Oncoids formed by Girvanella in the Cambrian of eastern China

The oncoids possess dual properties, namely “spherical stromatolite” and coated particles resembling ooids but with a larger particle size, which has garnered significant attention from the academic community. In this study, we focused on investigating the oncoids in the Zhangxia Formation of the Cambrian Miaolingian at the Jiguanshan section of Shandong Province, North China Platform applying sedimentological methods. Our findings reveal that these oncoids formed during the FRST (forced regression system domain) phase, representing products of a continuously submerged shallow sea high-energy environment. Two notable characteristics are observed: (1) the volume of the oncoids gradually increases from the bottom to the top; (2) the numerous clots between individual oncoid. Microscopic analysis demonstrates that laminae development is either undeveloped or indistinct in these oncoids. Morphologically speaking, they can be classified into giant and ordinary types based on their appearance. Abundant Girvanella fossils found within the oncoids and clots suggest that the interactions between microbes and the sedimentary environment played a crucial role in oncoid formation. Studying oncoids can reveal their depositional conditions and the role of microbial activity in their formation. Consequently, our study provides compelling evidence supporting microbial involvement in oncoid formation while also serving as a valuable reference for future studies exploring similar sedimentary environments of oncoids.