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Abstract

The shoreface translation model (STM) incorporates advances in the theory for coastal responses to changes in relative sea level, exposing some well-entrenched misconceptions about the formation of transgressive and regressive strata at chronosomal scales. The STM is a mass-conserving, morphological-behavior model that provides added generality to the updated theory by allowing for open sediment budgets (on the shoreface and in the lagoon) and time-dependent changes in shoreface and barrier geometries. Both the theoretical basis and application of the STM give neutral transgression for balanced sediment budgets on gently sloping surfaces undergoing a marine transgression. Under these conditions, no transgressive strata are formed, and the land surface being transgressed is not disturbed en masse. Consequently, shoreface-ravinement surfaces are not necessarily inherent by-products of transgression as assumed previously. Simulated transgressive strata are laid down (aggradational transgressions) only if there is a positive net littoral sediment supply (from deltaic sources or erosion of shoreline promontories), significant deposition in the lagoon (due to trapping of fine marine sediments or direct fluvial inputs), or both. Shoreface-ravinement surfaces are produced only under conditions of negative littoral sediment budgets or if the land surface being transgressed is steeper than the shoreface (degradational transgressions).

For negative sediment budgets, simulated shoreface ravinements form on low-gradient surfaces without seaward sediment displacement or genetically related aggradation of the seabed farther offshore. Ravinements also can develop during progressive deepening of the shoreface during transgression and highstands. Simulated highstand ravinements are consistent with, and provide an alternative explanation for, coarse-sand lags found on the lower shoreface of many accommodation-dominated shelves today. Simulated forced regression results in massive in-situ reworking of the highstand shelf surface, inevitably producing a strandplain stratum characterized by (1) an unconformity at its base and (2) shoreface isochrons, as opposed to the landward-dipping, backbarrier isochrons that characterize transgressive barriers (which consist of washover and tidal-delta sand deposits).

The revised approach to simulating each of these intrachronosomal-forming processes has significance for sequence models and the interpretation of stratigraphic data at basin-fill scales.

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