Geochemistry of Neoproterozoic shales of the Rabanpalli Formation, Bhima Basin, Northern Karnataka, southern India: implications for provenance and paleoredox conditions

Abstract

The Rabanpalli Formation exhibits two types of shales, viz. grey and calcareous shales. These\nshales (grey and calcareous) have been analysed for major, trace, and rare earth elements to fi nd out their\nsource rocks characteristics and paleo-oxygenation conditions. The grey shales have higher concentration\nof SiO2, Al2O3, Fe2O3, K2O, Zr, Th, U, V, Cr, La, Ce, and Y than calcareous shales, whereas calcareous\nshales are enriched in CaO, Mn, Sr, Ba, Cu, and Zn, which indicate that the carbonate phase minerals\nare higher in calcareous shales. The positive correlation of K2O with other elements, and abundance of\nAl2O3, Ba, Th, and Rb suggest that these elements are primarily controlled by the dominant clay minerals.\nLa/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th ratios of shales were compared with those of sediments derived\nfrom felsic and basic rocks (fi ne fraction), upper continental crust (UCC) and post-Archean Australian\naverage shale (PAAS) ratios, which reveal that these ratios are within the range of felsic rocks. The La/Sc\nvs. Th/Co plot also suggests the felsic nature of the source rocks. The shales show slightly light rare earth\nelement (LREE) enriched and fl at heavy rare earth element (HREE) patterns with negative Eu anomaly,\nand are similar to the granitoids from Dharwar Craton, which suggest that the Archean Dharwar Craton\ncontributed the sediments to the Bhima basin. The geochemical parameters such as U, authigenic U, U/Th,\nV/Cr, Ni/Co, and Cu/Zn ratios indicate that these shales were deposited under oxic environment.