Floodplain topography and avulsion pathfinding control stratigraphic architecture in a numerical model of a fluvial fan

Abstract

Buried channel sand bodies are important reservoirs of subsurface water and energy resources, but their arrangement and interconnectedness are difficult to predict. The dominant process that distributes channels and their sediments in alluvial basins is river avulsion, which occurs when a channel seeks a new location on the adjacent floodplain. Floodplain sedimentation, incision, and channel levee growth influence channel pathfinding during avulsion, and should control key aspects of the stratigraphic arrangement of channel bodies, including compensational (spatially and temporally even) deposition, stratigraphic completeness, and facies distributions; however, this impact has been difficult to isolate in natural and experimental basin fills. To test how different avulsion pathfinding parameters influence stratigraphic architecture, we use a numerical model of a fluvial fan to produce synthetic fluvial stratigraphy under seven different runs with progressively more complex channel pathfinding rules. In the simplest models where pathfinding is set by a random walk, the channel rapidly changes position and avulsions spread across the fan surface. The corresponding deposit is dominated by channel facies, is relatively incomplete, and the compensation timescale is short. As rules for pathfinding become more complex and channels can be attracted or repulsed by pre-existing channels, lobe switching emerges. Deposits become more diverse with a mix of channel and floodplain facies, stratigraphic completeness increases, and the compensation timescale lengthens. Previous work suggests that the compensation timescale is related to the burial timescale and relief across the depositional surface, yet we find that compensation approaches the burial timescale only for model runs with high morphodynamic complexity and relatively long topographic memory. Our results imply that in simple systems with limited degrees of freedom, the compensation timescale may become detached from the burial timescale, with uniform sedimentation occurring quickly relative to long burial timescales.

Additional details