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Evolution, Architecture and Stratigraphy of Froude Supercritical Submarine Fans


Sandy submarine fans from steep tectonically active margins were once considered a universal fan type, but improved geophysical imaging has revealed that the motif is not universal and differs from passive margin fans dominated by muddy levees developing on lower slopes. ‘Steep’ fans are usually small (< 10km), a fact attributed to coarse sediment sourced from steep short catchments, but could alternatively be explained by hydraulic jumps which limit channel extension or short avulsion lengths associated with steeper slopes. Typical geometries include large-scale backsets as a lobe is propagated upstream by a migrating hydraulic jump. In cross section this process produces a simple mounded lobe superimposed on a channel geometry typically referred to as a ‘steershead’. The model is tested on examples of ‘steep’ fans from outcrop and high resolution seismic including Brushy Canyon (Texas), Ainsa (Spain), Tabernas/Sorbas (Spain), Karoo (RSA), Golo (Corsica) and East Breaks (GOM). Supercritical bedforms and bedform sequences observed are consistent with the interpretation of supercritical fans. Interesting questions with important implications for reservoir architecture include: do supercritical fans represent an architectural archetype?; to what degree does submarine fan architecture depend on source grain size versus differences in local slope and related hydraulic regime (e.g. Froude regime)?; to what degree can fans be described by steady and more continuous mesoscale morphodynamic processes rather than discontinuous (unsteady) flows?; is it possible that the structure of ‘steep’ fans is more a function of supercritical flow and is relatively robust to different grain sizes so long as there is sufficient bedload to amplify hydraulic instabilities like hydraulic jumps?; conversely, is levee growth typically related to subcritical flow in the channel and backwater that pushes suspended sediment overbank?