Multistage, progressive slope failure in the Pleistocene pro-deltaic slope of the West Nile Delta (Eastern Mediterranean)

Abstract

Abstract The vast majority of submarine mass transport deposits (MTDs) have been attributed to retrogressive slope failure processes whereby an initial rupture zone migrates in a generally upslope direction towards the position of the ultimate headwall. Here we use 3D seismic data from the prodelta slope of the Western Nile delta to describe a series of volumetrically extensive MTDs (termed MTDs A, B and C) that are part of a linked sequence that failed progressively but with retrogressive modifications of the lateral and headwall margins. The MTDs have c. 500–1000 ms of relief on their marginal scarps with a minimum total volume of remobilised sediments of 750 km3. By comparing their motion histories, and by correlating their basal surfaces, we demonstrate that MTDs B and C are remnants of a single original body that was later cut by MTD A. This sequence is confirmed by cross-cutting relationships at the lateral boundaries between the three MTDs and the absence of any significant infill and burial of residual topography at the tops of MTDs B and C prior to the incision of MTD A. This implies that these two major submarine failures (MTD B/C, and then MTD A) were closely grouped in time. We suggest a mechanistic model whereby rapid load redistribution resulting from the initial failure led to localization of a deeper cutting failure, and the unloading in the headwall region then led to expansion of the deeper failure in an upslope direction until a new merged headwall was formed. Correlation with MTDs identified downslope in previous studies suggests a date between 117 and 105 ka for this giant slope failure. This was a period of relative sea level fall in the Nile region which may have contributed to increased pore pressure. Slope failure on this scale was probably preconditioned by high sedimentation rates and under-compaction in a mud-rich succession, leading to local increases of pore pressure.