川东卧龙河地区长兴晚期自然伽马异常及其环境启示意义

Natural Gamma-Ray Anomaly and Environmental Implication in Late Changhsingian,Wolonghe Area,Eastern Sichuan Basin

目前研究表明,深水环境缺氧团向浅水扩散是导致二叠纪末生物危机事件的重要诱因,然而其作用过程仍存在较大争议。四川盆地东部晚二叠世长兴期主要为台地环境,通过对研究区内卧龙河构造连续沉积的井剖面对比研究发现,在二叠纪末生物大灭绝界线之前的碳酸盐岩地层中自然伽马值整体出现异常正偏。当碳酸盐岩地层中泥质含量、钍和钾的含量相对较少时,自然伽马主要反映铀含量变化,进而指示海水氧化还原条件。长兴晚期区内台地环境显现出不同程度的缺氧效应,并持续发展直至生物大灭绝的来临。区内长兴组自然伽马值异常记录了长兴晚期海洋氧化还原过程,同时也见证了重大地质转折期将至时环境的不稳定性。

The present study shows that deep water environment＇s hypoxia group diffusing to shallow water environment is the most important reason for the biological crisis at the end of Permian.However,there is still a considerable controversy on its mechanism.Eastern Sichuan Basin is mainly platform environment in Changhsingian,Late Permian.Based on the correlation of the continuous deposition of well profiles of Wolonghe structure in the study area,we find that the natural gamma-ray value of the carbonate formation became anomaly positive excursion before the end of the Permian extinction boundaries.Natural gammaray can reflect the variation of uranium content,when the content of argillaceous,thorium and potassium is low in carbonate formation,indicating the redox conditions of sea water.The platform environment in the study area shows different degrees of hypoxia effect in Late Changhsingian,which continues to develop until the coming of the biological extinction.The natural gamma-ray value in the study area recorded oxidation-reduction process of sea water in Late Changhsingian,and also witnessed the instability of the environment in major geological transitions period.