Petrography and whole-rock geochemistry of the Tertiary Sylhet succession, northeastern Bengal Basin, Bangladesh: Provenance and source area weathering

Detrital modes and the geochemical compositions of sandstones and mudrocks from the Paleocene to Plio-Pleistocene Jaintia, Barail, Surma, Tipam and Dupitila Groups of the Sylhet basin in Bangladesh have been examined to identify their provenance and source area weathering conditions. Barail and Surma Group sandstones are quartz-rich, feldspar-poor, and are rich in sedimentary and metamorphic lithic fragments, indicating a recycled orogen source. Geochemically the succession is divisible into three parts. Paleocene-Late Eocene Jaintia Group mudrocks form separate trends, and have very high Chemical Index of Alteration (CIA) values, suggesting derivation from a stable cratonic source. The second part, the Late Eocene to Miocene Barail and Surma Groups, forms single trends on variation diagrams. Maximum CIA ratios decrease from the Barail to the Surma Group, suggesting rapid influx of detritus from a common felsic source undergoing active uplift, compatible with weathering and erosion proxies from the Bengal and Indus fans. Average bulk geochemical and REE characteristics of these two groups are identical, as are A-CN-K trends. These features and comparison with potential Himalayan sources support derivation of both the Barail and Surma Groups from that orogen, and hence supply of Himalayan detritus to the Sylhet basin as early as the Late Eocene, rather than from the Neogene. The third part, the non-marine Tipam (Late Miocene-Pliocene) and Dupitila (Pliocene-Pleistocene) Groups, forms slightly differing geochemical trends and has higher maximum CIA values. REE fractionation between sandstones and mudrocks is more marked, but REE patterns and parameters remain unchanged, suggesting that the overall source remained unchanged. Higher CIA and greater REE fractionation in the Tipam and Dupitila Groups may be a product of their fluvial deposition and more intense weathering on the floodplain during intensification of the South Asian monsoon from 4 Ma onwards.