Geomorphological evolution of palaeosinkhole features in the Maltese archipelago (Mediterranean Sea)

Palaeosinkhole features characterise the Maltese islands, some of them reaching relevant dimensions, especially in the Island of Gozo. They show varied morphostructures: sub-circular bays, large depressions and rounded buttes or mesas (due to relief inversion) are the resulting geomorphic expressions. A detailed geological and geomorphological study of the palaeosinkholes located in the NW sector of the Island of Gozo has been carried out with the aim of investigating the evolution of the associated landforms. A field survey and a mapping campaign at a 1:5000-scale, coupled with aerial-photo interpretation, has been carried out and a spatial database has been implemented within a GIS software. Existing evolutionary models have been critically analysed and new models proposed. Thanks to the detailed exploration of the collapse palaeosinkholes, it has been deducted that at a certain stage, selective erosion has become the main factor influencing their geomorphological evolution. Positive reliefs (rounded buttes or mesas) were formed where more resistant terrains were located inside the sinkholes; conversely, depressions and sub-circular bays developed where the sinkholes infill was surrounded by more resistant rocks. Collapse structures do not seem to be active at present and their activity probably ceased during the Miocene, suggesting that karst subsidence processes are not the only responsible for the final shaping of the above-mentioned subcircular depressions and bays. The Gozitan rounded bays related to the palaeosinkholes have been compared with similar sub-circular coastal landforms located along the southern coast of the Island of Malta. The investigation showed that the latter are not caused or influenced by karst processes, but linked to the attitude of strata and to their different resistance to erosion. The bays analysed in Gozo and Malta are morphologically similar but genetically different, representing a relevant example of equifinality. The Gozitan sinkhole-related landforms have also been compared with those of the Island of Malta, generally much smaller in size, which allowed their different geomorphological evolution to be pointed out.