Studio geologico dell’Altopiano di Poirino (Torino)

Geological study of the Altopiano di Poirino

Abstract

During a reapraisal of the recent evolution of the Piedmontese sector, as part of the Carta Neotettonica d’Italia promoted by the Progetto Finalizzato Geodinamica it was felt desirable to examine the Altopiano di Poirino (Poirino Plateau), which lies immediately to the South of the Collina di Torino relief. It has been thought that the drainage of the S. Piedmontese basin (now to the N of the Collina di Torino) lay across this area in pre-Wurmian times (CARRARO, 1976), though this has not yet been demonstrated for the plateau itself, which has been regarded as substantially the result of erosion of the substratum with a slight wind-borne cover. By contrast, this paper shows that the area as a whole includes fluvial deposits, together with the remains of meanders with kilometer-order bends (FORNO, 1980). These form a 10-30 meter layer on the “Villafranchian” substratum: their pedological features, supported by radiometric dating of the Moncucco Torinese fossilbearing bed (ALESSIO & alii, in press), have shown that they consist of two sequences (A and B) with different paleosols and a different areal distribution. Sequence A (Unit A1 and Unit A2) is referable to the upper part of the Middle Pleistocene, while sequence B belongs to the Upper Pleistocene. Unit A1 is composed of prevalently gravel deposits in the S sector of the plateau. These include poorly spherical, distinctly heterometric pebbles (quartzites and conglomerate quartzites), whose average size is about one decimetre. In addition a scanty sand matrix and a weak planoparallel stratification enable a distinction between these deposits and the underlying “Villafranchian” substratum, with which they have hitherto been assimilated, and also show that they are of fluvial origin. The substratum, in fact, is primarily composed of much smaller, highly spherical, homometric pebbles, with oblique-laminar stratification, and its matrix is much more abundant. Unit A1 is evidently attributable to a powerful stream flowing from the S. Piedmontese basin (from SW) as shown by the lithology, and draining towards to E, though this is uncertain, since the deposits have been removed by erosion. The silt and clay deposits of Unit A2 (S and centre of the plateau) come next. Their texture (poor sorting and a coarse sand fraction), and their gradual transition from the underlying pebbles suggest that they, too, are fluvial, and not wind-borne as indicated in the literature. The remains of the meander pattern on the surface and the fact that these deposits continue eastwards in the direction of the Asti hills, show that the watercourse ran from W to E to carry the outflow from the S. Piedmontese basin to the central Po plain. Sequence B, too, displays poor selection, and its sandy fraction is even more abundant than that of Unit A2, suggesting that these deposits are of fluvial origin. They lie in the N sector of the plateau and in the “paleo-Tanaro” cutting in the W sector. As in the case of Unit A2, the surviving meander pattern and the continuation of these deposits to the E suggest that the watercourse ran from W (where, in fact, the original bed can be discerned) to E. The present drainage pattern is very different. It is of local significance, its rate is low, and it runs mainly to SSW sud NW, whereas that responsible for these deposits was of regional importance, had a high rate, and run from W to E. The presence of these deposits over an area several tens of kilometres wide, and the fact that their features show them to be the relicts of several bands of meanders, indicate that the drainage pattern migrated over a considerable distance in the course of time. In the upper part of the middle Pleistocene (Sequence A), there were two distinct watercourses: a S one that flowed through the S and central sectors of the plateau and gradually migrated northwards, and a N one that flowed through the Collina di Torino and gradually shifted southwards. In the upper Pleistocene (Sequence B), these rivers became a single in the N sector of the plateau. The “paleo-Tanaro” cutting to the W is the morphological relict (referable to this interval) of the S watercourse before its confluence with the N. Contrary to what has always been supposed, therefore, this ancient bed was not an affluent of the present N river (F. Po), but of its abandoned course through the tableland. The present plateau is deformed: its N sector slopes to the SSW, and its central and S sectors slope to the NNW and NW respectively. It is also cut by escarpments to the E and W, and distinctly incised by the drainage pattern. This complicated morphological evolution is substantially due to its tectonics. The overall effect of such deformation has been to impose the present gently synclinal with an approximately E-W axis weakly dipping to the W. To the N and S, this synclinal disappears into the monoclinal of the Collina di Torino and the Langhe respectively. To the E, on the other hand, it is shut off from its original continuation into the Asti hills by a deep flexure escarpment, which has lowered the Asti area by about 60 meters. To the W, it is separated from the S. Piedmontese plain by an escarpment the height of which decreases from N to S. Both the make-up and the truncation of this syncline took place between the upper part of the middle Pleistocene and the upper Pleistocene (i.e. the period covered by Sequences A and B). Recognition of this situation in the plateau, in the other words an area that has remained relatively stable in the hill sector, shows the presence of a recent component in the deformation that the entire hill area has undergone. The age of these fluvial deposits on the surface of the plateau also make it clear that both diversion of the S. Piedmontese basin collector to form the present course of the Po to the N of the Collina di Torino, and the shift of the Tanaro to its present position S of the tableland, occurred in the upper Pleistocene, and not in the “Rissian” period as claimed in the literature.