Genetical analyses of natural cavities in the Messinian Evaporites of the Bologna area (Italy): 1st, the Spipola Cave (Spipola-Acqua Fredda karst system)

This work presents a genetic interpretation of the Spipola gypsum cave (Croara area, Bologna, Italy) based on structural and stratigraphic data collected inside the cavity. We have studied the relationship between the trend of fractures and faults and the preferential cave’s development and we have related the various cave’s levels to the main thick gypsum beds. The cave has formed in a sequence of thick soluble gypsum beds in alternation with thin layers of euxinic shales that constitute impermeable seams. Two principal systems of fractures have been recognized, both related to the major faults occurring in the cave. The cave originated along the principal system of fractures, parallel to the strike of bedding. The other system of fractures (parallel to the dip of bedding) has not developed important karst phenomena. This fact is related to the presence of euxinic shales that cap the permeable rocks and interrupt the water flow in the strata dip direction. Further cave development was produced by enlargement by water action, collapses of ceiling, detachments of blocks and other mechanical phenomena. The various morphologies due to water action or to mechanic collapses are described and interpreted.