Decadal architecture and morphodynamics of modern, river-fed turbidite systems: Bute Inlet and Congo Fan

ABSTRACT Modern active turbidite systems transport and store more sediments than any other sedimentary system. However, the depositional architecture which links these stored sediments to this short-term morphodynamics is not well understood due to the limited available monitoring data. Time-lapse bathymetric surveys were analyzed over a period of 20 years for two river-fed turbidite systems: eleven surveys for Bute Inlet (West Canada) and five for the Congo Fan (West Africa). Time-lapse maps show the evolution of erosion and deposition zones, which are related to the upslope migration of erosional and depositional features including knickpoints. Erosion–deposition zones (EDZs) occur as large, several-km-size steps with increasing length towards the lower channel. These intra-system deposition zones form large turbidite bodies described as macroforms, which onlap the adjacent, upslope erosion zone. The erosion and deposition zones are covered with mesoscale (meters scale high and tens to hundreds of meters long), crescentic and wavy bedforms extending down to the lobe. From the time-lapse maps, it is inferred that both macroscale and mesoscale bedforms are formed synchronously by ordinary seasonal events occurring once or a few times a year. Extreme seasonal events, occurring once in a decade, produce longer-wavelength EDZs. Four different, decadal architectural styles of turbidite macroforms are described from map views and cross sections. These reflect a continuum from confined-flow to unconfined-flow-related deposition. Calculated internal sediment budgets over EDZs indicate that about 50% of the sediment is stored in the transfer-zone macroforms, while only the remaining 50% reaches the terminal deposition zone over the two-decade monitoring period. About 40% of the total sediment erosion occurs distally in the lower channel and plays an important role in delivering sediments which build the terminal turbidite deposition zone. The short-term morphodynamics of EDZs represent an autogenic sand detachment mechanism in both the transfer zone and terminal deposition zone, which potentially create isolated depositional units meters to tens of meters thick and some kilometers in length in ancient turbidite systems.