Siliceous deposition and hydrothermal contributions in the Lower Cambrian Yurtus Formation, Tarim Basin

The Lower Cambrian Yurtus Formation in the Tarim Basin preserves important evidence of hydrothermal activity, microbial processes, and seawater chemistry that affected silica deposition and organic matter enrichment during the Ediacaran–Cambrian transition. Using field observations, petrography, redox-sensitive geochemical data, and biomarkers, this study examines how silica formed and what environmental conditions controlled the accumulation of black shales. The Yurtus Formation was deposited on a passive continental margin that was affected by extensional tectonism and occasional hydrothermal discharge. Geochemical data indicate that bottom waters were saline, acidic, and mainly anoxic, and that reducing conditions increased at times when hydrothermal H₂S and other reduced fluids entered the basin.

The siliceous layers show several ways through which silica was added or precipitated. Hydrothermal fluids supplied dissolved silica, while upwelling brought silica-rich deep water and nutrients into the basin. Microbial activity also contributed to silica precipitation. The presence of amorphous silica, barite nodules, and chert–mud alternations, together with microbial mats, radiolarians, and sponge spicules, shows strong interactions between microbes and minerals and the influence of early diagenesis. Acidification caused by hydrothermal gases and microbial metabolism played an important role in forming SiO₂ quickly. Differences between the siliceous units relate to changes in the balance between hydrothermal input and upwelling. Layers rich in phosphate and barite suggest increased nutrient supply and fluid mixing. Continuous barite beds and chert–mud layers also indicate silica delivery from distant volcanic and hydrothermal sources.

Organic-rich shales in the upper Yurtus Formation contain Type I–II kerogen from plankton, algae, and bacteria. Their biomarker features match those of Bashituo oils, showing that the Yurtus Formation is an important regional source rock. These results show that hydrothermal fluids were the main source of silica, and that microbial processes and upwelling influenced how silica and organic matter were preserved.