Marine Redox Environment and Organic Accumulation in NE Sichuan Basin during the Late Permian

Key words: 
• Late Permian /
• northeastern Sichuan Basin /
• redox environment /
• primary productivity /
• organic matter enrichment

Abstract: The Late Permian was a critical interval in geological history witnessing dramatic changes in tectonics, paleoclimate, paleo-oceanic environment, volcanic activities, and biosphere. Reconstructing the paleogeography, paleoclimate, and paleo-oceanic environment during this interval could provide a more complete picture of the interactions and relationships among different geological factors, as well as their influence on organic matter enrichment and the evolution of the biosphere. Organic-rich rock series were widely deposited during this period in the Sichuan Basin of the upper Yangtze Block, providing the insights into the co-evolution of the geosphere and biosphere. To better address the coeval temporal and spatial changes in paleogeographic settings, paleo-oceanic environment, primary productivity, and organic enrichment, we conducted a detailed sedimentological investigation integrated (major and trace) element contents, organic carbon isotopes and total organic carbon contents in Jianfeng section, located in an intrashelf basin in northeastern Sichuan Basin. The redox-sensitive trace elements data (MoXS, UXS, VXS contents , MoXS/UXS ratios and MoEF-UEF relationship) show that the redox environment of the Jianfeng section has experienced four intervals (I-IV). Interval I (0-10.35 m, spanning from the upper part of Wuchiaping Formation to the lower part of Dalong Formation) was predominated by an oxic environment during deposition, Interval II (10.35-23.35 m, the middle Dalong Formation) experienced a ferruginous-dominant anoxic condition, Interval III (23.35-27.00 m, the upper Dalong Formation) was mainly in a euxinic environment, Interval IV (27.00-34.05 m, from the top of the Dalong Formation to the bottom of the Changhsing Formation) was dominated by an oxic or suboxic environment. Comparing with the Xibeixiang section deposited in deeper water in the Kaijiang-Liangping trough, it was found that the redox environment in the intrashelf basin exhibits spatio-temporal heterogeneity. The micronutrient-limiting elements (ZnXS, CuXS, NiXS) contents demonstrate that primary productivity is high in both intervals II2 and III, coincident with the increasing terrestrial inputs (dedicated by Al、Zr、Ti contents and K/Al ratios). However, the Co×Mn values and Cd/Mo ratios indicated that the upwelling currents arose in intervals II2, II3, III, and IV, and the volcanic activities were only frequent in the Interval IV, which were both not in accordance with the eutrophication and the organic matter enrichment in the middle part of Dalong Formation. Therefore, the high primary productivity of Dalong Formation might be principally affected by a large number of terrestrial inputs. Furthermore, the upwelling currents also made contributions to the prosperity of plankton, but the volcanic activities have poor relationships with it. Combined with the Xibeixiang section in the study area, it is considered that the formation of euxinic water mass was closely related to the high primary productivity, while its spatiotemporal fluctuations were generally controlled by the sea-level fluctuations which could drive its upslope incursion in intervals II and III, downslope retreat in the Interval IV and even demise in the Interval V. In turn, the primary productivity could be further controlled by the upwelling currents particularly on the basinal slope, and terrigenous inputs on the more updip basinal margin. In this case, the primary productivity played a leading role in the enrichment of organic matter.