Ichnofabrics of the Cambrian Zhangxia Formation, North China: insights into bio-environmental dynamics of an epeiric sea

Epeiric seas served as critical depositional settings for ancient carbonate successions; however, the absence of modern analogues has significantly hindered their palaeoecological reconstruction. Well-preserved Cambrian epeiric carbonates of the Zhangxia Formation crop out along the southern margin of the North China Block. An integrated sedimentological and ichnological analysis has established six facies associations in the formation, representing six distinct depositional environments, with six corresponding ichnofabrics identified to characterize biogenic-sedimentary interactions. The deep subtidal zone, near the storm wave base, lacks bioturbation structures. Intra-platform oolitic shoal and microbial reefs complex host Balanoglossites triadicus Ichnofabric. Landward marginal zones of oolitic shoal exhibit Planolites montanus-Skolithos linearis ichnofabric and Thalassinoides horizontalis ichnofabric. Open shallow subtidal settings are dominated by Arenicolites isp.-Taenidium isp. ichnofabric. The barrier island oolitic shoals show abundant Planolites montanus, whereas restricted subtidal settings behind barrier islands are characterized by dense Thalassinoides bacae. The results demonstrate that benthic metazoans had pervasively colonized the entire epeiric sea during Cambrian Miaolingian. This ecosystem-wide colonization reflects the establishment of tiered infaunal communities in the Miaolingian epeiric sea, where microenvironmental stress gradients controlled both the diversity of bioturbation styles and the dominance of r- versus K-selected trace makers.