Geomorfologia e fenomeni di instabilità nei dintorni di Lioni (alta valle del Fiume Ofanto): prime considerazioni

Abstract

This paper presents the preliminary results of a geomorphological study on the Ofanto River valley (southern Apennines, southern Italy) within the project "Landslide evolution controlled by climatic factors in a seismic area - Prediction methods and warning criteria", sponsored by the European Community (contract EV5V-CT94-0451). Three orders of alluvial terraces were identified and mapped: The highest terrace is at an elevation of 130-150 m above the river bed. Although the slopes are characterized by low gradients, outcropping materials are highly prone to land-sliding: Superficial slides - up to approximately 1 m in depth - are the most typical feature. Old slope movements (rotational and trans-lational slides, and flows) are also suggested by the hummocky topography, steps and scarps in transversal profiles and by counterslopes. The most active slope movements are usually represented by partial or total reactivations of larger phenomena, the origin of which probably coincided with the deepening of the Ofanto river. Two areas of particular interest have been chosen for more detailed analyses. The first one includes the town of Lioni and its surroundings; this is an important zone from a paleoenvironmental point of view because of the presence of Pleistocene deposits (mostly silty clays, passing upward to subordinate sands and gravels), which are often reported as lacustrine sediments in the literature. The present field study showed that the areal extent of the lacustrine deposits related to the "Lioni basin" is probably much more limited than previously stated; this has been confirmed by the lithostratigraphy of a borehole drilled at the northern outskirts of the town, in an area where the presence of lacustrine deposits was previously indicated. The macrofauna found in the borehole cores is attributable to an infralittoral environment; moreover, the lack of continental fauna and organic remains suggests that a lacustrine origin of the sediments can be excluded. The second area ("La Cascata") is located near a narrow reach of the Ofanto River where the blockage of the river likely occurred and allowed the deposition of continental sediments. A landslide affecting 6 ha of land occurred in this area in 1988, causing severe damage to roads and facilities. Controls performed over the last two years indicate a slow evolution of the slide (displacements of a few centimetres/year). It appears that the present activity of the landslide is limited to its middle-lower portion, where very shallow piezometric levels were measured during the last winter/spring seasons.