Origin and depositional environments of mounded, banded and bedded dolostones in the clastic deposits of the Paleoproterozoic Chuanlinggou Formation in Beijing, North China

Various studies have indicated that microbes were involved in the precipitation of dolostones in the Proterozoic. The identification of ancient microbial dolostones remains challenging due to the rare preservation of microbe fossils. In this study, we found ancient microbial dolostones in Paleoproterozoic Chuanlinggou Formation in Beijing, North China, employing thin-section observations, trace and rare earth elements as well as carbon and oxygen isotope analyses to reveal the origin. The paleoenvironment has also been reconstructed. The results indicate that dolostones found in the Chuanlinggou Formation are formed through primary dolomite precipitation induced by cyanobacterial activity. These dolomitized geobodies exhibit various morphologies of mounded, banded and bedded, and all dolostones are microcrystalline textures. The geochemical characteristics of these dolostones show a relatively low δ¹⁸O value (average = −7.34‰), Fe(II) enrichment, Ce anomaly and comparable patterns of REY. The specific geomorphology (mounded, banded and bedded) and mottled structures of the dolostones indicate that they are formed through in-situ dolomite precipitation induced by cyanobacterial activity. The high Fe(II) content suggests a reducing environment, whereas the Ce anomaly indicates anoxic conditions within the water body. The REY patterns provide evidence for the microbial origin of the dolostones. These dolostones were formed in clastic off-shore shelf environments. The seawater was warm, normal in salinity, and the sea floor was slightly agitated to quiet and reducing. The geomorphology of the dolomitized geobodies changes from mounded to banded and bedded in correlation with depth increase and hydrodynamic energy decrease. This study may provide another type of environments for microbial dolostones and the geochemical characteristics of these dolostones may serve as a valuable reference for identifying similar ancient microbial dolostones.