Development of a partially-avulsed submarine channel on the Niger Delta continental slope: Architecture and controlling factors

Abstract

On the Niger Delta slope a partially-avulsed channel, named Tombia Channel (TC), was recognized. This work used high-resolution 3D seismic data to investigate its external morphology, internal architecture and the detailed association with the feeder channel, i.e. Bukuma Channel System (BCS). TC emanated from a slope transition zone of BCS where a sharp bend as well as the channel-floor aggradation increased the instability of BCS and triggered by an outsize flow event, the avulsion happened. The initial flow path of TC appeared to follow discontinuous linear trains of small scours and residual pockmarks; as it evolved, TC developed a very low sinuosity and did not have levees. Preserved cross-sectional profiles of TC changed from dish, deep-U, V to shallow-U shape downstream, which was mainly controlled by increasing then decreasing background slope. Three channel forms were recognized within TC, forming a fining upward sequence, which may be associated with the progressive abandonment of BCS; each channel form also showed a trend of increasing grain size along the flow direction. TC evolved through four stages: Stage 1 was characterized by the interaction of overspilling flow stripped off from BCS with the inherited overbank topography; Stage 2 was featured by the full establishment of a channelized flow pathway and then by deposition of early Channel form 1; Stage 3 was dominated firstly by further development of the flow pathway and then by deposition of latter Channel form 2; Stage 4 occurred in the abandonment phase and referred to the hemipelagic draping on the residual negative relief (Channel form 3). If TC commonly develops so as to fully capture the sediment gravity flows from BCS, architectural records of this type would have low preservation potential. However, the shut-down of BCS make that preservation possible and it provides potentially important insights into submarine channel avulsion processes.