Spongy-like porosity in peritidal carbonates: An interaction of cyclic sea-level oscillations, fresh water supply and sediment texture

This paper focuses upon the analysis of a complex paleokarstic system recorded within uppermost Triassicperitidal cycles in northwestern Sicily. Besides documenting spectacular karstification at the Triassic/Jurassicboundary, it provides an example of stratabound ‘spongy’ or ‘swiss-cheese’ dissolution. On the base of field observations,microfacies analysis, transmitted-light and cathodoluminescence petrography and stable-isotopeanalyses we put forward an original model for the formation of this peculiar stratabound dissolution. It impliesa complex interaction of several controlling factors at the interface between the marine and meteoric diageneticrealms during the relative cyclic oscillations of sea-level.The presence of a fresh water supply from an adjacent emerged area is the key for the periodic formation of amixing water lens during the relative sea level lowstand that brought about the subaerial exposure of the platform.The resulting dissolution pattern in the subtidal unit of a specific cycle is strongly controlled by the texturalfeatures of the sediments. In the case of bioturbated wackestones the ‘spongy’ or ‘swiss-cheese’ pattern develops,while inmollusk-rich beds biomoldic porosity occurs. Inwell-sorted subtidalmembers, such as algal grainstones,the dissolution originates as randomly distributed vuggy porosity. During periodic flooding of the platform, a newsubtidal unit is formed and the dissolution stops as fully marine phreatic conditions are re-established.