Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria

Terraces on the modern seafloor are defined as topographically flat areas above the active submarine channel thalweg but within the confines of the channel-belt. They have been described from many modern submarine channels, but the controls on terrace distribution, evolution and stacking patterns are not well understood. In this study, we describe the architecture of the Mahin and Avon channel-belts and their associated terraces, located offshore Nigeria towards the northwest of the Niger Delta. The studied channel sections are < 10 km apart up-dip and converge downslope. They are on slopes with similar gradients, yet they have significantly different morphologies indicating that the interplay between sedimentary processes and channel evolution must be different. 

The surfaces defining the bases of the terrace bodies have been mapped along both channels using high-resolution 3D seismic data. Spectral decomposition of the data reveals subtle variations in seismic character that highlight sedimentological detail that can otherwise not be recognized, allowing us to suggest the processes responsible for terrace formation and terrace body composition. 

The contrasting evolution of the two channels is reflected in the morphology and architecture of their terraces. While the Mahin terrace bodies show a predictable pattern, typically consisting of stacked channel-fill and overbank deposits in a circular planform shape (shape controlled by cut-off channel bends), the deposits of the Avon terrace bodies up-dip of the confluence with the Mahin are dominated by overbank deposits, with the planform terrace shape heavily controlled by the topography of the underlying channel-belt deposits.

This study shows how spatially and temporally associated channels can have markedly different architectures. The evolution of the channel and the abundance and stacking patterns of compositional elements within terrace bodies are shown to be linked.