Depositional Environment and Basin Development During Early Palaeogene Lignite Deposition, Western Kutch, Gujarat

Exploration for lignite by drilling in the early Tertiary Supratrappean paralic sequence on the onshore of western Kutch,Gujarat, provides detailed information on the basement characteristics and palaeoenvironment of lignite deposits. A comprehensive study of outcrop sequence and its dip directional subcrop counterpart reveal that lignite bearing sediments were deposited in three distinct domains; (a) fringe areas of exposed trap rock on mainland, (b) adjacent dip directional subsurface areas on trap basement and (c) areas located beyond the subcrop limit of trap in a basinward direction. Domain (b) is considered most favourable locale for exploration as it contains palaeotopographic lows. A synthesis of outcrop and suberop data helped in identifying three depositional environments for the lignite bearing sediments, namely a prograding delta, back barrier environment and lagoons of foraminiferal bank environments. Vegetation growth and accumulation took place in all the three environments but sizable peat matter are found to occur in transitional lower delta plain, and in the back barrier deposits. These depositional environments and various stratigraphic criteria are used to reconstruct six conceptual models to depict the evolution of early Tertiary basin and accumulation of lignite therein. The significance of stratigraphic markers i.e. Lower Eocene Assilina marl-moll usca coquina bed and the Middle Eocene large Nummulities bed, in locating lignite deposits is also discussed.