Numerical forward modelling of peritidal carbonate parasequence development: implications for outcrop interpretation

The mechanisms responsible for formation of peritidal parasequences have been a focus of debate between proponents of contrasting autocyclic and allocyclic models. To contribute to this debate a three-dimensional numerical forward model of carbonate production, transport and deposition has been developed. Shallowing-upward parasequences are produced in the model via carbonate island formation and progradation, with an element of self-organization, and no external forcing. These autocyclic parasequences have characteristics comparable with peritidal parasequences observed in outcrop. Modelled parasequence thickness and duration depend primarily on subsidence rate and sediment transport rate, illustrating the significance of sediment flux in formation of peritidal parasequences. Adding an element of stochastic variation of sediment transport rate and transport path leads to formation of nonuniform-thickness parasequences that generate Fischer plots showing apparent hierarchies similar to those often interpreted as evidence of eustatic forcing. The model results do not rule out allocylic mechanisms, but suggest that shoreline and island progradation are also plausible mechanisms to create variable-thickness, shallowing-upward peritidal parasequences and should be considered in interpretations of such strata.