Evolution of sedimentary environment of the Lower Cambrian Xishanbulake-Xidashan formations in the Tarim Basin

Abstract

The Cambrian is a critical period in the evolution of life and environment. Reconstructing the marine sedimentary environment, notably redox conditions during this period is key to explore the spatial and temporal evolutionary characteristics of seawater chemistry and the relationship between biological evolution and environmental changes. In this study, the major elements, trace elements and organic carbon content (TOC) of the Lower Cambrian Xishanbulake-Xidashan formations in well Tadong 2 in the Tarim Basin were analyzed, and the sedimentary environment of the formations was reconstructed. The results show that the water body was in weak restriction condition when the formation was deposited. The redox state of bottom waters was dominated by anoxia, but the degree of anoxia varied obviously. Specifically, the middle part of the Xishanbulake Formation showed elevated anoxia (euxinia) in bottom waters compared to the lower and the upper parts, while the Xidashan Formation showed a decrease in anoxia from the middle and the lower parts to the upper part and an expansion of bottom water oxidability. The results are consistent with the observation in previous studies that the Early Cambrian seawaters was characterized by dynamic changes in the redox state. The analysis of organic matter enrichment mechanism of Xishanbulake-Xidashan formations in well Tadong 2 shows that organic matter enrichments in the formations is not controlled by a single factor. The anoxic/euxinic environment in the middle and the lower parts of the Xishanbulake Formation is more conducive to the preservation of organic matter, while the higher level of primary productivity played a key role in the sedimentation of the upper part of the Xishanbulake Formation and the Xidashan Formation.

Keywords

• redox conditions
• productivity
• organic matter
• sedimentary environment
• early cambrian
• tarim basin