11:45 AM - 12:00 PM
[BCG06-11] Sedimentological and geochemical characteristics of Mesoproterozoic Chitrakoot Formation, lower Vindhyan Supergroup, central India
The Mesoproterozoic (1600-1000 Ma) era have significant importance for two distinct reasons: (i) delay in diversification of early eukaryotes causing evolutionary stasis and (ii) very low O2 concentration in hydrosphere and atmosphere. Significant research highlighting evolution of early eukaryotes and redox condition of Earth's hydrosphere of that time is required. Recently, some workers reported well preserved early eukaryotic fossils including red algae, i.e. crown group from early Mesoproterozoic Chitrakoot Formation, lower Vindhyan Supergroup, India. However, more research is required to ascertain the paleoenvironmental condition along with redox condition of the Chitrakoot Formation, to know about the habitability of reported early eukaryotes. In this backdrop, this study attempts to decrypt the paleoenvironment (including redox condition) of Chitrakoot Formation using field investigation, petrography, REE geochemistry and C and O isotopes. Detailed field investigation is carried out in Chitrakoot Formation that exposed along isolated hillocks named Sangrampur, Bihara, Kamtanath, Lodhwara, Muradpur and Jankikund river section in Chitrakoot town. Petrographic and geochemical investigation a total of 35 samples (52 subsamples) have been conducted and the results are summarized here.
Complete lithopackage of Chitrakoot Formation consists of Lower Glauconitic Sandstone(LGS), Stromatolitic Dolostone(SD), Pisolitic Dolostone(PD), Upper Glauconitic Sandstone(UGS), Massive Dolostone(MD) with sheet cracks and teepee structure and Plane Bedded Dolostone(PBD). All the lithopackage are dipping in E-SE direction with a degree varying from 3° to 9°. Sangrampur hill (considered as type section) and Jankikund river section (presence of phosphatic oncolites and stromatolites) have been studied in detail. The Sangrampur section is composed of basement Bundelkhand Granite, PD, UGS, MD with sheet cracks, tepee structure and PBD. Depositional features (i.e. pisoids, herringbone cross bedding, wave and current ripple, HCS and sheet cracks) indicates deposition in a shallow and high energy environment above the storm wave base. Pisoids are composed of dolomite, silicious matrix and pendant cement in the lower part and dolomitic matrix and mosaic cement in the upper part. The Jankikund section consists of PBD and phosphatic stromatolites, indicates relatively deep and low energy and high nutrient condition. PBD showing mosaic dolomitic cement and remnants of marine radiaxial fibrous cement. The phosphatic oncolites shows presence of filamentous microfossils and gas bubble features, indicating biological activity during phosphate precipitation.
Geochemically, Sangrampur carbonates shows negative δ13C values varying from -3.59 to -0.04 and δ18O values from -8.42 to -5.65 and Jankikund carbonates shows δ13C values from -5.50 to +0.34 and δ18O values from -9.94 to -5.65, which falls within the anticipated Mesoproterozoic range. The PAAS normalized REE pattern of Sangrampur and Jankikund sections are almost flat with Y anomaly in latter. Mixed result of both positive and negative La anomaly, positive Gd anomaly, Y/Ho (≧27), variable (Pr/Yb) SN value, enrichment of detrital components (Ti and Al) and positive Eu anomaly suggests probable marine origin with significant clastic contamination and possible hydrothermal influence. The absence of true negative Ce anomaly from carbonates of both sections indicates no dissolved O2 was present to oxidize Ce (III) in shallow sea. Interestingly, pisoids from Sangrampur section shows positive Ce anomaly, which indicates reduced condition for their formation or diagenesis(?)
References
Anbarasu, K.. (2001) Jour. Geo. Soc. India, 58, 341-350; Bengtson, S., Sallstedt, T., Belivanova, V., & Whitehouse, M. (2017) PLoS Biology, 15(3), e2000735; Singh, I. B. & Kumar, S. (1978) Jour. Geol. Soc. India, 19(8), 359-367; Singh, V. K., & Sharma, M. (2014). Journal of the Paleontological Society of India, 59(1), 89-102
Complete lithopackage of Chitrakoot Formation consists of Lower Glauconitic Sandstone(LGS), Stromatolitic Dolostone(SD), Pisolitic Dolostone(PD), Upper Glauconitic Sandstone(UGS), Massive Dolostone(MD) with sheet cracks and teepee structure and Plane Bedded Dolostone(PBD). All the lithopackage are dipping in E-SE direction with a degree varying from 3° to 9°. Sangrampur hill (considered as type section) and Jankikund river section (presence of phosphatic oncolites and stromatolites) have been studied in detail. The Sangrampur section is composed of basement Bundelkhand Granite, PD, UGS, MD with sheet cracks, tepee structure and PBD. Depositional features (i.e. pisoids, herringbone cross bedding, wave and current ripple, HCS and sheet cracks) indicates deposition in a shallow and high energy environment above the storm wave base. Pisoids are composed of dolomite, silicious matrix and pendant cement in the lower part and dolomitic matrix and mosaic cement in the upper part. The Jankikund section consists of PBD and phosphatic stromatolites, indicates relatively deep and low energy and high nutrient condition. PBD showing mosaic dolomitic cement and remnants of marine radiaxial fibrous cement. The phosphatic oncolites shows presence of filamentous microfossils and gas bubble features, indicating biological activity during phosphate precipitation.
Geochemically, Sangrampur carbonates shows negative δ13C values varying from -3.59 to -0.04 and δ18O values from -8.42 to -5.65 and Jankikund carbonates shows δ13C values from -5.50 to +0.34 and δ18O values from -9.94 to -5.65, which falls within the anticipated Mesoproterozoic range. The PAAS normalized REE pattern of Sangrampur and Jankikund sections are almost flat with Y anomaly in latter. Mixed result of both positive and negative La anomaly, positive Gd anomaly, Y/Ho (≧27), variable (Pr/Yb) SN value, enrichment of detrital components (Ti and Al) and positive Eu anomaly suggests probable marine origin with significant clastic contamination and possible hydrothermal influence. The absence of true negative Ce anomaly from carbonates of both sections indicates no dissolved O2 was present to oxidize Ce (III) in shallow sea. Interestingly, pisoids from Sangrampur section shows positive Ce anomaly, which indicates reduced condition for their formation or diagenesis(?)
References
Anbarasu, K.. (2001) Jour. Geo. Soc. India, 58, 341-350; Bengtson, S., Sallstedt, T., Belivanova, V., & Whitehouse, M. (2017) PLoS Biology, 15(3), e2000735; Singh, I. B. & Kumar, S. (1978) Jour. Geol. Soc. India, 19(8), 359-367; Singh, V. K., & Sharma, M. (2014). Journal of the Paleontological Society of India, 59(1), 89-102