Rivers of the Variscan Foreland: fluvial morphodynamics in the Pennant Formation of South Wales, UK

Abstract

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