Il bacino Plio-Pleistocenico del Valdarno superiore: Eventi deposizionali, paleomagnetici e paleoclimatici

Abstract

The structural setting of the Valdarno Basin in the Apennines is conformable to the NW-SE trend common of Apenninic intermontane basins. The Valdarno Basin is an extensional asymmetric basin 35 km long and 15 km wide produced by the differential uplifting of the Apennines during late Pliocene-middle Pleistocene times. Major normal fault systems characterize the steep NE margin that was affected by marked vertical displacements, whereas minor antithetic faults mark the gentler SW margin. Alluvial and palustrine-lacustrine sediments fill the basin up to a depth greater than 550 m. Sharp erosional surfaces, depositional hiatuses and local angular unconformities define three main sedimentary successions, (a) Deposition of the lowermost succession - outcropping at Castelnuovo dei Sabbioni - started in small depressions with fluvio-deltaic sands and gravels; the NE flank of these depressions is bordered by the major normal fault systems. Later, environmental conditions suitable to peat deposition occurred with thick lignite seams forming in the south-western margin of the basin. This was followed by rapid and intense subsidence causing the deepening of the lake which, in late Pliocene, was filled by deltaic sediments and whitish to light-yellow alluvial sands. Ongoing palynological analyses indicate a subtropical/warm-temperate climate at the bottom of the sequence and cold-humid to cold-dry climate in the upper part of it This latter episode is probably related to the major cooling occurred in late Pliocene and may be correlated with the Praetiglian of northern Europe. Dry climate conditions, during the final phase of the basin infilling, favoured ephemeral and fluctuating perennial flows producing terminal fan deposits. On the top of the sequence a new warm-humid episode occurred. At the end of late Pliocene, major tectonic movements displaced mainly the north-eastern margin of the basin, which was markedly tilted and enlarged, (b) The second succession (oucropping at Montevarchi) was deposited during latest Pliocene-early Pleistocene; lacustrine and fan-delta sediments from the uplifted flank of the basin form the succession. The fan-delta deposits consist of fan-head coarse-grained disorganized gravels with an abundant sandy matrix; down-fan, these deposits pass laterally to lenticular bodies of gravels and massive sands, whereas transitional and subaqueous portions of fan-delta deposits consist of massive silts interbedded with lenses and layers of sands. The lacustrine deposits are predominantly clayey with sand intercalations. A basin-wide level of peaty clay occurs in the upper portion of the lacustrine sediments. Palynological analyses show a sequence of cold and warm phases, indicative of well established glacial/interglacial climatic cycles. In the upper part of the sequence a cold, prevalently dry phase seems to be correctable to the beginning of Eburonian in northern Europe. The following period of tectonic and depositional stability is recorded by pedogenetic processes and partial erosion of the upper portion of the Montevarchi sequence, (c) Deposition of the third succession started under subaerial conditions during middle (early?) Pleistocene with fluvial and alluvial fan sediments (sequence of Monticelio and Ciuffenna). Fluvial deposits are poorlysorted gravels and medium- to coarse-grained sands capped by overbank massive silt and clay displaying a widespread pseudogley pedogenesis. Alluvial fan deposits are formed of poorly stratified clast-supported gravels and sands covered by intensely pedogenized silty clays. Results of a palynological analysis carried out in the basal part of the succession show that the climate was cold/cool to temperate with humid and dry phases. Mammal faunas are present in several levels of the Upper Valdarno basin allowing the biochronological dating of tectonic and sedimentary stages. A palaeomagnetic study carried out in the first and second successions allows to relate the biochronological dates to the Geomagnetic Polarity Time Scale (GPTS). Palaeomagnetic data from the lacustrine clayey-silt portion of the first sedimentary cycle indicates the polarity sequence belonging to the late Gauss chron. The lowermost silts show a normal polarity followed by a moderately short reversed interval covering most of the lower lignite seam; throughout the following sequence magnetization shows a normal polarity. The faunal assemblage recovered in the lignite seam is early Villafranchian in age, which fits a late Gauss correlation in the GPTS. The second sedimentary succession contains the Matassino local fauna and a slightly younger Tasso l.f. assemblages: these were found in beds lying within the second and third reversed polarity interval respectively, of the composite sequence measured along several outcrops. The late Villafranchian age of the fauna dated as latest Pliocene and early Pleistocene allows to assign the first bed to the top of the Olduvai subchron marking the Pliocene-Pleistocene boundary, and the second bed, which is just above the first one, to the following Matuyama chron. In particular, two other more isolated samples, which were collected in the silt beds of the third sedimentary cycle of mid-late Pleistocene show again a reversed magnetization.