Sequence stratigraphic and petrological analyses of the Cambrian oncoids exposed in the Liaoning Province, North China Platform

Abstract

The Cambrian Miaolingian Series strata are continuously exposed in the North China Platform and contain diverse sedimentary phenomena, including substantial with oncoid-rich units. Limestone samples from the Zhangxia Formation in the Sandaogou section were collected to characterise unique macroscale to microscale sedimentary characteristics of the marine carbonate oncoids. In the oncolitic beds (∼4.5 m thick), the individual size of oncoids gradually increases from bottom to top. The oncoid morphology and the presence of a nucleus and laminae in thin-sections are used to classify these oncoids into three types (concentric laminar, lateral growth and multicore oncoids). As evidenced by scanning electron microscopy and energy-dispersive X-ray observations, Girvanella are abundant inside these oncoids and are associated with nanospheres, framboidal pyrite, and spherical and filamentous microbial fossils, confirming the biogenicity of the studied oncoids. These results suggest that the oncolitic–oolitic limestone formed in an upward-shallowing marine environment caused by a forced regression process that resulted in a decrease in accommodation space in a fourth-order sequence. Thus, the conditions became increasingly suitable for the development of cyanobacteria-dominated microbial mats and large oncoids. The late stage of the depositional setting and elevated solar radiation resulted in the formation of the growth termination surface. KEY POINTS Evidence of microbial origin from the oncoids of the Cambrian Miaolingian Series in the North China Platform. Vertical variation in oncoid size and distribution coincide with geochemical data. A shallowing depositional environment is interpreted as the reason for variation in oncoid size and formation of oncoid growth termination surfaces.