Large-scale mass wasting processes in the Messinian Ciminna Basin (northern Sicily)

Facies analysis indicates the perfect correspondence in terms of facies association and number of cycles of the primary massive selenite deposits characterizing the lower and upper evaporitic units characterizing the Messinian succession of the Ciminna Basin (Sicily). Based on their peculiar facies characteristics, it can be documented that both units include the 1st to 7th evaporitic cycles of the Primary Lower Gypsum (PLG) unit recognized throughout the Mediterranean and marking the onset of the Messinian salinity crisis. In other words, massive selenite deposits of the lower and upper unit are identical and belong to a largely incomplete, top-missing PLG unit. As a consequence, we argue that their vertical superposition is not stratigraphic and a suitable tectonic or gravitative duplication mechanism of the local Messinian succession has to be envisaged.The lower evaporitic unit actually consists of an array of PLG tilted blocks which are onlapped by mainly clastic gypsum deposits. We interpret this succession as a base of slope complex formed in front of a thrust-related anticline undergoing uplift and progressive dismantling through large-scale mass-wasting processes during a intra-Messinian phase of tectonic activity. This unit is a perfect equivalent of the Resedimented Lower Gypsum (RLG) deposits found in many other deep Mediterranean basins. In this context, also the upper evaporitic unit is here considered a huge PLG slab emplaced on top of the lower one through mainly gravitative processes related to retrogressive sliding; however, a purely tectonic superposition of the two units cannot be ruled out.This interpretation rules out the occurrence of a precursor evaporitic event in the Ciminna basin and may solve a long-lived controversy by framing this succession in the regional context of highly dynamic tectonic and sedimentary evolution of the Apenninic-Maghrebian thrust belt during the Messinian.