Preserved Fluvial Cross Strata Record Bedform Disequilibrium Dynamics

• Published in: Geophysical research letters 2020-01, Vol.47 (2), p.n/a
• Main Authors: Leary, Kate C. P., Ganti, Vamsi
• Format: Article
• Language: English
• Subjects: Bed forms; bedform hysteresis; Coefficient of variation; cross‐stratified sandstones; deep time; Dunes; Dynamics; Earth; Evolution; Flood control; Flood discharge; Flood variability; Floods; Fluvial sediments; Fluxes; Hydrodynamics; Hydrologic cycle; Hydrological cycle; Hydrology; Laboratory experiments; Mars; Mars surface sediments; Morphology; Mountains; Organizations; Physiographic features; Preservation; Recession; Recessions; River beds; Rivers; Rocks; Sandstone; Sediment; Sediment discharge; Sedimentary structures; Sedimentation; Sedimentation rates; Steady flow; Strata; Unsteady flow; Variability; Water discharge
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL085910

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. Plain Language Summary Undulating topographic features such as ripples, dunes, and bars—called bedforms—abound on riverbeds worldwide. Bedform migration results in cross‐stratified rock units, which are ubiquitous sedimentary features on Earth and Mars. Cross‐stratified sandstones provide a window into the hydrological cycle as well as the mass fluxes that rivers carried from mountains to the sea in ancient environments. Consequently, controls on the preservation of cross strata have been studied for over a century, but the majority of this work focused on bedform evolution in steady flows. Floods, however, cause water and sediment discharge to vary continuously in rivers, which significantly affect bedform evolution. Despite this recognition, we currently lack a quantitative understanding of the influence of flow variability on fluvial cross strata. We addressed this knowledge gap using theory and laboratory experiments. We find that the preservation of cross strata preferentially occurs during flood recession, even if this duration is a small fraction of total time. We uncovered statistical metrics that can help decipher the nature of flood discharge variability from the geometry of cross‐stratified rock units, with implications for quantifying river dune dynamics and flood variability on ancient Earth and Mars. Key Points Cross strata are dominantly built dur