LEMBI DI DEPOSITI FLUVIALI PROVENIENTI DAI BACINI ALPINI NORDOCCIDENTALI SULLA COLLINA DI TORINO PRESSO VILLA GUALINO (NW ITALY)

Abstract

Remnants of fluvial deposits from Northwestern Alpine basins in the Turin Hills at Villa Gualino (NW Italy). Recent investigation on the
Western slope of the Turin Hills reveals a series of flat terraced surfaces produced by fluvial modelling occurred when morphological
conditions were very different from the present. Intense human occupation of the hillsides and diffusion of aeolian loess hinder the
study of these surfaces and make it hard to determine whether they still retain the original alluvial deposits or not.
Stratigraphic examination of some boreholes made during rectructuring of Villa Gualino on a terraced surface (130 m above the Po
Plain level) points out the presence of fluvial deposits in the immediate subsoil between the marine substratum and the aeolian cover.
Evidence in favour of this view is provided first of all by penetrometric tests of a cover 10-15 m thick of incoherent sediments, unusual
for these hills.
The stratigraphic data show the presence of an approximatively 5-m thick lenticular sandy to silty fluvial body that is distinct from the
substratum and separated from it by an evident erosion surface. The concave shape of the base indicates the presence of a channel.
The subhorizontal upper surface, on the other hand, points to the development of a strip of alluvial plain subjected to overspill flooding.
Moreover in the alluvial deposits fossils, which are abundant in the marine substratum, and bedding are lacking.
Textural and mineralogical data show that the grain size of fluvial deposits is much finer than substratum and less selected than loess,
and that their mineral composition qualitatively and quantitatively is different.
Pedological data reveal a marked weathering different from that of both the substratum and the overlying loess. These observations point to a surface erosion earlier than the formation of the alluvial body and that significant time gap occurred before the loess deposition.
In the absence of other stratighraphic data, the fluvial deposits can be assigned to the upper part of Middle Pleistocene in agreement
with the accepted, late Upper-Pleistocene age of the aeolian loess.
These observations are confined to a single strip of terraced surface and must be supplemented with further evidence from its surroundings. These preliminary results suggest that the fluvial deposits are derived from the facing mountain Alpine area, especially from the hydrographic basin of the Dora Riparia River, prior the establishment of the Po River