鄂尔多斯盆地延长组长7段沉积岩元素地球化学特征及沉积环境分析

Geochemical Characteristics and Depositional Environment Implications of Sedimentary Rocks in the Chang 7 Member of Yanchang Formation in the Ordos Basin

沉积岩的地球化学特征可以反映盆地沉积环境。为了探讨鄂尔多斯盆地延长组沉积时期古气候、氧化还原条件、古盐度等特征,对鄂尔多斯盆地东南部铜川延长组长7段的5个砂岩、8个页岩样品进行主量、微量以及稀土元素地球化学分析。结果显示,长7段细粒沉积物Al2O3/SiO2值与K2O/Na2O值指示样品成分成熟度差别不大,黏土矿物含量较高。∑REE平均值为132. 65×10-6,明显低于北美页岩平均稀土元素含量,也低于全球平均大陆上地壳值;LREE分异程度较高,呈微弱的Eu正异常,无Ce异常。'C值'、Sr/Cu、Rb/Sr、Mg/Ca等指标显示延长组沉积期整体处于温暖潮湿的气候区。研究区样品富集微量元素U、Sc、Cs(平均20. 11×10-6、9. 66×10-6、6. 08×10-6),结合V、δU、Th/U、V/Cr、V/Sc、V/(V+Ni)等指标分析,结果指示延长组沉积期以底层水体呈强分层的还原环境为主。Li、Sr、Ni、Sr/Ba、(MgO/Al2O3)×100等值显示沉积期水体总体为淡水-微咸水。

Geochemical characteristics of sediments can reflect the depositional environment.In order to study its paleoclimate,redox conditions,and paleosalinity in the deposition period of the Yanchang Formation,geochemical(major,trace and rare earth elements(REEs))characteristics of 15 sandstone and shale samples from the Chang 7 Member of the Yanchang Formation in the southeastern Ordos Basin were studied.The Al2O3/SiO2 and K2 O/Na2 O values of fine-grained sedimentary rocks in the Chang 7 Member indicate similar compositional maturity and high clay contents.The average∑REE value(132.65×10-6)is significantly lower than that of the NASC and the UCC.The rocks also display relatively high LREE contents,weakly positive Eu and no Ce anomalies.The'C value',Sr/Cu,Rb/Sr,Mg/Ca indices suggest that the entire deposition period of the Yanchang Formation was in a warm humid climate.Samples in the study area are rich in certain trace elements,such as U,Sc and Cs(average 20.11×10-6,9.66×10-6 and 6.08×10-6,respectively).This result and the values of V,δU,Th/U,V/Cr,V/Sc and V/(V+Ni)altogether indicate that the deposition period of the Yanchang Formation was dominated by a reducing,strongly stratified subsurface water layer.The values of Li,Sr,Ni,Ga,Sr/Ba,CaO/(CaO+Fe)and(MgO/Al2O3)×100 indicate that the water body of the deposition comprised mainly freshwater-brackish water.