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Controls on the Distribution of Channel Sands in a Fluvial Fan System: The Effects of Tectonic Deformation and Discharge Variation on the Stratigraphic Architecture of the Brahmaputra Fan Delta

Abstract

The Brahmaputra River bends sharply around the Himalayan syntaxis and again around the edge of the Shillong Plateau (SP), resulting in a tectonically-controlled anfractuous flow path. Understanding the role of allogenic forcings, like changes in slope induced by tectonics, in modulating the river course and construction of fluvial stratigraphy is essential for modeling stratigraphic architecture. Extensive drilling and dating of the Brahmaputra system has recently made it possible to document the river location before and after uplift of the SP. Prior to Plio-Pleistocene uplift, the Brahmaputra occupied the region east of the incipient plateau. Presently the river is routed along the western margin of the SP. Based on the interpretation of dozens of exposures along the Shillong margin and hundreds of well bores in the adjacent delta, the distribution of channel sands is divided into 3 distinct fan lobes. Depositional ages of each of these lobes were measured by luminescence dating, chronicling the history of river steering in response to uplift of the plateau margin over the past few hundred ka. The easternmost sediment lobe records the location of the river during incipient uplift; the western lobe documents deflection of the river to its westernmost position as recently as 100 ka; and the central lobe tracks the river course as it eroded eastward into the plateau margin toward its present course. This stratigraphy indicates that uplift of the plateau provides the first order control on channel sand distribution in the fan delta at this temporal resolution (105 years). Despite ongoing uplift of the SP, the river has been able to erode eastward into the plateau margin, the timing of which is coincident with a period of paleoflood events. Paleohydrological reconstructions suggest that some of these were megafloods with discharge >106 m3/s. Because these events coincide with eastward erosion of the river into the plateau margin, we propose that repeated increased discharge events contributed to the river's ability to overcome the uplift-induced deflection. Occurring over a period of 104 years, these high discharge events are a second order control on the channel sand distribution in the fan delta. Results document fan-scale changes induced by allogenic forcings and can be used to inform analog models for this and other tectonically-influenced river systems, as well as improve our understanding of sedimentation patters in the linked Bengal fan system.