The Boka Bill Formation of the Surma Group of Barogang-Hari section, NE Sylhet, Bangladesh is constituted by blue to black shale, yellowish gray to light yellow medium to fine sandstone, gray to grayish siltstone, gray to blue as well as dark colored mudstone and brownish gray conglomerate. Twelve distinct lithofacies have been recognized within the Boka Bill Formation. The facies are grouped into three facies associations based on genetic aspects: the Fluvial Facies Association (FFA) composes the Lower Boka Bill member; the Tidal Facies Association (TFA) forms the Middle Boka Bill member; and the Marine Facies Association (MFA) represents the Upper Boka Bill member. The FFA represents incised valley, braided channel and over bank fines; the TFA is formed due to the processes of estuary, tidal creek, and tidal flat; and the MFA is the result of deep marine hemipelagic sedimentation with little progradation of distal turbidite. The unimodal-unidirectional paleocurrent pattern of the FFA indicates mono-directional dispersal path of fluvial environment. The bipolar-bidirectional paleocurrent with some deviations of the TFA strongly shows flow reversal, which is common in tidal environment. The monodirectional-unimodal paleocurrent rose diagram of the MFA indicates unidirectional flow of turbidity current in the deep marine environment.

The Boka Bill Formation of the Surma Group is constituted by blue to black shale, yellowish gray to light yellow medium to fine sandstone, gray to grayish siltstone, gray to blue as well as dark colored mudstone and brownish gray conglomerate (Reimann, 1993). It is well-exposed along the banks of the Barogang River, a tributary of the Hari River, Sylhet district, Bangladesh, where the upper part of the Formation is regarded as upper marine transgression in the Sylhet Trough of the Bengal Basin (Reimann, 1993). Considerable controversy surrounds some aspects of the sedimentology and environments of deposition of the Boka Bill Formation of the Sylhet Trough.

Facies Analysis of the Boka Bill Formation as Exposed along the Barogang-Hari River Section, Northeastern Sylhet, Bangladesh,Formation, Sylhet, Association, paleocurrent, aspects, brownish, medium, mudstone, conglomerate, represents, sandstone, siltstone, district, bipolarbidirectional, estuary, braided, Fluvial, genetic, hemipelagic, lithofacies, Marine, composes, BarogangHari, monodirectional, Considerable, recognized, progradation, controversy