Reconstructing Conditions Driving Bar Migration and Preservation in Alluvial Rivers

Abstract:

Channel planform evolution is largely influenced by the feedback between channel deposit morphology and flow hydrodynamics. This feedback drives changes in the migration, deformation, and bifurcation of bar features in modern braided and meandering systems. Understanding how paleo-sediment flux and channel planform dynamics can be reconstructed from ancient fluvial bar deposits would allow for the development of important constraints on paleochannel dynamics that are, at present, difficult to quantify. Previous work has shown that the preservation and geometry of fluvial bar deposits are useful for reconstructing paleochannel kinematics and the influence of avulsion on stratigraphic architecture. Additionally, studies have shown that flow variability can influence the preservation of channel-bed features. In this work, we use numerical modeling to explore how bar migration and deformation under equilibrium conditions sets baseline probabilities of bar preservation in braided channel networks. We use Nays2DH to simulate channel evolution under a range of sediment-supply and discharge regimes and use modeled channel-bed evolution to generate synthetic stratigraphy, including approximations of channel facies based on local flow depths and bed shear stress conditions. We map synthetic channel deposits in a manner analogous to what can be accomplished in field settings – where stratal terminations and channel facies delineate individual bar packages – and document the degree of bar preservation and persistence in model runs. We compare our model results to mapped bar facies distributions and stratal geometries documenting bar preservation and persistence in several ancient alluvial deposits. Our results indicate that there are distinct differences in bar preservation among ancient deposits that correspond to different flow conditions in our models. In particular, preservation appears highest in systems with variable discharge conditions and in systems where channel avulsion period is short relative to bar turnover timescales.