I depositi lacustri delle valli Chiasone e Pellice (Alpi occidentali, Provincia di Torino)

Abstract

The review of Quaternary sediments outcropping in the lower Chisone and Pellice valleys (inner sector of the western Alps), carried out as part of the new urbanistic plan for "Comunità Montana Val Pellice" and "Comunità Montana Valli Chisone e Germanasca", forms the basis for a new hypothesis on the Quaternary evolution of these Alpine valleys. One of the most important results of this project is the acquisition of data from boreholes and wells on the thick lacustrine (l.s.) sediments filling the lower portion of the two valleys. Sediments are cut by erosional surfaces and are buried by younger deposits of Quaternary age, while their base is currently unknown. In detail, the lacustrine sediments show different lithostratigraphic sequences of different age, and can be subdivided into two complexes: 1. the "Chisone Valley Lacustrine Complex" is a > 300 m thick polygenic sedimentary body more than 8 km long, which fills the lower Chisone Valley. Stratigraphic relationships are uncertain given the presence of very few outcrops with several lithofacies, further complicated by folds of up to 1 metre and local brittle deformations; subsoil data are moreover heterogeneous and have proven difficult to correlate. The complex can be subdivided into four main lithofacies, characterized by heterotopic relationships: a) a typical lacustrine lithofacies, formed by usually varved blue-greyish clayey fine sands and silts; b) similar to (a) but with scattered pebbles and boulders or gravel layers probably derived from the edge of the basin; c) a typical fan-delta facies characterized by yellowish or greyish sands and silts; d) a fan-delta facies formed by gravel layers with a sandy matrix and subordinate blocks and boulders in a sandy-silty-clayey matrix alternating with sand layers. 2. The "Pellice Valley Lacustrine Complex" Is 100 m thick and extends more than 10 km to fill the lower Pellice Valley; only three outcrops have been identified. its stratigraphy is simpler than that of complex 1, with only two main lithofacies: a) frequently varved, blue-greyish fine sands and silts with rare layered gravel of medium to fine grain size up to > 80 m thick; b) yellowish silty sands and silts and medium to fine-grained layered gravel with a preserved average thickness of 15÷20 m. On the basis of stratigraphic relationships with other Quaternary deposits and of palynological data, the "Lacustrine Complex of the Chisone Valley" is assigned to the Lower Pleistocene whereas the "Lacustrine Complex of the Pellice Valley" is thought to date to the early Lower Pleistocene. No data are currently avail-able to indicate the origin of the lacustrine basins which, at the valley mouths, exhibit a reverse gradient sill modelled onto the rocky substratum. With no data indicating an erosional origin of depressions upstream of the sills, an interpretative hypothesis is proposed which sees differential uplift occurring in the transition hills-plain area between the Susa and Po valleys, hampering drainage at the mouth of valleys which had already assumed their general configuration. Subsequent differential uplifting movements prob-ably occurred before sedimentation of the "Pellice Valley Lacustrine Complex" took place. Uplifting may also have been responsible for the general N-NE 10°÷20° tilting of sediments in the "Chisone Valley Lacustrine Complex" and of their considerable deformations.