Speleogenesi nelle dolomie della Formazione di Dorgali (Sardegna centro orientale).

The inner margin of coastal Supramonte (central-east Sardinia) is bordered by the outcrops of palaeozoic granites and/or phyllites underlying the Jurassic sedimentary covers and outcropping because of tectonics and erosion. The Jurassic successions starts with clay layers, marls, conglomerates and sands of different thickness (generally some metres), followed by sandy dolostones increasing their carbonate content upwards (Dorgali Formation). In the central-western sector (Marghine - Su Canale area), a series of karst systems develops along the contact between these silicate and carbonate rocks. With their streamlets, these underground networks drain the allogenic recharge of small depressions formed on the impermeable basement. The typical morphologies of these caves is characterised by the lack of dissolutional morphologies and by the presence of wide voids that increase their size with depth, with volumes incompatible with the available discharge. The ceiling is generally flat and relatively low, while the floor is masked by a pile of graviclastic deposits. Parts of these vadose passages are sometimes developed for several metres in granite or metamorphic rocks. The mineralogical analysis of the weathering products at the contact between the palaeozoic basement and the mesozoic sediments have shown the presence of typical silicate hydrolysis minerals such as montmorillonite, kaolinite, and chlorite. The speleogenesis of these karst systems can thus be described through a first dissolutional phase that affects the carbonate cement of the basal sandy dolostones. This stage produces loose material that can easily be removed by water erosion. Once the critical dimensions of the void have been reached huge chambers can form in a few thousands of years.