Engineering-geological insights into the gypsum-bearing deposits of Punta delle Pietre nere (Puglia Region, Italy)

In a very limited area near the Punta delle Pietre Nere, NW of Lesina Lake, which, in turn, is close to the Gargano headland (Puglia region, Italy), chalky deposits crop out along with black limestones interspersed with bituminous marls and magmatic rocks. Chalks are traced back to evaporitic formations (of the Upper Triassic) present below successions 3000 m-thick of Jurassic-Miocene carbonate rocks. Their presence in outcrop can be explained by referring to three different mechanisms: a) diapirism, b) thrust upwards due to tectonic compression, and c) diapirism associated with tectonics. These chalks were affected (in the years from 1927 to 1930 and for almost a kilometre) by works to modify the route of the canal (called Acquarotta) which connects Lake Lesina to the sea so as to facilitate its flow conditions. Over time, the strip of land close to the Acquarotta canal has presented sinkhole phenomena linked to the dissolution of the chalky rocks, phenomena which have also affected the nearby town of Lesina Marina. Over time, and for different purposes, numerous investigations have been carried out in the area (mechanical surveys, geo-electrical tomographies, groundwater surveys, remote sensing through InSAR techniques, etc.) where, also thanks to new, specific site investigations, the ground instabilities have been studied in greater depth. In this study results of recent studies, which allowed to better understand the buried limits of the chalks, their depths, the geological characteristics of the covering soils and the flow conditions of groundwater. In summary, the distribution of gypsum-bearing deposits in the subsoil of the area is now better defined than in the past, when the uncertainty of the data represented a factor strongly limiting the local possibilities of settlements’ development.