Simulation models of salt-marsh morphodynamics: some implications for high-intertidal sediment couplets related to sea-level change

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A salt marsh composed of a vegetated platform dissected by creek networks has at any time characters determined by the previous history of the system, because forcing factors constantly change in rate. Results from numerical simulation models of marsh growth are combined with engineering regime theory (channel stability) to form a comprehensive conceptual model which qualitatively describes the three-dimensional character of marshes and the stratigraphic sequences they generate. The morphostratigraphic evolution of marshes is examined for two main patterns of regional relative mean sea-level change. Where the level fluctuates smoothly about an underlying upward trend, there develops a stacked sequence of silt-peat couplets, each of which is symmetrical in facies states but asymmetrical in facies thicknesses. Palaeochannel forms and fills distributed through each couplet show that creek networks expand and densify as sea level rises and the hydraulic duty of the marsh platform increases to its maximum, but shrink and silt-up as sea level temporarily stabilises or falls and the duty declines. Great earthquakes, and also the abandonment of manmade flood defences on aseismic marshes, create catastrophic-episodic increases in relative sea level. Where there is also an underlying upward trend of sea level, stacked silt-peat couplets result that are strongly asymmetrical in both facies states and thickness. The creek networks enlarge rapidly under strong forcing within a short time of the sea-level increase but later, as the hydraulic duty declines, shrink and infill. Under each scenario, the tidal creek networks that functioned during the accumulation of one couplet are unlikely to be inherited by the next, except in the case of channels that also transmit fresh water from the hinterland.