Shallow-marine sequences as the building blocks of stratigraphy: insights from numerical modelling
Two types of depositional sequences can be defined within the sequence stratigraphic framework: the parasequence and the high-frequency sequence. Both sequences consist of stacked regressive and transgressive deposits. However, a parasequence forms under conditions of overall sea-level rise, whereas a high-frequency sequence forms as the sea level oscillates which results in typical forced regressive deposits during sea-level fall. Both depositional sequences may develop over comparable temporal (10-100 kyr) and spatial (1-20 km wide and 1-40m thick) scales. Numerical modelling is used to compare the architecture, preservation potential, internal volumes, bounding surfaces, condensed and expanded sections and facies assemblages of parasequences and high-frequency sequences. Deposits originating from transgression are less pronounced than their regressive counterparts and consist of either preserved backbarrier deposits or shelf deposits. Shoreface deposits are not preserved during transgression. The second half of the paper evaluates in detail the preservation potential of backbarrier deposits and proposes a mechanism that explains the occurrence of both continuous and discontinuous barrier retreat in terms of varying rates of sea-level rise and sediment supply. The key to this mechanism is the maximum washover capacity,which plays a part in both barrier shoreline retreat and backbarrier-lagoonal shoreline retreat. If these two shorelines are not balanced, then the retreat of the coastal system as a whole is discontinuous and in time barrier overstep may take place.