Sinead J. Lyster, Alexander C Whittaker, Alex Farnsworth, Gary J Hampson

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.