Significato geologico di depositi fluviali ghiaiosi pleistocenici medi nella Collina di Torino

Abstract

As part of a research on the recent evolution of the Turin Hills, the authors report and examine the fluvial gravel deposits cropping out on the hills southern slope. These deposits are very discontinuously distributed above the Tertiary marine substratum, as lenticular bodies a few meter thick and several hundred meter long. They are covered by fluvial silt deposits, which are distributed over a wider area. Both gravel and silt deposits fill depressions incised by a meandering river of high discharge and a main eastward trend. Their degree of pedological evolution and their relations with the fossiliferous fluvial deposits of Moncucco Torinese (Alessio et ai, 1982) suggest an uppermost Middle Pleistocene age. The gravel clasts consist of serpentinite, peridotite, gabbro and metagabbro (reequilibrated under eclogite, greenschist and blueschist facies conditions), rare blueschist fades metabasalts, rodingite and quartzite and some other very rare lithotypes. They are not derived from substratum reworking, but from the Alpine sector comprising the Sangone, the Dora Riparia, and the Stura di Lanzo valleys, where the ultramafic Lanzo Massif (which includes the most non-serpentínized ophiolitic peridotites of the Western Alps) and the eclogitic to biueschistic meta-ophiolites of the Piedmont Zone crop out. The absence or extreme scarcity of other rock types, such as granite, gneiss, micaschist, calcschist, and limestone, now exposed in the supposed sector of provenance, is most likely due to their suscertivity to abrasion, weathering or dissolution. Futhermore, the absence of other resistant lithotypes, such as conglomeratic quartzite ("anageniti") and kinzigite and mafic granulite, suggests that these alluvial deposits do not derive from either the more southern or more northern sectors of the Western Alps, respectively. The collected data support the view that during the Middle Pleistocene the outflow of the Southern Piedmont basin occurred in the area now occupied by the Turin Hills (Forno, 1982).