Geochemistry of an intercalated unit of arkose and shale of the Dhandraul Formation belonging to the Vindhyan Supergroup, Eastern India: Insights from provenance, depositional environment, and geodynamic set-up

Abstract

In this study, we have presented a hitherto unreported mappable member of an intercalated unit of arkose and shale of the Dhandraul Formation of the Kaimur Group exposed in Kaimur district, Bihar, India which was not reported by earlier workers in the Vindhyan basin. It is a contribution for understanding the possible provenance, source area weathering, depositional environment, and geodynamic set-up of this member. Based on field characteristics and petrography study, three lithofacies units have been identified viz. (i) coarse to medium-grained arkose, (ii) intercalated sequence of arkose and shale, and (iii) shale. Geochemically, these lithofacies predominantly occupied the field of arkose and shale except samples fall in the field of sub-arkose. These lithofacies display relatively an enrichment of SiO2 and Al2O3 and show low concentrations of MgO, CaO, Na2O, K2O, and TiO2. The values of ∑REE in arkose are varying from 78.74 to 128.81 ppm whereas the values of fractionation indicate (La/Sm)N (3.73-4.22), (La/Yb)N (7.33-15.59), (Gd/Yb)N (1.32-2.30), and Eu/Eu* (0.58-0.66). In shale, ∑REE ranges from 354.02 to 382.11 ppm while the fractionation contents of (La/Sm)N, (La/Yb)N, and (Gd/ Yb)N, and Eu/Eu* are ranging from 3.82 to 4.82, 7.65 to 11.85, 1.38 to 1.73, and 0.56 to 0.81. On the basis of rock fragments and paleocurrent direction, the possible sources are presumed to be the Chhotanagpur Gneissic Complex and the Mahakoshal Group of rocks, which lie towards the south and southwest. In the binary and ternary plots, most of the samples of arkose and shale have mostly occupied the field of passive margin tectonic setting except one sample comes in the field of the continental island arc. Based on mineralogical and textural maturity, sedimentary structures and the overall sequence of these lithofacies from sandstone to shale member of the Dhandraul Formation show fining upward sequence which attributes the deposition in a shallow coastal fluvial-marine environment in a transgressive phase. This discovery has opened a new opportunity to relook at the depositional environment of the Dhandraul Formation in other parts of the Vindhyan basin.