New chronological constraints on the sedimentary evolution of the Quaternary Agri Valley Basin (Southern Apennines, Italy)

The sedimentary record of the Agri Valley Basin is of great importance for understanding the Quaternary tectonic activity and climatic variability in Southern Apennines. In the south-eastern portion of the valley, the exposed part of the syntectonic basin fill (more than 100 m thick) has been subdivided into four allostratigraphic units, by mapping the major basin-wide unconformities. The basin-fill architecture is characterized by large wedge-shaped alluvial-fan gravel and sandy gravel bodies (Valle del Nasillo Alloformation, II and Torrente Casale Alloformation, IV), prograding-aggrading from both the northern and the southern borders of the basin and thinning towards the depocenter. They pass basinwards to lacuo-palustrine silty-clay and silt with locally interbedded fan-delta lens-shaped gravel bodies (Lago di Pietra del Pertusillo Alloformation, I) or, alternatively, to braided alluvial bodies extended parallel to the basin axis (Vallone dell’Aspro Alloformation, III). A centimetre thick volcanic ash-fall deposit is locally preserved in the intermediate portion of this latter unit. Chemical and mineralogical composition of the volcanic fraction, suggest to correlate this layer with the Tufo Verde Epomeo of Ischia and the pyroclastic fall deposit Y-7 recognised in the central Mediterranean cores, dated as 56 ka. The ability to constrain the age of the alluvial-lacustrine succession allows to determine timing of the sedimentary events and rates of sedimentary processes. The chronological control of continental coarse-grained clastic succession is poor (irregularly alternating deposition, weathering and erosion stages) and is limited by the lack of organic materials for radiocarbon dating. In order to produce a detailed chronostratigraphy and sedimentary evolution, optically stimulated luminescence (OSL) dating were applied. Samples were collected from a variety of environments (sandy bars, crevasse splays and sheetflood deposits); quartz extracted was analyzed using the coarse-grained, single-aliquot regenerative-dose (SAR) technique. The estimated optical ages range from 50 to 5 ka, consistent with the available independent (radiocarbon, tephrocronology) and relative age constraints, provide the first contribution to the chronology of the sedimentary evolution of the upper part of the Agri Valley Basin fill. The data presented for the younger phase of sediment deposition suggest an Upper Pleistocene age for the upper part of Vallone dell’Aspro Alloformation (Unit III) and an Upper Pleistocene-Holocene? age for Torrente Casale Alloformation (Unit IV). As a consequence, the Middle-Late Pleistocene age of Valle del Nasillo Alloformation (Unit II) at present can be only inferred.