MIGRATION REVERSALS IN GRAIN-SIZE TRANSITIONS TO SHORELINE

The migration in grain-size transition (lithofacies boundary) recorded in the sedimentary record is key to interpreting changes in depositional environments. Change in grain size in the stratigraphic record is one of the fundamental and most recognizable physical lithological features. Advance and retreat of lithofacies and their boundaries are attributed to variation in external controls, e.g., climate variation, sea-level change, and tectonic subsidence. While most models focus on predicting the response of the fluviodeltaic shoreline to these basinal forcings, none have thoroughly incorporated the migration of grain-size transitions (GST) that coevolve within the fluviodeltaic system. We present a delta evolution model that treats both the shoreline and GST as moving boundaries to provide understanding of the dynamic interaction between the external boundary (shoreline) and the internal lithofacies boundaries under relative sea-level rise. A range of relative sea-level rise rates was tested 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 retreats were different, resulting in a counterintuitive case for a quicker retreat of GST while the shoreline still continued to advance. A series of experiments with a sand and crushed walnut sediment mixture captured the same behaviors of these two moving boundaries. We found that GST experienced higher relative sea-level rise rates that scale with the downstream river slope and the shoreline progradation rate, which caused earlier GST retreat timing in comparison to the shoreline. Time series data from the experiments show higher variability in migration rate of GST compared to that of the shoreline. Therefore, final recorded stratigraphy displayed a GST trajectory as a shazam line that shows zigzag fluctuations. This study investigates autogenic processes acting on the fluviodeltaic surface and tests the stratigraphic architecture recorded in the trajectories. The fundamental understanding of migration of both internal lithofacies transitions and shoreline in fluviodeltaic systems from this study will aid in accurately assessing the trajectories of GST in sedimentary strata as a proxy for environmental change.