The morphodynamics of ancient rivers can be reconstructed from fluvial stratigraphy using quantitative techniques to provide detailed insights into the driving forces behind the sedimentary systems. This work explores how these drivers can be evaluated from Paleozoic stratigraphy. Field measurements are taken in fluvial sediments from the Westphalian (Bolsovian and Asturian; 315.2–308 Ma) Pennant Formation of South Wales, UK, to reconstruct the hydrodynamics and morphologies of these Carboniferous rivers, which were sourced from the Variscan (Hercynian) Mountain belt located south of the study area. Field data consist of cross-set heights, grain size, palaeocurrent directions, and the dimensions of fluvial architectural elements. Hydrodynamic properties, including flow velocities and discharge rates, are reconstructed using a suite of numerical approaches. Results suggest median formative flow depths of 2–3 m and palaeoslopes of 4-5 × 10−4 (0.02–0.03°). Quantitative planform prediction suggests these rivers were likely anastomosing but with distinct single-threaded reaches. Mean single-thread width is 55 m, while mean channel-belt widths of 100–200 m are reconstructed, suggesting bankfull discharges of 390–560 m3 s−1. This study resolves contrasting palaeohydrological interpretations for Pennant rivers, and demonstrates how sophisticated reconstructions of morphology, slope and planform can be obtained from fluvial stratigraphy.
Floods determine river behaviour in time and space. Yet quantitative measures of discharge variability from geological stratigraphy are sparse, even though they are critical to understand landscape sensitivity to past and future environmental change. Here we show how storm-driven river floods in the geologic past can be quantified, using Carboniferous stratigraphy as an exemplar. The geometries of dune cross-sets demonstrate that discharge-driven disequilibrium dynamics dominated fluvial deposition in the Pennant Formation of South Wales. Based on bedform preservation theory, we quantify dune turnover timescales and hence the magnitude and duration of flow variability, showing that rivers were perennial but prone to flashy floods lasting 4–16 h. This disequilibrium bedform preservation is consistent across 4 Ma of stratigraphy, and coincides with facies-based markers of flooding, such as mass-preservation of woody debris. We suggest that it is now possible to quantify climate-driven sedimentation events in the geologic past, and reconstruct discharge variability from the rock record on a uniquely short (daily) timescale, revealing a formation dominated by flashy floods in perennial rivers.
Floods determine river behaviour in time and space. Yet quantitative measures of discharge variability from geological stratigraphy are sparse, even though they are critical to understand landscape sensitivity to past and future environmental change. Here we show how climate-driven floods in rivers in the geologic past can be quantified, using Carboniferous stratigraphy as an exemplar. Mass-preservation of woody debris coupled with the geometries of dune cross-sets demonstrate discharge-driven disequilibrium dynamics dominated fluvial deposition. Based on preserved bedforms, we quantified the magnitude and duration of flow variability, showing that rivers were perennial but prone to flashy floods lasting 4-16 hours. This is the largest stratigraphic interval over which disequilibrium bedform preservation has been documented and it demonstrates how climate-driven sedimentation events can be quantified in the geological past. We argue that signals of flooding may be ubiquitous but under-recognised in the rock record, indicating a significant preservation bias.
The morphodynamics of ancient rivers can be reconstructed from fluvial stratigraphy using quantitative techniques to provide insights into the driving forces behind sedimentary systems. This study explores how these drivers can be evaluated from Paleozoic stratigraphy. Field measurements are taken in fluvial sediments from the Westphalian (Bolsovian and Asturian; 315.2–308 Ma) Pennant Formation of South Wales, UK, to reconstruct the hydrodynamics and morphologies of these Carboniferous rivers, which were sourced from the Variscan (Hercynian) mountain belt located south of the study area. Field data consist of cross-set heights, grain size, palaeocurrent directions and the dimensions of fluvial architectural elements. Hydrodynamic properties, including flow velocities and discharge rates, are reconstructed using a suite of numerical approaches. Results suggest median formative flow depths of 2–3 m and palaeoslopes of 4–5 × 10 −4 m m −1 (0.02–0.03°). Quantitative planform prediction suggests that these rivers were probably anastomosing but with distinct single-threaded reaches. Mean single-thread width is 55 m, whereas mean channel-belt widths of 100–200 m are reconstructed, suggesting bankfull discharges of 390–560 m 3 s −1 . This study resolves contrasting palaeohydrological interpretations for Pennant rivers, and demonstrates how quantitative reconstructions of morphology, slope and planform can be obtained from fluvial stratigraphy. Supplementary material: Field data, a field localities KMZ file, analysis of flow depth scaling methods and fluvial facies analysis are available at https://doi.org/10.6084/m9.figshare.c.6131975
<p>The extent to which the stratigraphic archive preferentially preserves the record of large events such as floods remains contentious. While qualitative approaches exist to address this problem, the way in which disequilibrium morphodynamics is preserved quantitatively in fluvial strata has only recently begun to be investigated. While existing process&#8211;product relations for bedform preservation often assume that fluvial cross strata reflect steady-state formative conditions, i.e., bedform evolution equilibrated with the prevailing flow, theory indicates that bedforms may be preferentially preserved in unsteady, or disequilibrium, conditions. Here we test this concept using field data collected from fluvial stratigraphy in the Upper Cretaceous of Utah, USA (Ferron Sandstone and Blackhawk & Castlegate Formations) and the Upper Carboniferous of South Wales, UK (Pennant Formation).</p><p>For the US field site, we systematically measured preserved cross-set heights (n = 417) for all three formations, and we observed unanimously low coefficients of variation (<em>CV</em>) across individual co-sets and at a population level (<em>CV</em> = 0.25&#8211;0.5). These values are inconsistent with bedform preservation in steady-state conditions (<em>CV</em> = 0.88&#177;0.3), and instead point to bedform preservation in disequilibrium conditions. Similarly in the UK field site, the CV of cross-set height distributions average 0.4, significantly less than the theoretical value for steady-state deposition. In both cases these low values are ubiquitous throughout the stratigraphy studied.</p><p>Two independent hypotheses can explain our field observations: (1) short flood recessions, relative to bedform turnover timescale, in flashy flood hydrographs (<em>flood hypothesis</em>); (2) dune evolution in the presence of barforms (<em>hierarchy hypothesis</em>). However, in the Pennant Formation qualitative facies-based evidence such as storm beds containing large woody debris independently demonstrate that flood events clearly did occur. We therefore used our constraints on cross-set size and grain-size to calculate dune height, wavelength and unit bedload flux, in order to quantify bedform turnover timescale. Under the flood hypothesis, our field data are consistent with enhanced bedform preservation driven by flashy flood hydrographs with a duration of a few hours to a few days for both data sets. These durations are consistent with perennial rivers subject to torrential rains and storms. Under the hierarchy hypothesis, our field results would suggest bedform climb angles of 10<sup>&#8722;</sup><sup>2</sup> to 10<sup>&#8722;</sup><sup>1</sup>, and would require rapid bar migration relative to dune migration. We use architectural and palaeohydrological techniques to estimate the size and discharge of the floods that may have formed these deposits and we evaluate the extent to which it is now possible to extract information on flood variability from ancient sedimentary rocks.</p><p>&#160;</p>
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