鄂尔多斯盆地东南部晚二叠世地球化学特征及古环境一一以韩城薛峰川剖面石千峰组为例

LATE PERMIAN GEOCHEMICAL CHARACTERISTICS AND PALEOENVIRONMENT OFSOUTHEASTERN ORDOS BASIN:A CASE STUDY OF THE SHIQIANFENG FORMATION IN THE XUEFENGCHUAN SECTION,HANCHENG

晚二叠世是全球气候变化的重要转折时期,鄂尔多斯盆地二叠世石千峰组为一套典型的陆相沉积,前人对该时期较连续的海相地层揭示的古气候研究较为成熟,而对同时期陆相地层的研究缺乏系统性,相关研究程度低,研究资料需要进一步完善。文章通过盆地东南部薛峰川剖面石千峰组地质剖面的详细观察、测量及系统采样,开展了系统的岩石学和元素地球化学分析,分析了该时期沉积期的古气候、古盐度和古环境特征,揭示了其演化规律。结果表明:(1)样品化学蚀变指数CIA的变化范围为15.3578.28,大部分样品的CIA值变化范围在65~85之间,风化程度中等;碳同位素偏移明显,偏移幅度达3.47%,气候出现突变。总体为半湿润-半干旱气候,部分时期干旱程度增大;(2)通过碳氧同位素相关性分析和古盐度Z值和S值计算,表明了自下而上古盐度值总体上呈增加趋势,上部层段的Z值和S值分别为119.98%和31.36%,可达半咸水-咸水环境;(3)古氧化还原指标U/Th、Ni/Co、V/(V+Ni)比值的范围分别为0.15~1.00,1.35~2.43,0.73~0.88,沉积期整体处于强氧化沉积环境;(4)上段的钙质结核、粉砂质灰岩以及紫红色砂泥岩呈韵律层出现表明沉积环境向干旱演化。综合分析认为,盆地东南部晚二叠世地球化学元素变化剧烈,沉积相也变化大,表明该时期古气候、古环境发生重大变化,水体盐度变高,气候变得更为炎热干旱。

Previous studies on the paleoclimate inferred from the relatively continuous deposition of Per-mian marine strata in the Ordos Basin are well-developed.However,research on the continental strata de-posited during the same period remains limited and lacks systematic exploration,requiring further en-hancement to comprehensively understand the sedimentary characteristics and environmental conditions of the region during this crucial phase of global climate change in the late Permian.Through meticulous ob-servation,precise measurement,and systematic sampling of the geological sections of the Shiqianfeng For-mation in the Xuefengchuan area southeast of the basin,coupled with systematic petrological and elemen-tal geochemical analyses,this study aims to investigate the paleoclimate,paleosalinity,and paleoenviron-mental characteristics of the Shiqianfeng Formation's sedimentary period.The goal is to elucidate its evo-lutionary patterns and contribute to a deeper understanding of this transitional period in Earth's history.The study results indicate the following key findings:1.The chemical alteration index(CIA)ranges from 15.35 to 78.28,with most samples falling between 65 to 85,suggesting a moderate degree of weathering.The carbon isotope deviation(3.47‰)reflects abrupt climate shifts from semi-humid to semi-arid condi-tions,indicating increased aridity during certain periods.2.Correlation analysis of carbon and oxygen iso-topes,along with Z and S values calculations,reveal a trend of increasing paleosalinity from bottom to top.The Z and S values in the upper layers are 119.98‰ and 31.36‰ respectively,indicating a transition to a brackish water environment.3.Paleoredox indices(U/Th,Ni/Co,V/(V+Ni)ratios ranging from 0.15 to 1.00,1.35 to 2.43,and 0.73 to 0.88,respectively)indicate a strongly oxidizing sedimentary en-vironment throughout the area during the deposition period.4.Additionally,the presence of calcareous nodules,silty limestone,and purplish red sand mudstone in the upper part of the Shiqianfeng Formation suggests an evolving sedimentary environment towards aridity during the Permian period.In summary,comprehensive analysis reveals significant changes in geochemical elements and sedimentary facies in the southeastern part of the basin during the Late Permian.These changes indicate a shift to hotter and drier climates and an increase in salinity,highlighting notable shifts in paleoclimate and paleoenvironment.