鲕粒滩中光合作用生物膜构建的高能核形石:以辽西葫芦岛三道沟剖面寒武系张夏组为例

核形石是一种具有双重属性而较为特别的微生物碳酸盐岩:①作为一种包覆颗粒类型，核形石以不平滑的圈层和较大的大小被解释为微生物成因区别于成因存在着较大争议的鲕粒；②作为一种形成在围绕着核心的序列式纹层化作用过程中的球形或椭球型的生物沉积构造，核形石还常常被解释为微生物碳酸盐岩的一种类型而被归为球状叠层石。在辽西葫芦岛三道沟剖面寒武系苗岭统张夏组顶部的块状鲕粒滩相灰岩之中，密集发育着厘米级别大小的高能泥晶核形石。另外，以下重要特征将张夏组鲕粒灰岩中的核形石特征化，包括:①厘米级别的大小；②椭圆形为主的形态学特征；③特征性的纹层状构造；④核形石皮层中普遍发育而且较高密度保存的丝状蓝细菌鞘的钙化化石等等。尽管穿越成岩作用过滤器去解释古代核形石复杂的形成机理将存在着巨大的挑战，也尽管形成这些核形石复杂的生物膜钙化作用机理还没有得到精确的了解，但是，辽西葫芦岛三道沟剖面张夏组中的核形石，直接的微生物化石证据，尤其是在核形石内较为丰富的丝状蓝细菌鞘化石，使其成为一个了解光合作用生物膜建造核形石的典型实例；同时，也为进一步了解形成在水柱中的放射鲕粒以及形成在海底的泥晶核形石之间复杂的形成机理的差异性，提供了一个较为典型的实例。

High- energy oncoids within the ooid- grained bank built by photosynthetic biofilms: a case study of the Cambrian Zhangxia Formation at the Sandaogou section of Huludao City in the western part of Liaoning Province

The oncoid is a variety of microbial carbonate with double attributes: (1) amongst the coated grains the oncoid is distinguished from the ooid by its relatively large size and non- smooth lamina within its cortex; (2) it is a variety of spherical stromatolite with an obvious biosedimentation structure resulting from successive lamination processes. High- energy and micrite- dominated oncoids are developed within a massive ooid- grained bank in the Zhangxia Formation of the Cambrian Miaolingian at the Sandaogou section of the Huludao City in the western part of Liaoning province. Following features characterize these oncoids: (i) centimeter level size; (ii) chiefly oval morphology; (iii) the characteristic laminated fabric within the cortex; (iv) the common and relatively high- density preservation of calcified- sheath fossils of the filamental cyanobacteria within the cortex. Although the greatest challenge that remains is the interpretation of ancient oncoids and the details of their formation through the filter of the diagenesis, even the processes involved in calcification of biofilms dominated by the cyanobacteria are not well understood. The rich evidence represented by microbial fossils, especially for sheath fossils of the cyanobacteria within oncoids, in the top part of Zhangxia Formations at the Sandaogou section provides a good example of oncoids built by photosynthetic biofilms. They also provide an opportunity to understand the differences between processes forming the ooid within the water column and the oncoid growing on the seafloor.