RECONSTRUCTING PAST FLUVIAL FLOW AND SEDIMENT TRANSPORT CONDITIONS FROM THE GEOMETRY OF BEDFORM CROSS SETS IN THE STRATIGRAPHIC RECORD

The stratigraphic record is the principle archive of past Earth surface processes in depositional environments. Bedform cross sets are a nearly ubiquitous feature 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. Sediment transport intermittency (described by the shear velocity) and the flow’s ability to suspend sediment (described by the Rouse number) are shown to exert a dominant control on the distribution of deformation rates. 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. Ongoing 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.