Holocene Brahmaputra River path selection and variable sediment bypass as indicators of fluctuating hydrologic and climate conditions in Sylhet Basin, Bangladesh

Abstract

The Holocene stratigraphy of Sylhet basin, a tectonically influenced sub-basin within the Ganges-Brahmaputra-Meghna delta (GMBD), provides evidence for autogenic and allogenic controls on fluvial system behaviour. Using Holocene lithology and stratigraphic architecture from a dense borehole network, patterns of bypass-dominated and extraction-enhanced modes of sediment transport and deposition have been reconstructed. During a~3-kyr mid-Holocene occupation of Sylhet basin by the Brahmaputra River, water and sediment were initially (~7.5-6.0 ka) routed along the basin's western margin, where limited downstream facies changes reflect minimal mass extraction and bypass-dominated transport to the basin outlet. Sedimentdispersal patterns became increasingly depositional~6.0-5.5 ka with the activation of a large (~2250 km 2 ) splay that prograded towards the basin centre while maintaining continued bypass along the western pathway. Beginning~5.0 ka, a second splay system constructed an even larger (~3800 km 2 ) lobe into the most distal portions of the basin along the Shillong foredeep. This evolution from a bypass-dominated system to one of enhanced mass extraction is well reflected in (i) the rapid downstream fining of deposited sand and (ii) a change in facies from amalgamated channel deposits to mixed sands and muds within discrete depositional lobes. 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. This control is evidenced by (i) repeated, long-term preference for occupying a course along the basin margin rather than a steeper path to the basin centre and (ii) the persistence of an underfilled, topographically low basin despite sediment load sufficient to fill the basin within a few hundred years. The progradation of two 10-20-m thick, sandy mega-splays into the basin interior reflects an alternative mode of sediment dispersal that appears to have operated only in the mid-Holocene (~6.0-4.0 ka) during a regional weakening of the summer monsoon. The reduced water budget at that time would have lowered seasonal water levels in the basin, temporarily lessening the hydrologic barrier effect and facilitating splay development into the basin interior. Overall, these results place basin hydrology as a first-order control on fluvial system behaviour, strongly modifying the perceived dominance of tectonic subsidence. Such coupling of subsidence, fluvial dynamics and local hydrology have been explored through tank experiments and modelling; this field study demonstrates that complex, emergent behaviours can also scale to the world's largest fluvial system.Deciphering the response of depositional systems to changes in sediment transport dynamics, climate and Correspondence: R. Sincavage,

Coupling Mass Extraction and Downstream Fining With Fluvial Facies Changes Across the Sylhet Basin of the Ganges‐Brahmaputra‐Meghna Delta

“…The WB pathway has been described as a distinct splay deposit originating 150‐km downstream from the apex of the OB pathway (Sincavage et al, ). Our results indicate that it is actually two amalgamated lobe deposits both sourced from the OB pathway but at different downstream locations.…”

“…Due to the bypass dominance of the OB pathway in the first half of the mid‐Holocene, central Sylhet basin remained underfilled and primed for occupation by the SF and WB splay deposits in later stages of the occupation (Sincavage et al, ). We hypothesize that this increased accommodation in central Sylhet basin led to enhanced mass extraction, which should be expressed in higher relative mud fraction preserved within sediments along the SF and WB pathways.…”

“…An extensive drilling program on the Ganges‐Brahmaputra‐Meghna Delta (GBMD) over the past 5 years has provided uniquely dense, comprehensive data on the Holocene evolution of the system in terms of interactions of autogenic and allogenic processes (Goodbred et al, ; Pickering et al, ; Pickering, ; Reitz et al, ; Sincavage et al, ; Wilson & Goodbred, ). Sylhet basin, represents an ideal field‐scale basin for mass balance analysis due to relatively good constraints on sediment input/output, age, provenance, and rates of subsidence (Goodbred et al, ; Reitz et al, ; Sincavage et al, ; Steckler et al, ). A detailed Holocene avulsion history of Sylhet basin has demonstrated two main modes of fluvial system behavior: bypass dominance with minimal variance in facies and enhanced extraction with significant downstream changes in grain size and stratigraphy (Pickering et al, ; Sincavage et al, ).…”

“…Sylhet basin, represents an ideal field‐scale basin for mass balance analysis due to relatively good constraints on sediment input/output, age, provenance, and rates of subsidence (Goodbred et al, ; Reitz et al, ; Sincavage et al, ; Steckler et al, ). A detailed Holocene avulsion history of Sylhet basin has demonstrated two main modes of fluvial system behavior: bypass dominance with minimal variance in facies and enhanced extraction with significant downstream changes in grain size and stratigraphy (Pickering et al, ; Sincavage et al, ). These modes of bypass versus extraction were initially identified qualitatively based on downstream changes from sand‐dominated to mud‐dominated facies.…”

“…The shift to enhanced extraction toward the end of the mid‐Holocene is interpreted as a response to the weakening monsoon. This reduction in seasonal flooding removed a hydrologic barrier that had limited distal overbank transport and sedimentation to the central basin (Sincavage et al, ). The persistent bypass of sediment along the OB pathway during the first half of the mid‐Holocene left central Sylhet basin sediment starved and underfilled.…”