Mechanisms for Forming Discontinuity Surfaces Within Shoreface–shelf Parasequences: Sea Level, Sediment Supply, or Wave Regime?

A 2D process–response model was used to evaluate potential mechanisms involved in the formation of small-scale stratigraphic variability that is observed within progradational wave-dominated shoreface–shelf parasequences. Model experiments, which are based on scenarios that consist of a 25 ky progradational phase, suggest that discontinuity surfaces form when either sediment supply or sea level fall rapidly, or when wave-height regime increases over relatively short time periods (102–104 y) during coastal progradation. In these cases, mean deposition rate near the lower-shoreface drops, which, combined with coastal progradation, results in a discontinuity surface. Both observed and simulated discontinuity surfaces form predominantly in lower shoreface deposits and bound meter-scale stratigraphic cycles. Simulations show that wave-regime variability affects the shoreface–shelf to a much greater depth than minor sea-level change. Therefore, facies shifts along wave climate-induced bounding surfaces occur over wider reaches of the shoreface–shelf system than sea-level-induced facies shifts. High-frequency variability in sediment supply rate results in the formation of discontinuity surfaces across which there are no distinct facies shifts.