科学前沿

Giant ooids have been considered as abnormal sediment and distributed globally in the aftermath of the end-Permian mass extinction, and represent a marine carbonate supersaturated state which is unfavourable to the survival of metazoans. The marine environment returned to normal in the early Middle Triassic when abnormal sediments became rare. However, the discovery of giant ooids in the early Middle Triassic at the Poduan section, Ceheng, Guizhou Province, could give us more insights into the marine environmental conditions during that time. In this study, the sedimentary microfacies and fabrics of the carbonate rocks at the bottom of the Poduan section were analyzed. The sedimentary environments of the giant ooid-bearing beds were comprehensively studied based on the oolitic characteristics, oolitic content, bioclastic content and sedimentary microfacies. It can be considered that there are two oolitic enrichment periods. Four kinds of oolitic grains have been found at the Poduan section, including common concentric oolitic grains, giant concentric oolitic grains, polycrystalline oolitic grains and mud crystal oolitic grains. Four sedimentary microfacies were identified based on rock structure and carbonate particle characteristics and could represent the interbedded deposition of platform margin oolitic shoal and bioclastic shoal. The development of giant ooids in this area indicated that the marine carbonate saturation is still very high and the abnormal marine environment occurred in this area during the early Middle Triassic. It suggests that there were obvious regional differences in the global marine environment during the Middle Triassic, which is an important reason for the complexity of the model of biotic recovery and radiation.