Inquadramento stratigrafico-deposizionale delle faune a mammiferi villafranchiane (Unità Faunistiche Triversa e Montopoli) del Valdarno inferiore nella zona a sud dell'Arno (Toscana)

Abstract

Scope of the study is to set vertebrate finds discovered in the study area into a sequence-stratigraphic framework. Nine sections covering the uppermost 200 meters of Pliocene deposits and a stratigraphica! cross-section along a W-E transect of the central part of the lower Valdarno basin were studied in detail. Facies analyses based on the study of sedimentary fades and on mollusk paleoecology were carried out. It has been possible to subdivide the succession into several small-scale depositions sequences (10 / 30 m thick each). Well-developed sequences are formed (from bottom to top) by: 1) a basal erosional unconformity, 2) iowstand or early transgressive cross-bedded sand, filling the previously formed erosional vacuums, and 3) transgressive and highstand bioturbated mainly pelitic deposits. On the basis of a paieoenvironmental reconstruction and of the study of shallowing- and deepening-trends, the succession has been subdivided into three parts. The lower part (containing sparse remains of mammals from the Triversa faunal unit) outcrops on the eastern side of the basin in an area close to the north-eastern shoulder of the basin from which, during the Pliocene, coarse-grained fan deltas prograded into a shallow sea embayement. Deposits are Iowstand coarse sands and gravels of fluvial or fluvio-deltaic paleoenvironment, covered by transgressive flood-plain fine-grained deposits or lagoonai muddy sands. Marine influence increases upward, showing a deepeningupward trend. Deposits in the middle part of the succession are formed of basal fluvial or deltaic mouth-bar cross-bedded sands, overlaid by transgressive lagoona! muddy sands, and/or by bioturbated shoreface and shelf deposits. The deepest facies is represented by clays with a sparse inner shelf fauna. Thick and laterally continuous shell-beds often mark transgressive surfaces. The upper portion of the succession (containing the Montopoli local fauna) outcrops in the centra! and western parts of the Valdarno basin. Small sequences are composed of Iowstand fluvio-deltaic sands overlaid by transgressive marine inner shelf sands. The study of the stacking pattern of small sequences allowed us to interpret the succession as to be part of a composite large-scale depositional sequence. The lower part of the succession is thought to represent a late Lowstand Systems Tract - early Transgressive Systems Tract (late LST - early TST) bearing Triversa f.u. remains, while its middle part forms the late TST. The upper part of the succession (bearing the Montopoli local fauna) forms the Highstand Systems Tract (HST).