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 deposited in fresh water whereas the upper two formations, the Nodular and 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 signi ficantly. The marine transgression from the continental Nimar period to upper Limestone period was extremely rapid in all four sections (Mogra, Rampura, Acharkunda and Bariya) examined. However, the subsequent regression during the onset of the Lameta period was slow and fluctuating. The oxygen isotope ratios of some limestones from the Kawant area (Mogra section, Nodular Limestone Formation) are unusually low probably due to thermal alteration effected by intrusion of the nearby Ambadongar Carbonatite at a later date.