| Abstrakt: |
[Objective] The Late Permian was a critical interval in geological history, with dramatic changes in tectonics, paleoclimate, paleo-oceanic environment, volcanic activities, and the biosphere. Reconstruction of 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. [Methods] Organic-rich rock series were widely deposited during this period in the Sichuan Basin of the Upper Yangtze Block, providing insights into the co-evolution of the geosphere and biosphere. To better address the coeval temporal and spatial changes in the 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 (TOC) contents in the Jianfeng section, located in an intrashelf basin in northeastern Sichuan Basin.[Results] The redox-sensitive trace element 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 the Wuchiaping Formation to the lower part of the Dalong Formation) was dominated 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 a euxinic environment, and 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. Compared with the Xibeixiang section in the deeper water of the Kaijiang-Liangping trough, the redox environment in the intrashelf basin exhibits spatiotemporal heterogeneity. The nutrient-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, and Ti contents and K/Al values) . However, the CoxMn and Cd/Mo values indicated that the upwelling currents arose in intervals II2, II3, III, and IV, and the volcanic activities were only frequent in Interval IV, disagreeing with the eutrophication and organic matter enrichment in the middle Dalong Formation. Therefore, the high primary productivity of the Dalong Formation may 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. [Conclusions] Combined with the Xibeixiang section in the study area, the formation of euxinic water mass was found to be closely related to the high primary productivity, whereas its spatiotemporal fluctuations were controlled by the sea-level changes, 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 updipping basinal margin. In this case, the primary productivity played a leading role in the enrichment of organic matter. [ABSTRACT FROM AUTHOR] |
