CARBON AND OXYGEN ISOTOPIC COMPOSITIONS OF INFRATRAPPEAN LIMESTONES FROM CENTRAL AND WESTERN INDIA AND THEIR DEPOSITIONAL ENVIRONMENT

The lowermost formation of the Bagh Group, the Nimar Sandstone, was de posited in fresh water whereas the upper two formations, the Nodular a nd Coralline Limestones were deposited in marine environment. There is a distinct signature of diagenetic alteration in the oxygen isotopic ratios but the carbon isotopic ratios did not change significantly. Th e marine transgression from the continental Nimar period to upper Lime stone period was extremely rapid in all four sections (Mogra, Rampura, Acharkunda and Bariya) examined. However, the subsequent regression d uring the onset of the Lameta period was slow and fluctuating The oxyg en isotope ratios of some limestones from the Kawant area (Mogra secti on, Nodular Limestone Formation) are unusually low probably due to the rmal alteration effected by intrusion of the nearby Ambadongar Carbona tite at a later date.