Prime note sul rilevamento dell'Anfiteatro del Garda: metodi e unità provvisorie

ABSTRACT: Bini A. & Zuccoli L., .......... (IT ISSN 0394-3356, 2004). The Garda glacial Amphitheatre shows some peculiar geomorphological and petrographical features, which differ from those of the other alpine glacial amphitheatres. Therefore it has been always difficult to separate the end moraines based on the traditional classification of the Quaternary alpine glaciations. The most debated topic regarding the characterisation of the Garda Amphitheatre, is the correlation among the different end moraines, about which there is no agreement in literature. The results of the survey presented here are not comparable with the data by the previous authors, because different concepts and terminology are used and a conversion to older stratigraphic concepts is not available. For instance the term Würm is not coincident with the LGM concept, which is in use at present. The Würm Auct. includes probably more glaciations (sensu Richmond, 1986). If no widespread palesoils are available, other parameters are used to identify the different units, such as weathering and colour of colluvium, thickness of calcretes, geometry of depositional units. The colluvial deposits derive from flows which often mix materials from different sources; nonetheless in a lot of situations, within one amphitheatre, colluvium derives from one single unit, with own sedimentological features, or from a single moraine. Taking into account these situations there is a continuum of variations of the features of colluvial deposits, moving from the youngest units (close to the lake shores) to the eldest ones. In the Garda Amphitheatre, presence or absence of calcrete, and its thickness, represent discriminant features. Calcretes are present above all the depositional units, but the most internal one (Garda Alloformation), which represents the LGM. Thickness of calcretes increases systematically moving towards the external sector of the Amphitheatre, whose units are therefore completely cemented. In this way several allo- and lithostratigraphic units could be recognised, eight of which are formed by tills and therefore represent different glaciations.