A new look at modern carbonate shoals and coastal barrier systems

This review of published literature examines the nature of modern carbonate coastal barrier systems, concentrating on barrier islands and their related shoals and lagoons in light of concepts derived from their siliciclastic counterparts. The observations show that subaerial coastal (C-type) barriers are common along transgressing carbonate shorelines and are more abundant than largely subtidal shoals. The carbonate sediment composition and associated contemporaneous marine and meteoric cementation do not prevent landward migration during transgression, in contrast to R-type barriers that occur at platform margins to which they are genetically linked. As a result, carbonate C-type barrier systems act like siliciclastic barriers during transgression, showing both continuous (rollover) and discontinuous (overstepping) landward movement. The widely cited examples of immobile carbonate barriers in the United Arab Emirates, Persian Gulf, and the Coorong, Australia, are believed to be stable because of a recent sea-level fall. The sandy shoals on the Great Pearl Bank, Persian Gulf, and Faure Sill, Shark Bay, Australia, are likely the remnants of overstepped barriers that have been reworked after Holocene drowning; the transgressive history of Faure Sill is suggested by the presence of (lagoonal?) muds beneath the sandy shoal. Several examples of preserved, post-glacial overstepped coastal barriers have been reported from modern carbonate shelves, and evidence of preserved, back-barrier lagoonal deposits is reported from the Persian Gulf ramp and the southern Australian margin. This implies that transgressive successions that begin with tidalflat deposits overlying the sequence boundary, passing upward into more offshore sediments, should be more common than currently appreciated; the shoreline of NW Andros Island, Bahamas, is transgressing and provides an example of this succession. Such successions might contain two wave-generated erosional discontinuities: a lower one generated by a low-energy 'nested' beach-ridge barrier along the landward side of the lagoon (= the 'bay ravinement surface'), and a more significant surface formed by the passage of the main barrier shoreface (the 'master ravinement surface'). Preservation of the barrier's subaerial cap is not expected, except in rare cases, because it is removed by the master ravinement surface. Meteoric diagenesis of coastal-barrier grainstones is likely to be limited because of the presence of an underlying layer of impermeable lagoonal mud and the short residence time of any location within the freshwater lens due to barrier migration. The 'lag depth' or 'lag time' that is invoked to account for a delay in sediment accumulation is thought to be due to passage of the erosional shoreface, the depth of which is approximately equal to fair-weather wave base. Ancient ooid shoal complexes might be analogous to the amalgamated, highstand coastal barrier systems that have been stranded along many siliciclastic shorelines. The hypersaline conditions expected in arid coastal settings are most easily obtained if a continuous subaerial barrier exists; subaqueous shoals typically have too much tidal exchange to permit large departures from normal-marine salinity. These findings offer an alternative to the classic upward-shallowing model for coastal sedimentation in carbonate systems that might have been employed uncritically in some cases.