Late Miaolingian to Furongian Transition in Calcified Microbes from the North China Platform (Shanxi Province): Implications for Microbialite Development

This study examines the sedimentation of the calcified microbes in the late Miaolingian and Furongian microbialites in response to relative sea-level changes. The microbialites mainly bulge out as undifferentiated microbial boundstones observed as leiolitic bioherms in the upper reaches of four third-order depositional sequences, as recognized through the Gushan, Changshan and Fengshan formations. A flat-bedded biostromal thrombolite also developed in the lower Furongian strata, with a variety of poorly-preserved calcimicrobes. In geologic record, the variety of microbialites is widely attributed to the microbial mats of diverse size and bionetwork. The cyanobacteria are believed to predominate other mat-building microorganisms in mediating a sophisticated in-situ carbonate precipitation across the Miaolingian-Furongian boundary interval. A rapid shift in the microbialite microfabric from Miaolingian to Furongian is observed in the study area, which is marked by a substantial increase in the diversity and abundance of calcified microbes. The Miaolingian leiolitic bioherms hold abundant crust-forming structures in the mixed ground mass of micrite and microspar. With reduced occurrence, these possible microbial structures with uncertain biological affinity extend upward across Miaolingian-Furongian boundary. The Furongian leiolitic bioherms are unique as they developed in response to forced regression during third-order relative sea level fall. The well-preserved calcimicrobes recovered from these bioherms include Girvanella, Subtifloria, Epiphyton and Renalcis confirm the recovery phase of microbial carbonate abundance and the intense cyanobacteria calcification episode of Cambrian-Early Ordovician. The recovery of Epiphyton from the Furongian microbialites in the study area contradicts the possible decline in Epiphyton abundance at the end of Miaolingian series, as reported from other parts of the North China Platform. This study offers significant implications towards the sedimentation pattern in the shallow skeleton-deficient sea during Furongian before the metazoan radiation of the middle Ordovician.