A Case Study on the Co-Occurrence of Oncoids and Ooids in the Cambrian Miaolingian Series, North China Platform

Oolitic shoals widely developed in the North China Platform during the Cambrian Miaolingian Epoch, which coincided with the first episode of cyanobacterial calcification. This study reports the co-occurrence of oncoids with ooids in the carbonate sedimentary environment in four different sections. A comprehensive study involving outcrop description, microscopic observation and statistical analysis is carried out to determine the facies, morphology and internal structure of both oncoids with ooids. Field observations show that the co-occurrence of oncoids and ooids developed at the top part of the third-order depositional sequence that represents the depositional products of the relative sea-level decline process. Microscopic observation shows that both oncoids and ooids can be subdivided into five types on the basis of their internal structure and morphological features. The comparative study of oncoids and ooids in different stages shows the influence of microbial activity on the formation of oncoids and ooids. Moreover, the comparison of oncoids from four different sections shows that the paleogeographic position and microbial involvement impacted the deposition and morphological diversity of carbonate grains in this period. In addition, the presence of microbial fossils and constructive micrite envelopes around these ooids confirm a participatory role of microbial mats in carbonate grain formation during the first episode of the cyanobacteria calcification event in the Cambrian.