The middle-upper Quaternary of the "Asti basin"

Abstract

A description is given of fluvial deposits and shapes recognized and mapped for the first time at various altitudes in the hilly area between the River Tanaro and the Po watershed to the North, the course of the Borbore stream to the South, the escarpment that marks the eastern edge of the Poirino Tableland to the West, and a meridian line striking close to the city of Asti to the East. In view of their planimetric and altimetric distribution, and their varying degree of preservation and pedogenetic evolution, the fluvial deposits have been divided into two litho-pedomorphostratigraphic complexes (A and B). Complex B is further divided into Unit A and Unit B. It has proved possible to relate the entire series to the Lower Middle-Upper Pleistocene using a local pedostratigraphic scale devised according to Arduino et al. (1984), and appropriately calibrated with chronostratigraphic data derived from neighbouring areas. The fluvial nature of these deposits is revealed both by their distribution, which displays spatial continuity with those forming the Poirino Tableland surface immediately to the West, and by the presence of an rare and sporadically distributed coarse fraction (pebbles with a maximum diameter of 2+3 cm), whose lithology (green stones with subordinate gneisses and quartzites) is clearly distinct from the Upper Miocene-Villafranchian local substrate, characterized solely by quartz ± conglomerate pebbles. Further confirmation of this genesis is offered by the erosional nature of the basal surface of these deposits on the substrate, which is only visible locally. Air photos have also revealed variously reworked relic forms (mainly fluvial channels) in association with the deposits. The distribution of these fluvial forms suggests that the main lines of the local stream net were substantially the same as today in its earliest pattern, though its establishment took place under very different morphological conditions. At the beginning of the Middle Pleistocene, in fact, the Asti hills were a mainly flat country, corresponding to an erosionai fluvial plain, from which a series of slight, isolated rises protruded. It seems most likely that the ancient net was the eastward continuation of that whose traces have been recognized on the Poirino Tableland by Fomo (1980), which drained the South Piedmontese basin and flowed to the South of the Turin and Monferrato hills until it reached the Po plain through what are now the Alessandria flatlands. The fact that in the Asti reliefs between the Poirino Tableland and the Alessandria flatlands the individual terms of the sequence of the deposits linked to this net are distributed over terraced surfaces, whereas they are superimposed one upon another in the corresponding distribution districts of the two neighbouring area, is an outcome of lifting of the Asti Reliefs during the Middle Pleistocene. The differential nature of the mobility of this sector, with its greater uplifting in the Eastern and Southern sectors as opposed to the elongated, approximately WNW-ESE sector, is brought out by comparison between the present and original (reconstructed) geometrical configuration of their fluvial deposits. Furthermore, the pattern of the longitudinal profiles of the surfaces of the main collector in the time periods considered (area now occupied by the roughly E-W sections of the Borbore and Triversa stream), in which counterslopes are readily visible, suggests that differential lifting also took place along the E-W axis. The present morphological configuration of the Asti Reliefs, whose tops are little higher than the surface that can be thought of as linking the Poirino Tableland with the Alessandria flatlands, is attributed to the insertion of a large tectonic structure (Marginal Flexure of the Poirino Tableland) during the Holocene.