Constraining flow and sediment transport intermittency in the geological past

Abstract

Quantitative investigations of ancient rivers usually provide insights into either instantaneous or mean flow conditions. There is a critical gap between these timescales of investigation which reflects the intermittency of flow and sediment transport, and closing this gap is crucial to fully explore the dynamics and evolution of ancient fluvial landscapes. Here, we combine fluvial stratigraphic datasets, flow and sediment transport models, and paleoclimate general circulation model (GCM) results to develop new methods to estimate intermittency in the geological past, specifically flow intermittency factors (Iw) and sediment transport intermittency factors (Is). We illustrate these methods for the Upper Cretaceous Last Chance Ferron Sandstone, Utah, USA. For sand-transporting flow conditions in Last Chance Ferron rivers, we estimate Iw values of 0.54−0.90, which imply that channel-forming flows were sustained for the majority of the year, consistent with perennial systems in which relatively large discharges are sustained. In contrast, for gravel-transporting flow conditions, Iw values of 0.28−0.38 suggest that the largest formative flows may have occupied Last Chance Ferron rivers for nearly a third of the year, which could be explained by a monsoonal system in which high magnitude discharge events are sustained, or a subtropical system in which high magnitude discharge events have short durations but high frequencies. Meanwhile, Is values of 0.075−0.15 suggest that annual sediment budgets could have been transported in as little as 10 days, but up to 2 months, if channel-forming conditions were sustained, and highlight that small changes to the duration of channel-forming conditions could significantly impact sediment budgets. These results are consistent with independent facies- and proxy-based insights into Last Chance Ferron rivers, which point to a perennially wet system characterized by a monsoonal or subtropical discharge regime. Our results highlight new opportunities to use paleoclimate GCMs to constrain intermittency in the geological past. Going forward, paleoclimate GCMs will be particularly useful where the rock record is incomplete or inaccessible, or where stratigraphic approaches are limited, and will enable us to tackle pertinent research questions pertaining to past surface processes on both Earth and other planets.

Water discharge and sediment flux intermittency in the fluvial Escanilla Formation, Spain: Implications for changes in stratigraphic architecture

“…For instance, an intermittency of 120 days yields an intermittency factor of 0.33 or 33% of a year, meaning the river could transport its entire water or sediment budget in 0.33 of the time if channel‐forming conditions were continuously sustained (cf. Lyster et al., 2023). This is rarely the case and hence intermittency factors do not represent the temporal distribution of individual flow events which are typically not known for ancient river systems in the geological past.…”

“…However, rivers in semi‐arid to sub‐humid tropics display a wide range of discharge variability depending on the balance between precipitation and evapotranspiration, with several rivers showing pronounced run‐off seasonality and inter‐annual variability (Alexander et al., 1999; Fielding et al., 2009, 2011, 2018). Discharge and flux variability are further important as they can play a key role in controlling landscape evolution and the consequent sedimentary record of rivers (Allen et al., 2013; Plink‐Bjorklund, 2015, 2017; Fielding et al., 2018; Hansford et al., 2019; Hansford & Plink‐Björklund, 2020; Lyster et al., 2023). Seasonal discharge variability, in particular, has been found to influence preserved stratigraphy and sedimentology and thus plays a key role in the construction of fluvial bodies in both ancient and modern environments (Fielding et al., 2018; Lyster et al., 2023).…”

“…Discharge and flux variability are further important as they can play a key role in controlling landscape evolution and the consequent sedimentary record of rivers (Allen et al., 2013; Plink‐Bjorklund, 2015, 2017; Fielding et al., 2018; Hansford et al., 2019; Hansford & Plink‐Björklund, 2020; Lyster et al., 2023). Seasonal discharge variability, in particular, has been found to influence preserved stratigraphy and sedimentology and thus plays a key role in the construction of fluvial bodies in both ancient and modern environments (Fielding et al., 2018; Lyster et al., 2023). Discharge variability is also crucial to bedrock rivers through its role in controlling bedrock river incision and continental erosion thereby driving the resulting flux of sediments into depositional basins (Lague et al., 2005 and references therein).…”

“…In general terms, the temporal distribution of flow and sediment transport in a river is referred to as its intermittency. Two extreme cases can be identified: a highly intermittent system is characterised by repeated or prolonged intervals of no flow or sediment transport while a non‐intermittent system is characterised by continuous flow or sediment transport (Lyster et al., 2023). To study sediment fluxes and basin infilling over extended geological timescales, Paola et al.…”

“…(1992) introduced a specific dimensionless intermittency factor ranging from zero to one. This factor serves as a means of scaling instantaneous sediment transport rates which for ancient rivers can be reconstructed from palaeohydrological methods applied to channel outcrops (Lyster et al., 2021, 2023; McLeod et al., 2023; Sharma et al., 2023a, 2023b) to longer term sediment fluxes or depositional volumes which can be estimated for geological applications using mapping or from sediment thickness isopachs (Michael et al., 2013). The chosen averaging time should be considerably longer than individual channel‐forming events (instantaneous conditions) yet relatively shorter than the overall geological timescale (mean conditions).…”