Objective Oncoids occur in various kinds of marine and non−marine environments. Their morphology, size, and internal structure are controlled by various biological and physical factors, which are critical indicators for reconstructing paleogeography and paleoenvironments. This study aims to clarify the genesis and depositional settings of oncoids by analyzing their textures.

Methods On the basis of the field and polarized microscope, a comprehensive study was conducted on the morphology, internal texture, and depositional environment of oncoids from the upper part of Cambrian Xuzhuang Formation at the Fuzhouwan section, Dalian, Liaoning Province.

Results The oncolites of Xuzhuang Formation were deposited in the highstand systems tract of a third−order sequence and occur in micritic limestones. Some of the oncoids and thrombolites form a microbialite assemblage. The size of oncoids ranges from millimeter− to centimeter−scale and they are classified as giant oncolites. Based on macroscopic and microscopic characteristics of oncoids, they are divided into oncoids with nuclei and concentric layers, oncoids with nuclei and cortex, and full−cortex oncoids without nuclei. Oncoids are mainly composed of micrites. There are amounts of Girvanella filaments, dolomites and pyrites in the oncoids. Clots formed by Givanella, intraclasts formed by Bacinella, and cortoids characterized by a micrite envelope are around the oncoids.

Conclusions The oncoids developed in relatively low−energy sedimentary environments. Girvanella presenting cyanobacterial calcification, dolomite and pyrite associated with sulfate−reducing bacteria activity, and grains related to microbial activity surrounding the oncoids indicate microbial origins of oncoids. The final morphology of preserved oncoids is not only controlled by microbial activities and sedimentary settings but also is influenced by diagenesis. This study provides an important example of the complex extracellular polymeric substances (EPS) calcification in the microbial mat during the formation of oncoids and also provides important clues and ways of thinking for further research on oncoids in the future.