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

Abstract

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 echosounding). 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. Highlights ► 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 Coulomb critical taper theory applied to gravitational instabilities

“…For example, the critical taper theory could help better constrain the evolution of large submarine slumping, as is the case of the Storegga slide [Kvalstad et al, 2005], where the detachment surface is a low-resistance clay layer and where the contacts between stratigraphic layers intersect the wedge-shaped surface of the unstable mass (i.e., where no downslope buttress is found; Figure 8). Mass transport deposits occurring along passive margins subjected to fluid overpressure, as described recently by Pattier et al [2011Pattier et al [ , 2013, are also a natural example where our model could be applicable. This model would also be valid for onshore landslides, such as the gravitational instabilities of the Waitawhiti complex [Lacoste et al, 2009].…”

“…Mass transport deposits occurring along passive margins subjected to fluid overpressure, as described recently by Pattier et al . [, ], are also a natural example where our model could be applicable. This model would also be valid for onshore landslides, such as the gravitational instabilities of the Waitawhiti complex [ Lacoste et al ., ].…”

Origin of an enigmatic regional Mio-Pliocene unconformity on the Demerara plateau

“…This partition also accounts for the slope values observed on the bathymetric data: (1) the upper plateau (slope values less than 1°), (2) the intermediate plateau (average slope value 3°), (3) the lower plateau (slope value < 1°), and (4) the continental slope (slope value on average 12°, with maximum values reaching 20°) . The boundary between the upper and intermediate plateaus is defined by a bathymetric slope break that corresponds to a main sediment slope failure headscarp ( Figure 2; Loncke et al, 2009;Pattier et al, 2013). The continental slope shows contrasting slope values between the northern segment (i.e., the transform boundary, average slope value 15°) and the eastern segment (i.e., the divergent boundary, average slope value 4°) .…”

“…Numerous elongated depressions were described on the seabed on top of remobilized deposits. There were interpreted as pockmarks elongated by deep-sea bottom currents Pattier et al, 2013). In order to understand the factors that control slide onset (Fluids ascents?…”

“…In order to answer to these questions that may have important academic and petroleum issues, we carried out a stratigraphic analysis of available academic and industrial seismic data from the area. We first updated the chronology and mapping of the different slope failures already described by Pattier et al, 2013. Then, we mapped the extent of the Mio-Pliocene unconformity and analyzed its characteristics using high-resolution relative stratigraphy.…”