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Impact of Non-Uniqueness for Sequence Stratigraphy


Understanding the controls on formation of stratal architectures is important, particularly for predictions of sediment bypass and reconstructions of relative sea-level histories. Sequence stratigraphic methods are commonly applied to extract information such as accommodation and sediment supply histories from observed stratal geometries. For example, stratigraphic surfaces are observed from subsurface and outcrop data, and controlling factors are deduced. However, sequence models are inverse methods containing simplifying assumptions such as constant sediment supply, therefore application of such models will commonly produce non-unique results. This is because different combinations of controlling factors can produce the same (non-unique) stratal geometry. Non-unique stratal geometries can also result from entirely different processes. In contrast, a unique stratal geometry is demonstrably different from other stratal geometries and results from only one set of controlling variables with specific parameter values. This work uses a diffusional stratigraphic forward model to demonstrate how four stratal geometries commonly recognised in the ancient record can be non-unique. For example, results suggest that a maximum flooding surface is an example of a non-unique stratal geometry. A flooding surface develops due to an increase in the rate of accommodation creation during steady sediment supply, and due to a reduction in sediment supply during a steady increase in accommodation. There is also evidence from modelling results to imply that topset aggradation is non-unique in the sense the significant fluvial deposition can occur during both relative sea-level rise and fall. Results suggest that shoreline trajectories and sequence boundaries are also important examples of non-uniqueness. Modelled stratal geometries have been tested and verified with additional three-dimensional model runs and supported by results from an alternative stratigraphic forward model. Non-unique stratal geometries presented in this work have important implications for sequence stratigraphy. The results suggest that correlation of stratigraphic surfaces and predictions of volume and style of sediment bypass may be more complex than is often assumed. In a broader sense, the results highlight the challenge of non-uniqueness for sequence stratigraphy and highlight the limitation of conventional sequence stratigraphic methods for extracting reliable information from the ancient record.