The thickness variability of fluvial cross‐strata as a record of dune disequilibrium and palaeohydrology proxy: A test against channel deposits

Abstract

Strata produced by fluvial dunes can provide insight into the hydrological regime of ancient rivers. Recent experiments indicate that conditions of disequilibrium between bedforms and formative flows may be inferred from the coefficient of variation of preserved dune cross‐set thickness, suggesting that this quantity may act as a proxy for the flashiness of river floods relative to the time required for full bedform translation. To assess whether this idea is applicable to interpretations of the stratigraphic record, this study examines published data relating to more than 2600 cross‐sets from 53 sedimentary units of 19 river systems. The presented analyses must not be over‐interpreted, because the considered rivers span different environmental settings, the data sources are heterogeneous in terms of type and dimensionality, and some variables were established by applying empirical relationships. Yet, significant findings are revealed. Larger rivers exhibit discharge and bedform characteristics that are more conducive to disequilibrium; however, a modest increase in the coefficient of variation of cross‐set thickness, CV(Dst), as opposed to the expected decrease, is seen as a function of river size. Crucially, smaller CV(Dst) values are not systematically associated with conditions that should favour dune disequilibrium. Meanwhile, only ca 25% of the studied examples demonstrate cross‐set thickness statistics compatible with quantitative formulations of the autogenic control by variable dune topography – the notion of ‘variability‐dominated’ preservation. These findings indicate that the variability in cross‐set thickness may be a poor predictor of discharge variability, perhaps because of the multiplicity of factors controlling dune preservation, such as bedform hierarchy, transport stage and depth‐dependent variations in dune disequilibrium. To improve interpretations of cross‐stratified deposits, further research is needed: (i) to establish the value of process‐to‐product models for reverse product‐to‐process interpretations; and (ii) to define representative samples for preserved dune deposits accounting for temporal and spatial variability in preservation potential.

Timescales of Autogenic Noise in River Bedform Evolution and Stratigraphy

“…Together, these results provide a mechanistic basis for the findings of Colombera et al. (2024), who, in their meta‐analysis of modern river deposits, identified a lack of correlation between T f normalized by $$\overline{{T}_{t,H}}$$ and preserved set thickness. This lack of correlation can be indicative of varying degrees of flood signal degradation, where floods with $$\overline{{T}_{t,H}}< {T}_{f}< \overline{{T}_{t,L}}$$ have a lower degradation potential than $${T}_{f}< \overline{{T}_{t,H}}$$, and only floods with $${T}_{f} > \overline{{T}_{t,L}}$$ are expected to be minimally degraded (Figure 2).…”

“…We anticipate this prolonged transition to the white-noise regime in preserved deposition rates because scours bracketing bedform groups can cause hiatuses in the preserved record, spanning multiple bedform-group turnover timescales. Together, these results provide a mechanistic basis for the findings of Colombera et al (2024), who, in their meta-analysis of modern river deposits, identified a lack of correlation between T f normalized by T t,H and preserved set thickness. This lack of correlation can be indicative of varying degrees of flood signal degradation, where floods with T t,H < T f < T t,L have a lower degradation potential than T f < T t,H , and only floods with T f > T t,L are expected to be minimally degraded (Figure 2).…”