L’Arenaria miocenica di Ponsano (Toscana, Italia): Evoluzione paleoambientale e Bio-cronostratigrafia

In this work benthonic and planktonic foraminifers of the Middle-Upper Miocene sediments of the Ponsano Sandstone have been studied. The investigated outcrops of this formation are located at Ponsano (SE of Siena) and Rencine (NW of Siena) in Tuscany. Some studies on these sediments have been done in the past years to define the age and the characteristic of the abundant fauna contained into the sandstones, but no environmental analyses have been done. The aim of this study is to define the thickness of this formation, the paleo-environmental changing through time, its regressive or trasgressive trend, and to controll its age. In Rencine area the formation lie on the bedrock represented by the Ligurian Unit and it is composed mainly by sandstones and marlstones, only in the upper part of the sequence pebble conglomerates occur. In this area the unit is referred to the Neogloboquadrina continuosa-Paragloborotalia siakensis Zone (Neogloboquadrina continuosa Subzone) and therefore it is late Serravallian in age. In the Ponsano area the base of the formation is not outcropping. On the contrary the top below the late Tortonian lacustrine sediments (“Serie lignitifera”) is well exposed. In this area, the outcropping sediments of the units are referred to Neogloboquadrina acostaensis Zone, therefore they are arly Tortonian in age. Seven stratigraphic sections have been measured and the total thickness of the outcropping sediments is about 500 m. In this area the Ponsano Formation is characterized by two marly and two sandy levels. . Sandstones layers are grey-yellow, medium to fine grained, well cemented, rich of macro fossils and trace fossils of the Skolithos ichnofacies; in the upper sandstone conglomerates occur. Marlstone are composed by grey and massive marls and sandy marls. Benthonic Foraminifers are present in all the lithologies. In Rencine outcrop the most frequent are: Ammonia beccarii, Nonion boueanus, Textularia sagittula, Lobatula lobatula and Elphidium crispum. This assemblage is indicative of the shallow part of the infralitoral zone. The Rencine succession may have the meaning of a trasgressive trend. In the Ponsano area, on the base of the foraminifers assemblages, it is possible to subdivide the succession in 4 part: A - with frequent Ammonia beccari, Nonion boueanum, Textularia sagittula, Ammonia inflata, Nonion commune and Ammomarginulina sp., and common Bigenerina nodosaria, Lagenammina atlantica and Elphidium crispum; B - with abundant Heterolepa bellincionii, Ammonia inflata, Lenticulina calcar, Textularia abbreviata, Valvulinerina bradyana, Hopkinsina bononiensis and common Nonion commune, Spiroplectinella carinata and Haplophragmoides spp.; C - with the same abundant taxa of B, but without Hopkinsina bononiensis (present in only one sample) and with very frequent Uvigerinidae (Hofkeruva rutila and Uvigerina hollicki) and Buliminidae (Bulimina elongata, B. costata, B. affinis) and common Fondbotia wullesdorfi, Pullenia bulloides, Sphaeroidina bulloides. The maximum values of species diversity are recorded in this assemblages; D - with rare Ammonia beccarii, Elphidium crispum, Florilus boueanus, Quniqueloculina sp. and Textularia sagittula. The qualitative and quantitative foraminiferal data indicate a neritic environment. The first marlstone level is referred to the circalitoral zone, while sandstones and the second marlstone levels are referred to the infralitoral. Sediments of the upper part of the succession (sandstones and conglomerates) have been interpreted as deltaic deposits. The succession of the Ponsano outcrop is a regressive trend. Sedimentation of such thick succession of marls and sandstones in a shelf environment is related to a constant tectonic subsidence, while the batimetric fluctuations (Fig. 5) could be related to eustatic changing of the sea level. In particular the boundary between the upper level of marl and sandstone is correlated with Mi7 event of the isotopic zonation. Instead the end of the sedimentation could be associated with a decrease of the subsidence rate and/or to an increase of the sedimentation rate related to the uplifting of near areas.