Geochemistry of Permian carbonaceous shales from Raniganj sub-basin, Damodar Valley, India: Implications for provenance, weathering, tectonics and source of organic matter

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Raniganj sub-basin, the easternmost depository of Damodar Valley encompasses the entire succession of Gondwana Supergroup, the lower and upper Gondwana sequences. The present investigation of Permian carbonaceous shales belonging to Barakar, Barren Measure and Raniganj formations of lower Gondwana sequence from this basin is an integrated study of inorganic and organic geochemical characteristics viz. major/trace elements, stable isotopic composition and kerogen type to ascertain source rock attributes such as provenance, tectonic setting, prevailing weathering/climatic conditions as well as source and type of organic matters. Trace element indices, such as La/Sc, Th/Co, Th/Sc, (Gd/Yb)CN, Eu/Eu*, Cr/V, Y/Ni, reveal that the shales are originated from felsic/granitic source rock. Prominent Eu-anomaly, enrichment of chondrite normalised light rare earth element (LREE) values and a flat heavy rare earth elements (HREE) pattern reflect post-Archean felsic source for the shales. The major element composition, especially relative enrichment of Al over readily soluble elements, Ca, Na and K yielding high CIA (74.9–84.6%) and CIW (87.1–97.5%) as well as low ICV (0.64–1.32) values, establishes significantly weathered and matured nature of the shales. The distribution of redox-sensitive elements (U/Th, V/Cr, Ni/Co, Ce/Ce*) as well as low ICV, depleted K2O/Al2O3 ratios and high CIA values point towards an oxic environment and warm-humid climatic condition during deposition of these shales. The conventional method for determining tectonic setting using major and trace element composition fails to clearly delineate the tectonic setting for the current shales. The samples show wide scattering across active and passive continental margin in the K2O/Na2O–SiO2 diagram, whereas the samples are concentrated near the active in the continental margin in the Th–Sc–Zr/10 diagram contradicting the geological framework of Raniganj sub-basin. Discriminant functions-based approach which categorises the tectonic setting into continental/oceanic arc, continental rift and continental collision settings has been proved to be suitable in the present case and reveals continental-rift setting for the shales from Raniganj sub-basin. The carbon isotopic signature (δ13Corg) of the shales reveals a range (−24.5 − −22.9‰) intermediate between that of marine and terrestrial (C3 plants) organic matters indicating possible mixed type of organic matters. The fluvio-marginal marine depositional condition obtained from K2O/Al2O3 and MgO/Al2O3 ratios further implies the possibility of mixed nature of organic matter. The geochemical parameters of the organic matter (Rock-Eval pyrolysis parameters S1, S2, TOC, HI, OI etc.) indicate that the studied shales contain thermally matured mixed Type III/II gas-prone kerogen and are derived predominantly from terrestrial sources with significant marine input.