Paleoenvironmental Evolution and Controls on Organic Matter Enrichment During the Precambrian–Cambrian Transition in the Southwestern Tarim Basin, NW China

Marine organic-rich shales deposited during the Precambrian–Cambrian transition represent important hydrocarbon source rocks, yet the mechanisms controlling organic matter enrichment across this boundary in the southwestern Tarim Basin remain poorly constrained. In this study, organic geochemical and major/trace element data from the Sinian Kuerkake Formation at the Yutang section and the Lower Cambrian Yuertusi Formation at the Dongergou section were integrated to reconstruct paleoenvironmental conditions and evaluate the mechanisms of organic matter accumulation. The results show that the Kuerkake Formation contains relatively low TOC contents (average 0.68%) and is characterized by higher Al and Ti contents, lower productivity-related elemental ratios, and lower Mo/Al values. In contrast, the Yuertusi Formation exhibits substantially higher TOC contents (up to 9.52%, average 5.75%), together with higher Ni/Al and Cu/Al ratios, stronger Mo enrichment, lower Al and Ti contents, and positive Eu anomalies. Taken together, these proxies are consistent with a change from the Kuerkake to the Yuertusi depositional system, including greater productivity-related metal accumulation and more reducing conditions in the latter; however, the proxies do not independently establish persistent euxinia. Fe-Mn enrichment and positive Eu anomalies suggest a possible hydrothermal contribution during early Yuertusi deposition, although detrital, volcanic, and diagenetic alternatives cannot be excluded. Overall, organic matter enrichment in the Yuertusi Formation resulted primarily from the coupled effects of enhanced primary productivity, efficient organic matter preservation under reducing conditions, and reduced terrigenous dilution, with possible hydrothermal input acting as a secondary, non-unique influence. These findings provide new insights into organic-rich source rock development across the Precambrian–Cambrian transition and its implications for hydrocarbon exploration in the southwestern Tarim Basin.