Geochemical Evidence for Oxic Depositional Conditions in the Early Palaeogene Akli Formation, Barmer Basin, India
Abstract
Integrated stable carbon isotope and geochemical analyses of the Palaeocene–Eocene Akli Formation, exposed at the Sonari Lignite Mine in the Barmer Basin, western India, reveal predominantly oxidising depositional conditions, interrupted by short-term reducing phases associated with lignite-bearing intervals during the Palaeocene–Eocene Thermal Maximum (PETM). δ13C values measured on bulk organic matter range from −21.3 to −29.0‰ and exhibit a distinct negative excursion of ~5‰, co-occurring with low total organic carbon (TOC < 2%), together recording the local geochemical expression of the PETM. PAAS-normalised rare earth element (REE) patterns show enrichment of light REEs and a moderate negative europium anomaly (Eu/Eu* = 0.60–0.87), consistent with derivation from upper continental crustal sources. Redox-sensitive trace-element ratios—V/Cr (< 2), Ni/Co (< 5), and Th/U (> 2)—fall within ranges characteristic of oxic depositional conditions. Immobile element provenance indices (Th/Sc, La/Sc, Cr/Th) indicate a mixed felsic–mafic sediment source derived from the Precambrian Indian Craton and Deccan Trap basalts. Collectively, these proxies indicate deposition in a shallow-marine, regressive, predominantly oxygenated setting, punctuated by short-lived reducing conditions recorded within lignite-bearing horizons. The Akli Formation thus constitutes one of the few tropical records of PETM-age oxidation and provenance variability from the Indian subcontinent.