Palynostratigraphy of the sediments formation of Assam: 3. Biostratigraphic zonation of the cherra formation of South Shilong plateau

Abstract

In the South Shillong Plateau, sediments of the Cherra Formation rest on the eroded surface of the Langpar Formation (Danian) and are in turn conformably overlain by the Lakadong Limestone (Lower Eocene) member of the Sylhet Limestone Formation. Based on the disposition of the Cherra Sandstone in three altitudinal belts, each characterized by a coal-seam, and on the distribution of commonly occurring palynological species, three biostratigraphic zones have been recognized in the Cherra Sequence.

The lower zone can be distinguished by the high frequencies of Nympheoipollis crassimurus, Retialetes emendatus and Polypodiisporites mawkmaensis, together with a few other forms restricted only to this zone. The middle zone can be recognized by the common occurrence of Corrugatisporites formosus, Sestrosporites dettmanii, Foraminisporis medius, Couperipollis rarispinosus, Couperipollis wodehousei, etc., and low frequencies or ± absence of Nymphaeoipollis crassimurus and Polypodiisporites mawkmaensis. The upper zone is distinct in the absence of palynomorphs characteristic of the lower and middle zones together with increased frequencies of Foveosporites pachyexinous, Polycolpites speciosus, Engelhardtioidites parvus, etc.

A distinct palynological change across the Cherra/Lakadong boundary is apparent, which can be fruitfully used for demarcating this boundary.