Un'area in evoluzione recente: Il versante settentrionale di M. Molinatico (Alta val Taro - Prov. di Parma) con carta alla scala 1:10.000 e sezioni

Abstract

Since the end of the last century, the geological literature has described the northern side of Mt. Molinatico as having forms attributable to a glacial morphology. However there is no agreement on the distribution and extent of the deposits and related forms. An original and detailed field survey was carried out for the "Carta tecnica regionale alla scala 1:5.000" (Technical Regional Map at the 1:5.000 scale) utilizing aerial photographs taken in different periods and from various heights. On the "Carta geologica 1:100.000 della provincia di Parma e zone limitrofe" (1:100.000 Geological Map of Parma province and neighbouring areas) (1965), the Mt. Molinatico area is shown as an almost continuous morainic cover. However the genetic attribution of deposits is doubtful, and they are also supposed to be linked to gravity phenomena. From the environmental point of view, the presence of an almost continuous morainic cover suggests conditions of geomorphological balance in the slope because the deposits date back to more than 10.000 years ago. On the contrary, widespread extended and deep landslide movements indicate a rapidly evolving environment tending to still unattained equilibrium conditions. No deposits and forms linked to glacialismare shown in the "1:100.000 Geological and Geomorphological Map" attached to this paper. The substratum, formed by the San Siro Clay and Ostia Sandstone formations, and by the Complex of Block Clay and Mt. Molinatico Sandstone, has been mapped in a way as to emphasize its outcrops and eluvial covers. Recent deposits -elsewhere shown as "covers"- and respective forms have been carefully identified in the field. The geomorphological study has shown that the Mt. Molinatico slope down up to Taro river, is affected by more or less wide landsliding areas independent of the substratum lithological characters. Landslides have been distinguished between active and inactive landslides, according to the classification proposed by Carrara et al. (1985). The slope hydrographic features have also been studied in detail, because rivers frequently vary their course as a consequence of landsliding movements that cause morphological changes both in the "main scarp" and in the "zone of accumulation" of mobilized materials. Those landslides, which have been quiescent for a long time, are limited by streams running parallel to the landslide boundary in contact with the outcropping substratum. The study has also taken into consideration the morphotectonics of the area. Although tectonics seems to take part in the Mt Molinatico recent evolution, no sure evidence of recent tectonic features have been identified on the ground. It is worth remembering that low intensity seismic events are frequent in the area. The valley bottom area is characterized by a geomorphological evolution governed by alternating erosion and deposition phases of the Taro River. The intense erosive river activity during Holocene times, caused a lowering of the riverbed of about 56 m with respect to the maximum height of fluvial sediments throughout the valley bottom. A progressive sedimentation phase followed, which deposited sediments up to a level greater than the present outrcropping altitude. Then a new erosive discontinous phase occurred, which is testified by five orders of terraced surfaces. At present the River Taro is again in erosion, although the hydraulic maintenance works in defense of the roads and the railway devoloping on the valley floor are kept under continuous control. In order to compare temporal events to continental features, chronostratigraphic data have been carefully examined. The presence of eluvial covers and of a landslide activity suggests an in-progress evolution of the area; however it is not indicative of the beginning of the phenomena. The last phase of glaciers retreat may be a reasonable date. On the basis of chronostratigraphic data, a fluvial sequence with substratified lenses of sand with gravel and sand with clay, has been identified in the area of "I Bastioni". The fluvial sequence outcrops just for an area of a few ten square meters, and has come to light at the main scarp of a landslide. In the median upper part of the outcrop, a carbonized trunk has been found within a clayey lens; a lignite sample from the trunk has been radiocarbon dated, the age being 41,000 ± 3,500 years. The trunk has been identified as a Gymnosperm. The sediments at the top of the fluvial sequence, are changed by weathering into a brown-red soil, which may be the product of an old pedogenetic process. The fluvial sequence may be a relic deposit preserved tillnow because younger sediments cover it.