--> Wavy morphologies on continental slope of Santos Basin (Brazilian Margin)

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Wavy morphologies on continental slope of Santos Basin (Brazilian Margin)


The presence of undulated sediment structures is observed in very different sedimentary environments from passive to active margins and from continental shelves to abyssal-plain environments. This diversity of environments accounts for different formation processes. This study aims to propose a scenario for the morphogenesis of undulated structures observed on the continental slope of the Santos Basin between 300-800 mwd, using bathymetry and high-resolution sub-bottom data collected during the Petrobras-Ifremer SanBa scientific cruise (December 2010/January 2011). A detailed observation of these wavy echoes shows evidence for deformations and creeping movements on the high-resolution sub-bottom profile as well as a collapsed structure and a structure in blocks separated by faults that all connect at their bottom to a surface of discontinuities. In addition the continental slope is also affected by pockmarks, buried pockmarks, and mounds associated with acoustic wipeout linked to fluid or gas escape. The origin of these undulated structures could therefore be associated to the presence of fluids or gas escape that can cause deformations and hydro-fracturing of the sediment. This creeping movement is occurring down slope along the internal discontinuities. This interpretation is comforted by the occurrence of an erosion scar on the upper slope.

These internal discontinuities could correspond to weakness surfaces, sliding surfaces, that have acted as a basal shear planes for creeping events. Those deformation events seem to occur around the last glacial maximum (end of MIS3 and MIS2). During the low sea level phases, a cannibalization of sediments from the shelf induces an overload of sediment on the slope and thus favor sediment failures. The slope destabilization might also be linked to the liberation of hydrate dissociation by variation of temperature and pressure, linked to glacial/interglacial phases. The influence of bottom currents and quaternary sea-level changes are also probably significant. The origin of this undulation geometry is therefore complex and is linked to the interaction of depositional and gravity deformation processes. However, more regional studies using deeper penetration seismic are needed in the area as well as coring and geotechnical studies to fully assess the timing of events, the existence of gas hydrates and to determine whether creeping is still active or not today.