What control the sediment export in the ultimate deep-sea sink : the example of the Zambezi submarine fan (Mozambique channel)
Résumé
Deep-sea fans are the ultimate area of sediment deposition along the source-to-sink systems. Since the definition of transport efficiency by Mutti (1979), i.e. the ability of gravitary flows over at least hundreds of thousand years to transport sediments far off the slope, little attention was paid to the factors (tectonics, eustasy, sediment delivery at the top of the slope) controlling this efficiency. We here test the possible controls on the transport efficiency of submarine fans looking for the distribution of both sedimentary facies and mainly the distance of the upstream part of the lobes from the slope and the sediment volumes.
This study was performed on the Zambezi deep-sea fan and its upstream delta since 160 Ma. The Zambezi catchment and little Madagascan feeders experienced several uplifts (Southern African Plateau, East African Dome, Madagascar Plateau) and two drastic climate (precipitation) changes between semi-arid and very humid conditions. The used data are seismic lines calibrated on dated wells. We defined depositional sequences that provided time-lines for measuring siliciclastic sediment volumes. Facies mapping was based on seismic reflectors characteristics (geometry, attributes).
The first obvious control is the topography, mainly in the early stage of the ocean opening, but also the growth of topographic ridges isolating ponded basins. The main factor seems to be an increase of the siliciclastic sediment flux due to uplift of the upstream catchment, i.e. at the apex of the sink that is transferred, at the scale of hundreds of thousand years, directly to the deep-sea fan. This is supported by the effect of the two steps uplift of the southern African Plateau (90-80 and 40-30 Ma) and of the Eat African Dome. Climate and eustatic changes are clearly second order processes. We tested the buffer effect of the upstream delta and associated ponded basins.
