Fluid seepage associated with slope destabilization along the Zambezi margin (Mozambique)

Abstract

Evidences for active fluid seepages have been discovered along the Zambezi continental slope (offshore Southern Mozambique). These seepages are mostly associated with pockmarks which are aligned along a trend parallel to the slope and running closely upstream of the headwall scarp of a wide zone of slope destabilization. Fluid seepages are interpreted as a potential trigger for the slope destabilization. Acoustic anomalies within the water column have been interpreted as related to moderate bubble seepages mostly located outside and only punctually inside the destabilization zone. Exploration with the SCAMPI towed camera system in the widest pockmark (diameter 200 m wide) has shown fluid seepages associated toauthigenic carbonate crusts and bacterial mats. These fluid seepages are also associated to the presence of chemiosynthetic organisms (Vesicomyidae and Thyasiridae bivalves, Siboglinidae tubeworms). The sampled gas in the sediment corresponds mainly to CH4 of microbial origin, generated by hydrogenotrophic methanogenesis from a substrate of organic origin, i.e. a conventional process of genesis of microbial gas in the marine domain. No evidence for thermogenic gas was detected. Another type of pockmarks has been observed within the core of the slope destabilization zone. Most of these pockmarks are inactive in terms of fluid seepage at present time and are associated to carbonate buildups forming chimney geometries. They probably correspond to diagenetic chimneys of former fluid migration pathways that have been exhumed during the mass sliding and the surrounding depression are related to recurrent activity of strong lateral slope currents which have scoured the sediments around. The spatial organization of the slope destabilization features is considered as representative of the temporal evolution of the landslide giving information about the dynamics of slope instability processes. This proposed evolution started by scattered seepages of formation water with dissolved gas. Then free gas seepages appeared notably in the upper part of the slope. This was followed by progressive shallow deformation in the sediments downslope of the main gas seepages. Finally, the whole slope was destabilized forming imbricated landslides exhuming locally former diagenetic chimneys. Highlights► Fluid seepages are associated with slope destabilization along the Zambezi continental slope ► Gas seepage has a microbial origin ► Carbonate diagenetic chimneys have been exhumed during the mass sliding ► The spatial organization is representative of the temporal evolution of slope destabilization J o u r n a l P r e -p r o o f Journal Pre-proof temperature along the SCAMPI track. In the study area, it was operated during two dives (SCB01 and SCB03) on two different areas where pockmarks are present (Fig. 2). Gas sampling and geochemical studyGas was sampled in three Calypso sediment piston cores (MOZ04-CS18 and MOZ04-CS19, and also MOZ04-CS17 made north of the study area; see supplementary material Fig. S1, S2, S3). The gas samples ...

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