Morphological Variability, Microbial Components and Accretionary Histories of Miaolingian (Middle Cambrian) Oncoids on the North China Platform

Oncoids are microbially influenced coated carbonate grains whose cortices preserve records of accretion and later modification, but their relationships to grain accumulation require evidence at both grain and bed scales. This study integrates facies analysis, petrography and morphometry of 118 two-dimensional profiles from four Miaolingian intervals on the North China Platform: the Maozhuang and Xuzhuang Formations at Xihuang (XH), and the Xuzhuang and Zhangxia Formations at Chafangzi (CFZ). A uniform classification based on outline and cortical organization identifies five descriptive morphotypes among 63 classifiable profiles. Within the sampled Xuzhuang intervals, CFZ profiles have greater mean apparent length than XH profiles. CFZ cortices preserve abundant Girvanella-like filaments within micritic laminae, supporting recurrent microbial participation in accretion. In the Xuzhuang cortices at CFZ, ooids, peloids and filament-bearing clots enclosed by laminae document particle incorporation during growth. Ooid-rich grainstones and micrite-rich floatstones at XH, together with lenticular rudstones and a mound-like body at CFZ, record contrasting accumulation textures and geometries. Calcite-filled cortical voids and localized sutured grain contacts document pore filling and pressure solution. These relationships distinguish evidence for microbial cortex accretion from records of final accumulation and subsequent modification in the sampled Cambrian oncoids.