Early Eocene carbonaceous shales of Tadkeshwar Formation, Cambay basin, Gujarat, India: Geochemical implications, petrogenesis and tectonics

Abstract We report the early Eocene Cambay sedimentation, petrogenesis and tectonics of a rift graben basin of western India using geochemistry. The carbonaceous shales of Tadkeshwar Formation from Tadkeshawar and Rajpardi lignite mines of Cambay basin were studied using major, trace and rare earth elements. The study reveals that the protolith of these shales is likely to be mafic to ultramafic in nature based on the high abundance of Fe 2 O 3 (avg. of 8.68%), MgO (avg. of 5.78%) and TiO 2 (avg. of 8.80%) with a lower SiO 2 content (avg. of 44.74%) when compared to PAAS values. PAAS normalized REE distribution patterns of carbonaceous shales indicate relatively depleted LREE with enriched HREE coupled with small magnitude of Eu/Eu* strongly supports the protolith of these shales to be mafic to ultramafic. Weathering indices confirm these shales were derived from a source subjected to intense weathering. Low Na 2 O and high perseverance of Al 2 O 3 are proxies for Smectite to Illite transformations of shales in a gaining temperature regime across the basin. We speculate that the high geothermal gradients prevailed during 60–65 Ma along the western margin of Indian plate might be responsible for the intense weathering of the marine mafic protolith. Redox-sensitive trace element ratios and negative Ce-anomalies suggest that the shales were deposited under suboxic to oxic environment. The link between early Eocene marine transgression followed by basin subsidence is inferred for the deposition of these carbonaceous shales in suboxic to oxic environments. The immobile trace element ratios and geochemical discrimination diagrams (K 2 O/Na 2 O vs. Si 2 O; La–Th–Sc) reveals, the protoliths of the Tadkeshwar Formation were derived from a transitional domain of intra ocean island arc to active continental margin followed by convergence, leading to the deposition of immature shales.