Abstract

Relict barrier deposits preserved on the shelf seabed from barrier transgression are common features of passive, sandy margins, with multiple deposits occasionally found evenly spaced several to tens of kilometers apart. The formation of these deposits has typically been ascribed to allogenic overstepping processes, most commonly to episodes of rapid sea-level rise, although changes in topography or external sediment supply have also been invoked. Here, we present a mechanism whereby autogenic feedbacks between shoreface dynamics and the landward overwash of sediments can form regularly spaced shelf deposits even with constantly rising sea level. A simple morphodynamic model of barrier transgression exhibits a mode of periodic retreat whereby alternating episodes of translation and aggradation arise from internal dynamics, generating low-relief cross-shore deposits of barrier sediment with kilometer-scale spacing that increases for more gradual shelf slopes. Modeled barriers develop autogenic partial overstepping on shelf slopes with gradients between 1 and 3 m/km and for rates of sea-level rise between 1 and 18 mm/yr. Deposits produced by the model correlate with field observations of overstepped barriers from around the world, sharing an inverse relationship between shelf slope and spacing (wavelength). This result suggests that discrete remnant barrier deposits may not be exclusively indicative of changes in external forcing and that currently stable barrier islands may be susceptible to rapid behavioral transitions.

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