Carbonate production during the Messinian salinity crisis

The carbonate deposits associated with the Messinian salinity crisis (MSC) have been historically included in the “Calcare di Base” lithostratigraphic unit, best known in Sicily, Calabria and in the Northern Apennines. A still open debate regards their i) origin (primary evaporitic, microbialitic or clastic) and ii) age (associated with the onset or the acme of the crisis). Here, an integrated multidisciplinary (stratigraphic, sedimentological and geochemical) study focused on the carbonate deposits cropping out in the Northern Apennines (Eastern Romagna basins; Gabicce Riviera, Northern Marche) and in Calabria (Rossano basin; Crotone basin) is presented. The carbonate deposits outcropping in the Eastern Romagna basins are stratigraphically, comprised between upper Tortonian-lower Messinan deposits below and shale deposits belonging to stage 3 (Lago-Mare) of the MSC above. Since their age is poorly constrained, they have been here labeled as incertae sedis carbonates (IS-C). The IS-C deposits, were compared with other carbonates outcropping in the Northern Apennines, and found: i) below (PRE-C), ii) within (PLG-C) the Primary Lower Gypsum (PLG) deposits of the Vena del Gesso basin, that accumulated during the first stage of the MSC, and iii) (RLG-C) associated with the Resedimented Lower Gypsum (RLG) deposits in the outer Adriatic foredeep, in the Gabicce Riviera. They are characterized by a brecciated texture, and by the presence of both hard and soft carbonate clasts, like those described for the Calcare di Base of Sicily (CdB type 3, stage 2), pointing to a clastic origin and emplacement from gravity-flow deposits. In Calabria, the studied carbonate deposits can be ascribed to the stage 2 of the MSC and included in the RLG unit. Differently from those found in the Rossano Basin, the carbonate outcropping in the Crotone Basin consist of hybrid deposits including clastic gypsum. Up to twenty-seven distinct facies have been identified and assigned to five major facies group. These facies are primarily characterized by carbonate mudstone and rudstone, hybrid sandstone ranging from very fine hybrid-grain sand to coarse-grained hybrid sandstone, and clastic gypsum represented by gypsarenite to gypsiltite. In the IS-C of San Cassiano and clastic carbonates of Calabria, no primary gypsum facies have been found. The identified facies are devoid of fossil content but exhibit abundance of fecal pellets occurring as peloidal grainstone clasts, suggesting a primary microbial carbonate factory. The IS-C geochemical signatures indicate deposition under strong to moderate evaporative conditions with minor dilute water input followed by moderate diagenetic processes. 87Sr/86Sr values fall within the range of stage 1 + stage 2 deposits (gypsum and carbonate) of literature. The IS-C are characterized by a higher fecal pellets content that could have been produced by opportunistic stress-tolerant invertebrates, likely Artemia salina as suggested by morphological analysis of the peloids. Since no in situ peloidal carbonate deposits has been found, we suggest that they derive from the penecontemporaneous dismantlement of an original carbonate factory, probably located on top of the uplifting Forlì line. The integration of outcrops and subsurface (borehole) data reveals that the carbonate deposits cannot be longer followed in the deeper portion of the basins that are replaced by gypsum and halite deposits; thus, suggesting their presence associated with intra-basinal carbonate factory. In Crotone basin outcrops, lateral and vertical facies variations are observed, with onlapping relationships against the underlying pre-MSC deposits. The facies heterogeneity and thickness variations reflect multiple episodes of sediment supply and variability in available accommodation space. In the early stage, the coarse-grained sediments infill the basin depocenter, followed by subsequent episodes leading to sediment accumulation in more proximal setting. The hybrid deposits contain peloidal grainstone clast, notably smaller in size, suggesting a greater transport distance from the source area. In the subsurface, the RLG contain clastic gypsum and halite. The occurrence of sedimentary features like erosional bases, incorporated clay clasts, normal bed gradation, variations in clast morphology and size, and soft-sediment deformation features indicate deposition by gravity-driven flows of varying intensity. In outcrop, the clastic gypsum topping the carbonates or hybrid carbonate siliciclastic deposits, while in subsurface, the clastic gypsum occurs either at the base or top of the halite bodies; thus, constraining the deposition of halite within the RLG unit of stage 2.