ORGANIC PETROGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE UPPER EOCENE KOPILI SHALE, BENGAL BASIN, BANGLADESH

The fossiliferous upper Eocene Kopili Shale occurs in northern (both east and west) Bengal basin, Bangladesh. The Kopili-equivalent mudrocks are also found in lower Assam, India, deposited in a shallow-marine to lagoonal environment. In the current study, thin section petrography, organic petrologic analysis, XRD and XRF techniques, as well as field observations were used to characterize the depositional environments of the Kopili Shale.

The lower part of the Kopili Shale comprises mostly dark gray mudrock containing subordinate marl streaks, representing deep to shallow marine facies. The middle and top part comprises mostly medium-gray shale with interbedded silt layers, reflecting a higher energy of deposition. Organic petrologic observations of the Kopili Shale indicate the presence of terrestrial organic matter including liptinite, bituminite, vitrinite, and inertinite. The modal analyses of petrographic macerals reveal higher proportion of inertinite and vitrinite as compared to liptinite in the northwestern core samples, suggesting that the Kopili Shale deposited in a lower deltaic environment. Similarly, the northeastern outcrop samples of the Kopili Shale show more liptinitic materials and falls above 50% vitrinite line, suggesting an upper deltaic environment. Petrographic thin section analyses reveal planktonic foraminifers, bioturbate fabrics, pyrite framboids, sand lenses and flame structures, suggesting deposition in shallow marine environments, characterized at least by periodically oxygenated bottom waters and sulfidic pore waters. The Kopili Shale from the northeastern Dauki River section shows diverse benthic and planktic foraminifer assemblages. XRD and XRF analyses reveal high volume of coarser quartz grains in Kopili Shale due to rapid marine transgression. The results from organic petrographic analysis also suggest provenance from a low-relief crystalline Indian craton located adjacent to the west, rather than the Himalayan or Indo-Burman ranges to the north and east of the Bengal basin.