CALIBRATION OF THE UPPER VALDARNO BASIN TO THE PLIO-PLEISTOCENE FOR CORRELATING THE APENNINE CONTINENTAL SEQUENCES

Abstract

The Upper Valdarno continental sequence is formed by 500 m thick sediments calibrated to Gauss and Matuyama chrons, for nearly
2.5 my history. Continuity in lacustrine and fluviatile deposition is interrupted by two major pulses of the Apennine uplift. One is dated to the boundary of the Gauss/Matuyama chrons at 2.58 Ma and lasts until 2.1 Ma, the other from shortly after the Olduvai chron at 1.77
Ma until 1.05 Ma (Jaramillo), or 0.78 Ma (Brunhes). Such interruptions are recorded differently. The former by a condensed sequence
containing two events of the magnetic polarity, the G/M boundary and the short normal chron Reunion within the early Matuyama, while the second missed any record. The condensed sequence is of wind-blown sands, with scanty levels of fine silt, used also for pollen analyses.
The deposition intervals, now dated in the lacustrine sequences, are similarly long. In the first basin filling, from Mammoth at 3.3 Ma,
the lake deposits of uniform silty clays, the Meleto clays, began at nearly 3.15 Ma, from shortly before Kaena through the latest Gauss.
This time span was separately measured by spectral content of the magnetic signal to last 400 ky; there, cyclostratigraphy enhanced
periods of various lengths accounting for either different rates of deposition through the time series or the occurrence of a bimodal cyclicity at 2.85 Ma. At this time, the pollen record started moving towards a reduction of species of the subtropical forest and an increase of the altitudinal coniferous vegetation; an oscillating pattern from open to forest vegetation took place in the early Matuyama. In the second lacustrine cycle the record is less continuous, and the highest percentage of herbs is reached at nearly the P/P boundary.
Key Villafranchian faunas, from Triversa or Villafranca d’Asti sequence of north-western Italy to Pirro faunas of south-eastern Italy, were grouped in the biochronologic sequence of different faunal units, and are now correlated with the Upper Valdarno chronostratigraphic framework. After the early Villafranchian, the Montopoli fauna, the only one with a date (2.58 Ma) before the calibration of the Upper Valdarno, marked the major change with the preceding fauna of Triversa, leading to the middle Villafranchian. The other faunas formed the late Villafranchian, from Olivola of north-western Apennines to Tasso of Upper Valdarno and Farneta in central Italy to Pirro, which were little affected by the global changes occurred at the P/P boundary. After the Villafranchian age, the new fauna starts to build the asset of the present day distribution at the end of the Matuyama, and possibly at the boundary with the Jaramillo chron, as recorded also in the Upper Valdarno.
Devoid of a direct calibration, these faunas containing the Mammal Neogene (MN) age units MN16a MN16b and MN17 of western
Europe faunal distribution, had always been very difficult to correlate with biostratigraphy, and even more with the geomagnetic polarity time scale GPTS. They are now placed relative to the Upper Valdarno magnetochronology, from Mammoth to Jaramillo. The high resolution of the Upper Valdarno is viable for finer enhancements in the Apennine sequences explored by direct magnetostratigraphies.
These will permit the dating of their complete faunal sequences and correlate them with the European late-Neogene mammal faunas.