Il “Loess” della Collina di Torino: revisione della sua distribuzione e della sua interpretazione genetica e cronologica

The distribution and genetic and chronological interpretation of the “loess” in the Collina di Torino

Abstract

The subject of the present paper is a reassessment of the distribution and genetic and chronological interpretation of the deposits described in generic terms as “loess” in the Northern Section of the Collina di Torino, and the NW edge of the Altopiano di Poirino (central Piedmont). For this purpose, use has been made of recently established criteria, especially those proposed by the INQUA Loess Commission, for the interpretation and classification of deposits of this type. The term loess l.s. is currently employed for sediments that have been carried by wind in suspension. Depending on the deposition modality, these are further distinguished into aeolian loess or loess s.s. (i.e. deponed by the wind straight into the ground), and loess deposed into water. Next, changes occurring after deposition, either in sito or as the result of subsequent transport, are used to differentiate weathered and non-transported loess from weathered and loess-derivates. The ambiental conditions under which sedimentation takes place serve to distinguish periglacial and desert loess. Lastly, differences in texture are expressed in the terms typical, clayey and sandy loess. The criteria underlying these distinctions are sedimentological (texture and structure), pedological (aggregation, carbonate content and color), and geological (distribution). Following an analysis of the previous literature on the Collina di Torino “loess” (especially the papers of F. Sacco), a detailed survey was made of the area. The results of this survey are illustrated on the accompanying 1:25000 map, and can be summed up as follows with reference to the “loess” distribution areas shown at the feet of the map itself. In central Sector aeolian loess forms a continuous cover (up to 6 m thick) on the Tertiary sediments, which locally outcrops under the aeolian sands in meridional Sector. It runs on into sudoriental Sector, where it becomes thinner and overlays alluvial clayey silts. In occidental Sector, aeolian loess occours locally only under the loess colluvium (loess-derivates). Wind-borne sands (these differ from loess because they are conveyed by saltation as opposed to suspension) form an extensive cover in meridional Sector and may be as much as 8 meters thick. The alluvial clayey silts form the surface of oriental Sector (to a depth of up to 7 meters), and outcrop locally below the aeolic loess in sudoriental Sector. They consist of two complexes with a different soil evolution. The loess colluvium forms a thick, continuous cover in occidental Sector, but it is discontinuous in septentrional Sector. Next, an assessment was made of these deposits in terms of their texture and mineralogy. Aeolian loess alone fell within the loess distribution range formulated by FERRARI & MAGALDI (in Grruppo di Studio del Quaternario Padano, 1976) and displayed a typical particle arrangement; qualitative and quantitative mineralogical differences were noted between these deposits. Account was also taken of other features: structure (only the aeolian sands retained relicts of stratification in the form of curved, non-parallel stratification surfaces ) aggregation (the aeolian loess and alluvial clayey silts displayed a vertical prismatic aggregation, which was more marked in the former), and pedological characters: these permitted to establish a relative succession, since decreasing evolution in the order silts, loess and sands pointed to a middle Pleistocene origin for the two silt complexes, Würmian for the loess, and post-Würmian for the sands. Paleontological confirmation in the case of the loess can be seen in the report of the remains of Elephas primigenius BLUMENBACH in the underlying colluvium by PARONA (1907). Furthermore, examination of the malacofauna observed locally in the loess (and mentioned by previous workers) indicated that it was deposited in a steppe environment, as in the case of comparable deposits elsewhere. Lasty, an attempt was made to determine the chronological succession of these deposits – in particular their parental materials were identified – and the part they played the climatic and tectonic events in the geological development of this area.