Grain size dynamics using a new planform model – Part 3: Stratigraphy and flexural foreland evolution

Abstract. Within the stratigraphic record, grain size fining has been commonly used to infer subsidence, rate and its variability has been interpreted as a signature of external forcing events. We have recently developed a model (Wild et al., 2025b) that predicts grain size fining within a two-dimensional Landscape Evolution Model to predict the effect of autogenic processes on grain size fining. Here, we couple it to a flexural model to predict the stratigraphic evolution of a foreland basin, the distribution of grain size fining, and which of subsidence or autogenic processes dominates in controlling the fining. We show that, throughout its evolution, the foreland basin experiences a gradual increase in the bypass ratio, F, that provokes a gradual shift from subsidence-dominated to autogenically dominated grain size fining but also progressively alters stratigraphic preservation. The amplitude, and therefore efficiency, of autogenic processes in controlling grain size fining processes is modulated by the shape of the surface topography that we control by changing the rainfall gradient and extent of the basin confinement compared to the orogen. We also show how the evolution of the basin can be mapped in the framework we recently developed (Wild et al., 2025c) to interpret grain size fining data. Finally, we demonstrate how the model results and our findings can be used to interpret the stratigraphy and grain size information stored in a real foreland basin, namely the Alberta Basin of Western Canada.