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Measure of sedimentary fluxes
Over the last 3 years, we have developed and validated a new approach to estimate sediment flux to basins (Rouby et al., 2009, Helm phd, Guillocheau et al., 2012). Most of the published data are from the shelf and the slope of the margins. Time lines are extrapolated along the abyssal plain using old public seismic lines and existing DSDP/ODP data. For each section we differentiate 3 alternative interpretations to include uncertainties on the basin distal geometry. On each section and for each time step, we measure the mean thickness and rate of sediment deposited. We then determine, from independent data, the spatial extension of the basins for the corresponding time increment and use it to extrapolate 2D sedimentation rates to volumes.
We have estimated the sedimentary flux along the West African passive margin. Preliminary estimates of the total sedimentary flux for this area (i.e a margin about 2 000 km in length) range between 5 000 and 135 000 km3/Myr. The maximum error reaches 25 % in very unconstrained areas, but is typically below 10 to 15 %. The North Western and Eastern African passive margin basins remain to be processed, as well as intracontinental basins. The 2D section database is however completed.
From total sedimentation volumes to siliciclastic flux
The sedimentary flux estimations will be corrected for sediments that are not the products of regional erosion on the continent: i.e. mainly carbonate but also evaporites and possibly volcano-clastic sediments. This relies on an additional phase of more detailed geological data compilation; mainly DSDP-ODP in the deep-sea plain, industrial wells on the shelf and published lithologic maps.
GUILLOCHEAU, F., ROUBY, D., ROBIN, C., HELM, C., ROLLAND, N., LE CARLIER DE VESLUD, C. & BRAUN, J. (2012) Quantification and causes of the terrigeneous sédiment budget at the scale of a continental margin : a new method applied to the Namibia-South Africa margin. Basin Research, 24, 3-30.
Extrapolation method for the initial cross-sections and measurement of the mean deposited thickness for each time interval. (a) The initial cross-section is (b) extrapolated across the whole sedimentary wedge using all available data (e.g. bathymetry, ODP wells, total isopach maps, subsurface data, etc.; see text for details). (c) For each initial section, several hypotheses are established within this extrapolation framework; three, in this case. (d) For each hypothesis and time interval Dti, the mean deposition length li and deposited section area Ai are measured. The mean deposited thickness hi is then deduced as hi5Ai/li (see Guillocheau et al., 2012 for more details).
Orange and Walvis Basins measurement of the terrigeneous sediment budget: location of the studied area and data used to constrain the geometry of the sedimentary wedge (see Guillocheau et al., 2012 for more details).
Orange and Walvis Basins terrigeneous sediment accumulated volumes and rates: variance corresponding to the variability between the extrapolation hypotheses, uncertainties on horizon age, carbonate content, sand/shale ratio and seismic velocities are shown (see Guillocheau et al., 2012 for more details).