STRATIGRAPHIC ARCHITECTURE AND SEDIMENTARY EVOLUTION OF THE AGRI INTERMONTANE BASIN (SOUTHERN APENNINES, ITALY)

The high valley of the Agri River, a NW-SE oriented intermontane basin located in the axial zone of the Southern Apennines, is filled by Quaternary continental deposits that cover pre-Quaternary rocks of the chain. The basin developed after strike-slip and extensional deformation which affected the pre-existing fold-and-thrust belt since Early Pleistocene (Cello & Mazzoli, 1999; Cello et al., 2000, with references therein). The basin consists of three tectonically controlled and diachronous depocenters, bounded by WNW-ESE and NE-SW faults; outcrops occur in the southeastern one (“Pertusillo depocenter”; Colella et al., 2004), which was deeply dissected by the drainage network during the Late Pleistocene-Holocene. In this area, close to the basin threshold, detailed mapping of lithofacies, architectural elements and their bounding surfaces and analysis of palaeocurrent directions have been selected as the primary criteria for stratigraphic subdivision and geological mapping. Physical stratigraphy, facies analysis (Miall, 1996) and palaeopedological characterization (mainly micromorphological) are the key to decipher the sedimentary evolution of the basin and the contemporaneous palaeogeographic and climatic changes. These data, in combination with OSL, 14C, AFTA dating methods, provide insights into the tectono-sedimentary evolution of the area during the Pleistocene. In this contribution a new stratigraphic framework is presented for the outcropping part of the Agri Basin fill, which has been subdivided into four allostratigraphic units, following the recognition and correlation of the major basin-wide unconformities. These units, accumulated in response to changes in intrabasinal subsidence, catchment relief, regional tectonics and climate, form the Agri Valley Allogroup, up to 100 m thick (Zembo, 2007). Their erosional boundaries can be associated with preserved weathering profiles and/or palaeosols (“palaeosurfaces”; Widdowson, 1997). In the all studied area, these profiles correspond to deeply truncated palaeosols, developed during geomorphological stability stages. The unconformable boundary with the tectonic substratum crops out only at places, close to the basin margins (Fig. 1A) and along the actual shore of Pietra del Pertusillo Lake, in the regions of minimum thickness of the clastic succession. The early basin fill was associated with Spinoso Conglomerates Formation (Unit I, Early Pleistocene ?) that is formed by stratified and weathered conglomerates, locally up to 20±30 m thick, which are strongly deformed and disconnected to the original source area. The conglomerates are probably cut by transtensional faults and are uplifted at different elevations only along the southeastern basin flank. These deposits have been interpreted as the result of coarse-grained alluvial fans that were probably dominated by braided channels. The lowermost Lago di Pietra del Pertusillo Alloformation (Unit II, Middle Pleistocene) is exposed in the axial sector of the all outcrop area and its total thickness is unknown at present; a minimum of 30-40 m can be figured out by the available exposures (Fig. 1). Unpublished subsurface data suggest that the maximum thickness of Unit II can reaches 200 m along the northeastern basin flank. At its base, this unit rests unconformably over the bedrock along the surface S2. The contact with the pre-Pleistocene deposits or Spinoso Conglomerates Formation is covered. Unit II 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 (calcrete), root traces and vertebrate remains occur locally. The erosional surface S3 between Unit II and III delineates the change from lacustrine to alluvial environment (Valle del Nasillo Alloformation; Unit III, Middle-Late Pleistocene; Fig. 1). The latter unit 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. Valle del Nasillo Alloformation reaches 40 m in thickness adjacent to the southeastern basin margin and thins to the northwest. In the upper part of this unit, a laterally continuous, truncated (surface S4) fersiallitic palaeosol indicates decreasing aggradation rate during a fan-surface stability stage and provides a key-surface for correlation at the basin-scale (Fig. 1A). This strongly weathered palaeosol appear to be related to long-cycle pedogenesis (i.e. some ten thousand years) rather than extreme palaeoclimatic conditions and suggests a rather humid climate with hot and dry seasons, alternating with semiarid ones. Subsequently the fans retreated and were overlain by the deposits of an axial braided alluvial system (Vallone dell’Aspro Alloformation; Unit IV, Late Pleistocene) that drained towards the SE. This unit, up to 70 m thick, is made up of multistory gravel/sand bodies (channel fills and gravel-sand bars; Fig. 1). The poorly drained environment of the alluvial plain is documented by hydromorphic palaeosols and vertisols (Fig. 1A). Transverse alluvial fan bodies interfinger with the axial unit from the southern and northern basin margins. A centimeter thick ash-fall deposit is locally preserved in the intermediate portion of this unit. Chemical composition and mineral assemblage 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 Ionian Sea cores, dated as 56±4 ka (40Ar/39Ar on sanidine; Allen et al., 1999; Kraml, 1997). This volcanic level is expected to offer a good geochronological marker as a result of the dating of the pyroclastic apatites currently in progress. The boundary between Vallone dell’Aspro Alloformation and the overlying Torrente Casale Alloformation (Unit V, Late Pleistocene-Holocene?) is an erosional surface (surface S5), locally underlined by the remains of a reddish brown palaeosol (Fig. 1B). Torrente Casale Alloformation crops out close to the marginal slopes of the basin and is represented by prograding-aggrading coarse alluvial fan deposits developing from both the northern and the southern borders of the basin, above the underlying axial braid plain (Unit UIV; Fig. 1). 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; Murray & Wintle, 2000 and 2003) 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 Basin fill. The data presented for the younger phase of sediment deposition suggest a Late Pleistocene age for the upper part of Vallone dell’Aspro Alloformation (Unit IV) and a Late Pleistocene-Holocene? age for Torrente Casale Alloformation (Unit V). As a consequence, the Middle-Late Pleistocene age of Valle del Nasillo Alloformation (Unit III) at present can be only inferred. Asymmetric subsidence in the Middle (?)–Late Pleistocene Agri Basin is indicated by the accumulation of a very thick pile of aggrading deposits, restricted to the northeastern faulted margin of the basin (Unit II and IV; unpublished subsurface data; Fig. 1B). The alluvial depocenter gradually moved towards the southwestern border of the Agri Basin during the latest Pleistocene, testifying to the shift of maximum subsidence towards the opposite (southern) basin margin. This is interpreted by Morandi & Ceragioli (2002) and Maschio et al. (2005) as reflecting a recent fault control at the southwestern side of the valley. Finally, during latest Pleistocene-Holocene time the Agri Valley Allogroup succession was incised and terraced by the modern Agri River and its lateral streams.