Simple model, complex strata; 2D numerical forward model analysis of heterogeneity and controls in submarine fan systems

Turbidite strata stacking patterns are interpreted as products of either allogenic forcing, autogenic processes, or a combination of both. However, the relative influence of each remains difficult to constrain. Here, a simple reduced complexity 2D numerical forward model (Lobyte2D) simulates gravity flows passing down a slope, running out and depositing across a flat basin floor. Flows can erode, bypass or deposit, cumulatively producing a variety of entirely autogenic retrograding, aggrading and prograding strata with stratigraphic completeness values ranging from less than 1% to around 40%. Complexity of modelled strata is measured with a spatial entropy metric, and sensitivity analysis indicates that grain size and flow acceleration parameters are the key controls on stratal complexity: larger grain sizes are associated with deeper erosion, which makes more rugged topography, and higher values of flow acceleration make flow speeds more sensitive to topography, which triggers localized deposition/erosion. Such complexity produced by a simple two-dimensional numerical forward model suggests even more complex behaviour is likely in natural systems, and this should be reflected in outcrop and subsurface interpretations. However, comparison of geometries in chronostratigraphic and cross-section plots of modelled strata shows that, due to a variety of cryptic bypass and erosion surfaces, stacking trends visible in chronostratigraphic plots are much more difficult to detect in outcrop and sub-surface cross-sections and vertical sections. These insights suggest that many outcrop and subsurface interpretations of submarine fan strata, particularly sequence stratigraphic interpretations, may be missing substantial complexity, and underestimating the uncertainty inherent in limited data.