Evaluation of provenance, tectonic setting, and paleoredox conditions of the Mesoproterozoic-Neoproterozoic basins of the Bastar craton, Central Indian Shield: Using petrography of sandstones and geochemistry of shales

We carried out petrographic analyses of the sandstones and geochemical analyses of the shales from the Mesoproterozoic-Neoproterozoic Chhattisgarh and Indravati Basins to determine their tectonic setting, provenance, and paleoredox conditions. Petrographic study shows that the sandstone samples have high amounts of quartz but are depleted in feldspar and lithic fragments. The shales have been classified into the calcareous and noncalcareous shales. The noncalcareous shales have higher concentrations of most of the major elements and trace elements, including the rare earth elements (REEs), in comparison to the calcareous shales. However, this difference in elemental concentrations between the calcareous and noncalcareous shales proved to be significant only for SiO2, TiO2, Al2O3, MnO, CaO, K2O, loss on ignition, Rb, Sr, Nb, Ce, Pr, Sm, Gd, Hf, and Ta using the Student's t-test at better than 95% confidence level. Upper continental crust (UCC)-normalized elemental ratios of the calcareous and noncalcareous shales suggest evolved sources similar to UCC. The sandstone petrology and Ni versus Cr diagram, chondrite-normalized REE patterns, and negative Eu/Eu* values of the calcareous and noncalcareous shale samples reveal that the sediments have been derived from felsic rocks (granites and gneisses) of the Bastar craton. The SiO2 versus K2O/Na2O and SiO2/Al2O3 versus K2O/Na2O tectonic-setting discrimination diagrams of the shales and the petrology of the sandstones indicate a passive-margin tectonic setting for Chhattisgarh and Indravati Basins. Geochemical parameters such as Ce/Ce* and Mn* suggest that the calcareous shales were deposited in a suboxic environment, compared to the oxic environment of the noncalcareous shales.