Characteristics and sedimentary evolution of late Ediacaran-early Cambrian microbial carbonates, chert, and phosphorite in South China

ABSTRACT

The Ediacaran-Cambrian (E-C) transition period witnessed one of the most important environmental changes in Earth’s history. The western margin of the South China Block (SCB), a shallow-water environment, shows a complete sedimentary record of the E-C transition. Section surveys revealed that dolostone, microbial carbonate, chert, and phosphorite predominantly compose the lithologies of the western margin of the SCB during the E-C transition. Petrological analysis indicates that microbial carbonates in this region exhibit stromatolitic, thrombolytic, cavitated, and sporadic microbial structures. Microbial carbonates with stromatolitic, thrombolytic/cavitated, and sporadic structures correspond to strong, moderate, and weak hydrodynamic conditions, respectively. Microcrystalline dolomites crystallize during the syngenesis stage, while meso- to macrocrystalline and sparry dolomites develop during the burial diagenesis stage. Analysis of major elements and rare earth elements anomalies (δCe: 0.580 ~ 0.753; δEu: 1.234 ~ 1.433) suggests that the siliceous component originates from hot-water sources and is transported into shallow-water environments by upwelling. The absence of a δEu anomaly and the morphology of peloids indicate that the phosphatic component is primarily influenced by biological activity and upwelling. During the Tongwan Movement (545–535 Ma) in the E-C transition period, cyclical upwelling transported increased amounts of deep-source materials into shallow platform waters, leading to higher concentrations of siliceous components and widespread microbial mortality. Following the widespread microbial mortality, the phosphatic component became enriched in deep-water environments. Subsequently, the upwelling transported siliceous- and phosphatic-rich fluids back into the shallow-water environments. The phosphatic component was ultimately assimilated by small shelly fauna and deposited in the phosphatic-rich strata of the early Cambrian period. This study elucidates the origins of complex lithological compositions and the sedimentary evolution model in shallow water environments of the E-C transition period in western SCB, offering evidence for global environmental changes during this period.

KEYWORDS:

• Ediacaran–Cambrian transition
• microbial carbonates
• chert
• phosphorite
• sedimentary evolution model

Acknowledgments

Our thanks go to the Associate Prof. Zhuang Ruan, Mr. Chang’an Liu and Yuxuan Wu for their support in field survey and geochemical tests. We are also grateful to the State Key Laboratory of Geological Processes and Mineral Resources of the China University of Geosciences, Beijing and the Analytical Laboratory of BRIUG for their support in sample testing and analysis. This work was supported by the National Nature Science Foundation of China [Grant No. 42102162], and the Major National Science and Technology Projects of China [Grant No. 2017ZX05005-002-003].

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2024.2364911

Additional information

Funding

The work was supported by the National Major Science and Technology Projects of China [2017ZX05005-002-003]; National Natural Science Foundation of China [42102162].