Transgressive coastal systems (1st part): barrier migration processes and geometric principles
AbstractCoastal processes during transgression have been explored through morpho-kinematic simulations using the Shoreface Translation Model (STM). Our STM experiments show that the landward migration of coastal system is controlled by the rate of sea level rise (SLR), the rate of sediment supply (Vs), the shelf slope (α), and the morphology of the coastal profile (M). Additionally, the geometric relationships between shoreface and plane of translation govern three kinematic modes of coastal barrier migration: (1) roll-over, (2) hybrid, (3) encroachment. Each mode exhibits differences along the coastal profile in relation to zones of erosion (cut) and redeposition (fill) and to the consequent sediment exchanges across the profile (from the cut to the fill). Each mode produces distinctive facies architectures and specific stratigraphic position of the shoreface-ravinement surface. Environmental conditions (rates of sea-level rise, sediment supply (±), barrier morphology) and kinematic modes both control stratal preservation. Transgressive roll-over, in particular, occurs on gently sloping shelves and involves erosion along the entire shoreface and landward sediment redeposition (by overwash and tidal inlet processes). Three different types of roll-over are possible depending on the conditions of sediment supply (Vs) to the coastal cell: neutral roll-over (Vs=0 m3), which produces no effect on the shelf; depositional roll-over (Vs >0) and erosional (Vs<0) roll-over, which modify the shelf through stratal preservation and erosion, respectively. These differences are quantified in simulations by tracking parameters that principally relate to the trajectory of a ‘neutral point’ (maximum depth of shoreface erosion). The shoreface-ravinement defines the trajectory in all the transgressions and in principle is preserved in the rock record, making it a much more useful tracking point than the shoreline trajectory analysed in other studies. Coastal migration in all kinematic modes includes state-dependent inertial effects, experimentally well evident when, after a perturbation, the drivers (SLR, Vs, α, M) are maintained constant for a long interval of time. Kinematic inertia appears as progressive geometric self-adjustments of the barrier until it acquires a shape that is stable under prevailing conditions (constant drivers). At this stage (kinematic equilibrium), which is unlikely ever to be attained in nature, simulated transgressions finally evolve with processes and geological products that remain invariant. Kinematic inertia is likely to be an additional factor that governs the real transgressions under most circumstances.
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