Multi-scale dissolution events in Western-Tethyan peritidal carbonates close to the Triassic/Jurassic boundary

The stratigraphical and sedimentological study of uppermost Triassic-lowermost Jurassic sections of peritidal carbonates, in north-western Sicily, allows to differentiate multi-scale dissolution events that affected the carbonate platform interior during the uppermost Rhaetian. The Triassic cycles show extensive phenomena of stratabound dissolution that can be differentiated in three main types: vug, moldic and “spongy-like”. The dissolution horizons are particularly concentrated in the subtidal members of cycles. The creation of different type of porosity seems to be mainly controlled by the textural characters of the host-rock. In particular the cm-scale bioturbation in muddy sediments, probably operated by crustacean decapods, results as the most important factor in determining the spongy-like pattern. Regarding the dissolution fluids responsible of the stratabound dissolution, the morphological comparison of the spongy-cavities with recent similar examples from modern carbonate platforms, coupled to geochemical data, supports the influence of a mixing water lens. In our case we think that a mixing water lens could have been established during the periodical exposure of the platform (documented by thick paleosols at the top of each cycles) thanks to a freshwater recharge from an adjacent exposed area. Later dissolution events, at a larger scale, are superimposed on the stratabound dissolution cavities. Giant caves up to 100 m in diameter, filled up by collapsed breccias, scattered over a distance of about 1 km, along a narrow stratigraphic interval. Among the breccia elements, the presence of clasts arising from the spongy-like horizons allow to postdate the creation of the cavern dissolution with respect to the stratabound dissolution. Up-section from the cave alignment, a karstified surface covered by a particularly thick red paleosoil, laterally correlated with a structure that is interpreted as a paleo-sinkhole, is considered as the subaerial surface connected to the cave system. The biostratigraphic data support a late Rhaetian age of this surface as the T/J boundary, based on the LO of Trasina hantkeni, falls about 100 m above the karstified surface. The amount of the exposed stratigraphic thickness during the formation of the cave system is estimated at about 130 m, a value that is difficult to explain only in terms of eustatic variation. So we consider, a possible influence of a tectonic control induced by the ongoing rifting relating to the opening of the adjacent Alpine Tethys.