Biostratigraphy of the Post Kopili Sediments of Garo Hills, Meghalaya

The lithounits of post Kopili sediments constitute the Simsang, Baghmara, Chengapara and Dupitila formations in that order of sequence. Based on the present study five biostratigraphic zones have been recognised in these rock stratigraphic units. The palyno-assemblage recovered from the Simsang formation consists of gemmate syncolpate grains, Spinospites acolporata, Schizeaesporites knoxi, Polypodisporites tuberculensis, Polypodeaceaesporites, Lycopodiumsporites sp., together with a host of angiospermic pollen grains. Since such an assemblage is indicative of Oligocene age, the Simsang formation is, therefore, considered as the time and rock stratigraphic equivalent to the Barail Group in Assam. From the Baghmara formation a palynozone of early Miocene age has been recorded, which is characterised by two winged conifer pollen, ribbed trilete spores referable to Striatriletes, Lycopodiumsporites, and frequent occurrence of fungal bodies and spores of Ascomycetae and Fungi Imperfectii. Two faunal biostratigraphic zones have been recognised in the fossiliferous beds occurring at the bottom of the Chengapara formation. The lower assemblage is characterised by the presence of Anadara, Cardium and Ostrea. The zone is unique for the occurrence of variety of pelecypods and gastropods. The upper assemblage zone is characterised by the frequent presence of Tellinaceans and the absence of most of the gastropods occurring on the lower assemblage. The lower and upper assemblage zones are of Aquitanian-Burdigalian age.

The fossiliferous horizon occurring well within the Chengapara formation is considered to represent probably the uppermost fossiliferous horizon in the Garo Hills. This horizon is characterised by the presence of elasmobranchs represented by Carcharadon carcharias and a few batoids. Carcharadon carcharias is not known to occur in the beds older to upper Burdigalian. Dupitila and Dihing formations have been found to be devoid of fossils.