sichuan Basin, Southwest China,effectively recorded paleoclimate and paleoenvironment changes. Thepaleoenvironment of the Shaximiao Formation is reconstructed by usingdetailed sedimentological and elemental geochemical analysis of themulticolored mudstones. The provenance, paleoclimate, paleosalinity,and paleoredox conditions are distinguished by using the discriminantindicators of CIA, C-value, Sr/Cu, Rb/Sr, Th/U, V/Cr, and V/(V + Ni).The results show that all samples derive primarily from felsic igneousrocks and intermediate rocks rather than recycled sediments. The mudstonesequences were deposited under semiarid and semihumid regions withpaleoclimate evolved to drier and cooler conditions from lower toupper Shaximiao Formation. Such a paleoclimate coincided with therecords of several basins in the lower paleolatitudes of the NorthernHemisphere and were possibly affected by the Middle Jurassic globalgeological events such as wildfire, paleogeographic reorganizations,and seaway dynamics change. The paleowater body belongs to a typicalterrestrial freshwater environment, although the paleosalinity increasedsignificantly during arid periods. The multicolored mudstones weredeposited in oxidation and weak-oxidation to weak-anoxic conditions.We also propose a detailed conceptual paleoenvironment model for ShaximiaoFormation, with a large perennial lake surrounded by limited alluvialplain during a period of high lake level and small ephemeral lakesscattering extensive alluvial plain during a phase of low lake level.