Depositional environments of early Cambrian marine shale, northwestern Tarim Basin, China: Implications for organic matter accumulation

The organic-rich lower Cambrian Yurtus Formation is considered to host one of the most important source rocks in the Tarim Basin of China. To investigate the paleoenvironment and mechanism of organic matter accumulation, we conducted petrological and geochemical analyses of the Yurtus Formation using samples obtained from three outcrop sections in the northwestern Tarim Basin. The Yurtus Formation can be divided into five units and contains two sets of shale (upper and lower). Both the lower and upper shales were deposited during transgressive phases and exhibit strong enrichment in total organic carbon (TOC). The mineralogy of the lower shale is dominated by quartz, feldspar, barite, and clay, whereas the upper shale consists mainly of quartz, feldspar, ankerite, and clay, with minor apatite and hematite. Field investigation, trace element enrichment and rare earth elements suggest that the lower shale was influenced by weak hydrothermal activity. The rock assemblage of organic-rich shale, chert, phosphate, and glauconite is well developed in the two transgressive-regressive cycles. Based on Co-EF x Mn-EF, Co (ppm) x Mn (%), and Cd/Mo, Mo/TOC ratios and enrichment factors of trace elements that are comparable with those of modern upwelling and restricted setting, upwelling is proposed to have been active and widespread during the deposition of the Yurtus Formation. Enrichment factors of redox-sensitive elements (e.g., U, V, and Mo) indicate that the lower shale was deposited in an anoxic conditions with less frequent euxinic conditions, whereas the upper shale was deposited in a suboxic environment. Excess silica and P2O5 contents indicate high productivity throughout the deposition of the lower shale, with lower and decreasing productivity throughout the deposition of the upper shale. During the early Cambrian, a global transgression led to the development of an extensive carbonate platform in the northwestern Tarim Basin, and the climate became warm and humid. During the deposition of the lower shale, widespread silica-rich hydrothermal fluids provided abundant trace elements (including U, V, Mo, Ni, Cu, and Zn) that are beneficial for the prosperity of biomass. Upwelling provided abundant nutrients that further enhanced paleoproductivity and expanded the vertical extent of the oxygen minimum zone, favoring the preservation of organic matter. During the deposition of the upper shale, paleoproductivity decreased owing to the termination of hydrothermal activity and an increase in carbonate content. Organic matter accumulation was possibly controlled mainly by decreased but still high paleoproductivity and oxygen-poor level bottom water.