Organic-walled microfossil assemblage and thermal evolution of the mid-Mesoproterozoic Taizi Formation of Shennongjia Group on the northern margin of the Yangtze Platform in southern China
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Abstract:
The Shennongjia Group on the northern margin of the Yangtze Platform consists of thick marine carbonate and clastic rocks of the middle and late Mesoproterozoic Era. The middle part of the Shennongjia Group, i.e., the Taizi Formation, consists of sandstone and silty shale in the lower part and fine-crystalline limestone in the upper part. According to previous investigations, organic-walled microfossils of the Taizi Formation are the most abundant among the formations of the Shennongjia Group. To further improve the understanding of the biodiversity during the deposition of the Taizi Formation, we carry out a micropaleontological investigation on the fine-clastic rocks of the Taizi Formation from the Tiechanghe section, Shennongjia National Park, Hubei Province. Our investigation reveals a mode- rately diverse assemblage of organic-walled microfossils dominated by spherical vesicles and aggregates with a few complex vesicle forms. In addition, the fossil specimens of the Taizi assemblage are very small, generally ranging from 20 to 50 μm. A total of 11 microfossil taxa belonging to seven genera have been identified. These taxa include simple spherical form (Leiosphaeridiacrassa, Leiosphaeridiaminutissima, Leiosphaeridiajacutica, Leiosphaeridiabicrura, and Leiosphaeridia sp.), cell aggregate form (Synsphaeridium sp. and Eomicrocystismalgica), and complex vesicle form (Navifusa sp., Satka sp., Germinosphaera sp., and Arctacellulariatetragonala). As for biological affinity, prokaryotes are the predominant components, whereas eukaryotes are scarce. The low diversity and small individual size of the microfossils of the Taizi assemblage, which might be caused by a harsh living environment or insufficient essential nutrients, are significantly different from those of the earlier and later biological communities. Furthermore, the Taizi Formation has experienced low- to middle-grade metamorphism with peak metamorphic temperatures of ~280°C based on Raman geothermometers of individual organic-walled microfossils, which is higher than those of other well-preserved Proterozoic microfossils. The high burial temperature may affect the preservation quality of the microfossils of the Taizi assemblage.
