Focused Fluid Flows and Seeps Offshore Southern Majunga Basin (NW Madagascar)

The offshore southern Majunga Basin (Madagascar) was investigated during the PTOLEMEE, PAMELA-MOZ01 (2014) and -MOZ04 (2015) marine expeditions as part of the PAMELA (Passive Margin Exploration Laboratories) scientific project. Multibeam and seismic surveys were conducted along the slope roughly between 400 and 1400 m of water depth. On board, following data processing, gas-related evidence were identified through water column and (sub-)seafloor acoustic imagery, including bathymetry, seafloor backscatter, subbottom and high-resolution seismic profiles. There are only a few tens of seeping sites scattered long the 2000 km² surveyed area. Based on a low amplitude signal and the small height of gas bubble «flares», these seeps were interpreted to be associated with weak gas flux. Most of the seeps are gathered in two main clusters (of less than a few square kilometres). These results guided the strategy adopted for the deep-towed camera surveys (Scampi © Ifremer) and core/dredge sampling. Seafloor seep-related morphologies correspond to pockmarks, slide scars and slightly outcropping methane-derived authigenic carbonate structures, partly if not entirely covered by sediments. Subcircular pockmarks are mainly inactive while seeping sites are located along destabilized flanks and in the close surroundings of irregular pockmarks that are lacking strong thermal anomalies. The seeps are characterized by metre-scale dark reduced sediment patches, with frequent bivalve shells or fragments. Siboglinid tubeworm fields were also observed locally. The not-well developed seeps at the seafloor, the presence of buried methane-derived authigenic carbonates together with the in-depth seismic evidence for pipes all question the connections to the sources, the sedimentary control on fluid migration and the possible impact of recent sedimentation from the Betsiboka and South Mahavavy rivers.