Deep-Water Fan Hierarchy: Assumptions, Evidence, and Numerical Modelling Analysis

Abstract

Submarine fan strata are commonly described and 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 rarely evidence that the model or method accurately reflects the true nature of the strata. To develop more robust understanding of hierarchy in submarine fan strata we developed two metrics, a clustering strength metric that measures how much clustering is present in the spatial distribution of beds, 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. The first model is a very simple geometric model with 10 realisations ranging from a perfectly clustered hierarchy to an indistinguishable-from-random arrangement of beds. The second model, Lobyte3D, is a reduced-complexity process model which uses a st eepest descent flow routing algorithm, combined with a simple but physically reasonable representation of flow velocity, erosion, transport and deposition thresholds, to generate 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 currently unidentified processes responsible for producing hierarchy, or that interpretations of hierarchy in submarine fan strata are not realistic.