Abstract: Black shale and chert suites were deposited during the Edicarian-Cambrian transition all around the world. The Early Cambrian Yurtus Formation in the Tarim Basin contains this rock suites and was considered to be one of the major Paleozoic source rocks in Tarim. However, the origin and depositional process of the rock suites are poorly understood. In this contribution, our field study shows that the Yurtus Formation can be divided into five stages and comprises two transgression-regression cycles. The cherts of stage 1 are characterized by low Al/(Al+Fe+Mn) (average 0.29) and high Fe/Ti (average 108.24) ratios. In AlFeMn and Fe/TiAl/(Al+Fe+Mn) diagrams, most of the samples show strong hydrothermal influence. Lacking pronounced positive Eu anomaly (average 0.84) and no LREE enrichment in the cherts REE pattern suggests seawater involvement to the sediment. Kule and Shiairike profiles near the hydrothermal vent are characterized by thicker chert and related deformation. In the Early Cambrian, the South Tianshan Ocean evolved into a mature ocean with silica-rich hydrothermal fluid spreading widely in the shallow and deep environment across northern Tarim Basin, reflecting strong extensional tectonic activities in this area.

Key words: depositional process, ramp, silica-rich hydrothermal activity, Northwest Tarim, Early Cambrian