Reverse migration of lithofacies boundaries and shoreline in response to sea-level rise

The migration of the lithofacies boundaries preserved in the sedimentary record is key to interpreting changes in depositional environments. Grain size is one of the most recognizable physical characteristics of lithofacies. The advance and retreat of grain-size breaks, as a proxy for lithofacies boundaries (e.g. gravel-sand transition), is commonly attributed to variations in external controls (e.g. climate, sea level and tectonic subsidence). While most models of fluviodeltaic systems focus on predicting the response of the shoreline to these forcings, none have thoroughly incorporated the migration of grain-size transitions (GST) that coevolve with the shoreline. We present a numerical delta evolution model that treats both the shoreline and GST as moving boundaries to provide quantitative understanding of the dynamic interaction between the downstream boundary (shoreline) and the upstream lithofacies boundaries (GSTs) of the fluviodeltaic system under relative sea-level rise. We tested a range of relative sea-level rise rates in the model. The shoreline and GST gradually reduced their progradation rates and eventually retreated landward as the fluviodeltaic topset and foreset elongated. However, their timings of retreat were different, resulting in a counterintuitive case for a quicker retreat of GST while the shoreline still continued to advance. A series of scaled flume experiments with a sand and crushed walnut sediment mixture captured the same behaviours of these two moving boundaries. We found that GST experienced higher relative sea-level rise (RSLR) rates than the shoreline. This additional RSLR rate scales with the downstream river slope and the shoreline progradation rate to cause earlier GST retreat in comparison to the shoreline. The fundamental understanding from this study of migration of both the GST and shoreline in fluviodeltaic systems will aid in accurately assessing the trajectories of GST in sedimentary strata as a proxy for environmental change.