DECADAL ARCHITECTURE AND MORPHODYNAMICS OF MODERN, RIVER-FED TURBIDITE SYSTEMS: BUTE INLET AND CONGO FAN

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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 analysed over a period of 20 years for two river-fed turbidite systems: eleven surveys for Bute Inlet (West Canada) and five for Congo Fan (West Africa). Time-lapse maps show the evolution of erosion and deposition zones, which are related to the upslope migration of erosive and depositional features including knickpoints. Erosion-deposition zones (EDZs) occur as large, several km-sized 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 (m’s scale high and 10’s m to 100’s m long), crescentic and wavy bedforms extending down to the lobe. From the time-lapse maps, it is inferred that both macro 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 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 significantly contributes to the bulk of the terminal turbidite deposition zone. The short-term morphodynamics of EDZs represents an autogenic sand detachment mechanism in both the transfer zone and terminal d eposition zone, which potentially create isolated depositional units of m’s to 10’s m thick and some kilometres in length in ancient turbidite systems.