RECONSTRUCTING FLUVIAL FLOW AND SEDIMENT TRANSPORT CONDITIONS FROM PRESERVED CROSS-SET GEOMETRIES IN THE SEDIMENTARY RECORD

Critical to understanding past Earth and planetary surface processes is an ability to quantitatively reconstruct surface kinematics from the sedimentary record. Fluvial strata comprise a significant component of the record of Earth’s terrestrial environments, and have also been recognized on Mars. Cross sets are nearly ubiquitous in fluvial strata and provide a detailed record of the short-timescale evolution of bedforms, which in turn respond to flow and sediment transport conditions at the bed. A lack of quantitative links between flow conditions, sediment and bedform dynamics, and the generation of stratigraphic architecture, however, has hindered the direct determination of fluvial conditions from preserved cross-set geometries. Here we present results of theoretical modeling and experimental investigations showing a direct relationship between sediment hydrodynamics, rates of bedform migration (which quantify the translation of bedforms) and deformation (change in shape of translating bedforms), and curvature of cross set bounding surfaces. We show that reach slope exerts a dominant control on bedform migration rate, and that the ability of flow to suspend sediment (parameterized by the Rouse number) exerts a dominant control on the distribution of deformation rates of bedforms. These relationships are combined with previous work towards a model for calculating skin friction shear velocity from curvature of bounding surfaces of preserved cross strata. Combining our findings with previous models for reconstructing paleoslope, given assumptions of normal flow, results in an inverse model from which flow depth (stage) variability can be calculated from the stratigraphic record. All model input variables are directly observable from outcrop, including grain size, bankfull flow depth (e.g., from preserved barforms), and cross set curvature. Further work is underway to describe the nature of this relationship under aggrading-bed conditions, such that this method will be generalizable to encompass the span of sand-bed fluvial conditions. Our findings will likely enable quantitative reconstruction of past landscape behavior and paleoflow conditions, including hydrograph variability, bedform dynamics, sediment transport rates, and short-timescale bed aggradation rates on Earth and Mars.