Competition Between Little Channels and Big Earthquakes to Drive River Avulsion Timescales

Abstract:

The Ganges-Brahmaputra Delta offers a rare opportunity to explore multiple drivers of river channel network evolution due to its extensive fluvial system and complex tectonic setting. Here, we use a newly vetted optically stimulated luminescence (OSL) dating approach that makes use of the quartz silt fraction of sedimentary deposits to determine the time of events captured in the Ganges-Brahmaputra Delta stratigraphic record. We apply this to two sedimentary archives of river activity: (a) a scroll plain of deposits in the western fluvial delta plain that provides a record of minor, highly mobile channels functioning as offtakes of the mainstem Ganges River, and (b) a rare clastic sand dike field that may record a major prehistoric earthquake, positioned on the bank of an immense and underfilled paleochannel scar. Results from 48 new OSL ages suggest that small channels operate in an episodic fashion to distribute sediment across the delta plain, thus aggrading the delta surface and inhibiting superelevation of the trunk channel. The aggradation rates we identify are on the order of a few mm/yr, akin to subsidence rates averaged over comparable timescales indicating that minor channels are effective in filling the accommodation generated by subsidence. Geochronologic reconstructions reveal that the timing of seismite dike emplacement corresponds to abandonment of the adjacent major paleochannel, and the underfilled nature of the channel is consistent with a rapid, seismic event-driven avulsion. Combined, our data indicate that multiple factors – both autogenic and allogenic – alter the background frequency of avulsion that is more typically thought to be set by rates of channel-braidbelt aggradation and floodplain subsidence. For the mainstem Ganges River, these factors include: (1) suppressed avulsion frequency caused by aggradation of the delta plain via small, ephemeral distributary channels (i.e., autogenics), and (2) enhanced avulsion frequency triggered by large, high-magnitude seismic events (i.e., allogenics). Such paleoreconstructions are critical to estimating when the big river may next avulse and thus to mitigating geohazards for the densely populated nation of Bangladesh.