Early Palaeogene in the south Shillong Plateau, NE India: local biostratigraphic signals of global tectonic and oceanic changes

The early Palaeogene in the history of the Earth is noted for large-scale plate reorganisation believed to be associated with a changeover from the Laramide tectonic regime (‘Old’ crustal configuration) to the Himalayan tectonic regime (‘New’ crustal configuration). This tectonic change possibly proved to be a turning point in the evolution of Lithosphere, and ushered in drastic changes in the physical environment and evolutionary course of Biosphere in the Cenozoic. The consequent changes in the sedimentary environment and biotic development during Early Palaeogene have been pointed out for the Indian Ocean region. This paper examines the biostratigraphic data of the Early Palaeogene (Late Palaeocene–Middle Eocene) succession of the South Shillong Plateau, in the light of climatic and oceanic changes, and explores the possible role the regional tectonic changes may have played in shaping the course of environmental and biotic development preserved in the rock record of the Sylhet Limestone Group. An attempt has also been made to propose a model to explain the underlying tectonic changes in the Shillong Plateau. The biostratigraphic data from the area of investigation indicate that the major stratigraphic sequences recognised in the Sylhet Limestone Group can be broadly correlated with the stratigraphic succession of the Early Palaeogene identified in the Indian Ocean region. The succession of the sedimentary facies in this region seems to be the result of sea-level and climatic changes controlled by major tectonic reshuffling. From this correlation, it appears that the litho and biostratigraphic markers of the Sylhet Limestone were created by sea-level and climatic events related to tectonic changes occurring in the adjoining Indo-Burmese orogen as part of the large-scale global plate reorganisation mentioned as above.