Note preliminari sul Neoautoctono dell'Area di Sassa (settore SW del Bacino di Volterra) Provincie di Pisa e Livorno

Preliminary results of multidisciplinary research (geological mapping, paleoecological and biostratigraphical analyses) on The neoautoctonous sediments exposed in the Sassa area (Tuscany, Italy) are presented. Some sedimentary cycles are evident throughout the stratigraphic succession, which ranges from Upper Miocene to Lower Pleistocene. During the Late Tortonian the northern sector of the area (Settore A) formed part of a subsiding continental basin characterized by interfingering fuvio-Iacustrine sands and conglomerates (and locally by lignitie). This fluvio-lacustrine succcssion (the so-called "serie Iignitifera") is absent from the southern part of the area (Settore B) either because no or negligible subsidence occurred, or because Settore B was a paleo-high. Conformably overlying the continental deposits is a predominantly argillaccous succession with paralic and marine microfauna respectively in the lawer and upper part. Biostratigraphical analyses enable correlation of the marine portion with the lower part of the Non-distinctive Zone (foraminiferal zonation), and with the uppermost part of the Amaurolithus primus / A. amplificus Zone (calcareous nannofossil zonation). This brief marine episode was terminated by an evaporitic event (ev), coeval with the beginning of the Mediterranean "salinity crisis", and leading to the deposition of primary gypsum. The evaporitic drawdown rejuvenated the basin's morphology, bringing fresh water, clastic material (clay, sand grave l and reworked gypsum) in the deepest parts of the basin. The so-called "lago-mare" environment was then established in the area. In Settore A the subsidence allowed appreciable sedimentation; in Settare B the "lago-mare" sediments are lacking, either because of non-deposition or subsequent erosion due to tectonic uplift; an angular unconformity between Messinian and Lower Pliocene deposits testify to the latter hypothesis. During the Early Pliocene a diachronous transgression restored the marine domain to the Sassa area, starting from Settore A and migrating southward in Settore B. In Settore A, mostly clayey sediments conformably overlie the uppermost Messinian " lago-mare" sediments and range from the Sphaeroidinellopsis seminulina s.1. Zone lo the Globorotalia puncticulata Zone (foraminiferal zonalion), and from the Discoaster variabilis s.1. Zone of the D. tamalis Zone, Reticulofenestra spp. Subzone (calcareous nannofossil zonation). The Lower Pliocene sediments are unconformably overlain by a Middle Pliocene calcarenitic argillaceous succession (G. aemiliana Zone, upper part of the Reticulofenestra spp. Subzone and Coccolithus pelagicus Subzone, D. tamalis Zone) and are transgressive over the praeneogene. In Settore B the succession is continuous from Lower to Middle Pliocene, reaching the lower part of G. aemiliana Zone (D. tamalis Zone, Reticulofenestra spp. Subzone) and rests unconformably on Messinian sediments and praeneogene units. During the early Pleistocene a new subsidence episode occurred, which determined the deposition of both conglomerates and sands; these deposits, unconformably overlying the Miocene and Pliocene sediments, only outcrop in the western and northern sides of the investigated area.