STABLE-ISOTOPE STUDIES OF FOSSILIFEROUS PALEOGENE SEQUENCE OF KUTCH, WESTERN INDIA - PALEOENVIRONMENTAL IMPLICATIONS

Stable isotope analyses have been carried out on rocks and fossils of the Palaeogene carbonate-elastic sequence of the Kutch area in Western India. Isotopic, lithological and palaeontological data indicate that Tertiary sedimentation started in this region in the early middle Eoc ene (Lutetian) with the development of semi-locked basins (lagoons) in the depressions of pre-existing Deccan Trap basements. The larger ben thic foraminifera Assilina was the dominant microbiotic species capabl e of surviving in this highly stressed environment. The basins were la ter cut off by the oscillatory withdrawal of the sea, which trapped la rge amounts of organic matter. Degradation of this organic matter exha usted the dissolved oxygen and biogenic methane was produced in anaero bic conditions. This methane, decomposed by sulphate reducing bacteria , produced some bioclastic limestones with extremely depleted carbon i sotope values. The oxygen isotope values of the limestones suggest a s ignificant contribution of freshwater into these lagoons. As a result, terrigenous clastics were formed; associated lignite beds developed i n a warm, humid climate. Probably the area was part of a global warm h umid belt which extended through Africa and Europe as far as 40 degree s N during the Lutetian. A renewed marine transgression is recorded by the formation of aerated tidal lagoons having a more seaward extensio ns. The process culminated in a relatively open shallow marine conditi on when late middle Eocene and Oligocene bioclastic limestones (with a vast array of tropical chlorozoan assemblages) were deposited.