ACROSS THE BACKWATER: SURFACE MORPHOLOGY AND SUBSURFACE STRATIGRAPHY IN THE GANGES-BRAHMAPUTRA DELTA

The backwater transition zone in river deltas can have a profound impact on bedload sediment mobility (Nittrouer et al., 2011) and channel avulsion styles and location (Chatanantavet et al., 2012; Edmonds, 2012). Only recent work seeks to elucidate the role these processes have in controlling the underlying stratigraphic architecture preserved in modern and ancient systems (Columbera et al., 2016).

We present an overview of the surface morphology and subsurface stratigraphy across the backwater transition in the Ganges-Brahmaputra (G-B) delta revealed from sediment boreholes. Where the G-B rivers discharge into the Bengal Basin, decrease in slope and increase in backwater influence trigger the mass extraction of sediment. This has resulted in the formation of fan deltas from the aggradation of laterally mobile braided channels, and the wide-scale deposition and preservation of bedload fine-to-medium sand with only a thin veneer of mud. Below the backwater transition, channel mobility is limited, and sedimentation is predominantly via overbank flooding. As a result, sand is confined to channel deposits and crevasse splays while fine-grained mud and organic-rich successions are concentrated within broad interdistributary basins. In the central delta, borehole analyses reveal these mud and peat deposits may be laterally extensive, but are only 5-30 m thick, superimposed above massive fine-to-medium grained sand similar to fan delta stratigraphy. In the tidal deltaplain, the stratigraphy similarly comprises 5–20 m of fine-grained silts, yet these are tidally deposited and bioturbated, and overlie river-mouth sands. Although peat and wood preservation occurs (Allison et al. 2003, Khan & Islam 2008), in general, little peat is found here.

We hypothesize the fan delta has transgressed with Holocene sea-level rise and migration of backwater influence, similar to bayhead deltas observed within incised Pleistocene valleys in the northern Gulf of Mexico along the TX coast (Anderson et al., 2015). In our case, fluvial aggradation occurred from 12 to 8 ka, during the period of enhanced monsoon, and adequately filled the incised valleys. Today the tidal deltaplain overlaps these deposits.