Deep-water clastic evaporites deposition in the Messinian Adriatic foredeep (northern Apennines, Italy): did the Mediterranean ever dry out?

A new genetic facies model for deep-water clastic evaporites is presented,based on work carried out on the Messinian Gessoso-solfifera Formation of thenorthern Apennines during the last 15 years. This model is derived from themost recent siliciclastic turbidite models and describes the downcurrenttransformations of a parent flow mainly composed of gypsum clasts. Themodel allows clearer comprehension of processes controlling the productionand deposition of clastic evaporites, representing the most common evaporitefacies of the northern Apennines, and the definition of the genetic andstratigraphic relationship with primary shallow-water evaporites formed andpreserved in marginal settings. Due to the severe recrystallization processesusually affecting these deposits, petrographic and geochemical analyses areneeded for a more accurate interpretation of the large spectrum of recognizedgravity-driven deposits ranging from debrisflow to low-density turbidites.Almost all the laminar ‘balatino’ gypsum, previously considered a deep-waterprimary deposit, is here reinterpreted as the fine-grained product of high tolow-density gravity flows. Facies associations permit the framing of thedistribution of clastic evaporites into the complex tectonically controlleddepositional settings of the Apennine foredeep basin. The Messinian SalinityCrisis occurred during an intense phase of geodynamic reorganization of theMediterranean area that also produced the fragmentation of the formerMiocene Apennine foredeep basin. In this area, primary shallow-waterevaporites equivalent to the Mediterranean Lower Evaporites, apparentlyonly formed in semi-closed thrust-top basins like the Vena del Gesso Basin.The subsequent uplift and subaerial exposure of such basins ended theevaporite precipitation and promoted a widespread phase of collapse leadingto the resedimentation of the evaporites into deeper basins. Vertical faciessequences of clastic evaporites can be interpreted in terms of the complexinterplay between the Messinian tectonic evolution of the Apennine thrust beltand related exhumation–erosional processes. The facies model here proposedcould be helpful also for better comprehension of other different depositionaland geodynamic contexts; the importance of clastic evaporites deposits hasbeen overlooked in the study of other Mediterranean areas. Based on theApennine basins experience, it is suggested here that evaporites diffused intothe deeper portions of the Mediterranean basin may consist mainly of deepwaterresedimented deposits rather than shallow-water to supratidal primaryevaporites indicative of a complete basin desiccation.