Organic petrography and depositional environments of the upper Eocene Kopili Shale, Bengal Basin, Bangladesh

Mudrock samples of the upper Eocene Kopili Shale, taken from the outcrops in the northeastern Sylhet Trough areas, and drilled cores from the Indian Platform in the northwestern Bangladesh, were investigated in order to present an overview of the depositional environment. Several techniques such as organic petrography, thin section petrography, X-Ray fluorescence (XRF) elemental analysis, X-Ray diffraction (XRD) mineralogical analysis, and lithologic observations were combined to draw the best possible conclusions.

Organic petrographic analysis shows higher proportion of vitrinite along with liptinitic materials in the northeastern samples, suggesting deposition in an upper deltaic environment, whereas the northwestern samples show abundance of inertinite, indicating deposition in a lower deltaic environment. Thin section petrographic analyses, which reveal planktonic foraminifers, bioturbated fabrics, pyrite framboids, sand lenses and flame structures; and XRD and XRF analyses reveal abundance of quartz and illite in Kopili Shale, all of which suggest deposition in shallow marine reducing environments.

The fauna in the Kopili Shale can be correlated to upper Eocene faunas of the Tethys Sea around the Indian subcontinent. Organic petrographic analysis 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. Taken together, the present results suggest that the Kopili Shale of the Bengal basin is a silty mudrock, which was deposited during rapid marine transgression and regression, in a deltaic near-shore setting.