Fluid Seepage in Relation to Seabed Deformation on the Central Nile Deep-Sea Fan, Part 1: Evidence from Sidescan Sonar Data

The central Nile Deep-Sea Fan contains a broad area of seabed destabilisation in association with fluid seepage: slope-parallel sediment undulations are associated with multibeam high-backscatter patches (HBPs) related to authigenic carbonates. During the 2011 APINIL campaign, a deep-towed sidescan and profiling system (SAR) was used to acquire high-resolution data along three transects across water depths of 1,700-2,650 m. Three seabed domains are distinguished, all developed within stratified sediments overlying mass-transport deposits (MTDs). Upslope of the undulations (<1,950 m), sidescan HBPs record focused fluid seepage via seabed cracks. In the western area of undulations, sidescan HBPs are distinct from intermediate-backscatter patches (IBPs) that extend up to 850 m parallel to the undulations, mainly along their downslope flanks; some contain sub-circular HBPs up to 300 m wide, three associated with smaller (<10 m) hydroacoustic gas flares. Focused fluid seeps are inferred to have shifted over time to form elongate carbonate pavements, preferentially along the footwalls of faults beneath the undulations that provide pathways for fluid flow. In contrast, in the eastern area of undulations, sidescan imagery reveal only slope-transverse furrows formed by turbulent flows, interpreted to indicate that fossil carbonates sampled during submersible operations have been exhumed by erosion.