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

Abstract

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, importance of 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.

Drainage area estimates for synorogenic clastic wedges in the Central Appalachian Basin (sink) with implications for terrane accretion in the hinterland (source)

“…The source is linked to the depositional sink via a transfer zone (Schumm, 1977), but the extent of the transfer zone and the nature of the source remain elusive as few geomorphic elements of the hinterland are preserved. During the past decade, source‐to‐sink studies have been used, for example, to: (1) reconstruct palaeogeography, compositions and ages of source regions; (2) estimate drainage basin areas (km 2 ) and bankfull discharge (m 3 s −1 ); (3) calculate sediment yields and denudation rate; and (4) document changing fluvial styles through time (e.g., Bhattacharya et al, 2016; Blum et al, 2013; Buller et al, 2018; Davidson & Hartley, 2010; Eriksson & McClung, 2021; Eriksson & Romans, 2017; Ielpi et al, 2017; Sømme et al, 2009; Walcott & Summerfield, 2009; Wang et al, 2020).…”

The thickness variability of fluvial cross‐strata as a record of dune disequilibrium and palaeohydrology proxy: A test against channel deposits

“…Leclair & Bridge, 2001) to make interpretations of river palaeohydraulics and palaeohydrology for ancient successions (cf. Wang et al ., 2020; Hartley & Owen, 2022; Lyster et al ., 2022; McLeod et al ., 2023).…”

Field evidence for disequilibrium dynamics in preserved fluvial cross-strata: A record of discharge variability or morphodynamic hierarchy?

“…Bedforms are readily formed on riverbeds across a range of grain sizes (e.g., Best, 2005) and their evolution generates cross-stratification -the resultant cross strata are a fundamental building block of alluvium on planetary surfaces (e.g., Allen, 1982;Edgar et al, 2018). Cross strata provide a window to formative conditions in ancient fluvial systems and are routinely used to reconstruct morphologies and hydrodynamics (Holbrook & Wanas, 2014;Wang et al, 2020;; for instance, measurements of cross-set heights provide a mechanism to estimate the sizes of dunes active on ancient river beds and, therefore, palaeoflow depths . Moreover, bedform kinematics and geometries respond to spatial and temporal changes in flow and sediment transport conditions (e.g., Ten Brinke et al, 1999;Wu et al, 2020), and recent research has highlighted that these changes may be recorded in preserved cross-set geometries ).…”

“…Further, these models predict that the coefficient of variation, CV, of cross-set heights has a constant value of 0.88 (Paola & Borgman, 1991;Leclair, 2002;, with Bridge (1997) suggesting that the steady-state model for bedform preservation can be applied so long as the CV of crossset heights is bounded by 0.88±0.30. While these insights have primarily been supported by numerical and experimental studies under steady-state conditions (e.g., Leclair, 2002;Ganti et al, 2013;, they are widely applied in field-scale palaeohydrological studies (e.g., Holbrook & Wanas, 2014;Wang et al, 2020;.…”

“…Despite advances in understanding bedform dynamics, the prevalence of bedform disequilibrium dynamics in preserved fluvial strata is currently unclear, partly because we lack detailed field measurements of cross-set geometries and their statistical nature. While a handful of field studies have documented low CV (0.3-0.7) in fluvial cross strata Colombera et al, 2017;Cardenas et al, 2020;Wang et al, 2020), consistent with bedform disequilibrium dynamics, these data are often limited to a few outcrop observations for individual geologic formations. Here, we systematically characterize the geometries and statistical nature of fluvial cross strata for three Late Cretaceous geologic formations, central Utah, USA (Figs 1,2), to assess the degree to which they reflect bedform disequilibrium dynamics.…”