Fluid Seepage in Relation to Seabed Deformation on the Central Nile Deep-Sea Fan, Part 2: Evidence from Multibeam and Sidescan Imagery

摘要

On the central Nile deep-sea fan, stratified sediments overlying mass-transport deposits (MTDs) are deformed into slope-parallel seabed undulations associated with fluid seepage. The western part of this system, in water depths of 1,950-2,250 m, is examined using multi-frequency data from hull-mounted and deep-towed swath/profiling systems. Sub-bottom profiles show sub-vertical fluid pipes that terminate both at and below seabed, and gas signatures along fault planes bounding the undulations. Fluid seepage is recorded by high- to intermediate -backscatter patches (HBPs, IBPs) that differ in appearance on multibeam imagery (30 kHz, ≤3 m penetration) and sidescan swaths (170/190 kHz, ＜0.1 m penetration). Comparison of the two suggests a distinction of (a) buried carbonates (0.1-3 m), (b) broad near-seabed (＜0.1 m) carbonate pavements elongate along the undulations, (c) sub-circular areas of seabed seepage up to 300 m across. Four of the latter have narrower gas flares at their edges rising 400-800 m above seabed. These results are consistent with an evolving system of narrow fluid conduits that support the growth and burial of carbonate pavements, shifting over millennial timcscales along linear zones parallel to fault planes rooted in MTDs. Sediment deformation above MTDs is inferred to provide pathways for fluid escape, but migration of gas-rich fluids from depth is likely to have facilitated slope destabilisation.