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Cycles sédimentaires dans le système turbiditique du Congo : nature et origine

Abstract : Turbidite systems are huge submarine sedimentary fans located off rivers, at the foot of the continental slope. They constitute the most distal terrigeneous deposits of a fluvial system. Their internal structure, defined as a stacking of channel-levee systems, show architectural changes through time. Forcing factors of these architectural changesare still poorly understood, and the respective role of internal (related to the own functioning of the system) or external forcing factors (climate, seal-level variations, tectonics) remains debated. To better understand the role of these controlling factors, a detailed study of the Congo system architecture was carried out. This study was conducted as part of the Reprezaï research project (Ifremer/IUEM, since 2006), following the Guiness and ZaïAngo projects (Ifremer/Total, 1992-2003), during which many oceanographic surveys allowed acquiring an important geophysical and geological data base. A quantitative analysis of different architectural parameters from the channels (e.g. total length, length built after avulsion,…) of the Congo Axial Fan (last 200 ka) reveals progradational-retrogradational sedimentary cycles of the depocenters, the highest retrogradations corresponding to avulsions located very upfan. Depocenters identified at the termination of the channels correspond to relatively elongated lobe complexes (according to the nomenclature of Prélat et al., 2009) with greatly variable volumes (from 3 to 196 km3) both in time and space, without any cyclicity like that identified by other parameters. The cumulative volume of these lobe complexes represents up to 31% of the Axial Fan volume. The multiproxies study (14C dating, δ18O, XRF measurements, Total Organic Carbon…) of cores sampled on the most prograding and retrograding channels provided a chronological framework to these sedimentary cycles. The temporal evolution of the architectural parameters was compared with changes in paleoenvironmental and paleoclimatic signals (pollens, TOC, XRF and MSCL measurements, clay mineralogy,…) recorded in the sediments of a reference core deposited simultaneously to the Axial Fan, but located outside the turbidite flows. Results highlight a strong link between the architectural evolution of the turbidite system and the Congo River discharge. These, as well as the expansion of the vegetation cover in the catchment area, control the sand/clay ratio and thus, at least partly, the transport capacity of turbidity currents that build the channel-levee systems. Both factors depend on rainfall related to the monsoon intensity variations in relation with precession (19-23 ka Milankovitch). A scenario for the deposition of channel-levee systems and lobes of the Axial Fan in link with arid/humid climate variations in the catchment area has been proposed thanks to good stratigraphic constraints for the last 40 ka. During arid periods, turbidity currents are infrequent and present a high sand/clay ratio which decreases their transport capacity. These turbidity currents are suitable to channel aggradation, generating favorable conditions for the subsequent creation of upfan avulsion. During periods of transition of arid to humid conditions, when the canopy is still underdeveloped, fluvial discharge and significant sediment load increases result in an increase of the turbidity currents volume, which favors upfan avulsions. Finally, during humid periods, Congo discharge is characterized by a strong liquid flow and a mainly clayey solid discharge which decreases gradually as the canopy extends. These conditions generate turbidity currents with decreasing transport capacity through time. However the transport capacity of these currents remains sufficient to reach distal portions of the systems and favor channels progradation. This climatic factor seems therefore to regularly disrupt the more perennial internal control which is also highlighted by the Congo Axial Fan architecture.
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Marie Picot. Cycles sédimentaires dans le système turbiditique du Congo : nature et origine. Sciences de la Terre. Université de Bretagne occidentale - Brest, 2015. Français. ⟨NNT : 2015BRES0049⟩. ⟨tel-01962553⟩

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