Vol.:(0123456789) 1 3 International Journal of Earth Sciences https://doi.org/10.1007/s00531-020-01946-1 ORIGINAL PAPER Sedimentology and basin‑fll history of the Cenozoic succession of the Sylhet Trough, Bengal Basin, Bangladesh Farida Khanam 1  · M. Julleh Jalalur Rahman 2  · M. Mustafa Alam 3  · Rashed Abdullah 2 Received: 25 March 2020 / Accepted: 12 October 2020 © Geologische Vereinigung e.V. (GV) 2020 Abstract The Sylhet Trough, a petroleum province of the Bengal Basin, accommodates a huge thickness of Eocene to Recent sedi- mentary successions. However, the basin-fll history of the trough is poorly understood; specifcally, constrains on timing of deposition of the individual units are yet to be established. Therefore, we aimed at establishing sedimentation and basin-fll history of the Sylhet Trough based on detailed lithofacies analysis of the outcropping Cenozoic succession. We have divided the entire Cenozoic succession into three megasequences which can be further sub-divided into nine lithostratigraphic units based on bounding discontinuities, such as transgressive erosion surface, regressive erosion surface, transgressive surface, marine fooding surface, and incised valley foor. The oldest is the Megasequence 1, comprised of shallow marine shelfal deposits overlain by shallow marine to nearshore deposits. In the middle, the Megasequence 2 is representing tide-dominated marine to coastal (deltaic) depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of incised channel, tidal inlet, tidal ridge/shoal, tidal fat and other tidal deposits are separated by shelfal deposits. The top of the Megasequence 2 is marked by a pronounced erosion surface interpreted as an incised valley foor indicating the fnal phase of marine regression followed by the gradual establishment of the overlying continental-fuvial depositional systems (i.e., the Megasequence 3). This youngest megasequence is characterized by stacked braided river sand bars that pass up-sequence into meandering river deposits. Based on the Cenozoic eustatic sea level curve, we suggest that the upper boundaries of the Megasequence 1 and Megasequence 2 are approximately at 39.5 Ma and 5.0 Ma, respectively. Keywords Bengal Basin · Cenozoic succession · Facies assemblages · Depositional environment · Bounding discontinuity · Sequence stratigraphy Introduction The Bengal Basin, located in the northeastern part of the Indian subcontinent, occupies a vital geographic position at the junction of three interacting plates, namely, the Indian, Burmese and Eurasian plates (Fig. 1a). The Sylhet Trough is a sub-basin of the Bengal Basin, occupies mostly northeast- ern part of Bangladesh (Fig. 1b) and comprises an approxi- mately 17-km-thick sedimentary column of Eocene to Recent age (Alam et al. 2003; Hiller and Elahi 1984; John- son and Alam 1991). The entire succession is dominantly composed of clastic sediments except for the Eocene Sylhet Limestone (Alam et al. 2003). Sedimentation in the Bengal Basin (including the Sylhet Trough) has been controlled by the uplift and erosion of the Himalayas and Indo-Burman Ranges, which were formed due to the collision of the Indian plate with the Burmese and Eurasian plates (Alam 1989; Maurin and Rangin 2009; Najman et al. 2008; Rahman et al. 2020; Yang et al. 2019, 2020). From a geological point of view, the Sylhet Trough is not only interesting but also of great economic value as being gas prone. Unfortunately, the Sylhet Trough is mostly cov- ered with younger alluvium, and, therefore, it has been pre- viously studied from drill hole or limited outcrop data that are mostly focused on Miocene reservoir zones (Khanam et al. 2017; Rahman et al. 2009). Limited attempts (Johnson and Alam 1991; Najman et al. 2012; Uddin and Lundberg * Farida Khanam khanamgeo@gmail.com 1 Geophysical Division, Bangladesh Petroleum Exploration and Production Company Ltd. (BAPEX), Dhaka, Bangladesh 2 Department of Geological Sciences, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh 3 Department of Geology, Dhaka University, Dhaka 1000, Bangladesh