インド中原生界，Vindhyan超層群Chitrakoot層の堆積相と真核生物微化石

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抄録

The eukaryotic clade is composed of stem and crown groups, and the emergence of latter is gathering particular attention because it diversified into all kind of eukaryotes that we see today. Based on eukaryotic fossils reported from worldwide, it is widely believed that they emerged in early Mesoproterozoic. Among them, Mesoproterozoic Chitrakoot Formation, lower Vindhyan Supergroup, India, recently gained attention for its well preserved early eukaryotic fossils including red algae, i.e. crown group (Bengtson, et al. 2017). However, there are still some uncertainties with Chitrakoot Formation: (1) the inhabiting environment of these eukaryotes is unclear, although it is important for understanding O₂ demand of the early eukaryotes, and (2) the depositional age is not well constrained because the upper part (1650 Ma; Bengtson, 2009) is older than the lower part (1406–1483 Ma; Kumar et al., 2001). In this presentation we will describe the lithological characteristics, depositional features, and eukaryotic microfossils from Chitrakoot Formation.The lower part of Chitrakoot Formation is composed of granular dolostone (it was called peloidal dolomite by previous studies), glauconitic sandstone and intraclastic dolostone. Some horizons of granular dolostone have chert matrix, which contains many microfossils including spherical with meshwork, smooth ornamented, and multicellular filamentous forms. The depositional setting of the lower part is tentatively considered as high energy setting and deepening upward.The upper part of Chitrakoot Formation is composed of bedded dolostone with overlying stromatolite-bearing dolostone and phosphorite. The latter is partly deformed by slump folding. The stromatolitic/oncolitic phosphorite contains filamentous structures representing probable mold of filamentous microorganisms. The depositional setting of the upper part is tentatively considered as a relatively low energy environment with flourished microbial mats. In the summary， we believe that detailed investigation is required to elucidate the depositional environment and possible eukaryotic assemblage of Chitrakoot Formation.

References

Bengtson et al. (2009) PNAS 106, 7729–7734; Bengtson et al. (2017) PLoS Biology, 15, e2000735; Kumar et al. (2001) Current Science, 81, 806–809