Franco GIANOTTI, Maria Gabriella FORNO, Susan IVY-OCHS, Giovanni MONEGATO, Roberta PINI, Cesare RAVAZZI
The Ivrea Morainic Amphitheatre (IMA) is the main evidence of Quaternary glaciations in the Dora Baltea basin (NW Italy). This end moraine system forms a 500 km2 wide complex of lateral and terminal moraines and kame terraces, dated from the end of the Early Pleistocene (on palaeomagnetic basis) to the end of the Late Pleistocene (14C radiometric and 10Be exposure ages). A synthesis of the state of the art on the IMA stratigraphy is presented, based on the previous literature and reinterpreted in the light of new surveys and dating. Some embedded palaeosols and palustrine deposits within the glacigenic sequences, interpretable the first as interglacial layers and the latter as interglacial or interstadial layers, crop out along stream incisions cutting the IMA outer and inner edges or were sampled through boreholes (Alice Superiore and Torre Canavese cores). The top of the deposits shows different degrees of weathering and they can be distinguished into three pedogroups based on the hue index color of the soil horizons. Buried stratigraphical markers and pedogroups together allowed the subdivision of the glacigenic succession into nine stratigraphic units (Mongrando, Bornasco, Montino and Zubiena synthems, grouped in the 2.5YR pedogroup; Parogno and Torrazzo synthems, grouped in the 5YR pedogroup; Magnano, Serra and Ivrea synthems, grouped in the 7.5YR pedogroup). Each synthem has been attributed to different glacial expansion. So a potentially complete glacigenic succession outcrops, which can be possibly correlated to the whole sequence of the main Quaternary glaciations of the marine oxygen isotope stratigraphy from MIS 22-20 to MIS 2. The widest unit (Serra synthem), related to the penultimate glaciation (MIS 6), comprises the Serra d’Ivrea lateral moraines and is subdivided into two sub-units (External Serra and Internal Serra subsynthems) on morphological grounds. The innermost unit (Ivrea synthem) related to MIS 2 is subdivided into 13 sub-units on morphological basis, corresponding to the height of the Last Glacial Maximum (Piverone subsynthem), to six LGM retreat stadials in amphitheatre (Palazzo, Quintas, Pavone, Bienca, Prà San Pietro and Germano subsynthem) and six LGM-Lateglacial stadials in the Dora Baltea Valley.
glacial stratigraphy, glaciations, end moraine system, Pleistocene, Western Alps, LGM.