渭河盆地古近系细屑岩的古盐度分析

The Palaeosalinity Analysis of Paleogene Lutite in Weihe Basin

渭河盆地位于鄂尔多斯地块与秦岭造山带之间,古近纪主要出露地层为红河组及白鹿塬组,由于近些年在盆地新生界地层中发现氦气前景,有必要对其沉积环境进行研究,本文通过野外样品的主量元素、微量元素的测定,运用Adams法、Walker相当硼法、B/Ga、Sr/Ba比值法定量—半定量地分析了渭河古近系沉积时的盐度特征,同时结合化学蚀变指数CIA值的变化特征,对古近系沉积环境进行了综合分析,结果表明渭河盆地古近系红河组、白鹿塬组盐度为2.2‰~11.7‰,为一内陆淡水湖泊,整体气候温暖湿润,局部地区干旱,并出现咸水沉积环境;同时发现蚀变指数CIA与盐度值成负相关关系,由红河组湖相环境至白鹿塬组河流及三角洲相为主的沉积环境,风化指数升高,而B/Ga比值所反映的盐度值降低。CIA值在一定程度上反映了盐度特征,由于生物富集作用导致样品出现高Sr值,使得Sr/Ba比值在反映盐度时出现异常点。

The main emergence stratum of Paleogene in the Weihe Basin,which hold in the Ordos block and Qinlingorogenic belt,include Honghe Formation and Bailuyuan Formation. In recent years,helium gas in the basin found in the Cenozoic strata has a prospect,it's necessary for us to research on sedimentary environment. Through the analysis of major and trace elements of field samples,the Paleogene sedimentary environment was revealed by quantitative semiquantitative analysis involving calculation using Adams formula,equivalent boron suggested by Walker,B / Ga ratio,Sr / Ba ratio methods and the variation characteristics of chemical index of alteration CIA. The results indicate the range of palaeosalinity of Honghe Formation and Bailuyuan Formation is 2.2‰ ~ 11.7‰,and display that Weihe basin belongs fresh water lake,which are warm,moist climate overall,regional drought and salt water sedimentary environment. At the same time,we find that chemical index of alteration have negative correlation with salinity value. By the lacustrine environment in Honghe Formation to river and delta facies environment in Bailuyuan Formation,CIA index increases,and the salinity of the B / Ga ratio is reduced,which show that CIA values,to some extent,reflect the characteristics of salinity. Because of bioconcentration the samples have high Sr values,which result in abnormal points when Sr / Ba ratio reflect salinity.