Carbonate sedimentation and early diagenesis of the Doushantuo Formation, South China: A window into the terminal Proterozoic

Precambrian marine carbonates with distinct sedimentary fabrics and structures provide valuable insights into the chemical and biological conditions of early oceans. To better understand shallow marine environments approaching the terminal Proterozoic, this study examines carbonate deposits from the Ediacaran Doushantuo Formation across the shoal-rimmed Yangtze platform in South China. These mimetically dolomitized rocks comprise facies of micropeloidal leiolite, grainstone-packstone, micropeloidal and microlaminated leiolites, stromatolite, and mudstone, representing a transition from the upper slope and platform margin environments to the back-shoal peritidal system. The grainstone-packstone facies features large intraclasts of incipient aragonite and high-Mg calcite cements, along with radial ooids that are <2 mm in diameter, all exhibiting varying degrees of microbial micritization. The peritidal facies, marked by abundant microbialites, are dominated by micropeloidal-microlaminated leiolites from the intertidal zone and stromatolites and dolomudstones from deeper waters. Abundant fibrous dolomites (fascicular and radial length-slow, up to 5 mm long) fill cavities in the peritidal facies, particularly within microbial mat cracks. Developed on a warm-temperate to tropical shelf, the rapid early lithification of Doushantuo sediments indicates a high degree of seawater supersaturation during the late Neoproterozoic compared to modern conditions. The remarkable diversity in the physical properties of the Doushantuo carbonates reflects an increased compartmentalization of processes governing carbonate formation and dissolution in shallow marine environments. This shift corresponds with the evolving chemical and biological conditions of the Precambrian ocean, which increasingly emphasized microbial roles in dolomite precipitation. By the terminal Proterozoic, platform carbonate formation became more susceptible to local fluctuations affecting saturation states and kinetic conditions, largely influenced by microbial activity.