Potential role of compressional structures in generating submarine slope failures in the Niger Delta

The study area, offshore Nigeria, is located in one of the compressional zones within the Niger Delta, which is characterized by imbricate thrust structures. Although the low mean slope angle (around 2°), bathymetry data from the study area have shown the existence of several submarine landslides which coincide with known subsurface faulted compressive features. In this paper, we have focused on a submarine slide occurring in water depths ranging between 1690 and 1750 m. Headwall scars, internal architecture and associated deposits have been characterized using a combination of 3D seismic data, near-bottom echosounder seismic profiles, Kullenberg cores and in-situ geotechnical measurements. The slide shows horseshoe shaped headwall scars and depositional lobes with positive relief. Monitoring of excess pore pressure for 12 months indicates the presence of negative hydraulic gradients, which is either an indication of a local present-day mechanical activity of subsurface faults or related to the regional extension of the Niger Delta and the possible creation of a regional depression in the hydraulic regime. In order to identify the triggering mechanism of the observed landslide, a three-dimensional slope stability model (SAMU-3D) based on the upper bound theorem of plasticity was used. Calculation results have shown that the gravity loading generated by the sediment weight alone is not sufficient to explain the observed submarine slide. Cylindrical cavity expansion theory was used to locally simulate the compressional structure movements and to evaluate the strength generated within the upper sediment layers. Slope stability assessment carried out by considering this additional structural strength has shown that regional compressional gravity driven deformation can explain the observed submarine failures.