TRANSGRESSIVE DOLOMITE: A MODEL FOR INTERBEDDED SUBTIDAL LIMESTONES AND INTERTIDAL DOLOSTONES

Subtidal limestones are commonly observed in the carbonate rock record to be cyclically interbedded with intertidal dolostones. Such successions are typically interpreted to represent repeated “shallowing-upward” peritidal cycles, whereby, following an “instantaneous” high-order sea-level rise, subtidal variably-restricted environments are gradually overridden by intertidal equivalents during regression, creating an overall progradational deposit. Dolomitization of the intertidal to supratidal cycle “caps” has often been attributed either to refluxing saline waters produced in restricted tidal-flat settings, or to the presence of colonial microbes that are commonly found within tidal-flat deposits. A recent review of modern coastal carbonate systems, however, indicates that an alternative depositional model may be more appropriate. In this model, which is based on direct observation of many classic carbonate systems, carbonate lagoons and their associated tidal flats are mostly formed during transgression behind landward-migrating coastal barriers. These barriers override their respective lagoonal deposits (a process termed barrier “roll-over”) during continued sea-level rise, such that the lagoons and associated tidal flats typically form cycle bases seaward of the migrating barrier, being sharply overlain by relatively open platform deposits with an erosive surface between (a ravinement surface), a situation documented at Andros Island. Dolomitization of these transgressive deposits may occur during oxidation of organic matter by well-oxygenated open platform waters introduced to lagoonal pore systems during wave ravinement. Such oxidation reactions acidify the porewaters, resulting in calcium carbonate dissolution, and the facilitation of low-temperature dolomite replacement, as the competition from kinetically favored carbonate minerals is removed. Because dolomite replacement is a lithifying reaction, it promotes the retention of carbonate lagoonal sediments during ravinement (if the mineralizing reaction occurs to some degree before significant wave abrasion). This early dolomitization may explain the apparent dichotomy that lagoonal and tidal-flat deposits are common in carbonate rocks but are generally absent in coastal siliciclastic successions.