塔里木盆地西北缘下寒武统玉尔吐斯组的元素地球化学特征及其对古沉积环境和物源的指示

Elemental geochemical characteristics of the Lower Cambrian Yurtus Formation in the northwestern margin of the Tarim Basin and their implications for ancient depositional environments and provenance

针对塔里木盆地寒武系盐下玉尔吐斯组沉积环境众说纷纭的情况,充分利用采集的野外露头剖面泥岩、粉砂质泥岩样品,开展微量元素、主量元素、碳氧同位素和稀土元素等分析。通过对标志性元素含量、特定元素比值,及其构造和沉积特征进行研究,从古盐度、古氧化-还原程度、古水深、古气候、古构造位置等方面,探讨了下寒武统沉积环境特征。研究发现:从垂向来看,与玉三段相比,玉一段表现出低Mg/Al值、高Sr/Ba值、高V/Cr值、高V/(V+Ni)值、高&U指数(&U=U/(0.5×(Th/3+U)))指数、低Sr/Cu值、高C值、低Z值、低Zr/K值和Ti/Al值、高Baau值和Porg值、高古生产力指数Pz和P/Ti值,体现出玉一段为低盐度、强还原、陆源碎屑输入较少、相对温暖湿润的古环境,导致该层段现今总有机碳(total organic carbon,TOC)含量很高,是有效的烃源岩发育层段;垂向上水体深度变化分为深—次浅—次深—浅4个演化阶段,平面上沉积水体深度具有昆盖阔坦>肖尔布拉克>苏盖特布拉克的特征,相对应沉积速率具有苏盖特布拉克>肖尔布拉克>昆盖阔坦的特征;沉积环境为塔西碳酸盐岩台地内由台内断裂控制的断陷沉积。该研究成果加强了塔里木盆地寒武系盐下油气勘探的地质理论基础,对研究区理论研究和生产实践均具有指导意义。

Facing the contentious and divergent views on the depositional environment of the Yurtus Formation beneath the Cam⁃brian salt in the Tarim Basin,samples of mudstone and silty mudstone from field outcrops were fully utilized to conduct comprehen⁃sive analyses of trace elements,major elements,carbon and oxygen isotopes,and rare earth elements.By examining the concentra⁃tions of signature elements,specific elemental ratios,and integrating structural and sedimentary characteristics,the research delved into aspects such as paleosalinity,paleoredox conditions,paleodepth,paleoclimate,and paleotectonic setting to elucidate the deposi⁃tional environmental traits of the Lower Cambrian strata.It is found that,in a vertical perspective,compared to the third Member,the first Member is characterized by lower Mg/Al value,higher Sr/Ba value,higher V/Cr value,higher V/(V+Ni)value,higher&U index,lower Sr/Cu value,higher C value,lower Z value,lower Zr/K value and Ti/Al value,higher Ba_(au)value and P_(org)value,and higher paleoproductivity indices Pz and P/Ti ratio,reflecting a paleoenvironment of low salinity,strong reduction,minimal ter⁃rigenous detrital input,and relatively warm and humid conditions.This led to high total organic carbon(TOC)content within this section,marking it as an effective hydrocarbon source rock interval.Vertically,water depth variations are categorized into four evolutionary stages:deep,sub-shallow,sub-deep,and shallow.Horizontally,the sedimentary water depths exhibit a pattern of Kungekuotan>Xiaorbulak>Sugaitblak,with corresponding sedimentation rates following Sugaitblak>Xiaorbulak>Kungekuotan.In conclusion,the sedimentary environment is identified as a fault-controlled rift deposition within the carbonate platform of the Tarim West.This research enhances the geological theoretical foundation for hydrocarbon exploration beneath the Cambrian salt in the Tarim Basin and offers valuable guidance for both theoretical studies and practical pro⁃duction in the region.