The basal unconformity of the Brahmaputra-Jamuna paleovalley, first identified during a 1976 geotechnical drilling campaign aimed at determining feasibility of bridge construction, has been imaged along a 255-km fluvial seismic survey. The valley floor is defined by a planar gravel unit up to 10 m thick and 40 km wide with boulders up to 30 cm in diameter, overlying Pleistocene-aged diluvial sands of Brahmaputra provenance. Timing of deposition of the basal unit is constrained by radiocarbon ages; the diluvial unit below the gravel layer contains material dated to 28 ka, and the valley fill alluvium above the gravel unit contains material dated to 9 ka at a similar latitude. This age bracket is coincident with paleoflood events that originated in a glacially-dammed lake that formed along the flow path of the Brahmaputra River in Tibet. These floods resulted in high discharge (up to 10⁶ m³/s) flows that were routed through the existing Brahmaputra-Jamuna lowstand valley, effectively excavating the valley and transporting and depositing the gravel unit. In a sequence stratigraphic framework, these findings are important because they reveal an observable example of an unconformity that formed in response to an upstream driver (i.e., incision happened in response to a forcing that originated in the upstream catchment rather than at the coastline in response to sea-level lowering) in a very short amount of time. As such, this unconformity represents a geologically instantaneous time surface that can be used as a marker for stratigraphic correlation.