寒武系张夏组树形石中的钙化蓝细菌——以山东淄博峨庄镇后紫峪剖面为例

Calcifying cyanobacteria in dendrolites of the Cambrian Zhangxia Formation:A case study of the Houziyu section in Ezhuang Town,Zibo City,Shandong Province

山东淄博峨庄镇后紫峪剖面寒武系张夏组顶部发育树形石,根据宏观露头岩性变化所反映的沉积相特征,研究区张夏组层序可以划分为三个四级亚层序。随着微生物影响树形石的形成过程的证据不断丰富,广为接受的“CCM机制引发蓝细菌鞘钙化作用”的观点正不断受到挑战。本文通过野外勘探采样、室内薄片分析相结合的方式,镜下识别刻画树形石的组构,并与现代实例进行类比进而分析其成因机制。树形石的主要组构为暗色泥晶基质、亮晶方解石胶结物和碎屑颗粒,显微镜下显示,树形石内部发育钙化蓝细菌生物化石,包括附枝菌(Epiphyton)、基座菌(Hedstroemia)、肾形菌(Renalcis),还可见孢网菌(Bacinella)、石松藻(Lithocodium)状组构。通过对研究区树形石中多种类型钙化蓝细菌和钙化蓝细菌微生物膜钙化残余物质的研究,为树形石内钙化微生物多样性做出了较为翔实的补充,明确了树形石是附枝菌等蓝细菌主导的微生物膜或微生物席钙化作用的产物,其中胞外聚合物(EPS)在树形石的发育和矿物沉淀过程中起关键作用,为树形石的“微生物影响成因”提供了一个典型实例。

Dendrolites develope at the top of the Cambrian Zhangxia Formation in Houziyu section,Ezhuang Town,Zibo,Shandong.According to the sedimentary facies characteristics reflected by the macroscopic lithology changes in the outcrop,the sequence of the Zhangxia Formation in the study area can be divided into three fourth-order sub-sequences.As more and more evidence indicating the microbial influence on the dendrolite formation process continues to accumulate,the widely accepted view of the"CCM mechanism triggering the calcification of cyanobacterial sheaths"is challenged.This paper integrates field exploration sampling and thin-section analysis to identify and subsequently describe the fabrics of the dendrolites under the microscope.Their genetic mechanisms are further analyzed by comparing them with modern examples.The main components of the dendrolites are dark micrite matrix,sparry calcite,and detrital grains.Microscopically,calcified cyanobacteria fossils,including Epiphyton,Hedstroemia,Renalcis,and Bacinella,as well as Lithocodium-like fabrics,are observed within the dendrolites.Through the study of various types of calcified cyanobacteria and their calcified microbial membrane residues in the dendrolites of the study area,a detailed supplement has been made to the diversity of calcified microorganisms found in dendrolites.It is clear that the dendrolites are the product of the calcification of microbial films or mats dominated by cyanobacteria such as Epiphyton.Extracellular polymeric substances(EPS)play a crucial role in the development and mineral precipitation of dendrolites,providing a typical example for the"microbial influence"genesis of dendrolites.