Deep-Water Fan Hierarchy Interpretations are Perhaps Unrealistic: Evidence from Numerical Modelling Analysis

Submarine fan strata are commonly interpreted assuming a nested, hierarchical organisation of elements, from beds, to lobe elements, lobes and lobe complexes. However, describing outcrop and subsurface strata following a particular conceptual method or model is never evidence that the model or method accurately reflects the true nature of the strata. To develop better understanding of robust hierarchy identification and measurement we developed two metrics, a clustering strength metric that measures how much clustering is present in the spatial distribution of beds on a submarine fan, and a hierarchy step metric that indicates how many clustered hierarchical elements are present in the bed spatial distribution. Both metrics are applied to two quantitative fan models, a very simple geometric model with 10 realisations ranging from a perfectly clustered hierarchy to an indistinguishable-from-random arrangement of beds, and Lobyte3D a reduced-complexity process model that generates detailed 3D representations of submarine fan strata. Application of the cluster strength and hierarchy step metric to the simpler model demonstrates how the metrics usefully characterise how much order and hierarchy is present in the fan strata. Application to four Lobyte3D models with increasingly complex basin-floor topography shows no evidence for true hierarchy, despite clear self-organisation of the model strata into lobes, suggesting that either Lobyte3D is missing key, as yet unidentified processes responsible for producing hierarchy, or, perhaps more likely, that hierarchal interpretations are not realistic.