Stratigrafia delle successioni sedimentarie evaporitiche e post-evaporitiche del messiniano superiore in Irpinia settentrionale (Appennino meridionale, Italia)

Stratigraphy of the upper Messinian evaporitic and post-evaporitic sedimentary successions outcropping in northern Irpinia (southern Apennines, Italy). The Mediterranean upper Messinian is subdivided into two main intervals (Ogniben, 1957; Selli, 1973; Montadert et alii, 1978; Patacca et alii, 1990, 1992): an evaporitic interval ("lower evaporitic" interval or "first evaporitic" cycle), and a post-evaporitic interval ("upper evaporitic" or "Lago-mare" interval or "second evaporitic" cycle). In the southern Apennine regions, evaporitic and terrigenous deposits referable to the upper Messinian are well exposed only in northern Irpinia-southern Daunia sector (Crostella & Vezzani, 1964; Di Nocera & Torre, 1987; Dazzaro et alii, 1988; Basso et alii, 1996) and in the southern Sannio-western Irpinia sector (De Castro Coppa et alii, 1969; Di Nocera et alii, 1981). Their sedimentary features can be correlated to the evolution of a foreland basins system, but the real sedimentary, tectonostratigraphic and palaeo-environmental meaning of these deposits is not yet fully clear. We have attempted to reconstruct the lithostratigraphic units and a first physical-stratigraphical and chronostratigraphic framework of the upper Messinian in the northern Irpinia-Daunia sector. Detailed lithostratigraphic investigations, biostratigraphical analyses (foraminifera, calcareous nannoplankton and ostracoda assemblages) and preliminary facies analysis have been performed on 11 stratigraphical sections, located in the area encompassing the Cervaro, Calaggio and Fiumarella rivers. As already recognised in the literature in the upper Messinian sequences outcropping along the Apennines (i.e. Roveri et alii, 1998), two sedimentary intervals have been identified in the latest Miocene in northern Irpinia; they are separated by the unconformity LM 1 . The lower interval represents the evaporitic stage, and is characterised by calcareous and gypsum evaporitic sequences, lying conformably upon pre-evaporitic diatomitic and pelitic sequences; the upper interval represents the post-evaporitic stage of the Messinian and is characterised by resedimented gypsum-bearing clastic sequences. The integrated analysis of the planktonic foraminifera and calcareous nannoplankton assemblages enables detailed chronostratigraphic resolution only within the lower Messinian. Biostratigraphical analysis does not permit a chronological attribution in the upper Messinian, because marine fauna was destroyed by the salinity crisis in the entire Mediterranean area. The relative stratigraphical position of the sequences has been inferred through the analysis of the physical stratigraphical relationships, the unconformities and the environmental signatures from ostracoda palaeoecology. Lithostratigraphic events and unconformities have been correlated and discussed and put into a chronological scheme, according to McKenzie et alii (1999), Krijgsman et alii (1999) and Sprovieri et alii (1998). Two stratigraphic units are distinguished within the evaporitic stage. The first unit, named Mt. Castello evaporites (Crostella & Vezzani, 1964; Dazzaro et alii, 1988) is formed by evaporitic limestone, mainly laminated to selenitic gypsum and reworked coarse-clastic gypsum of supratidal to subtidal environment. The second unit, named "Mezzana di Forte" argillites with gypsum, is made up of deep-water facies and is formed by thin bedded grey, red and green marly argillites, calcilutites, calcareous marls with hard-ground, carbon-rich diatomites with sulphur concretions, clays and marls with laminae of small gypsum crystals. These two units can be correlated to the "lower evaporites" or to the "first evaporitic cycle". The two evaporitic units lie conformably upon Middle-Upper Tortonian-lower Messinian pre-evaporitic diatomitic and pelitic sequences of marine and euxinic environment; they are represented respectively by euxinic diatomitic clay and marl ("Tripoli Fm.") and by thin-bedded limestone, clay and marl, named "Torrente Calaggio" polychromatic argillites (Ciaranfi et alii, 1973). Three successions are distinguished within the post-evaporitic stage: the T. Fiumarella unit, the "Anzano Molasse" formation and the Lago-Mare facies deposits. Within these stage a new unconformity (LM 2 ) has been recognised between the T. Fiumarella unit and the "Anzano Molasse" fm. The T. Fiumarella unit (Basso et alii, 1996) is characterised by clastic continental (lacustrine and alluvial) facies; the proximal facies are mainly coarse-grained, while the distal facies consist of clays with lenses of silty sands, conglomerates and reworked clastic gypsum. The "Anzano Molasse" formation (Crostella & Vezzani, 1964) is subdivided into two members. The lower member is composed of granular to pebbly conglomerates and medium to coarse grained turbidite sandstones. The upper member is composed of medium to coarse grained thinly stratified turbidite sandstones and by marly or clayey siltstones, rich of organic matter. The successions have been deposited through grain flows and turbiditic flows with variable distality in a lacustrine environment. A rhyolitic volcaniclastic layer is found in the high part of the Anzano Molasse sequence (Di Girolamo et alii, 1986). The "Lago-Mare" facies (Bonaduce & Sgarella, 1999) deposits are represented by quartz-lithic arenites, calcarenites, hybrid arenites, or silty-marly clays with abundant Ostracoda shells (Ilyocypris gibba, Cyprideis torosa and Candona sp.) exposed in scattered small outcrops....