Mass-transport deposits and fluid venting in a transform margin setting, the eastern Demerara Plateau (French Guiana)

• Cenozoic succession of the Demerara Plateau contains a complex of at least 12 MTDs.
• Sedimentary undulations and circular to elongated depressions occur on the seafloor.
• Faults provide fluid migration pathways through the MTDs (biogenic or thermogenic).
• Detachments occur on stratigraphic horizons that outcrop along the continental slope.
• Fluid overpressures and the distal transform free border seem to control MTD dynamics.

The eastern Demerara Plateau offshore French Guiana was surveyed in 2003 during the GUYAPLAC cruise (multibeam bathymetry and acoustic imagery, 6-channel seismic reflection and 3.5 kHz echo-sounding). The data show the “post-transform” Cenozoic that the series located on the outer part of the plateau (below c. 2000 m) contain at least twelve stacked mass transport deposits (MTDs) that have recorded a history of large-scale slope failure, as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series, reaching at high as the uppermost MTDs. Seabed data show that the area above the failures is characterized by circular-to-elongate (slope-parallel) depressions interpreted as fluid seeps (pockmarks), some of them have been modified by along slope currents. We suggest that the development of the MTDs to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin's deep structure, in particular the existence of a 'free borderlateral border' on the outermost plateau. Our results also emphasise the role of stratigraphic décollements within the Cenozoic series.