279 / 2019-07-29 13:25:00

Cambrian Stratigraphy and Sedimentology of the Cyanobacteria-Dominated Microbial Carbonates in the North China Platform

Calcified cyanobacteria; Miaolingian; Furongian; Microbialites; North China Platform.

The study describes the diverse macro-, meso- and microfabrics of microbial carbonates deposited on the northern margin of the North China Platform during the Miaolingian-Furongian interval. During the first transgression of Paleozoic in the Cambrian, shallow-marine facies were deposited in most areas of the platform, which favored diversified biological reproduction inside the framework of microbial carbonates. The microbial carbonates in the selected sections of Shanxi, Liaoning, Inner Mongolia and Hebei provinces occur in Xuzhuang, Zhangxia, Gushan, Changshan and Fengshan formations. Based on the facies stacking patterns and cyclicity in the chronostratigraphic context, these formations are divided into six third-order depositional sequences. The shift in sedimentary strata from mixed tidal flat facies at the base of the Xuzhuang Formation to the overlying ramp carbonate deposits shows retrogradation under the second-order transgressive process, which deposited plentiful carbonate microbialites during the Miaolingian-Furongian interval. Compared to the development and abundance of thick oolitic banks during the Miaolingian, the profusion of calcirudites and massive biohermal microbialites in the Furongian represents the sedimentation style of the skeleton-poor stormy sea of the North China Platform.
The studied microbialites deposited near or slightly below the fair weather wave base with no strong waves and currents. Meanwhile, the seafloor reached a water depth suitable for the microbial growth during sea-level rise accompanied with carbonate production. Among the identified microbialites, leiolite as undifferentiated microbial boundstone appears to be the abundant-most type recognized throughout the studied formations, followed by clotted or chaotic thrombolite confined to the lower Furongian Changshan Formation. The laminated stromatolite, and the cyanobacteria-bearing oncoid and ooid also occur in a relatively lower abundance. Diversification in microbialites can be generally attributed to variation in the size of contributory microbial mats and the bionetwork. Cyanobacteria predominate other mat-building microorganisms across the Miaolingian-Furongian boundary interval, and are believed to mediate a sophisticated in-situ carbonate precipitation. These aerobic calcimicrobes contribute to calcification of mucilaginous sheaths by uptake of CO2 and/or HCO3− during photo-assimilated photosynthesis. Besides, the extracellular polymeric substances (EPSs) are degraded by heterotrophic bacteria, and the alkalinity-increase triggered by sulfate reduction and release of Ca2+ leads to CaCO3 precipitation and subsequent formation of microbial mats.
Petrographic observation led to identification of eleven microfacies through the carbonate strata, which are grouped into six facies associations sited on a carbonate ramp. The high density preservation of calcimicrobes is chiefly observed in the microbial boundstone microfacies deposited in inner ramp open marine environments, which counts for the major proportion of the studied microbialites. The well-preserved calcimicrobes include Girvanella, Subtifloria, Botomaella, Renalcis, Tarthinia and Epiphyton, which confirm the recovery phase of microbial carbonate abundance and the cyanobacterial calcification episode of Cambrian-early Ordovician. A substantial increase in the diversity and abundance of calcimicrobes in the microbialite microfabric from Miaolingian to Furongian is observed in the study area. Furthermore, the recovery of Epiphyton from the Furongian microbialites refutes the possible decline of Epiphyton at the end of Miaolingian for this part of the North China Platform.