Quantifying the influence of transport stage on the geometry of preserved fluvial cross strata

Fluvial cross strata are a pervasive constituent of alluvium on Earth and planetary bodies. They record bedform evolution and encode paleoflow and sediment transport conditions. Existing work primarily describes the relation between preserved cross strata, bedform geometry and flow depth. Dune dimensions, however, vary significantly with transport stage (ratio of the dimensionless bed shear stress to the Shields stress) but our understanding of how sediment transport conditions affect bedform preservation remains limited. Here, we analyzed a series of 12 experiments of steady-state bedform evolution conducted in a 15-m-long, 1-m-wide flume, with flow depths of 15, 20 and 25 cm, and transport stages ranging from threshold-dominated (transport stage: 4.4) to suspended-load-dominated conditions (transport stage: 26.5). For each experiment, we used the high-resolution bed elevation data to quantify scour depthsthe bedform trough elevation relative to the mean detrended bed profile. We stacked the sequential bed elevation maps to generate synthetic strata, and measured the preserved set thickness as the vertical distance between two consecutive erosional boundaries. Results demonstrate that mean scour depth and the mean preserved set thickness have a parabolic functional dependence on the transport stage across all flow depths, with the maximum scour depth for a given flow depth occurring during mixed-load (transport stage: 13-17) conditions. We show that a two-parameter Gamma distribution approximates the trough depth distribution, and the variability-dominated preservation model of Paola and Borgman (1991) relates the formative trough depth distribution with the preserved set thickness distribution. Our results also reveal that the ratio of the mean set thickness to the mean bedform heights only asymptotically approach the theoretical value of 0.25 to 0.45 at high transport stages. The results demonstrate that cross-stratal geometry varies with transport stage and existing models can overestimate paleoflow depth at low transport stages. The results provide a new quantitative basis for refining paleoflow depth reconstruction from deposits and additionally enable, for the first time, an assessment of bed material transport rates in ancient rivers from the preserved fluvial cross strata.