Nuovi aspetti dell’evoluzione paleogeografica e tettonica al Plio-Quaternario della Piana di Lucca

New aspects of the paleogeographic and tectonic evolution of the plain of Lucca during Pliocene and Quaternary (Tuscany)

Abstract

The paleogeographic and tectonic evolution of the Plain of Lucca (Lower valley of the Serchio river, in the north-western Tuscany) during the Pliocene and the Pleistocene are shown. In the investigated area, extended between Lucca, Vinci and the Arno river, sediments referred to some sedimentary cycles, that are both continental and marine, outcrop: in particular between Lucca and Vinci there is a first lacustrine cycle, in the past referred to an Early Villafranchian age; the Author takes a new dating of the basal clays of this cycle, indicating a Late Ruscinian, as a starting point in order to revise and to sketch the picture of the main stages of the evolution in this area: - after the paroxysmal phase, in Northern Apennine an extensional tectonics takes place; lacustrine basins form in Garfagnana (Middle-Upper Serchio Valley) between Barga and Castelnuovo (Early Villafranchian) and between Lucca, Montecarlo and Vinci (Late Ruscinian). A transgression in the South-East of Lucca occurs and is confined by a barrier beach that separates it from the lacustrine basin; - during Middle Villafranchian in the South-East area an upilft begins and a regression occurs; this uplift is not remarkable between Lucca, Montecarlo and Vinci and so there lacustrine sedimentation continues; - during Late Villafranchian lacustrine sedimentation spaces itself with fluvial events. The area corresponding to actual upland named «Le Pizzorne», which separates the Plain of Lucca from Garfagnana, begins to uplift and a Stream that flows down from Le Pizzorne captures the Serchio river; the river, that previously had an endorheic drainage confined to Garfagnana, so enters the plain of Lucca and the lake. Towards the end of Villafranchian, the lacustrine sedimentation between Lucca, Montecarlo and Vinci terminates; an erosion occurs and then a fluvial cycle takes place between Lucca and Montecarlo. In the South-East area a new transgression occurs; - during Middle Pleistocene another fluvial cycle takes place in the area now named «Le Cerbaie» and a second lacustrine cycle, confined in an area close to Lucca, takes place. Owing to further uplift of Le Pizzorne the Serchio river diverts its course to South-West; - during the Riss-Würm interval the Serchio river and the Arno river overflood their valleys; particularly the Serchio, after eroding the sill of the lake of Lucca, at this point extinguished, goes towards South and becomes a tributary of the Arno; - during Würm II, with a new phase of overflooding, the Serchio diverts the course of the Arno towards SouthWest.