Anthropogenic sinkholes in the Marsala area (western Sicily) linked to underground quarries

Marsala territory (western Sicily) is characterized by the presence of a Lower Pleistocene (Calabrian) calcarenite succession (Marsala Calcarenite Fm). It can be divided into three lithofacies that show the regressive evolution of the depositional system: a) coarse to fine yellow bio- and lithoclastic calcarenites, b) sands, and c) gray sandy clays. At least 80 m-thick, this succession gently dips (5-10) towards the south and the south-west. At some locations the Marsala Calcarenite is covered by Middle and Upper Pleistocene marine terraced deposits. Since the Roman period, due to the great abundance of calcarenite rocks, and to the facility of extraction, the Marsala area has been characterized by a high number of quarries for the extraction of this building materials. Many of them were excavated underground, at depth varying from a few meters to about 25 m, and are arranged in one or two levels, following the galleries and pillars excavation technique. With time, the underground quarries have been progressively abandoned for the decay of the physical and mechanical properties of the calcarenite rock mass, the interaction with the groundwater, the high costs of extraction, and the dangers and difficulties encountered in working underground. Since the 1960’s the quarries have been affected by instability processes, visible through collapses and deformations of vaults and pillars. These phenomena often propagate upward reaching the topographic surface and forming sinkholes which affect and severely damage the built-up area. In particular, two case studies of sinkholes related to different underground quarries will be analyzed in this paper. The aim is to provide a description of the most significant processes and factors responsible of the instability processes based on field surveys, as well as to understand the generation mechanisms of these anthropogenic sinkholes by means of numerical modeling, based on rock laboratory testing data, that represents in these cases a remarkable tool for the investigation of the cause-effect relationships, as already performed in other areas of Italy

DETECTION OF PLIO-QUATERNARY FAULTS IN MADONIE MOUNTAINS (SICILY) BY USING QUANTITATIVE GEOMORPHIC AND STRUCTURAL GEOLOGY ANALYSES

The Northern Madonie Mountains (Northern-Central Sicily), sector of the Sicilian Maghrebian chain, consist of a tectonic thrust system developed through two subsequent main contractional events: 1) a shallow-seated compressional event developed during the Middle-Upper Miocene; 2) a deep-seated transpressional event occuring since the Late Miocene. Lower Pliocene (Trubi) to Quaternary clastic deposits unconformably lie on the tectonic units and are partially involved by deformation. In the selected area, syntectonic sedimentary basins characters are able to define the timing of deformation only until the Lower Pliocene; to resolve this gap of information application of quantitative geomorphic techniques, based on relationships between tectonics and hydrographic network development could contribute to recognize and characterize Quaternary structures in areas where clayey/marly deposits, widely outcropping, are not marked by pervasive tectonic deformations. In order to define the geological setting of the study area and to detect Quaternary tectonic structures, geological, structural and geomorphological analyses have been carried out. Geological and structural analyses have shown: 1) characters and style of deformation of fold structures: two main systems of folds have been recognized - the early system NW-SE-trending is refolded by a later system (trends in the E-W to NE-SW range); 2) orientation and kinematics of faults related to superimposed compressional events: an early thrust system characterized by SW-ward tectonic transport; a later transpressive system consistent with a maximum compression oriented N-S ± 20°, and nearly horizontal. Although the occurence of two compressional deformation events, interplaying in the construction of the Sicilian chain, is well-known, the field data, here collected, help to better characterize the relationship between shallow-seated and deep-seated structures. Due to rare and thin Quaternary deposits, quantitative geomorphic analysis has been performed on the hydrographic network of the study area, because the river drainage of Sicily is believed to have developed during the Quaternary age. In particular, have been carried out: 1) azimuthal distribution analysis, by cumulative length, of stream channels related to different orders, taking into account structurally and lithologically homogeneous areas to evaluate the influence of Quaternary tectonics on the geometry of drainage patterns; NNW-SSE, NNE-SSW, E-W and N-S domains have been evidenced in lower orders of channels; 2) “azimuthal transect method”, performed along 16 suitable segments crossing previously inferred fault zones, able to detect possible Quaternary strike-slip kinematics. Progressive apparent rotations of stream channels have been found, documenting the occurence along the main rivers of Quaternary faults and suggesting both right-lateral (NNW-SSE oriented) and left-lateral (NE-SW oriented) kinematic components. The multidisciplinary approach used suggests the geological/geomorphological setting of the study area is influenced by Quaternary faults with strike-slip component, highlighting a general congruency between hydrography and tectonics

Genetic models of poljes in Sicily

Geomorphological and geological studies have been carried out to contribute to the recognition of controlling causes and to the definition of genetic models for poljes of Sicily. A polje is a kilometric closed depression developed mainly on karst rocks, with a conspicuously flat and alluviated bottom affected by intermittent flooding. A polje is usually characterised by relatively steep slopes enclosing an almost perfectly horizontal floor, caused by lateral solution planation related to flooding events. The origin of a polje is due to dissolution of the land surface, although geological structure generally influences its genesis. These large depressions are often elongated according to the direction of main faults, in consequence of a control due to tectonics or to differential erosion. The performed researches have shown the existence of at least seven poljes located along the north-western (chain zone) and the southern (deformed foredeep zone) areas of Sicily. These large karst depressions are developed on Mesozoic limestone/dolomitic rocks within the chain zone and on Messinian gypsum rocks within the deformed foredeep zone. They are up to 4 km in length, can reach surfaces of 3-8 km2 and are around hundred metres deep, with steep slopes and a flat bottom. Generally, they are open, occasionally active depressions and their genesis seems to be strongly controlled by structure. In particular, the studied poljes occur in two different geological/geomorphological settings: a) in graben-like tectonic depressions, where important fault slopes/scarps border the flat bottom; b) in complex depressions controlled by structure, where wide fault line slopes/scarps or large inclined degraded structural surfaces mark the poljes. Finally, landscape analysis leads to the proposition of two main genetic models in which the development of poljes is primarily due to tectonics or differential erosion followed by dissolution

Some Considerations on 3-D and 2-D Numerical Models for the Assessment of the Stability of Underground Caves

The application of numerical modeling to the analysis of the stability of both natural and manmade underground caves is rapidly increasing due to the availability of powerful numerical codes, that can account for either continuum or discontinuum behavior of the rock masses. Numerical methods allow to overcome traditional methods for cave stability analysis that assume too simplified geometrical, geological and geomechanical conditions. Further, they are also able to assess the potential failure mechanisms of underground systems. On the other hand, the application of numerical methods requires availability of a detailed geo-structural survey of the cave, as well as a proper geomechanical characterization of the rock and joint material properties for the pre-processing stage. The present contribution is aimed at describing some aspects related to implementation, and comparison of the outcomes, of two-dimensional or three-dimensional finite element analysis as regards artificial caves with complex geometries excavated within calcarenite rock masses. This type of man-made caves is very common in Southern Italy, and is at the origin of frequent sinkholes, often threatening the built-up areas. In particular, the results from 2-D and 3-D analyses of a case study represented by a cave with complex geometry are presented, showing that 3-D analysis leads to more stable conditions of the rock mass surrounding the cave. This is supposed to be the consequence of the different stress state as calculated by the two analyses, the 2-D one assuming plane strain conditions while the 3-D analysis assumes more general stress conditions

Surface denudation rate of gypsum in Sicily

Studies on surface denudation rate of karst rocks were carried out for many years with different methods, although researches on limestones are much more numerous than those on gypsum. In Sicily the most large and complete Messinian evaporite succession of Gruppo Gessoso \u2013 Solfifero outcrops and since 1993-1994 surface denudation measurements were performed on different types of gypsum by the Micro-Erosion Meter (M.E.M.) method. MEM stations were placed on natural sites representing different lithological features of gypsum outcrops of the Island: 1) selenite gypsum with centimetre-sized crystals; 2) selenite gypsum with sub-centimetre crystals; 3) gypsum arenite; 4) microcrystalline gypsum; and 5) gypsum laminite (balatino type). The measuring stations are positioned in three localities in western and central Sicily: Santa Ninfa (Trapani), Ciminna (Palermo) and Campofranco (Caltanissetta). The average lowering rates vary in the different lithofacies: from 0.25 mm yr1 in microcrystalline gypsum to 0.74 mm yr1 in selenite gypsum with centimetre-sized crystals. The average surface denudation rates are 0.40 mm yr1 in balatino gypsum and gypsum arenite, and 0.37 mm yr1 in selenite gypsum with sub-centimetre crystals. These different values are connected to several factors such as: rock texture, dip of gypsum surfaces, climatic conditions, troubles on the measurement sites (e.g.: presence of lichens, soil, remains of vegetation, etc.). The aim of this paper is to show the results of roughly twenty years of experimental measurements, and to compare the surface denudation rate of gypsum in Sicily with those of other evaporite areas characterised by different climatic settings

Morphology and evolution of sulphuric acid caves in South Italy

Sulphuric acid speleogenesis (SAS) related to the upwelling of acid water enriched in H2S and CO2 represents an unusual way of cave development. Since meteoric infiltration waters are not necessarily involved in speleogenesis, caves can form without the typical associated karst expressions (i.e. dolines) at the surface. The main mechanism of sulphuric acid dissolution is the oxidation of H2S (Jones et al., 2015) which can be amplified by bacterial mediation (Engel et al., 2004). In these conditions, carbonate dissolution associated with gypsum replacement, is generally believed to be faster than the normal epigenic one (De Waele et al., 2016). In Italy several SAS caves have been identified, but only few systems have been studied in detail: Frasassi and Acquasanta Terme (Marche)(Galdenzi et al., 2010), Monte Cucco (Umbria) (Galdenzi & Menichetti, 1995), and Montecchio (Tuscany) (Piccini et al., 2015). Other preliminary studies have been carried out in Calabria (Galdenzi, 2007) and Sicily (De Waele et al., 2016). Several less studied SAS cave systems located in South Italy, and in particular in Apulia (Santa Cesarea Terme), Sicily (Acqua Fitusa, Acqua Mintina) and Calabria (Mt. Sellaro and Cassano allo Ionio) have been selected in the framework of a PhD thesis on SAS caves and their speleogenesis. Using both limestone tablet weight loss (Galdenzi et al., 2012) and micro erosion meter (MEM) (Furlani et al., 2010) methods the dissolution rate above and under water in the caves will be quantified. Geomorphological observations, landscape analysis using GIS tools, and the analysis of gypsum and other secondary minerals (alunite and jarosite) (stable isotopes and dating) will help to reconstruct the speleogenetic stages of cave formation. Preliminary microbiological analysis will determine the microbial diversity and ecology in the biofilms