No abstract

Three main rivers-the Ganges, Brahmaputra, and Meghna-coalesce in the Bengal basin to form the world's largest delta system, which serves as fi lter and gateway between the Himalayan collision and vast Bengal fan repository. New insights into the Holocene construction of the Ganges-Brahmaputra-Meghna delta, with a focus on river sedimentation, channel migration, and avulsion history, are presented here using the Sr geochemistry of bulk sediments as a provenance tracer. The sediment load of each river transmits a distinct Sr signature owing to differences in source rocks from the Himalaya, Tibet, and local regions, allowing for effective tracking of river channels and stratigraphic development within the delta. In the early Holocene, vigorous delta aggradation occurred under rapid sea-level rise and high river discharge and supported the construction of sand-dominated stratigraphy by laterally mobile, braided-stream channels. However, the vertically (i.e., temporally) uniform, but geographically distinct, Sr signatures from these deposits indicate that the Ganges, Brahmaputra, and Meghna fl uvial systems remained isolated from one another and apparently constrained within their lowstand valleys. By the mid-Holocene, though, delta stratigraphy records spatially and temporally nonuniform Sr signatures that hallmark the onset of avulsions and unconstrained channel migration, like those that characterize the modern Ganges and Brahmaputra fl uvial systems. Such mobil-ity developed in the mid-Holocene despite declining discharge and sea-level rise, suggesting that earlier channel behavior had been strongly infl uenced by antecedent topography of the lowstand valleys. It is only after the delta had aggraded above the valley margins that the fl uvial systems were able to avulse freely, resulting in numerous channel reorganizations from mid-Holocene to present. These channel-system behaviors and their role in delta evolution remain coarsely defi ned based only on this initial application of Sr-based provenance tools, but the approach is promising and suggests that a more complete understanding can be achieved with continued study.

Major part of the Holocene Ganges-Brahmaputra delta occupies the southern and southwestern part of Bangladesh with a smaller part extending beyond the international boundary in the west. Five facies assemblages are documented in the lower deltaic plain in five different depositional environments: levee or levee complex, bil or depression, abandoned meander belt, interdistributary plain and estuarine plain. The thickness of the Holocene sediments ranges from 30 m to 70 m in the deltaic plain, usually floored by the Pleistocene stiff clays, with the exception of the abandoned meander belt deposit where Holocene channel sand deposited directly on the Pleistocene sand. Radiocarbon dates indicate that low-rate sedimentation has occurred in the northern part, where 4-6 m thick sediments were deposited since the midHolocene, whereas 10-30 m thick sediments were deposited in the southern part during the same span of time. In addition, significant coastal subsidence (3 mm/a on average), added by sea-level rise (1.5 mm/a, conservative rate) occurs in the study area, which serves as a negative factor in degrading the coastal plain of Bangladesh in the future, while taking into consideration the weaker sedimentation in the area.

Analogs-ancient and modern-are key to our understanding and interpretation of the stratigraphic record, which is too often incomplete and sparingly exposed. Here we describe an upwardcoarsening Holocene delta sequence that sits unconformably on another, remarkably comparable, delta sequence of Pleistocene age. Such a complete and well-preserved Pleistocene example is rare given extended periods of sea-level lowstand and fluvial incision during the past 200 ka.These stacked delta sequences allow us to consider how analogous our well-studied Holocene analogs are.The comparison reveals a nearly identical facies succession, with modest differences only in the relative timing of delta response to rising sea level. One key difference, though, is a unique facies in the Pleistocene sequence suggesting that major floods from the Himalayas impact the Bengal margin, perhaps periodically, during glacial-interglacial climate transitions.

The Swatch of No Ground (SoNG) canyon in the Bay of Bengal is a shelf-incising submarine canyon that is actively aggrading in its upper reaches despite regular gravity-driven transport and mass wasting. Although the canyon lies 150 km downdrift of its main sediment source, the Ganges-Brahmaputra-Meghna (GBM) river mouth, high sedimentation rates (5-50 cm year −1 ) are sustained by both progradation of the subaqueous delta into the canyon head and the conveyance of shelf-generated hyperpycnal flows to the canyon floor. This rapid accretion appears to be largely balanced by mass failures triggered by regularly occurring storms, and less frequently by major earthquakes. Here we use high-resolution sub-bottom sonar data to elucidate dominant sedimentdispersal pathways and their transport-related morphology at the canyon head; these include: 1) a laterally prograding clinoform that intersects the canyon head at water depths of 20-120 m; 2) several shelf-incising bypass gullies that originate in b 20 m water depth above the rollover point and connect the inner shelf to the canyon floor, and 3) numerous U-shaped slide valleys formed by deep-seated mass failures initiating at water depths N 50 m. The clinoform deposits reflect westward progradation of the GBM subaqueous delta into the upper canyon, where its axis-normal orientation leaves it dissected by cross-cutting gullies and mass failures. The morphology and acoustic stratigraphy of the gullies, coupled with strong bed shear and high suspended sediment concentrations on the inner shelf, suggest that these features are sustained by the regular conveyance of gravity-driven fluid muds that are formed in shallow water (b 20 m) where the gullies originate. The downslope termination of the gullies coincides with a break in slope at the canyon floor, indicating that gullies serve as sediment conduits linking shallow water processes with sediment deposition in the lower canyon. Together these locally interacting shelf, clinoform, and canyon features form a unique composite morphodynamic system that more broadly defines the highstand connection between this large fluvial delta system and its shelf-indenting canyon.