Reconstructing Past Fluvial Flow Conditions from the Geometry of Bedform Cross Sets in the Stratigraphic Record

Abstract:

The stratigraphic record is a principle archive of past Earth surface processes in depositional environments. Bedform cross sets are nearly ubiquitous in sandy fluvial strata and provide a detailed record of bedforms as a function of flow and sediment transport conditions over short timescales. Here we present results of theoretical development and empirical relations linking the streamwise curvature of cross set bounding surfaces to (A) reach-averaged sediment hydrodynamics, (B) rates of bedform migration, and (C) rates of bed deformation, the stochastic changes in shape of bedforms in their downstream-migrating reference frame. Empirically, reach slope explains approximately 75% of the variance of characteristic dune velocity under normal flow conditions. Theoretically, the ability of flow to suspend sediment exerts a dominant control on the standard deviation of deformation rates; a finding supported by some empirical validation. Combining these relationships results in a forward model defining the curvature of cross set bounding surfaces as a function of formative sediment hydrodynamic conditions. When applied in conjunction with previous inverse models for estimating paleoslope from strata, a potential inverse model is developed from which flow stage variability and sediment transport conditions may be reconstructed from the stratigraphic record. Further experimental work is underway to constrain constants and uncertainties in the proposed models, as well as to describe the nature of these relationships under three-dimensional transport and aggrading-bed conditions. Empirical validation of inverse models will assess the potential for quantitative reconstruction of past landscape behaviour and paleoflow conditions, including hydrograph variability, bedform dynamics, sediment transport conditions, and short-timescale bed aggradation rates.The stratigraphic record is a principle archive of past Earth surface processes in depositional environments. Bedform cross sets are nearly ubiquitous in sandy fluvial strata and provide a detailed record of bedforms as a function of flow and sediment transport conditions over short timescales. Here we present results of theoretical development and empirical relations linking the streamwise curvature of cross set bounding surfaces to (A) reach-averaged sediment hydrodynamics, (B) rates of bedform migration, and (C) rates of bed deformation, the stochastic changes in shape of bedforms in their downstream-migrating reference frame. Empirically, reach slope explains approximately 75% of the variance of characteristic dune velocity under normal flow conditions. Theoretically, the ability of flow to suspend sediment exerts a dominant control on the standard deviation of deformation rates; a finding supported by some empirical validation. Combining these relationships results in a forward model defining the curvature of cross set bounding surfaces as a function of formative sediment hydrodynamic conditions. When applied in conjunction with previous inverse models for estimating paleoslope from strata, a potential inverse model is developed from which flow stage variability and sediment transport conditions may be reconstructed from the stratigraphic record. Further experimental work is underway to constrain constants and uncertainties in the proposed models, as well as to describe the nature of these relationships under three-dimensional transport and aggrading-bed conditions. Empirical validation of inverse models will assess the potential for quantitative reconstruction of past landscape behaviour and paleoflow conditions, including hydrograph variability, bedform dynamics, sediment transport conditions, and short-timescale bed aggradation rates.