Mechanisms of Formation of Oncoids of Miaolingian Zhangxia Formation at Chafangzi Section, Shanxi province, North China

Abstract

Abstract As a variety of microbial carbonates, oncoids exhibit characterstics of coated grains that have relative large size than ooids with microbial origin, and features of successive lamination similar to stromatolites. The present study is aimed at investigation of formation of the Cambrian oncoids that developed in top part of the Zhangxia Formation at Chafangzi Section of Shanxi province, North China. All three subsequences of the Zhangxia Formation associated with calcareous mudstone in lower whereas oolitic limestone in upper part of the subsequences. Further top part of third subsequence of the Zhangxia Formation comprises a bed of oncolite overlying the oolitic beds at the studied section which was taken up for this investigation. The oncolites exhibit the characteristcs of coated grains as well as laminated stromatolites on large scale whereas small scale portrays the abundance of twisted and non-twisted filamentous cyanobacteria in the cortex and matrix among the oncoids that provide the concrete evidences of direct or indirect involvement of filamentous cyanobacteria in the formation of the Cambrian oncoids. This study reveals the Cambrian oncoids of the Zhangxia Formation provide a typical example of their formation that was associated with complicated calcification of EPS in the microbial mats dominated by cyanobacteria.