![]() | Mass-transport deposits and fluid venting in a transform margin setting, the eastern Demerara Plateau (French Guiana) Auteur(s) : Pattier, France Loncke, L. Gaullier, V. Basile, C. Maillard, Alexandre Imbert, Patrice Roest, Walter Vendeville, B. C. Éditeur(s) : Elsevier Sci Ltd Résumé : 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 decollements within the Cenozoic series. Marine And Petroleum Geology (0264-8172) (Elsevier Sci Ltd), 2013-09 , Vol. 46 , P. 287-303 Droits : 2013 Elsevier Ltd. All rights reserved. http://archimer.ifremer.fr/doc/00157/26850/25316.pdf DOI:10.1016/j.marpetgeo.2013.06.010 http://archimer.ifremer.fr/doc/00157/26850/ | Partager |
![]() | Fluid flow regimes and growth of a giant pockmark Auteur(s) : Marcon, Yann Ondreas, Helene Sahling, Heiko Bohrmann, Gerhard Olu, Karine Éditeur(s) : Geological Soc Amer, Inc Résumé : Pockmarks are seafloor depressions commonly associated with fluid escape from the seabed and are believed to contribute noticeably to the transfer of methane into the ocean and ultimately into the atmosphere. They occur in many different areas and geological contexts, and vary greatly in size and shape. Nevertheless, the mechanisms of pockmark growth are still largely unclear. Still, seabed methane emissions contribute to the global carbon budget, and understanding such processes is critical to constrain future quantifications of seabed methane release at local and global scales. The giant Regab pockmark (9 degrees 42.6' E, 5 degrees 47.8' S), located at 3160 m water depth near the Congo deep-sea channel (offshore southwestern Africa), was investigated with state-of-the-art mapping devices mounted on IFREMER's (French Research Institute for Exploitation of the Sea) remotely operated vehicle (ROV) Victor 6000. ROV-borne micro-bathymetry and backscatter data of the entire structure, a high-resolution photo-mosaic covering 105,000 m(2) of the most active area, sidescan mapping of gas emissions, and maps of faunal distribution as well as of carbonate crust occurrence are combined to provide an unprecedented detailed view of a giant pockmark. All data sets suggest that the pockmark is composed of two very distinctive zones in terms of seepage intensity. We postulate that these zones are the surface expression of two fluid flow regimes in the subsurface: focused flow through a fractured medium and diffuse flow through a porous medium. We conclude that the growth of giant pockmarks is controlled by self-sealing processes and lateral spreading of rising fluids. In particular, partial redirection of fluids through fractures in the sediments can drive the pockmark growth in preferential directions. Geology (0091-7613) (Geological Soc Amer, Inc), 2014-01 , Vol. 42 , N. 1 , P. 63-66 Droits : 2013 Geological Society of America http://archimer.ifremer.fr/doc/00186/29731/28293.pdf DOI:10.1130/G34801.1 http://archimer.ifremer.fr/doc/00186/29731/ | Partager |