Submersible study of mud volcanoes seaward of the Barbados accretionary wedge: sedimentology, structure and rheology

In 1992, the Nautile went to a mud volcano field located east of the Barbados accretionary wedge near 13 degrees 50N. Using nannofossil analysis on cores, we determined the sedimentation rate, and provided a new estimation of the age of the mud volcanoes (750,000 years for the oldest one). Six structures have been explored with the submersible Nautile, and manifestations of fluid venting (chimneys, carbonate cementation and chemosynthetic communities) were observed on all. Sedimentological analysis identifies two sources of diapiric mud. Most mud volcanoes expel mud containing Late Miocene to Quaternary faunae that have the same composition as sediments drilled above the Barbados wedge decollement. One volcano also contains older Oligocene taxa, with a mud composition corresponding to the sedimentary sequence below the decollement. We use diving observations to map the fine-scale morphology, the distribution of chemosynthetic fauna and define two end-member types of structures: mud-pies (flat topped mud volcanoes) and conical mounds. Mud-pies (Atalante and Cyclops) are characterised by the presence of a lake of high-porosity mud (70% to 75%) in their central parts. Chemosynthetic benthic communities (Calyptogena colonies and sponge bushes) are concentrated in the outer parts. Contrasting morphologies of the two mud-pies indicate different stages of activity: Cyclops is growing whereas Atalante is collapsing. Expulsion of water and methane occurs mostly through the mud lake and may be stronger during the collapse phase. On conical mounds there are no mud lakes, fluid venting concentrates near the summit and occurs through carbonate cemented chimneys which form within the sediment. Viscosity measurements have been carried out on mud samples from the two mud-pies and one conical mound. All mud samples have a plastic fluid behaviour, the plastic threshold decreases with porosity, and thixotropy is observed for a porosity of more than 70%. An analogue experiment shows that for this thixotropic mud, shearing in the feeding conduit liquefies the mud which then spreads to form a mud-pie. Conical mounds form when the mud remains plastic. We show that the dissociation of methane hydrate is the cause of the high porosity in mud-pies and confirm that these structures are a consequence of large-scale dissociation of methane hydrate at the base of stability field. Dissociation of hydrates before and during ascent is only slightly contributing to the pore fluid in conical mounds, but solid hydrates still present in the mud may contribute to its buoyancy, (C) 1998 Elsevier Science B.V.