Palynofacies Study Of Lakadong Limestone (Late Paleocene) Of Mawsynram Area, Shillong Plateau, India: Implications For Sequence Stratigraphy

Subtle paleoenvironmental fluctuations are difficult to detect in sedimentologically uniform carbonate rocks, however, study of relative increase and decrease of marine and terrestrial organic matter content provides a useful tool for deciphering the deepening and shallowing cycles in these successions. Lakadong Limestone Member in Khasi and Jaintia hills in South Shillong Plateau is late Paleocene in age and forms part of the carbonate platform. It represents deposition during High Stand Systems Tract. In the present study palynofacies analysis were carried out in two Lakadong Limestone sections i.e., Dohsniang (Kurtinsiang) (KPL) and Laitmowksing (LTL) from Khasi hills for detailed palaeoenvironmental interpretations, for the correlation and for the identification of higher order sea level cycles. Both the sections have been dated as late Paleocene based on the characteristic larger benthic foraminiferal assemblages belonging to the Tethyan Shallow Benthic Zones SBZ 3. For palynofacies analysis various type of organic matter were characterized and counted. The study shows cyclicity pattern in the organic matter types in both the sections which may be linked to the sea level changes of higher order cycles. Based on the variation in the organic matter content both the sections were subdivided into distinct palynofacies units. Each palynofacies unitsstart with high proportion of black oxidized palynomaceral along with dinoflagellate cysts representing a transgressive surface, followed by high quantity of degraded brown and cuticle organic matter from terrestrial source. Each palynofacies unit thus represent progradational deposit of High Stand Systems Tract starting with the deepening facies followed by shallowing facies of more terrestrial origin. Four progradational sequences have been identified in the KPL and three in the LTL section that can be correlated. Palynofacies study thus offers a logical approach for the study of uniform carbonate facies. Based on the present palynofacies criteria it has been possible to identify higher order sea level cycles in the Lakadong Limestone exposed at KPL and LTL section in Khasi Hills.