Presentation Information
[T7-O-12]Sedimentological and geochemical characteristics of Mesoproterozoic Chitrakoot Formation, lower Vindhyan Supergroup, central india: Implication on paleoenvironment
*Abhishek Kumar Pandey1, Partha Pratim Chakraborty2, Ryoichi Nakada3, Akihiro Kano4, Fumito Shiraishi1 (1. Hiroshima University, 2. Delhi University, 3. JAMSTEC, 4. Tokyo University)
The Mesoproterozoic (1600–1000 Ma) era have significant importance for two reasons (ⅰ) delay in diversification of early eukaryotes causing evolutionary stasis and (ⅱ) very low O2 concentration (0.1% PAL) in atmosphere which was below Pasteur point (1% PAL). One common ground that links both reasons is the paleoenvironment along with redox condition, hence, paleoenvironmental interpretation becomes crucial to knowing the habitability of early eukaryotes during Mesoproterozoic. Recently, well preserved early eukaryotic microfossils including red algae (crown group eukaryotes) have been reported from early Mesoproterozoic Chitrakoot Formation, lower Vindhyan Supergroup, India. However, the previous research did not discuss about the paleoenvironmental condition along with redox condition in which those early eukaryotes were living. In this backdrop, this study attempts to decrypt the paleoenvironment and redox condition of Chitrakoot Formation using field investigation, petrography and geochemistry (REE and C and O isotopes).
Based on field evidence three lithofacies i.e. Peloidal dolostone, Glauconitic sandstone and Plane bedded dolostone are consistently exposed throughout in ascending order. Presence of sedimentary structures like herringbone cross bedding, trough cross bedding, wave ripples and plane lamination suggest environment was near shore environment with tide and wave activity. Petrography suggest peloidal dolostone lithofacies experienced vadose zone cementation near coastal environment. This study also found some well preserved eukaryotic microfossils from the silicified matrix of peloidal dolostone. Some peloids are internally silicified and show coccoidal cyanobacterial community within it. The upper plane bedded dolostone contains localized phosphate deposits that contains phosphatic oncolites. The phosphatic oncolites contains filamentous microfossils and gas bubble features, indicating biological activity during phosphate precipitation.
The PAAS normalized REE patterns are almost flat and lack open sea like patterns. Mixed result of variable La and Gd anomaly, Y and Ho ratio (≧27), variable shale normalized Pr and Yb ratio value with high detrital components (Ti and Al) suggests probable mixing of end members freshwater and marine water in near coastal environment (i.e. estuary or coastal lagoon). The absence of true negative Ce anomaly from carbonates indicates no dissolved O2 was present to oxidize Ce (Ⅲ) in shallow sea (suboxic–anoxic). Interestingly, peloids from Sangrampur section shows strong positive Ce anomaly, which indicates reduced condition for their formation and or later effect of diagenesis. However, the presence of oxygenic prokaryotes i.e. coccoidal cyanobacteria within the peloids reckons to provide small oxygen oasis in which early eukaryotes would have sustained and evolved.
Based on field evidence three lithofacies i.e. Peloidal dolostone, Glauconitic sandstone and Plane bedded dolostone are consistently exposed throughout in ascending order. Presence of sedimentary structures like herringbone cross bedding, trough cross bedding, wave ripples and plane lamination suggest environment was near shore environment with tide and wave activity. Petrography suggest peloidal dolostone lithofacies experienced vadose zone cementation near coastal environment. This study also found some well preserved eukaryotic microfossils from the silicified matrix of peloidal dolostone. Some peloids are internally silicified and show coccoidal cyanobacterial community within it. The upper plane bedded dolostone contains localized phosphate deposits that contains phosphatic oncolites. The phosphatic oncolites contains filamentous microfossils and gas bubble features, indicating biological activity during phosphate precipitation.
The PAAS normalized REE patterns are almost flat and lack open sea like patterns. Mixed result of variable La and Gd anomaly, Y and Ho ratio (≧27), variable shale normalized Pr and Yb ratio value with high detrital components (Ti and Al) suggests probable mixing of end members freshwater and marine water in near coastal environment (i.e. estuary or coastal lagoon). The absence of true negative Ce anomaly from carbonates indicates no dissolved O2 was present to oxidize Ce (Ⅲ) in shallow sea (suboxic–anoxic). Interestingly, peloids from Sangrampur section shows strong positive Ce anomaly, which indicates reduced condition for their formation and or later effect of diagenesis. However, the presence of oxygenic prokaryotes i.e. coccoidal cyanobacteria within the peloids reckons to provide small oxygen oasis in which early eukaryotes would have sustained and evolved.