Abstract
Three different types of carbonate deposits are included within the “Calcare di Base,” commonly envisaged to record the Messinian salinity crisis onset: type 1 consists of sulfur-bearing limestones, representing the biogenic product of bacterial sulfate reduction after original gypsum; type 2 comprises dm-thick, laminated dolomitic limestones interbedded with diatomites, sapropels, and marls found at the top the Tripoli Formation; type 3, the most common variety, consists of m-thick, brecciated limestones interbedded with shales and clastic gypsum.
Type 3 shows sedimentary features suggesting a clastic origin and deposition from high- to low-density gravity flows; thus, these deposits can be regarded as an end-member of a large variety of evaporite-bearing, gravity-flow deposits, with a dominant carbonate component.
The genetic and stratigraphic characterization of these carbonates has strong implications for a better comprehension of Messinian events; the three types of Calcare di Base seem to have formed during different stages of the Messinian salinity crisis (MSC). Type 2 formed in the first stage (5.96–5.60 Ma), and is the only type that can be regarded as the Lower Gypsum time-equivalent. Type 3 was deposited in the second stage (5.60–5.55 Ma), and its base is associated with a regional-scale hiatus and erosion (Messinian erosional surface). Type 1 formed even later, likely in post-Messinian time, through diagenetic processes affecting resedimented gypsum deposited during the second stage of the MSC.
It follows that not all the Calcare di Base deposits record the onset of the Messinian salinity crisis, as commonly thought. Thus, a detailed facies characterization of these carbonate deposits is fundamental for both stratigraphic reconstructions and a better comprehension of Messinian events.
