Stratigraphy and sedimentary evolution of the Quaternary Agri Valley Basin (Southern Apennines, Italy)

The high Agri River valley is a WNW-ESE oriented intermontane basin located in the axial part of the Southern Apennines. The basin developed after strike-slip and extensional deformation which affected the pre-existing fold-and-thrust belt since the lower Pleistocene. In the south-eastern portion of the Agri Valley Basin, the Pertusillo depocenter exposes the Quaternary succession, more than 100 thick. The palaeogeography and the basin evolution were reconstructed by a new field survey of the intermontane basin fill, closely integrating physical stratigraphy, facies analysis, geopedology methods with different dating techniques (14C, OSL and AFTA). We present a new stratigraphic framework for the outcropping part of the Agri Valley Basin fill, which has been subdivided into four allostratigraphic units, following the recognition and correlation of the major basin-wide unconformities. The lowermost Lago di Pietra del Pertusillo Alloformation (I, Middle Pleistocene) is composed of lacuo-palustrine silty-clay and silt with interbedded fan-delta lens-shaped gravel bodies, prograding into the lacustrine area. Sources were from both the western and the southern slopes of the basin. Upward-coarsening sequences are common in the fan fringe lacustrine deposits where carbonate layers, root traces and vertebrate remains occur locally. The erosional surface between Alloformations I and II delineates the change from lacustrine to alluvial environment (Valle del Nasillo Alloformation; II, Middle-Late Pleistocene). The latter includes coarse-grained conglomerates and gravels with subordinate silts and fine sands deposited in coalescent alluvial fans. The fans prograded north-northeastwards, forming wedge-shaped bodies that filled the lacustrine area. In the upper part of Valle del Nasillo Alloformation, a laterally continuous, truncated fersiallitic palaeosol indicates decreasing aggradation rate during a fan-surface stability stage. This palaeosol suggests a rather humid climate with hot and dry seasons, alternating with semiarid ones, and provides a key-surface for correlation at the basin-scale. Subsequently the fans retreated and were overlain by the deposits of an axial braided alluvial system (Vallone dell’Aspro Alloformation; III, Late Pleistocene) that drained towards the SE. This unit is made up of multistory gravel/sand bodies (channel fills and gravel-sand bars). The poorly drained environment of the alluvial plain is documented by hydromorphic palaeosols. Transverse alluvial fan bodies interfinger with the axial unit from the southern and northern basin margins. A centimetre thick ash-fall deposit is locally preserved in the intermediate portion of this unit. Chemical composition and OSL chronology, 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 proposed correlation suggests an Upper Pleistocene age for the upper part of Alloformation III. As a consequence, the Middle-Late Pleistocene age of Alloformation II at present can be only inferred. The boundary between Alloformations III and IV (Torrente Casale Alloformation, Late Pleistocene-Holocene?) is an erosional surface, locally underlined by the remains of a reddish brown palaeosol. Alloformation IV is represented by prograding-aggrading coarse alluvial fan deposits developing from both the northern and the southern borders of the basin. Asymmetric subsidence in the Middle(?)–Late Pleistocene Agri Valley Basin is indicated by the accumulation of a very thick pile of aggrading deposits, restricted to the north-eastern faulted margin of the basin (alloformations I and III; unpublished subsurface data). The alluvial depocenter gradually moved towards the south-western border of the Agri basin during the latest Pleistocene, testifying to the shift of maximum subsidence towards the opposite (southern) basin margin.