Integrated Stratigraphy of the Early Cambrian Qiujiahe Formation in Northwestern Sichuan Province, and Its Evolutionary and Industrial Implications

Resolving the stratigraphy of the Ediacaran-Cambrian (E-C) transition in deep-water slope settings is not only crucial for investigating the emergence of early ecosystems but also beneficial for the petroleum exploration associated with the lower Paleozoic source rocks. Although the lower Cambrian strata in the Deyang-Anyue trough of the Sichuan Basin have recently become a focus of natural gas exploration, the stratigraphy of the lower Cambrian deep-water settings in the northern end of the trough remains poorly constrained, due to the lack of biostratigraphic and chemostratigraphic evidence. This research provides a comprehensive stratigraphic analysis of the Cambrian Qiujiahe Formation and the underlying Shuijing Formation at Qiujiahe Village, Guangyuan City, integrating information from high-resolution carbonate carbon (δ13Ccarb) and organic carbon (δ13Corg) isotopes chemostratigraphy, lithostratigraphy, microfacies analysis, and microfossil assemblage analysis. The δ13Ccarb data exhibit a negative excursion in the upper part of the Shuijing Formation, which can be correlated with the Ediacaran DOUNCE/SHUIRAM excursion, while signals possibly correlative with the Basal Cambrian Carbon Isotope Excursion (BACE) are recovered at the top of this formation. The δ13Corg profile in the Qiujiahe Formation can also be correlated with those reported from other lower Cambrian slope deposits in South China. Critically, we report the first findings of a diverse early Cambrian chert-hosted assemblage from the Qiujiahe Formation, consisting of mineralized skeletal and carbonaceous filmy fossils. This assemblage corresponds to the recently established Terreneuvian microfossil assemblage, which is broadly distributed in other parts of South China. The integration of these datasets establishes the E-C boundary at the Shuijing-Qiujiahe transition. The non-silicified black shale above a nodule-containing level in the upper Qiujiahe Formation could be correlated with the start of the Cambrian Stage 3. This study demonstrates the usefulness of the microfossil assemblage in the stratigraphic correlations of the Terreneuvian cherts. The integrated bio- and δ13Corg chemostratigraphic methods seem to be a practical tool for the stratigraphic studies of slope successions in the lower Cambrian of South China.