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A Big Fan of Signals? Exploring Autogenic and Allogenic Processes in Lobyte3D, a Numerical Stratigraphic Forward Model of Submarine Fan Development

Abstract

Distinguishing an allogenic signal from trends and patterns produced by autogenic processes is a critical element in interpreting, understanding and predicting strata. Lobyte3D is a new reduced-complexity model of dispersive flow over an evolving topography on fan systems that produces surprisingly complex strata despite a simple formulation. Two submarine fan model scenarios are run, one with constant sediment input, and one with a sinusoidal variation in sediment input. Both model scenarios show that flows cluster to produce lobes which migrate and can rapidly switch location. Runs tests and spectral analysis show strata can be ordered, even in the absence of any allogenic signal, with cycles and trends in bed thickness, but no single characteristic frequency. In the oscillating supply scenario, an allogenic signal is present in places, particularly in the axial mid fan, but may be difficult to distinguish from the autogenic signal without knowing a priori how the allogenic signal is likely to be preserved in complex and incomplete strata. Analysis of mid fan vertical sections, where stratigraphic completeness is relatively high and many flows are likely to be recorded, using simple power spectrum analysis and counting of the significant peaks present across a range of frequencies, may allow identification of a “signal bump” that could be evidence of the presence and nature of allocyclic forcing. However, this also requires a volume of stratigraphic data beyond what is typically collected from outcrop studies.

Even a reduced complexity numerical stratigraphic forward model like Lobyte3D produces stratigraphic behavior more complex than many stratigraphic conceptual models and interpretations account for. Almost certainly real depositional systems are even more complex. This deficit in the complexity of our stratigraphic interpretations and analysis methods needs to be addressed, by revision of existing conceptual models, and perhaps by more integration of outcrop and experimental modelling analysis