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Cambrian oncoids were deposited in the upper part of the Miaolingian Xuzhuang Formation at the Wuhai Section, Inner Mongolia, China in the late highstand systems tract of the third-order depositional sequence. Microscopic studies show an abundance of dark micrite in the dolomitic, bioclastic and micritic oncoids that possibly indicate the presence of microbes such as filamentous cyanobacteria and sulfate reducing bacteria either in the cortices and/or nuclei of these oncoids. These rounded to elliptical oncoids, forming packstone-to-grainstone facies in the Xuzhuang Formation, existed in a high energy setting with an abundance of filamentous fossils of cyanobacteria. These observations demonstrate a complicated mechanism of oncoid formation associated with calcification of extracellular polymeric substances in the cyanobacteria dominated microbial mats. The present work provides evidences about the active mediation of cyanobacteria in the formation of the Cambrian oncoids in the Xuzhuang Formation. It offers a reference example for the development of microbial carbonates in the North China Platform during the Cambrian period. [ABSTRACT FROM AUTHOR]