四川盆地西南缘YY1井五峰组—龙马溪组元素地球化学特征及古环境意义OA

Most outcrops of the Wufeng-Longmaxi formations along the southwestern margin of the Sichuan Basin have been affected by surface weathering,resulting elemental migration that compromises the accuracy of geochemical analyses to some extent.To minimize weathering effects,this study conducted geochemical analysis on organic-rich shales from the Wufeng-Longmaxi formations in Well YY1.Major element analyses reveal that the average concentration of SiO2 in the shale sample is 49.99%,and the average concentration of Al2O3 is 11.78%,both mostly lower than those of the North American Shale Composite(NASC).In contrast,CaO concentrations are higher than those in NASC and in other regions of the Sichuan Basin,exhibiting low SiO2/Al2O3 values and high CaO concentrations.Trace element data exhibit relative enrichment in Th,U,Pb,Nb,Sr,Ba,and Mo,and depletion in Ti,Hf,and Ta.Redox-sensitive proxies indicate that the U/Th ratio ranges from 0.09 to 4.31,δU from 0.44 to 1.86,V/(V+Ni)ratio from 0.53 to 0.88,Ni/Co ratio from 3.02 to 23.29,Th/Mo ratio from 0.07 to 41.00,and MoEF/UEF ratio from 0.2 to 5.58,with clear vertical segmentation.The upper Wufeng Formation to the first submember of Long1 Member(S1ln11)represents weakly reducing to reducing depositional environment conducive to the preservation of organic matter.High V and biological Ba concentrations reflect high paleoproductivity.Simultaneously,the Wufeng-Longmaxi formations were deposited within a paleogeographic framework confined by paleo-uplifts,reflecting restricted water circulation and limited influence from upwelling currents.