Spongy-Like Porosity in a Carbonate Platform From Sicily (Italy) Close to the T/J Boundary: An Interplay Between Textural Characters, High Frequence Sea-Level Fluctuations and Mixing Water Lens

Abstract

The analysis of a complex paleokarstic system recorded by the Uppermost Triassic peritidal cycles in Northwestern Sicily provides a spectacular example of karstification at the Triassic/Jurassic boundary and offers an example of stratabound ‘spongy’ or ‘swiss-cheese’ dissolution. Although these textures have been recognized in other systems, they are generally not well described or understood, but provide the opportunity to understand diagenetic processes at the interface between the marine and meteoric diagenetic realms during a fall in relative sea level. The database for this study includes field observations, microfacies analysis, transmitted-light and cathodoluminescence petrography and stable-isotope analyses. The results point to differential dissolution controlled by mixing of marine and meteoric porewaters within bioturbation cavities created by crustacean decapods during high-frequency lowstand stages. The fresh-water supply was provided by an adjacent exposed hinterland and/or by infiltration of rainwater. Sediments and microcrystalline calcite, that usually filled tunnels and vugs, were replaced at this time by several generations of fibrous and blocky calcite and polychrome silts. Sedimentological analysis of the cyclic stacking pattern shows that this particular type of dissolution concerns only some subtidal muddy units while, in subtidal units dominated by mollusk shells (Megalodonts), the dissolution is essentially biomoldic. Moreover, in subtidal units consisting of algal grainstones a vuggy dissolution is recorded. Our data suggest that it is possible to explain the presence of different types of stratabound dissolution (spongy-like, moldic, vuggy) in a carbonate platform by the development of a mixing zone lens during relative sea-level lowstand stages that are responsible of the cyclic subaerial exposure of the platform. Our data also highlight the strong textural control exerted by the bioturbation and the presence or absence of skeletal debris on the resulting solution pattern.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015