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 (10(2)-10(4) 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.