Quantitative Constraints on Paleohydrology and Flood Variability in Ancient Rivers

Reconstructing the morphodynamics of ancient river systems is vital to evaluate the movement of water and sediment across the surface of the Earth in the geologic past. At the same time, quantification of discharge variability is necessary to constrain how sedimentary systems record environmental change. Here we show how these drivers can be evaluated for Paleozoic fluvial stratigraphy, represented by the regionally-significant upper Carboniferous Pennant Formation of Wales, UK, which drained the Variscan (Hercynian) mountain belt located south of the study area. We collected a wide range of measurements including cross-set heights, grain size, paleocurrent directions and the dimensions of architectural elements. Hydrodynamic properties are reconstructed using a suite of numerical approaches. Results suggest formative flow depths of 2-3 m and paleoslopes of 0.02-0.03°. Quantitative planform prediction suggests these rivers were likely anastomosing but with distinct single-threaded reaches and bankfull discharges of 390-560 m3 s-1. We also show the formation contains abundant fossilised woody debris in channel floor deposits, attributed to flood events which entrained logs (Calamites and Lepidodendron) up to 2 m long. Furthermore, the low coefficient of variation of dune cross-set height (CV) throughout the formation is inconsistent with the theoretical range for steady-state deposition (CV = 0.88 ± 0.3), suggesting that disequilibrium bedform dynamics prevailed in these rivers. We estimated the magnitude and duration of flood events, with bedform turnover timescales reconstructed as 1 - 3 days, implying prevailing flow durations of 2 - 7 hours. We conclude Pennant rivers were likely perennial but prone to flashy flood hydrographs, which resulted in enhanced bedform preservation and the accumulation of large volumes of woody debris. Our data represents the largest stratigraphic interval (>1.3 km) over which flood-driven discharge variability has been quantified, and we suggest that disequilibrium bedform preservation may be under-recognised in the geologic record.