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
Abstract:The oncoid is a variety of microbial carbonates with double attributes: ① as a type of coating grains the oocid is distinct from the ooid by its relatively large size and non-smooth lamina within its cortex; ② as a variety of stromatolites that is further defined as a spherical stromatolite by an obvious biosedimentation structure resulted from the successive lamination process. High-energy and micrite-dominated oncoids are developed within 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: ① the size of centimeter level; ② the chiefly oval morphological feature; ③ the characteristic laminate fabic within the cortex; ④ the common and relatively high-density preservation of calcified-sheath fossils of the filamental cyanobateria within the cortex. 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 get better understanding, the richness of direct evidence represented by microbial fossils especially for sheath fossils of the cyanobacteria within oncoids in the top part of Zhangxia Formations of the Cambrian Miaolingian at the Sandaogou section in the western part of Liaotung province make them become a good example of oncoids built by photosynthetic biofilms, which is also an example for the further understanding of the difference of forming mechanism and process between the ooid forming within the water column and the onciud growing on the seafloor.