Dominic A. Armitage

Postavulsion channel evolution: Niger Delta continental slope

Channel avulsion is fundamental in defining submarine fan morphology yet, as a process, is poorly understood. The postavulsion evolution of five channel-levee systems, documented from both the shallow subsurface and the sea floor, is marked in the early stages by relatively wide axial channel belts containing sinuous channel elements. The axial channel belt in each system narrowed through time in association with levee aggradation, which resulted in increased channel confinement. Of the five systems studied, four avulsed from a radial avulsion node at the mouth of the basin feeder-channel complex, which is the entry point to the basin. Only one avulsion occurred at an avulsion node downflow of the mouth of the feeder-channel complex. The degree of channel instability in three of the four systems before an avulsion event was increased by channel-floor aggradation caused by the backfilling of channel-confined turbidity current deposits. Channel-floor aggradation reduced the confinement relief of the systems, thereby increasing the probability of avulsion during an outsized flow event. The backfilled deposits in the channel belts display relatively high seismic-reflection amplitudes inferred to be coarser grained (more sand rich) than their surroundings, that is, out-of-channel deposits. Overbank cyclic steps are exceptionally well preserved on subsurface levees, and their potential function in promoting an avulsion event is discussed. The actual process of avulsion is caused by the flow itself instead of a reduction in confinement relief, and although outsized flows are the likely trigger, depending on the degree of this relief in the channel, multiple small flows could also be responsible for levee breaching, resulting in avulsion. The process of channel-system evolution resulting in avulsion can be applied to other subsurface data where compensating high-amplitude channel belts are recognized. In the context of hydrocarbon exploration, investigating up depositional dip to identify avulsion nodes increases the chance of locating sand-rich deposits, especially where multiple channels converge on one point.