2022 CSDMS meeting-080

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The influence of geologically instantaneous, event-scale processes on basin-scale sediment dispersal patterns

Ryan Sincavage, Radford University Radford Virginia, United States. rsincavage@radford.edu
Man Liang, Indeed.com Seattle Washington, United States.
Jennifer Pickering, University of Kansas Lawrence Kansas, United States.
Steven Goodbred, Vanderbilt University Nashville Tennessee, United States.
Paola Passalacqua, University of Texas Austin Texas, United States.

The Jamuna Valley of the Bengal basin was in part developed by an early Holocene (~10.5 ka) Tibetan-sourced glacial lake outburst megaflood. This same event scoured a smaller, tangential channel east of the Jamuna valley into Sylhet Basin. This flood-carved channel on the western margin of the basin remained unoccupied until delta aggradation allowed the Brahmaputra River to re-occupy it ~7.5 ka. Strong topographic and tectonic influences suggest that the river was primed to occupy the topographically low basin interior. In spite of these conditions, the Brahmaputra remained largely restricted to this marginal paleo-flood course for the next ~2500 years. We use numerical modeling to investigate two possible scenarios driving the persistence of this channel course: (1) local backwater effects from a semi-permanent 10,000 km2 lake within the basin due to enhanced early Holocene Indian Summer Monsoon conditions, and (2) antecedent morphological control of the paleo-flood channel form. We simulate mid-Holocene conditions in Sylhet Basin by perturbing several physical parameters within a 1-D channel profile model and a 2-D depth-averaged hydrodynamic model to determine preferential flow path selection between two possible pathways. Neither a local backwater effect nor a reduction of the topographic slope to simulate pre-subsidence topography along two pathways appear to be plausible explanations for exclusion of flow to the central basin. Instead, the introduction of a scour along the western margin flow path is the only mechanism tested that induces a strong preference for bypass of the basin. Thus, both field and modeling evidence indicate that Himalayan-sourced megafloods modified the lowstand surface of the Bengal basin, creating antecedence that strongly influenced Holocene delta evolution and river channel behavior. These results suggest that geologically instantaneous, event-scale processes may exert long-term control on sediment dispersal patterns and thus preserved stratigraphy at the basin scale, even in large systems with pronounced tectonic and climatic influences.