UNDERSTANDING THE IMPACTS OF AUTOGENIC PROCESSES ON STRATAL ARCHITECTURE USING A 2D DIFFUSIVE SEDIMENT TRANSPORT MODEL

Classical sequence stratigraphy defines sequences and systems tracts based on allogenic controls (eustasy, sediment supply, and subsidence) on accommodation and deposition. However, more recent findings show that autogenic processes (e.g., channel avulsion and delta lobe switching, shoreline auto-retreat) can interact with, modify, and even eliminate allogenic signals. Here, we identify specific ways autogenic processes affect the timing, extent, and architectural evolution of systems tracts relative to allogenic processes. Our findings are based on experimental strata generated in the St. Anthony Falls Laboratory’s Experimental EarthScape basin (XES-02) under constant sediment supply and subsidence rates but numerous cycles of varying base level, which we compare against numerically modeled strata simulated under the same allogenic initial and boundary conditions but assuming wide-spread, non-autogenic 2D diffusive sediment transport as in classical sequence stratigraphy. We find that the autogenically-influenced strata created in the XES basin differs from the allogenically-driven, diffusively-modeled strata primarily due to lateral channel movement and sediment storage and release on the basin topset region. We then succeed in getting the numerical model to reproduce this same style of strata formation by altering the diffusion coefficients such that (1) diffusion increases with cell steepness, (2) downstream diffusion is four times greater than cross-stream diffusion, and (3) there is no diffusion outside of topographic lows. Similar to the experiment, the revised diffusion model produces avulsions over base level change; timings of systems tracts development that can differ from the classical sequence stratigraphic model (e.g., transgression begins earlier, in place of lowstand normal regression, because autogenic processes store sediment in updip channels such that progradation does not occur at the shoreline); and diachronous stratal surfaces from sediment storage and release events that create localized variations in deposition and erosion across the basin topset.