Palaeohydrological characteristics and palaeogeographic reconstructions of incised‐valley‐fill systems: Insights from the Namurian successions of the United Kingdom and Ireland

Namurian (Carboniferous) eustatic fluctuations drove the incision and backfill of shelf‐crossing valley systems located in humid subequatorial regions, which are now preserved in successions of the United Kingdom and Ireland. The infills of these valleys archive the record of palaeoriver systems whose environmental, hydrological and palaeogeographic characteristics remain unclear. A synthesis of sedimentological data from fluvial strata of 18 Namurian incised‐valley fills in the United Kingdom and Ireland is undertaken to elucidate the nature of their formative river systems and to refine regional palaeogeographic reconstructions. Quantitative analyses are performed of facies proportions, of geometries of incised‐valley fills and related architectural elements, and of the thickness of dune‐scale sets of cross‐strata. Reconstruction of the size of the drainage areas that fed these valleys is attempted based on two integrative approaches: flow‐depth estimations from dune‐scale cross‐set thickness statistics and scaling relationships of incised‐valley fill dimensions derived from late‐Quaternary examples. The facies organization of these incised‐valley fills suggests that their formative palaeorivers were perennial and experienced generally low discharge variability, consistent with their climatic context; however, observations of characteristically low variability in cross‐set thickness might reflect rapid flood recession, perhaps in relation to sub‐catchments experiencing seasonal rainfall. Variations in facies characteristics, including inferences of flow regime and cross‐set thickness distributions, might reflect the control of catchment size on river hydrology, the degree to which is considered in light of data from modern rivers. Palaeohydrological reconstructions indicate that depth estimations from cross‐set thickness contrast with observations of barform and channel‐fill thickness, and projected thalweg depths exceed the depth of some valley fills. Limitations in data and interpretations and high bedform preservation are recognized as possible causes. With consideration of uncertainties in the inference of catchment size, the palaeogeography of the valley systems has been tentatively reconstructed by integrating existing provenance and sedimentological data. The approaches illustrated in this work can be replicated to the study of palaeohydrological characteristics and palaeogeographic reconstructions of incised‐valley fills globally and through geological time.