CSDMS 2015 annual meeting poster RyanSincavage
Comparing morphologic and stratigraphic field data from a tectonic basin on the Ganges-Brahmaputra River delta with results from a reduced-complexity model for river delta formation
Carol Wilson, Vanderbilt University, Tennessee, United States.
Man Liang, University of Texas Austin Austin Texas, United States.
Paola Passalacqua, University of Texas at Austin, Texas, United States.
Steven Goodbred, Vanderbilt University, Tennessee, United States.
Abstract:
Sylhet Basin in northeastern Bangladesh contains an archive of Holocene sediment deposited by the Brahmaputra River during episodic occupations of this subsiding basin. Geochemical and lithologic analyses of samples from more than 300 closely-spaced (3-5 km) boreholes have been used to delineate three discrete channel occupations confined to two sediment “fairways” within Sylhet Basin over the past ~10,000 years. A well-defined break in topography has been identified along these fairways, distinguishing a steeper (~10–4) upper fan delta from a lower gradient (~10–5) backwater-reach delta. This break in topography corresponds to a change from rapid to slow rate of downstream fining in Holocene sediments, perhaps representing the distal limit of mass extraction of bedload material. Furthermore, this break in slope roughly correlates with start of the backwater transition as calculated by a variety of methods. Stratigraphy along this reach of Sylhet Basin is characterized by 20-30 m thick early Holocene muds overlain by 30 m of amalgamated channel sands deposited during a prolonged (2-3 kyr) occupation of the basin during the mid Holocene. This stratigraphic succession is consistent with recent reduced-complexity model results of fluvial systems avulsing to infill an existing topographic basin. Similar model runs involving an actively subsiding basin (rate high enough to accommodate all incoming sediment) generate an attraction of channels towards the subsidence maximum not observed with the static basin. Based on modern sediment discharge of the Brahmaputra River, volume of the amalgamated channel sands (30 m thick x 80 km wide) formed during the mid-Holocene occupation of Sylhet basin is insufficient to account for the entire sediment budget, likely indicating bypass of the basin to its downstream outlet along the Meghna River, or that discharge was reduced during a the mid-late Holocene weakening of the monsoon. We therefore conclude that the subsidence rate of Sylhet Basin is sufficiently slow relative to sediment input such that the preserved stratigraphy and channel avulsion history are more characteristic of models exhibiting static basin topography than of active subsidence at rates comparable mass deposition. Future modeling scenarios will use the Sylhet Basin stratigraphic data to investigate the role of backwater on morphodynamic channel behavior and the resulting influence on downstream changes in net to gross values and stratal architecture.
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