Abstract

River avulsion, or the rapid relocation of a channel, is a fundamental process in alluvial basins that can proceed in different styles, including via progradation (where a wedge of sediment is built out onto the floodplain) or incision (where floodplain erosion channelizes flow and captures the main channel). The tendency for a floodplain to erode or deposit sediment exerts control over avulsion style, but the influence of floodplain topography on avulsion styles has not been quantified, in part because floodplain topography is rarely preserved in the rock record. Gilgai topography is a patterned mound-and-depression topography associated with Vertisol floodplains that has a unique stratigraphic signature due to bowl-shaped slickensides that form in the subsurface. In this study, we analyze gilgai topography and avulsion style in two ancient deposits (the Devonian Catskill Formation, Pennsylvania, USA, and the Cretaceous Cedar Mountain Formation, Utah, USA). We show that 100% of incisional avulsions observed in both formations occur over paleogilgai topography, whereas most progradational avulsions are not associated with gilgai topography.

Additionally, we build on a published Delft3D-FLOW avulsion model to test how gilgai topography could influence avulsion style. Control models contain floodplain topography with randomly distributed Gaussian bumps, which we compare to gilgai runs, which have uniform mound-and-depression topography in the model floodplain. Noncohesive sediment size (125–750 μm) and critical shear stress of cohesive sediment (0.275–0.185 N/m²) are varied between model runs to simulate a range of progradational or incisional baseline conditions. Model results show that gilgai models deposit less sand on the floodplain than control models, due to increased topographic roughness from the gilgai mounds and because avulsion channels have high, inefficient bifurcation angles compared to control runs. Model results show that the avulsion style is less progradational in models with gilgai, which is consistent with the outcrop results. Together, these lines of evidence suggest that organized floodplain topography can measurably influence avulsion style.