Preserved Fluvial Cross Strata Record Bedform Disequilibrium Dynamics

Fluvial cross strata are an archive of ancient river morphodynamics. Controls on their preservation are well documented for bedforms equilibrated with prevailing flows; however, variability in water and sediment discharge results in bedforms that are routinely out of phase with flow conditions-termed bedform disequilibrium. Using theory and experiments of dune evolution in steady and unsteady flows, we show that preservation of cross strata is greater during unsteady flows-particularly during flood recession-when bedform disequilibrium is prevalent. The enhanced preservation of disequilibrium dynamics is associated with a high bedform preservation ratio, which results from localized increase in sedimentation rates relative to bedform migration rates due to the hierarchical bedform organization during flood recession. We find that the coefficient of variation of cross sets reflects the ratio of the time scales of formative-flood variability and bedform adjustment. We parameterize bedform disequilibrium for rivers globally and suggest that flood variability is a key control on cross set preservation.