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 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 reconstructions of morphology, slope and planform can be obtained from fluvial stratigraphy.