南襄盆地泌阳凹陷核桃园组三段纹层状泥页岩地球化学特征及其成因解释

Geochemical Characteristics and Genetic Interpretation of Laminated Shales of the Third Member of Hetaoyuan Formation in the Biyang Depression of Nanxiang Basin

黑白相间纹层状页岩是沉积岩中非常重要且常见的一种岩石类型。采用X射线荧光岩心扫描仪对泌阳凹陷5号页岩层中纹层状页岩样品进行测试,分析结果显示湖相纹层中一些指标,如Sr、CaO、K_2O、SiO_2、TiO_2、Fe_2O_3、Al_2O_3、Cu含量在纹层中表现出明显规律性变化,即白色纹层中Sr和CaO含量明显高于黑色纹层,而K_2O、SiO_2、TiO_2、Fe_2O_3、Al_2O_3、Cu含量明显低于黑色纹层。硼元素以及古氧化还原和古气候指标表明纹层状页岩主要发育在半干旱-干旱的半咸化-咸化分层水体。镜下鉴定揭示,黑色条纹主要为高有机质泥质沉积物,白色条纹主要为方解石、石英以及白云石等矿物,黑白纹层的物质成分组成与陆源供给和水体咸化后自身形成的矿物密切相关。黑白纹层中元素差异变化揭示了不同矿物组成的差异,也揭示出陆相湖盆季节性韵律变化。

The occurence of black and white stripes of laminated shales is a very important and common phenomenon in sedimentary rocks. The X-ray fluorescence core scanner has been used for testing laminated shale samples from the fifth layer in the Biyang Depression. The analysis results show that some chemical elements of the lacustrine lamina,such as Sr,CaO,K_2O,SiO_2,TiO_2,Fe_2O_3,Al_2O_3 and Cu,show obvious regular changes in the black and white laminae. That is,the Sr and CaO contents in the white laminae are significantly higher than those in the black laminae,and the contents of K_2O,SiO_2,TiO_2,Fe_2O_3,Al_2O_3 and Cu in the black laminae are significantly higher than those in the white laminae. The formation machanism of laminae is analyzed combined with the boron element and paleoredox proxies and paleoclimate proxies,and the result suggeststhat these laminated shales would have mainly developed in the brackish-saline and semiarid-arid stratified water. Microscopic investigation also reveals that the black stripes are mainly composed of a high proportion of organic matters and argillaceous sediments,while white stripes are mainly composed of the calcite,gypsum,quartz,dolomite and other minerals. The black and white laminated material components are closely related with a terrigenous supply and authigenic minerals formed in a saline enviroment. The occurence of different elements between the black and the white reveals the different mineral compositions as well as seasonal rhythm changes in the continental lake basin. Therefore,core scanning provides a new effective method to evaluate geochemical characteristics of the laminated shale and its formation mechanism.