CARBON ISOTOPE STRATIGRAPHY AND ENVIRONMENTAL CHANGES IN THE LATE PALAEOCENE - EARLY EOCENE SUCCESSION OF MEGHALAYA, NE INDIA

Lakadong Formation, the lowermost unit of the Sylhet Limestone Group, represents the earliest phase of a major marine transgression during the Palaeogene in the South Shillong Plateau. It consists of medium to thick bedded, occasionally nodular, highly fossiliferous limestone with thin interbeds of shale, overlain by siliciclastic rocks comprising sandstone, shale and coal. The present study emphasizes the environmental changes that occurred during the late Palaeocene and early Eocene, documented from microfacies, carbon isotope and mineralogical investigations of samples of the Lakadong Formation exposed in Mawmluh quarry. Petrological study indicates that the limestones are primarily packstone to grainstone facies and show evidences of marine, meteoric and burial diagenesis. Based on the foraminiferal assemblages and their associations with calcareous red algae and green algae, Lakadong Limestone has been divided into various microfacies types. These microfacies indicate inner to middle ramp deposition of the carbonates, with a general deepening trend towards the upper part. The detrended correspondence analysis of foraminiferal data supports the overall deepening trend from the bottom to the top of the succession. The overlying coals form in peatlands within lagoonal environments. Carbon isotope data of bulk organic carbon displays a negative excursion of a large magnitude (~3 ‰) towards the top of the limestone. Mineralogical study of the section suggests a change from arid climate before the CIE to a relatively humid climate during and after the CIE. The CIE lies within the assemblage Miscellanea miscella, Ranikothalia nuttalli, Lakadongia tibetica and Orbitoclypeus schopeni group of forms. The truncation in upper ranges of these taxa is likely due to a major facies shift post-CIE from a carbonate to shale-coal dominated facies.