Stable isotope studies of fossiliferous Palaeogene sequence of Kutch, Western India: palaeoenvironmental implications

Abstract

Stable isotope analyses have been carried out on rocks and fossils of the Palaeogene carbonate-clastic 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 Eocene (Lutetian) with the development of semi-locked basins (lagoons) in the depressions of pre-existing Deccan Trap basements. The larger benthic foraminifera Assilina was the dominant microbiotic species capable of surviving in this highly stressed environment. The basins were later cut off by the oscillatory withdrawal of the sea, which trapped large amounts of organic matter. Degradation of this organic matter exhausted the dissolved oxygen and biogenic methane was produced in anaerobic conditions. This methane, decomposed by sulphate reducing bacteria, produced some bioclastic limestones with extremely depleted carbon isotope values. The oxygen isotope values of the limestones suggest a significant contribution of freshwater into these lagoons. As a result, terrigenous clastics were formed; associated lignite beds developed in a warm, humid climate. Probably the area was part of a global warm humid belt which extended through Africa and Europe as far as 40°N during the Lutetian. A renewed marine transgression is recorded by the formation of aerated tidal lagoons having a more seaward extensions. The process culminated in a relatively open shallow marine condition when late middle Eocene and Oligocene bioclastic limestones (with a vast array of tropical chlorozoan assemblages) were deposited.