辽东半岛复州湾剖面寒武系第二统光合作用生物膜建造的核形石

Oncoids built by photosynthetic biofilms: An example from the Series 2 of Cambrian at Fuzhouwan section in Liaodong Peninsula

作为一种经常大于鲕粒的包覆颗粒类型,核形石以其不平滑的圈层被解释为微生物成因,区别于成因存在较大争议的鲕粒,而且常与鲕粒和其他类型的碳酸盐颗粒相互共生;作为一种在围绕着生物碎屑和非生物碎屑核心的序列式纹层化作用过程中形成的球形或假球型生物沉积构造,核形石还常常单独产出和分布,所以又被解释为微生物碳酸盐岩,或者被归为球状叠层石。在辽东半岛寒武系第二统碱厂组和馒头组之中,厘米级别大小的核形石密集发育在三级层序的顶部,成为一种时间特化的相。另外,以下重要特征将辽东半岛寒武系第二统的核形石特征化,包括:(1)与凝块和微凝块共生;(2)多为球状和椭球状;(3)由不均一的非纹层状致密泥晶和微亮晶构成;(4)核形石皮层以及核形石间凝块中发育特别的蓝细菌鞘钙化化石等。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存在着巨大的挑战,也尽管形成这些核形石的复杂生物膜钙化作用细节需要更加深入地研究才能得到更好地了解,但是,辽东半岛第二统碱厂组和馒头组核形石中直接的微生物化石证据,尤其是核形石内较为丰富的钙化蓝细菌鞘化石,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。

As a type of coated grains with the size of more than that of ooids,oncoid is marked by non-smooth circles and is commonly interpreted as a microbial origin,so different from ooid whose origin remains uncertain.Also as a variety of spherical or pseudo-spherical biosedimentary structures formed as successive laminations around both biogenic and nonbiogenic nuclei,its widespread distribution and development by oneself imply that the oncoid can be grouped into a type of microbial carbonates or a kind of spherical stromatolites.Within the Jianchang and Mantou Formations of the Series 2 of Cambrian in the Liaodong Peninsula,oncoids with the size of centimeter level as a time-specific facies concentrate in the top part of third-order sequence.In addition,several features characterize these oncoids of the Series 2 of Cambrian in the Liaodong Peninsula:(1)the symbiosis with clots or microclots;(2)the spherical and ellipsoid morphology;(3)the inhomogenous or non-layer composition of dense micrite and microspar;(4)particular calcified sheath fossils of the cyanobacteria within the cortex of oncoids and the clot among oncoids and so on.Although the greatest challenge that remains is the interpretation of ancient oncoids and their detail forming processes through the filter of the diagenesis,also the detail of the calcification of biofilms dominated by the cyanobacteria has still not got better understanding,the richness of direct evidence represented by microbial fossils especially for sheath fossils of the cyanobacteria within oncoids of both the Jianchang and the Mantou Formations of the Series 2 of Cambrian in the Liaodong Peninsula make them become a good example of oncoids built by non-layer photosynthetic biofilms.