Geology of the high plains of Castelseprio and Tradate (Northern Italy)

Abstract

The geology of a significant portion of the High Po Plain is here analized. In keeping with the tradition handed down from the previous geological works the term "ferretto" indicates the most weathered sediments that outcrop in the Sornbardian region. Those sediments deposited during the Mindel Glaciation and weathered during the Mindel-Riss interglacial period. As many authors criticized the model that explain the quaternary climatic variability with the classical four glaciation framework, it becomes necessary a revision of the problem. New methodologies (a well detailed scale map used for the survey and the use of formal geological - aliostratigraphic and litostratigraphic - units) are responsible for the quite different interpretation of the studied area. The new framework, if compared with the previous interpretations, is more complex and teemed with data. The Valle Olona, and the outern portions of the Verbano and Lario glacial amphiteatres are studied here. On the basis of the analisys of the units recognized in the two glacial amphiteatres it become possible to put to evidence some identities (E.g., the Alloformazione di Cantu is the same in the Lario and in the Verbano sectors because there is no break in the moraines proceeding from one amphitheater to the other.), some similarities (E.g., the Alloformazione di Binago and the Alloformazione di Golasecca have similar weathering profiles and occupie comparable stratigraphic positions.), and some differences (E.g. the first evidence of a Pleistocene glaciation is represented by a completely weathered till in the Verbano area and by a tillite in the Lario sector.). Another interesting point is the analisys of the conglomerates (so called Ceppo in the lombardian region) that outcrop in the territory. The aim of the study is to recognize similarities and differences among cemented bodies and to reconstruct the different palaeogeographical settings witnessed by these sediments. Two main cemented bodies of gravel are recognized: the Ceppo della Bevera, the older, has a petrographical composition that is related with a local (prealpine) feeding whilst the Ceppo detl'Olona, the younger, is related with a greater catchment as indicated by his petrographical composition that clearly includes alpine pebbles. It is possible to know the oldest phases of the development of the territory analyzing the buried sedimentary bodies. These units outcrop in most of the little and deep canyons present in the area and are also recognizable in the boreholes. The buried bodies allow us to put to evidence both the paleogeographical setting during the Pliocene and the evolution of the Valle Olona starting with the Messinian age. During the Late Pliocene the territory was affected by the two erliest great glacial advance. Tills are preserved both in the Verbano and in the Lario amphitheaters. No morphological elements (moraines, sandur, etc.) connected to those galciations are still preserved. In fact, a detailed survey allows to put to evidence a lot of geological data concerning this area that were disregarded in the past. Working with the aim of the reconstruction of the geological setting analyzing the amphitheaters one by one, and looking for the correlations only when the single frameworks are clear allow to avoid the mix up of sediments that look alike even if they are geologically different (e.g.: no similar stratigraphical positions, petrographical compositions, weathering profiles, etc.). As consequence, the "ferretto", that is the "appearance" of some sediments, cannot be regarded as a diagnostic element for building up geological units, because different units can be equally weathered. The "ferretto" should be compared with a fades, and no more. The revision here proposed for the lombardian area is useful in order to look at the classical quadriglacialist model from a new perspective. The classical model looks wrong in this area and that is enough to put to evidence the necessity of work in the future, using new tools and new ideas, for the understanding of the evolution of the whole Po Plain.