Elementi di geologia

The Montello hill is an uplifting anticline structure located in the front of the Neogene- Quaternary Venetian Alps chain. It’s made of Messinian (ca 6.5-5.3 Ma) rocks deposited in a transitional marine to continental environment. These are conglomerates and sandstones with calcite cement alternating with mudstones (Montello Conglomerate). The Montello Conglomerate belongs to the South-Alpine Molasse deposited in a foredeep which development started from the Serravallian onward. The maximum thickness of the Messinian unit is 1800 m and the depositional coarsening and shallowing upward trend points to a rapid filling of the foredeep. The clast composition of the conglomeratic levels is mainly carbonatic (Mesozoic limestones and dolomites) but older magmatic and metamorphic pebbles testifying the erosion level of the northern source areas are also present. The lowermost Messinian deposits are represented by a number of cycles consisting of shallow-marine to brackish-water mudstones and sandstones, associated with fluvial/alluvial-fan conglomerates and sandstones. These deposits represent the last record of a marine influence. The continental succession mainly consists of an alternation of lacustrine deposits and alluvial-fan conglomerates. Major fan bodies entered from the north in a lacustrine basin probably hydrologically closed, i.e. without waters outflowing into the Mediterranean. Four megasequences were recognized at a regional scale. They may be explained as cyclic pulsating tectonic episodes or interaction of relatively continuous deformation with climatically modulated cyclical changes in the regional base level. Recent investigations by means of seismic profiling (TRANSALP) show that the Montello anticline lies over a ramp of a south-vergent blind thrust (Montello thrust), which is the frontal thrust of the Venetian belt. The back limb of the fold is deformed by an antithetic reverse fault (Montello backthrust) cropping out on the south slope of the Col Cesen. Therefore, the overall geometry of the Montello structure is that of a pop-up. The Montello hill is located at the mid point between the Schio and Gemona lateral tips of the eastern South Alpine belt front, which shortening absorbs part of the convergence between Africa and Europe. The 1976 destructive earthquakes of the Friuli are the last prominent episode of such fault activity. Although the Montello thrust exhibits little evidence of Quaternary activity (778?, 1268, 1857-60 A.D.), the history of its folding and uplift may be unravelled by the analysis of the seven Quaternary terraces mainly developed on the eastern side of the Biadene valley through the erosion of the palaeoPiave. The present-day Piave river flows eastward along the north side of the hill. Instead of the anticline growth, responsible for the westward shift of the river course recorded by the westward younging set of terraces, the eastward river deviation has been ascribed to the glacial history of the region.