锶同位素地层学在奥陶系海相地层定年中的应用--以塔里木盆地塔中12井为例

根据锶同位素地层学的基本原理,从国际上已有的锶同位素演化曲线建立了奥陶系87Sr/ 86Sr比值 年龄数据库。利用塔里木盆地塔中 12井海相碳酸盐的锶同位素分析结果,确定了该井中、上奥陶统和中、下奥陶统的界线,两个界线分别位于井深 5 12 0m( 87Sr/ 86Sr≈ 0.70 88,年龄为 4 6 5Ma)和 4 880m处 ( 87Sr/ 86Sr≈ 0.70 82,年龄为 4 5 5Ma),中、上奥陶统的界线为本文首次确定,中、下奥陶统的界线与前人利用牙形石资料确定的界线一致。塔中 12井的锶同位素演化曲线说明,尽管构造运动造成的盆地抬升和近地表的大气水作用影响了上奥陶统顶部碳酸盐的锶同位素组成,但塔中 12井奥陶系与上覆地层的关系基本上是连续的,奥陶系顶界的井深在 4 6 5 0m附近 ( 87Sr/ 86Sr≈ 0.70 79,年龄为4 35Ma)。

Application of Strontium Isotope Stratigraphy to Dating Ordovician Marine Sediments--An case study from the well Tazhong 12 in Tarim basin

According to the principle of strontium isotope stratigraphy (SIS), we establish the Ordovician data baseof 87 Sr/ 86 Sr ratio through the geological timefrom the digitized global marine strontium isotope curve. We have confirmed the boundaries of middle upper and middle lower Ordovicianusing the analytic data of strontium isotope of the Well Tazhong 12 in Tarim basin.The two boundaries are located atthe depth of 5 120 m ( 87 Sr/ 86 Sr≈ 0.708 8,with theage of 465 Ma)and4 880 m( 87 Sr/ 86 Sr≈ 0.708 2,with theage of 455 Ma),respectively, of the Well Tazhong 12. The verificationof middle upper Ordovicianboundary is just the first time for this well,and the location of the boundary of middle lower Ordovician confirmed is consistent with the depth of middle upper Ordovician boundary calibratedby conodontdata.This variation of strontium isotope curve for the Well Tazhong 12 shows the following facts: despite the tectonic movementresulted in the rising of Tarim basin, leading to the leaching of meteoricwater to the upper most part of Ordoviciancarbonate, and to the increase of 87 Sr/ 86 Sr of some samples, the relationship between Ordovicianand its upper strata of the Well Tazhong 12 is transitionary. The top boundary for Ordovicianis located at the depth of 4 650 m ( 87 Sr/ 86 Sr≈ 0.707 9,with theage of 435 Ma).