The new geological map of the area between Mendrisio (Switzerland), Como and Varese: 300 million years of geological history

Ambrosi, Christian and Bernoulli, Daniel and Scapozza, Cristian and Schenker, Filippo and Dall'Agnolo, Stephan (2016) The new geological map of the area between Mendrisio (Switzerland), Como and Varese: 300 million years of geological history. In: Abstract Volume 14th Swiss Geoscience Meeting 14 th Swiss Geoscience Meeting, Geneva, Switzerland.

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The new sheet Mendrisio of the Swiss Geological Atlas of Switzerland 1:25 000 covers an area that is crucial for the understanding of the sedimentary and tectonic history of the western Southern Alps of Lombardy (northern Italy) and Ticino (Switzerland). Three major tectono-stratigraphic units can be distinguished: 1. The area of Monte San Giorgio that is characterized by a Mesozoic high exposing a stratigraphic succession from the Hercynian basement to the Cretaceous Flysch Lombardo; 2. The Generoso area, separated from the Monte San Giorgio area by a Liassic syn-sedimentary listric fault of several kilometres throw and dominated by 4 km basinal syn-rift (Lower Liassic) and post-rift (Middle Liassic–Cretaceous) sediments, and 3. The unit of the Gonfolite Lombarda Group, a Oligo–Miocene clastic wedge that was back-thrusted onto the Mesozoic successions in late Miocene times. The map documents the evolutionary steps from Permian transtension and volcanicity, Triassic basin formation accompanied by the deposition of hydrocarbon source rocks, late Triassic to middle Liassic rifting along syn-sedimentary crustal faults, and ocean-wide palaeo-oceanographic chances during post-rift thermal subsidence. Early Alpine pre-collisional orogenic movements are testified to by late Cretaceous flysch sediments; Oligo–Miocene post-collisional ones by the deep-water clastic wedge of the Gonfolite Lombarda Group hat is part of the subsurface Milano fold belt below the Po Plain. Deep valleys, incised into the fold belt and flooded by the early Pliocene transgression are related to the Messinian salinity crisis of the Mediterranean. Numerical dating of Quaternary sediments and climate proxies reflecting content-wide or global events allow establish a regional stratigraphy integrating the different glacial events into a coherent picture. Four major Quaternary chronostratigraphic units are distinguished from top to bottom: 4. Postglacial deposits (“Depositi del Postglaciale ”; 0–0.0117 Ma) are referred entirely to the Holocene. 3. Deposits of the Last Glacial Maximum and the Late-glacial (“ Depositi dell’Ultimo Massimo Glaciale e del Tardoglaciale ”; 0.0117–0.029 Ma). 2. Deposits preceding the Last Glacial Maximum (“Depositi precedenti all’Ultimo Massimo Glaciale ”; 0.029–0.781 Ma), both referred to the Middle and Late Pleistocene. 1. Deposits referred to the Early Pleistocene (“Depositi del Pleistocene inferiore”; 0.781–2.588 Ma). In the detailed legend of the map, we represent both age and facies of the Quaternary deposits. This representation of the chronostratigraphical units enables a ‘harmonised cartography’ in a larger regional framework because it allows for a quick visualisation of the spatial extent of the main morphoclimatic events, as, for example, the maximal glacial extent during the Last Glaciation compared to the previous ones. It also facilitates the understanding of the dynamics and the recognition of the main glacial flow directions in the area, in particular of the important role played by the Larian lobe of the Adda glacier with respect to the glacial lobes of Lake Lugano.

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