Paleoenvironmental study of the Barail succession exposed in north-eastern Sylhet, Bangladesh

The Barail succession (Oligocene) in north-eastern Bangladesh consists of three major lithofacies: (1) Facies A: sandstone- dominantly cross bedded, medium-to fine grained, well-sorted, subangular to rounded and quartz-rich; (2) Facies B: heterolithic-interstratified sandy siltstone, siltstone and silty shale and (3) Facies C: shale. These lithofacies are laterally persistent and occur in a predictable vertical sequence: Facies A -Facies B- Facies C, representing a spectrum of physical energy conditions. They also develop multiple fining upward cycles of facies assemblage in the study area. Study of the lithofacies types and vertical facies association, texture of the sediments and overall geometry of the sandstone bodies strongly suggest a shallow marine environment of deposition. The sediments most probably accumulated in tectonically subsiding tide dominated shelf with some influence of wave-driven and/or wind induced longshore currents.