The Holocene sedimentary archive of Sylhet basin, Bangladesh: Linking surface processes to the stratigraphic record within a mass balance framework

Abstract

The removal of sediment mass to deposition in sedimentary systems creates predictable patterns of downstream fining and facies changes. Using Holocene lithology and stratigraphic architecture from a dense borehole network, the mid-Holocene sedimentary record of the Ganges-Brahmaputra-Meghna Delta (GBMD) is examined in a mass balance framework to demonstrate a shift from bypass-dominated to extraction enhanced modes of sediment dispersal. A prolonged (~3000 year) occupation of the tectonically-influenced Sylhet basin occurred from ~ 7.5-4.5 ka, producing thick (20-30 m) packages of channel sands and splay deposits. Sediment was initially (~7.5-6.0 ka) routed along the western edge of Sylhet basin, where confinement in antecedent valleys promoted bypass to downstream depocenters. At ~6.0-5.5 ka, the system shifted to an extraction enhanced mode of sediment dispersal, as a large (3500 km²) splay developed northeast of the main sediment delivery pathway. This pattern of deposition persists through the end of the occupation (~4.5 ka) as reflected in another large (~4000 km²) splay along the Shillong Foredeep at the basin’s northern margin. The persistence of sediment bypass suggests that seasonal flooding of the basin by local runoff exerts a hydrologic barrier to overbank flow and is thus a principal control on river path selection. The progradation of two 10-30 m thick, sandy mega-splays into the basin interior reflect an alternative mode of sediment dispersal that appears to operate only in the mid Holocene (~6.0-4.0 ka) during a regional weakening of the summer monsoon. The reduced water budget at this time would have lowered seasonal water levels in the basin, temporarily lessening the hydrologic barrier effect and facilitating splay development into the basin interior. The spatial distribution of sediment mass contained in each of the three pathways was apportioned spatially to calculate 𝒳, a scale-independent dimensionless downstream distance that represents the total percent of supplied sediment flux lost to deposition at any given point in the system. Both of the splay deposits contain distinct facies shifts and a reduction in net:gross at a 𝒳 value of ~0.7. A statistical analysis of the field data yields another quantitative measure of downstream facies shifts associated with mass extraction, as well as a measure of stratigraphic order. The runs metrics 𝑐 and 𝑓 are employed to describe thickness and grain size trends within vertical sequences of borehole data. A Monte Carlo simulation estimates the probability that a given succession of strata could be generated by chance. The results of both the statistical analyses and Monte Carlo simulations indicate spatial patterns in order and completeness, whereby areas associated with enhanced mass extraction exhibit more stratigraphic order. These findings are consistent with experimental and field results from a variety of depositional settings that demonstrate the utility of a scale-independent mass balance framework for the prediction of downstream facies changes.