| Abstract/Notes | The Neogene sedimentary successions, cropping out in the Casino Basin and the northern and southern region of the Siena and Val d’Elsa basins respectively, have been extensively investigated (see also Lazzarotto & Sandrelli). In most of the study area, is located between the “Dorsale Medio Toscana” (DMT) and the “Dorsale del Chianti” (DdC), Neogene sediments unconformably overlain the Ligure Complex(see attached map, scale 1:25.000). Their deposition is mainly related to a distensive tectonics starting from the Middle Miocene (Carmignani et al., 1994 e Elter & Sandrelli, 1995). Formations of the Ligure Domain are exposed only in the periphery of the investigated area and are briefly discussed herein. The first distensive phase started after the late Early Miocene and led to the deposition of the marine late Serravallian Arenaria di Ponsano (Mazzei et al., 1981; Bertini et al., 1991; Elter & Sandrelli, 1994). This unit is composed of inner shelf sandstones. Its base is marked by a transgressive conglomerate. In the late Tortonian a second distensive (Bertini et al., 1991; Bossio et al., 1993; Elter & Sandrelli, 1994) phase produced tectonic depressions with NW-SE (Appenninic) trend, such as the Volterra-Chiusdino and Elsa-Siena basins. Deposition of late Tortonian lacustrine sediments, i.e. Conglomerati di Pod. Luppiano and Argille del T. Fosci p.p., is associated with the formation of these basins. During the deposition of Argille del T. Fosci, the starting rising DMT was not yet an obstacle for the connections between the Volterra-Chiusdino and Elsa-Siena basins (Bossio et al., 1995) and, therefore, that the first Messinian transgression, which is well recorded in the westermnost basins, reaches also the study area. This event led to a gradual transition from a lacustrine to a brackish-lagoonal domain, as clearly showed by the microfaunal content of the uppermost part of the Argille del T. Fosci. The progressive rise of the DMT caused the definitive closure of the connections between the Volterra-Chiusdino and Elsa-Siena basins, the latter being not interested by the second, more important Messinian transgression. As a consequence, the marine and evaporitic Messinian formations, which extensively crop out in the westernmost basins, are not present in the studied area. The early Messinian units are succeeded unconformably by late Messinian deposits comprising the Argille del Casino, the Conglomerati di Lilliano and the Breccia di Grotti. The outcropping area of the Argille del Casino is wide, extending all over the length of the Casino Basin. This formation consists of clays, silty clays and mans with lenses and horizons of lignite, with a microfaunal content characteristic of freshwater or meiomesohaline water. The uppermost levels of the Argille del Casino are characterized by ostracofaunas typical of the Parathetys (Bossio et al., 1993b). These assemblages are indicative of the “lago-mare” biofacies which is distinctive of those Mediterranean areas where the Pliocene trangression took place “water on water” leading to a change from Late Miocene continental to Pliocene open marine environment (laccarino & Bossio, 1999). The fluvio-lacustrine Conglomerati di Lilliano, which are intercalated in lenses within the Argille del Casino and interfingered to their uppermost part, crop out in the westernmost and easternmost areas of the basin. The Breccia di Grotti, deposited in an alluvional and/or deltaic system, is exposed in the south-western area of the basin. Available data indicate that the Late Miocene lacustrine domain extended deeply southwards reaching at least the Grotti area, but they shed no light about the areal extent of this domain northwards. Pliocene sediments (comprising the Argille azzurre, Conglomerati di C. Stieri, Conglomerati di Gaggiano and Sabbie di Tolciona) are well represented in the north-west part and the southern extremity of the studied area. The Pliocene succession varies according to location. In the north-west the Argille azzurre record two distinct cycles separated by a continental event (Bossio et al., 1993b). The first cycle is represented by fossiliferous clays of early Zanclean age, which rest conformably (“water on water” transgression) upon the Argille del Casino. The fluvio-lacustrine Conglomerati of C. Stieri separate the deposits of the first cycle from the Zanclean Piacenzian, fossiliferous silty-clays of the second cycle. In the southern outcrop the base of the second cycle is marked by the transgressive Conglomerati di Gaggiano. The silty-clays are followed by the Sabbie di Talciona in both the southern and northern areas. This unit consists mainly of yellow-ochreous sands and sandstones, which include a level of conglomerates. The Sabbie di Talciona shows a quite variable stratigraphical position, occurring above (as regressive deposits), below (as transgressive deposits) and laterally to the Argille azzurre. These geometric relationships suggest a Zanclean p.p.-Piacenzian p.p. age for the Sabbie di Talciona. |
|---|
