Early Miocene (Burdigalian) Dinoflagellate Cysts from the Kerala Basin, Southwest India: Indicators of Marine Ingression at the Onset of the Mid-Miocene Climate Optimum (MMCO)

In the present study, the assemblages of organic walled dinoflagellate cyst (dinocysts) and calcareous nannofossils from Quilon Formation, Kerala Basin, south-western India are analyzed for age interpretation and reconstruction of the depositional environment. The integrated biostratigraphy based on dinocysts and calcareous nannofossils from the same sediment samples indicates that the studied succession was deposited during the early Miocene (middle Burdigalian, ∼19-17.5 Ma) time. The dinocyst assemblages contain abundant taxa ( Cleistosphaeridium, Polysphaeridium, Lingulodinium, Homotryblium and Spiniferites ) that range from shallow inner to outer neritic marine zone. The abundance of autotrophic, thermophilic taxa suggests relatively high surface water temperatures and high nutrient availability. The dinocyst assemblage in the lower part of the studied succession is dominated by Cleistosphaeridium spp., Polysphaeridium spp., Spiniferites spp., Homotryblium spp. and Lingulodinium spp. An increase in the relative abundances of outer neritic dinocyst taxa such as Spiniferites, Achomosphaera and Hystrichokolpoma in the upper part of the succession suggests a change towards more open marine conditions created during marine ingression. The interpreted marine ingression along the south-western Indian coast probably corresponds to the initial small pulses of transgression at the onset of the long-term global sea-level maximum related to the Mid-Miocene Climatic Optimum (MMCO).