Autogenic cycles of channelized fluvial and sheet flow and their potential role in driving long-runout gravel progradation in sedimentary basins

The paleoslope estimation method uses a threshold-shear-stress criterion, together with field-based measurements of median grain size and channel depth in alluvial gravel deposits, to calculate the threshold paleoslopes of alluvial sedimentary basins. Threshold paleoslopes are the minimum slopes that would have been necessary to transport sediment in those basins. In some applications of this method, inferred threshold paleoslopes are sufficiently steeper than modern slopes that large-magnitude tectonic tilting must have occurred in order for sediments to have been transported to their present locations. In this paper, we argue that autogenic cycles of channelized fluvial and sheet flow in alluvial sedimentary basins result in spatial and temporal variations in the threshold slope of gravel transport that can, under certain conditions, cause gravel to prograde out to distances much longer than previously thought possible based on paleoslope estimation theory (i.e., several hundred kilometers or more from a source region). We test this hypothesis using numerical models for two types of sedimentary basins: (1) an isolated sedimentary basin with a prescribed source of sediment from upstream, and (2) a basin dynamically coupled to a postorogenic mountain belt. In the models, threshold slopes for entrainment are varied stochastically through time with an amplitude equal to that inferred from an analysis of channel geometry data from modern rivers. Our models show that when local threshold slope values vary stochastically and sediment supply is relatively low compared to transport capacity, alluvial gravels can persistently prograde at slopes far below the threshold slopes predicted by paleoslope estimation theory. The result holds whether the stochastic changes in threshold slope are autocorrelated along the entire channel profile or occur in localized sections of the channel profile. As such, our model results suggest that long-runout gravels do not require steep regional slopes in order for transport to occur. We conclude that the minimum progradational slopes of fluvial sedimentary basins adjacent to postorogenic mountain ranges are functions of both the mean and coefficient of variation of water-flow depths as well as the density and texture of the bed sediment.