Sedimentology of the Bengal shelf, Bangladesh: comparison of late Miocene sediments, Sitakund anticline, with the modern, tidally dominated shelf

Abstract

Characterisation of latest Miocene facies of the Bay of Bengal reveals a range of tidal and fluvially dominated sediments that represent sedimentation on a broad, shallow shelf that was strongly influenced by tidal and seasonal cyclicity. These tidally influenced sediments are excellently exposed within growth anticlines of the westward-migrating Chittagong–Tripura Fold Belt (CTFB), and permit detailed study of the late Miocene Bengal shelf for comparison with the modern subaqueous deltaic sediments. At the outcrop scale, facies associations record a subtidal shelf, migrating intertidal/subtidal bedforms, tidal and incised channels. The subtidal shelf comprises extensive sand sheets interbedded with thick clay horizons, while the migrating intertidal/subtidal bedforms exhibit a decrease in bedform size from large dunes to small ripple-scale cross-stratification that reflect a decrease in current velocity and sand/silt grade sediment supply as the bedforms migrated. Rootlet horizons within these intertidal/subtidal bedforms may represent a lower delta plain environment. The tidal channel facies consists of metre-scale sandstone beds that are rich in locally derived clay clasts, with up to 0.5 m of incision into a rhizolith horizon below. Incised channel facies records a coarse, pebbly sandstone with clasts derived from a reworked clastic source, with metre-scale scour at the base of this facies association into laminated and rhizolith clays. A 534-m one- and two-dimensional section within the Sitakund anticline, north of Chittagong, is characterised by 20–100-m thick, tidally dominated, progradational parasequences. Mudstones with isolated sand lenses and abundant trace fossils of the Cruziana ichnofacies, coarsen upwards to fine–medium-grained sandstones deposited in shallower, high-energy environments. These sands were deposited under both uni-directional and bi-directional currents resulting in dune-scale cross-stratification, and are overlain by 2–5-cm thick clay drapes, considered to be of a thickness greater than that typical of deposition from a single slackwater episode. The main controls on sedimentation are the seasonally influenced river and sediment discharge onto the shelf from the Himalayan source region, combined with the influence of daily, spring–neap tides, channel and bedform migration and minimal wave action over the shallow shelf. The modern Bay of Bengal shelf provides an analogue for interpreting processes recorded within the Miocene succession, allowing construction of a depositional model for the late Miocene shelf of the Brahmaputra/Ganges delta system, with sedimentation dominated by tidal and seasonal cyclicity.