Embracing Non-Stationarity – Hysteresis and Coherence in the Ganges-Brahmaputra Source-to-Sink System and Its Imprint on the Stratigraphic Record.

Abstract:

The transport and deposition of Himalayan sediment since the last lowstand has led to 10,000 km³ of fluvio-deltaic sands and muds infilling the tectonically active Bengal basin. The considerable dimensions of this late Quaternary sequence provide an expanded geologic record of source-to-sink processes from glacial to interglacial boundary conditions. Recent coring, seismic surveys, and lab analyses of basin sediments now provide an unprecedented view of mass transport history, depositional controls, and resulting stratigraphic architecture. During the glacial period, river hydrology was strongly affected by ephemeral ice-dammed lakes and associated sediment sequestration in bedrock valleys of the upper catchment; downstream, though, reduced sediment flux and stochastic discharge led to scour and incision of the lower alluvial valleys. Specifically, in the Bengal basin these glacial-period influences led to development of a vast, exhumed valley system floored by a prominent basal gravel and boulder layer. The legacy of such sediment starvation and valley growth presented an apparent mass-balance problem and a potential hysteresis in the evolutionary path of the delta system as it developed during ensuing post-glacial sea-level rise. However, the hysteresis effect is ultimately mitigated by coherence between post-glacial sea-level rise and the abrupt onset and peak intensity of the summer monsoon. In this case, the mass-balance problem is offset by intensified summer monsoon rains and enhanced discharge of exhumed bedrock-valley and lake-bed sediments, leading to infilling of the downstream margin valley in pace with sea-level rise. As a result, any hysteresis effect in the Ganges-Brahmaputra system had been erased by the mid-Holocene deceleration of sea level. In the stratigraphic record, the imprint of these prominent shifts in the source-to-sink system is best reflected in the sand grain size and sand-mud ratios of preserved sediment, and by mass-balance derived estimates of total sediment flux. Although such major shifts in this sedimentary system are recorded by relatively subtle stratigraphic indicators, fidelity in these interpretations is greatly enhanced by consulting multiple proxy records on the controlling variables between source and sink.