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

Abstract

Quantitative interrogation of grain sizes in sedimentary systems has the potential to improve predictions of stratigraphic architecture, facies distributions, and downstream reservoir characteristics. To quantify these relationships, downstream fining data are coupled with rates of mass extraction, with input grain-size distribution, accommodation, and sediment input from multiple transport pathways providing primary controls on resulting sediment dispersal patterns. We spatially apportioned mass distribution along three sediment delivery pathways with distinct accommodation characteristics within the Ganges-Brahmaputra-Meghna Delta to calculate chi (χ), the total fraction of supplied sediment flux lost to deposition at any given point. Low rates of downstream fining and sand-rich channel facies characterize a bypass-dominant pathway along the western margin of Sylhet basin, whereas two splay deposits that prograde into the underfilled basin interior exhibit higher rates of fining and preservation of mud-rich facies. Both splay deposits show a shift from sand-dominated to mixed sand and mud facies and increased mud preservation (above 30%) at a χ value of ∼0.8. No comparable increase in mud preservation occurs along the bypass-dominated pathway, suggesting that this course operated in an inherently different extraction mode due to limited mid-Holocene accommodation. A similarity solution model effectively reproduces most of the spatial patterns of mass extraction observed in Sylhet basin, except in one location receiving lateral sediment input from a distributary channel. These field and modeling results indicate that grain-size data and sediment volume measurements can be used to not only reconstruct paleodynamics of transport networks and resulting stratigraphy but also lead to predictive insights on subsurface heterogeneity, and thus improved reservoir and aquifer characterization.