Carbonate deposition during the Messinian salinity crisis in the Romagna Apennines, Italy

Abstract The upper Miocene succession of the Romagna Apennines includes a variety of carbonate deposits that have been included in the “Calcare di Base” lithostratigraphic unit and usually interpreted as primary evaporites recording the onset of the Messinian salinity crisis (MSC). We carried out an integrated multidisciplinary (stratigraphic, sedimentological, and geochemical) study on a Messinian carbonate deposit cropping out in the Romagna Apennines, whose origin and stratigraphic position are still not well defined and for this reason here labeled as incertae sedis carbonates (IS-C). In this study we compare the IS-C with other well-known carbonate deposits of the Northern Apennines that differ in stratigraphic position, age, and origin: i) the PRE-C, interbedded within the euxinic shale unit underlying the primary evaporites (pre-MSC); ii) the PLG-C, hosted in the Primary Lower Gypsum (PLG) deposits of the Vena del Gesso Basin (MSC stage 1); and iii) the RLG-C, associated with the Resedimented Lower Gypsum (RLG) deposits of the Gabicce Riviera in the outer Adriatic foredeep (MSC stage 2). The IS-C carbonates, which are stratigraphically sandwiched between upper Tortonian–lower Messinian deepwater deposits below and uppermost Messinian Lago-mare deposits above, are characterized by a brecciated texture, and by the presence of both rigid and deformed carbonate clasts, similar to those described for the Calcare di Base of Sicily (CdB type 3), pointing to a clastic origin and emplacement from subaqueous gravity-flow deposits. Petrographic analysis enabled the recognition of eight main lithofacies. No carbonate extraclasts and primary gypsum facies have been found, but locally some secondary nodular gypsum beds occur lateral to the carbonate deposits. The stable-isotope compositions are facies dependent, as reflected by the peloidal grainstone clasts showing more positive δ18O values and more negative δ13C values, suggesting deposition under strong to moderate evaporative conditions with minor diluted water inputs, similar to that found in the PLG deposits. Conversely, the matrix shows negative values for both isotopes that could probably be related to moderate diagenetic processes. With respect to PRE-C, PLG-C, and RLG-C types, the IS-C carbonates are characterized by a higher content of fecal pellets whose morphological analysis suggests that they could have been produced by assemblages of opportunistic planktonic metazoans adapted to schizohaline conditions, likely including Artemia salina. No clear in situ peloidal carbonate deposits have been found, suggesting the complete penecontemporaneous dismantlement during MSC stage 2 of carbonate factories developed on the culmination of anticlines related to active thrust faults.