New models for submarine channel deposits on structurally complex slopes: Examples from the Niger delta system

摘要

Submarine channel complexes are often described as having a two-phase stratigraphic evolution where an initial phase of migration is followed by aggradation, generating a 'hockey-stick shaped' channel trajectory. However, the role of tectonic forcing in modifying time-integrated sedimentary architectures remains poorly understood. Here, we evaluate how tectonically driven changes in slope modify the evolution-both in terms of morphology and stratigraphic architecture-of submarine channels across a range of spatial scales from the fundamental architectural unit, a channel element, to the scale of a channel complex set, using examples from the Niger Delta system. From a 3D, time-migrated seismic reflection volume, we use amplitude extractions, frequency decomposition and RGB blending to determine channel stratigraphic architectures. These observations are used systematically to evaluate the development of cross-sectional and planform architectures as the channel systems interact with a range of active and pre-existing structural bathymetry. Our results indicate that while a channel complex's stratigraphic architecture may be captured by a two-phase evolution on unstructured slopes, this model fails on structurally complex slopes. Unstructured slope channel complexes display a repeated arrangement of migration dominating the early stratigraphic record and subsequent aggradation. The late aggradational phase signals a decrease in the rate of growth in channel complex width and the rate of change in sinuosity relative to aggradation throughout the complex's development. However, tectonically driven changes in sinuosity and the relative rates of channel migration and aggradation modify complex development significantly. We identify three end-member styles of channel-structure interaction, determined by the timing of bathymetry development and its associated style: (1) pre-channel structural bathymetry; (2) coeval positive relief, and (3) coeval negative relief. Where structural relief pre-dates channel inception, a principal adjustment is in the initial channel course with early channel elements being forced around positive relief of the structure, generating long-wavelength bends in the complex's course. Where structure continues to modify slope creating positive and negative bathymetry during complex development, migration and bend development continue with complex width and channel element sinuosity increasing until abandonment. These observations demonstrate that submarine channel architecture and planform are highly sensitive to tectonic perturbation and we use these results to generate graphical models that show predicted architectural evolution of submarine channels on structurally complex slopes in general.