The deep-water counterpart of the Messinian Lower Evaporites in the Apennine foredeep: The Fanantello section (Northern Apennines, Italy)

Abstract

A possible deep-water non-evaporitic unit equivalent of the Messinian Lower Evaporites has been recognised in the Northern Apennine foredeep. This unit, whose existence is implicitly admitted in the two-step model of the Mediterranean Messinian salinity crisis is documented here for the first time; it occurs throughout the Apennine foredeep basin below a thick horizon of resedimented gypsum deposits, usually ascribed to the Lower Evaporites. Actually, the Lower Evaporites of the Apennine foredeep basin include both shallow-water, in situ precipitated facies and deep-water, resedimented facies deposited in distinct depocenters. The usually envisaged coeval nature of the two deposits has been recently challenged in several works based on a) physical–stratigraphic considerations about the downbasin correlation of the Messinian erosional surface cutting on top the in situ evaporites with the sharp base of the resedimented evaporites unit and b) the common occurrence of a barren unit below the resedimented evaporites which has no obvious equivalents in pre-evaporitic successions underlying the in situ precipitated evaporites. In this work a multidisciplinary study has been carried out on a 140 m-thick composite section (Fanantello section) to definitely assess the age and palaeoenvironmental characteristics of the unit underlying the resedimented gypsum deposits. The topmost part of the section comprises a 60 m-thick succession of barren euxinic shales falling entirely within a reversed polarity chron and characterised by a) the complete disappearance of Foraminifera, b) the occurrence of high salinity tolerant thecosomatous pteropods ( Creseis sp.) c) the abundance peaks of the calcareous nannofossil Sphenolithus abies (interpreted as related to highly stressed likely hyperhaline conditions), d) the appearance of hyperhaline and/or stratified water molecular indicators (gammacerane, < 0.2 pristane/phytane ratio, < 1 n-alkanes odd–even predominance). Based on a multidisciplinary integrated stratigraphic approach this barren interval, is here proposed to be a deeper-water time-equivalent of the in situ primary Lower Evaporites, thus allowing to assess the events connected to the onset and the first phase of the Messinian salinity crisis from a basinal perspective. These data lend support to the hypothesis that the resedimented evaporite deposits of the Apennine foredeep basin postdate the in situ evaporites, thus implying that the term Lower Evaporites actually comprises rock bodies with different origins and separated in time and space by the Messinian erosional surface and its correlative conformity. This could be true also for other Mediterranean basins where the stratigraphy of deep-water successions should be carefully re-examined.