Calcified cyanobacteria fossils from the leiolitic bioherm in the Furongian Changshan Formation, Datong (North China Platform)

Abstract

Microbialites are studied for their sedimentary features in stratigraphic relationship to the surrounding facies to determine the depositional environments suitable for dissemination of calcimicrobes. This study examines the stratigraphic position and distribution of microbial fabric in the Lower Furongian Changshan Formation exposed in Kouquan section of Datong City in the North China Platform. The formation is comprised of the first third-order carbonate depositional sequence of the drowning unconformity-type in Furongian. A variation tendency of sedimentary facies characterized by a generally shallowing upward succession can be noticed across the formation, ranging from the calcareous mudstone and shale of shelf facies at the bottom to massive micrites in the middle, and to thick-bedded oolitic grainstone of shallow-ramp facies in the top part. Inside a bed of microbial carbonate composed of massive micrites in the upper part of the formation, developed the dense undifferentiated microbial boundstones as a result of forced regression in response to the third-order relative sea-level falling, which can be grouped into leiolitic bioherms. A variety of well-preserved calcified cyanobacteria fossils recognizable as Epiphyton, Girvanella, and Renalcis developed inside the leiolite. These aerobic calcified microorganisms contribute to the calcification of mucilaginous sheaths by uptake of CO2 and/or HCO3− during photo-assimilated photosynthesis and participate in the first episode of cyanobacteria calcification event in Phanerozoic. The current macroscopic and microscopic analyses provide significant implications for the future understanding of the origin of leiolitic bioherm and their paleodepositional environment in the North China Platform.