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The east-central part of Sardinia (Italy) is characterised by Jurassic dolomitic mesas (Tacchi, or “table mountains”) that overlie a Palaeozoic basement mainly composed of metavolcanics and phyllites. These mountains are the remnants of a continuous carbonate cover, dissected by faults and river erosion, and are now completely isolated hydrological systems. Most of these rivers have cut valleys more than 200 metres deep into the Palaeozoic basement rocks, whose slopes are often characterised by landslides, suggesting their recent oversteepening. Some valleys, on the contrary, have not reached the base of the carbonate sequence and appear to be suspended above the deeper incisions, apparently disconnected by them. Several subhorizontal surfaces can be distinguished on the table mountains, related to local base level stillstands. Also water table caves, scattered along the flanks of the mountains over an altitudinal range of about 200 m, show several stillstands in base level lowering. 26Al and 10Be burial dating of sediments in four caves located at different elevations on the flanks of the suspended Taquisara Valley show an Upper Pliocene or Lower Pleistocene age. Thus, this valley appears to be of Late Tertiary age. The deeper valleys, such as Riu Pardu, that dissect the Tacchi mountains completely, cutting deeply into the basement rocks, are much younger, as their unstable slopes suggest. Knickpoint retreat in Riu Pardu and estimated valley erosion rates suggest the capture of Riu Pardu by Rio Pelau to have occurred in the last 100 ky.

Quartz sandstone of the Sarisarinama massif in Venezuela hosts the world biggest collapse dolines in quartz-rich lithologies, with volumes up to some millions of cubic meters. Due to extremely complex logistics required to reach the massif, the genesis of these depressions and of the underlying caves has never been studied in detail. The lack of field campaigns and extended data has fostered a decade-long scientific debate on whether their origin was due to epigenic or hypogenic processes. This study integrates petrological, structural and hydrochemical observations, including analyses of silica concentration, pH, conductivity of surface and cave waters (EC), to investigate the speleogenetic processes acting underground. Petrographic and compositional analyses of the host rock (Mataui Formation) show that in the Sarisarinama region quartz sandstones are regularly characterized by clay interlayers with significant content of pyrophyllite and kaolinite and minor amount of iron hydroxides. Compared to surface waters, subsurface infiltration along vertical fractures and fault planes show enrichment in silica, higher pH and lower EC, confirming that chemical weathering is effective underground provoking intergranular silica dissolution along structural discontinuities. The weathering of the clay and iron hydroxide interlayers guides the speleogenesis, weakening specific stratigraphic levels and causing the collapse and fragmentation of the more resistant quartz sandstone strata. The initial void, created by piping of the loose sand released by quartz sandstone weathering, can migrate upwards by means of roof and wall breakdown; this chain of events eventually triggers a collapse at the surface, which generates a circular or squared sinkhole. The weathering acts mainly along the dominant fracture networks, showing a clear guidance by regional tectonics. These speleogenetic controls rule out the hypothesis of a hypogenic origin of the simas, suggesting a primary role of long-term epigenic chemical weathering and mechanical erosion guided by joints, weak clay and iron hydroxide interlayers, followed by subsequent massive collapses.

none 16 si Although outcropping rarely in Italy, evaporite (gypsum and anhydrite) karst has been described in detail since the early 20th century. Gypsum caves are now known from almost all Italian regions, but are mainly localised along the northern border of the Apennine chain (Emilia Romagna and Marche), Calabria, and Sicily, where the major outcrops occur. Recently, important caves have also been discovered in the underground gypsum mines in Piedmont. During the late 80s and 90s several multidisciplinary studies were carried out in many gypsum areas, resulting in a comprehensive overview, promoting further research in these special karst regions. More recent and detailed studies focused on the gypsum areas of Emilia-Romagna and Sicily. Sinkholes related to Permian-Triassic gypsum have been studied in Friuli Venezia Giulia. This article reviews the state of the art regarding different aspects of evaporite karst in Italy focusing on the main new results. open De Waele, J.; Piccini, L.; Columbu, A.; Madonia, G.; Vattano, M.; Calligaris, C.; D’Angeli, I.M.; Parise, M.; Chiesi, M.; Sivelli, M.; Vigna, B.; Zini, L.; Chiarini, V.; Sauro, F.; Drysdale, R.; Forti, P. De Waele, J.; Piccini, L.; Columbu, A.; Madonia, G.; Vattano, M.; Calligaris, C.; D’Angeli, I.M.; Parise, M.; Chiesi, M.; Sivelli, M.; Vigna, B.; Zini, L.; Chiarini, V.; Sauro, F.; Drysdale, R.; Forti, P.

Buso della Rana and Buso della Pisatela are two karstic caves located in north-east Italy. They are part of the same karst system and are developed in the Castelgomberto calcarenitic marine sediments, which were deposited in a shallow Caribbean-type sea during the Eocene. The Buso della Rana-Pisatela system developed mostly at the contact between the Castelgomberto calcarenite and underlying volcanic rocks. The system of caves is ~ 37 km long and has only three entrances, two of which are semi-artificial. The overlying karst plateau is not directly connected to the Buso della Rana-Pisatela system and, with the exception of one deep abyss, exhibits a rather poorly developed karst. This is unexpected considering the presence at depth of such a large and long cave. Gypsum (CaSO 4 ·2H 2 O) has locally been observed on the walls of the Buso della Pisatela cave. Energy dispersive X-ray spectroscopy (EDS), performed with a scanning electron microscope (SEM), reveals the presence of sulfur-bearing minerals within the host rock. Gypsum was formed by oxidation of these minerals as indicated by negative δ 34 S values. The oxidation of sulfide minerals forms a sulfuric-acid solution that dissolves the Castelgomberto calcarenite and, once it is oversaturated in calcium, precipitates as gypsum. The lack of well-developed karst on top of the plateau and the analyses suggest that the formation mechanisms for the Buso della Rana-Pisatela system differ from classical epigenic speleogenesis. The “pyrite-effect” has been recognized in other caves and described in literature. In our case pyrite is responsible of two hypo-speleogenetic processes: i) the dissolution of a portion of the host rock and ii) the enlargement of the karst voids as a consequence of the haloclastic effect.

Although hypogene cave systems have been described since the beginning of the 20th century, the importance in speleogenesis of ascending fluids that acquired their aggressiveness from in-depth sources has been fully realized only in the last decades. Aggressiveness of waters can be related to carbonic and sulfuric acids and the related corrosion-dissolution processes give rise to different types of caves and underground morphologies. The abundance of hydrothermal springs and associated travertine deposits, and the widespread interaction between volcanic or sub-volcanic phenomena and karst in many sectors of the Italian peninsula are a strong evidence of hypogene speleogenesis. Furthermore, researches on secondary minerals have allowed to discover hypogene caves formed by highly acidic vapors in subaerial environments, also showing that most of these caves have extremely rich mineral associations.