Calcite Ooids Formed By Photosynthetic Biofilms

Ooids and oncoids are two of the most common carbonate coated particles,but different from the widely recognized biogenic oncoids,ooids genesis has always been a fascinating sedimentology problem.In the study of the Bahamas modern ooids,two opposing viewpoints have been obtained: the one is a new chemical model based on the previous ooids chemical origin,which denied the basic role of microbes in the process of ooid formation,and suggested the formation of ooid should be attributed to complex chemical processes;And with the deep research of the Bahamas modern ooids,emphasize the genetic relationship between the ooid formation and complex microbial activity.Moreover,Cambrian ooids are not the same as modern marine concentric aragonite ooids,it mainly develops radial fibrous calcite ooids.Although the genesis model of modern marine aragonite ooids cannot be used to explain the Cambrian ooids completely,the microscopic characteristics of Cambrian ooids also show the close relationship between ooids formation and biofilm calcification.A large number of oolitic-grain bank limestone are generally developed at the upper part of the third-order sequence of Xuzhuang formation and Zhangxia formation of the Miaolingjian of Cambrian in the Shijing section,Xinan,Henan Province,especially in giant ooids at the top of Xuzhuang formation.Very clear and abundant calcified cyanobacteria sheath fossils have been found.The ooids shows obvious bright and dark laminations,the bright laminations are mainly composed of slightly bright calcite,while the dark laminations are mainly composed of Girvanella filaments and micritic calcites.These Girvanella filaments can also existed as the core of ooids.The existence of Girvanella filaments was also found in the ooids of Zhangxia formation,and the existence of dark mud crystal cortex and the existence of pseudo-ooids also shows the close relationship between ooids and microbial activities.These characteristics indicate that it is not the microorganisms that form ooids directly,it is the calcification of biofilms composed of microorganisms that form these complex carbonates.Different from the modern marine aragonite concentric ooids formed by non-cyanobacteria biofilm,the ancient cambrian is mainly developed the radialfibrous calcite ooids formed by the photosynthetic(mainly cyanobacteria)biofilms.Although the existence of “the filter of the diagenesis”make it more difficule to analyze,and it is not possible to determine the detailed process of ooids formation mechanism.However,the evidence of direct microbial fossils in ooids and a series of other indirect characteristics represent the new progress in the study of the genesis of Cambrian calcite ooids.The pursuit of these new developments will help us to further understand the exact mechanism of ooids formation in various environments,it also helps to understand the genesis of oolitic particles in different geological historical periods and provides useful clues and ways of thinking for the further study of more comprehensive theories.