Environmental setting of the Cambrian Terreneuvian rocks from the southwestern Yangtze Platform, South China

Abstract

Terreneuvian strata from the western Yangtze Platform host diverse assemblages of small shelly fossils (SSFs), which are key for understanding the Cambrian Bioradiation Event and have been the objects of numerous geochemical studies to understand environmental factors during Fortunian and Cambrian Age 2. The region has been proposed to host the vacant stratotype of Cambrian Stage 2. However, a comprehensive study of paleogeography and evolution of sedimentary facies for the Terreneuvian is still lacking. Here we present a detailed sedimentological and petrographic study of six stratigraphic sections from the western Yangtze Platform. Eleven depositional environments (facies associations) are proposed based on the characterization of twenty-five facies. We further reconstruct palaeogeographic model based on the evolution of depositional environments and discuss the formation of phosphorites. The Zhujiaqing Formation deposited diachronously on the Ediacaran strata, and facies associations range from tidal flats, shoal, back-shoal to semi-restricted and energetic subtidal. As sedimentation continued, protected shallow subtidal became widespread. Evidence presented from the Zhujiaqing Formation indicates that strata in the laolin section are more continuous than those on other parts of the Yangtze Platform. However, stratigraphic condensation and the protected and partly restricted environment should be taken into consideration when discussing a proposed global GSSP “Laolinian Stage” (Cambrian Stage 2). In-situ phosphates formed in various environments, which argues against the previous assumption that primary phosphate formed only in isolated embayments. Concentration of granular phosphorites occurred in shoal areas by winnowing and reworking of in-situ phosphate and phosphatized skeletons, while in the intra-platform basin, repeated alternations of syndepositional phosphogenesis, current reworking, and amalgamation of storm-generated phosphatic event beds contributed to the concentration of granular phosphorites.