Dinoflagellate cyst and nannofossil imprints in the middle Eocene Kirthar transgressive strata from the western Garo Hills, South Shillong Plateau, India

One of the global characteristic features of the Eocene is the extensive development of carbonate platforms in various regions of Tethys. Paleogene sequences of the Shillong Plateau from west to east are exposed in the steep Garo, Khasi and Jaintia hills. In the Khasi and Jaintia hills, Paleogene successions are represented by alternating carbonates and siliciclastics and provide datable micropaleontological fossils, whereas in the Garo Hills, which is in the south-western part of the Shillong Plateau, the Paleogene succession is mainly represented by siliciclastics and fewer carbonates. As the carbonates in this area are mostly marly, the recovery of carbonate microfossils is difficult. Accurate age assignment is important to correlate the sedimentary successions of the Khasi, Jaintia and Garo hills, which are mostly time transgressive and different lithologies were deposited at the same time. A detailed palynological investigation of 65 samples from the 25-m-thick sedimentary succession of the Siju Formation, Western Garo Hills, has yielded well-preserved and diverse organic-walled dinoflagellate cysts and calcareous nannofossils. Dinoflagellate cysts ( Heteraulacacysta porosa, H. leptalea ) and nannofossils ( Discoaster barbadiensis , D. saipanensis , Reticulofenestra bisecta , R. erbae , and R. reticulata ) indicate a middle Eocene age (Bartonian) for the studied section. The synthesis of nannofossils and dinocysts with records from India and Australia reveals a major late Bartonian transgression in the Indo-Pacific known as the ‘Kirthar transgression’.