Marcellina e Gissi, origine naturale e antropica dei sinkhole

Abstract

The main goal of this work is to study the geological and structural setting of two different geological areas, Marcellina close to Rome in Latium and Gissi in Abruzzi, to identify the possible mechanism of the generation and evolution of these collapses. Close to Marcellina town, a sinkhole suddenly formed in Pozzo Grande area on 2OO1, sinkhole have been formed in Holocene lacustrine sediments. For this reason microgravity prospecting has been applied in Marcellina town in order to identify the sinkholes risk areas and any other geological conditions similar to those leading to the 2OO1 collapse. The underground geological features of Marcellina town shows some sinkholes risk areas due to the nature of the sediments and sometimes to the presence of anthropic cavities. Both microgravity and geoelectrical tomography data have been proposed to explain the causes of some future collapse: the results obtained by geoelectrical tomography, made it possible to establish the presence of a low resistivity (4O Ohm/m) that corresponds a negative anomaly zone by microgravity survey (due to the presence of a considerable thickness of clay and sand formation, and sediments with lower density and resistivity) and a high resistivity (3320 Ohm/m) corresponds a maximum gravity zone (due to low deep of calcareous bedrock in underground, with higher density and resistivity). A profile is irregular: in the south-east side of city hall, the presence of a high resistivity corresponds a minimum gravity zone, due to the effect of the cistern or natural or anthropic cavity origin. Recently many sinkholes have been formed in some peculiar areas of the Abruzzi, Gissi. A microgravity survey of the area was carried out in 2002 and defined the extend, cause and nature of subsidence. It showed the subsidence resulted from underground dissolution of evaporitic rocks which presents karstic processes and active faults; the presence of a great underground water flow with its physico-chemical properties, ascending liquids and gases, tectonic faults and anthropic causes that can develop fastest phenomena. The rapid dissolution of evaporitic rocks in subsurface conditions can result in caves and cavities that rapidly expand, become unstable and start to collapse. In Marcellina town where the bedrock is buried by thick alluvial successions there are a human geohazards associated with the lower density sediments. Abstraction from sediments aquifers can yield excessively hard water, accelerate removal and cause aggravated subsidence. In the Gissi area where evaporitic rocks occurs there are a natural or induced geohazards associated with the rock, but awareness of them is low.