08-Monteleone 187-192
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Geogr. Fis. Dinam. Quat. 33 (2010), 187-192, 5 figg. SALVATORE MONTELEONE (*), MARIA SABATINO (*) & ANTONIO BAMBINA (*) GENESIS AND GEOMORPHOLOGICAL EVOLUTION OF DEEP-SEATED GRAVITATIONAL SLOPE DEFORMATIONS: AN EXAMPLE OF DATING IN WESTERN SICILY ABSTRACT: MONTELEONE S., SABATINO M. & BAMBINA A., Genesis RIASSUNTO: MONTELEONE S., SABATINO M. & BAMBINA A., Genesi ed and geomorphological evolution of deep-seated gravitational slope deforma- evoluzione geomorfologica di D.G.P.V.: un esempio di datazione nella Sici- tions: an example of dating in Western Sicily. (IT ISSN 0391-9838, 2010). lia Occidentale. (IT ISSN 0391-9838, 2010). The study area lies in the westernmost part of the Sicani Mountain L’area oggetto della presente nota ricade nel settore più occidentale range where a superposition of SE-trending, stacked, geological bodies dei Monti Sicani dove è possibile riscontrare una sovrapposizione di cor- crops out. this structure is the result of transpressive tectonic phases of pi geologici impilati e vergenti verso i quadranti meridionali, quali conse- Miocene and Lower Pliocene age. The neo-autochthon outcrops are guenza delle fasi tettoniche traspressive riferibili al Miocene e al Pliocene made up of clay-marly deposits of the Middle-Upper Pliocene, and of cal- inferiore. In quest’area il neoautoctono è rappresentato dai depositi di carenites of Pleistocene age. natura argillo-marnosa del Pliocene medio-superiore appartenenti alla The extensional Quaternary tectonic phases later dissected and dis- Fm. Marnoso-arenacea del Belice e dalle calcareniti del Pleistocene. Le membered the original structure, thus involving the calcarenites that crisi tettoniche quaternarie di tipo distensivo hanno ulteriormente sezio- show clear, mainly NE-SW oriented dislocations. nato e smembrato l’originario edificio strutturale, coinvolgendo anche le Such calcarenites, which have a total thickness of about 80 metres, calcareniti che manifestano evidenti dislocazioni orientate, principalmen- overlap the Belice marly-arenaceous formation. This determined favourable te, NE-SW. conditions for Deep-seated Gravitational Slope Deformations (DSGD), La sovrapposizione delle calcareniti, il cui spessore complessivo si which affected the western plateau of St. Margherita Belice causing deep aggira intorno agli 80 metri, sulla porzione a comportamento duttile della landslides. The trigger agent of these landslides has been correlated with Formazione Marnoso-arenacea del Belice determina condizioni favorevo- the seismic activity that has affected this area since ancient times. li per una evoluzione geomorfologica del settore occidentale del pianoro The Deep-seated Gravitational Slope Deformations considered here di S. Margherita Belice per frane profonde del tipo DGPV. Il loro inne- have peculiar geomorphologic characteristics such as disjointed blocks, sco va correlato con l’attività sismica che ha interessato l’area fin dai tem- scarps, landslides and areas with double ridges. On top of the tilted bod- pi più remoti. ies, wide trenches are frequently observed, whereas the base of the col- Le Deformazioni Gravitative Profonde di Versante prese qui in esa- lapsed masses sometimes shows no real area of accumulation. As a result me hanno caratteristiche geomorfologiche peculiari; si osservano blocchi of the partial rotation of the masses involved in the collapse, the cal- disarticolati, gradini, terrazzi di frana ed aree con doppie creste. Al coro- carenitic blocks show counter-tilting, and in some cases are covered by namento dei corpi tiltati si osservano quasi sempre ampie trincee, mentre «talus»-type deposits of uneven thickness. alla base delle masse collassate può non essere presente una vera e pro- These deformations can currently be considered inactive. The dis- pria zona di accumulo. Per effetto della parziale rotazione delle masse covery of an archaeological site of late-Roman and Byzantine age (VII- coinvolte nel dissesto, i blocchi calcarenitici presentano una immersione VIII centuries AD) has proved that the landsliding activity has been in- verso monte e in alcuni casi risultano ricoperti da uno spessore variabile active since before the area was inhabited by humans. However, the di depositi tipo talus. dislocation of rocky tombs belonging to the middle Bronze Thapsos Tali deformazioni possono essere considerate attualmente inattive; il culture (XV-XIII century BC) suggests that these landslides occurred ritrovamento di un sito archeologico di età tardo-romana e bizantina del after the necropolis was built. Data from an archaeological study of the VII-VIII secolo d.C. porta a dedurre che lo stato di attività dell’intero earthquakes involving the city of Selinunte, located just south of the corpo franoso debba essere stato quiescente già prima della realizzazione study area, has allowed us to date the onset of the above-mentioned dell’insediamento stesso. La dislocazione, invece, di tombe rupestri asse- instability. gnate alla cultura del medio Bronzo di Thapsos (XV-XIII secolo a.C.) porta a considerare tali fenomeni franosi comunque successivi alla realiz- KEY WORDS: Neotectonics, DSGD, Western Sicily. zazione della necropoli stessa. L’utilizzo dei dati derivanti da un approc- cio archeologico ai terremoti che hanno coinvolto la città di Selinunte, ubicata poco più a sud dell’area su cui insistono i fenomeni franosi ana- lizzati, ha permesso, infine, di datare l’innesco della instabilità stessa. (*) Dipartimento di Geologia e Geodesia, Università di Palermo, via Archirafi, 22 - 90123 Palermo (Italy) - [email protected] TERMINI CHIAVE: Neotettonica, DGPV, Sicilia Occidentale. 187 INTRODUCTION The geomorphological characteristics of slopes involved in Deep-seated Gravitational Slope Deformations (DSGD) have been the object of study for many years. The defor- mation mechanisms that characterize these landslides are particularly affected by the stratigraphic-structural and tectonic sequences that can trigger them even in normally stable situations. In depth discussions have defined Deep-seated Gravita- tional Slope Deformations as «the deformation of a mass for which it is not necessary to postulate the presence of a de- fined deformation surface» (Dramis & Sorriso Valvo, 1994). Several authors have studied these phenomena, and they have all tackled the problems of defining the their characteristic morphological elements (Agnesi & alii, 1978; Dramis, 1984; Forcella & Orombelli, 1984), and of identi- fying the geological-structural conditions that are a prereq- uisite for their occurrence (Agliardi & alii, 2001; Di Luzio & alii, 2004). In this paper we describe the results of a detailed geo- logical-structural and geomorphological survey effected on the north-west slope of the S. Margherita Belice (AG) plateau. This was performed with the aim of ascertaining what the prerequisite conditions and triggering factors were that caused the landslides involving the Caliata and Saiarotto areas. FIG. 1 - Geological-structural scheme in the Montevago (AG) area (from The results show that there are some atypical aspects to Monteleone & alii, 2006 - modified). these landslides when compared with the classical models of DSGD. crops mainly consist of clay and clay-marly rocks. Where the slopes are steep, carbonate and calcarenitic rocks crop REGIONAL SETTING out. Areas with gentle to sublevel slopes can be found on the calcarenitic substrata of the St. Margherita Belice The study area is represented by the western sector of plateau belonging to the Wide Upper Terrace (WUT) the Sicani Mountain range and is characterised by out- (Ruggieri & Unti, 1977). The WUT develops at an altitude crops of carbonate, clay-marly, clay-sandy and calcarenitic of about 400 m asl and dates back to 700 ka. This means rocks of Jurassic through Pleistocene age (Ruggieri & that it has undergone an uplifting rate of about 0.6 mm/y. Torre, 1974; Catalano & D’Argenio, 1978; Mascle, 1979; This is evidence of neotectonic activity in this part of west- Di Stefano & Vitale, 1992; Monteleone & alii, 2006). ern Sicily. Holocene alluvial and marsh deposits are present at the bottoms of the valleys of the main streams, while along the The diffusion of prevalent pelitic substrata, mainly the hilly slopes there are terraced alluvial deposits belonging marly clays of Medium-Upper Pleistocene age, helped the to the Upper Pleistocene-Holocene and to recent accumu- development of extensive alteration layers, which are fre- lations from landslides (fig. 1). quently involved in surface landslides of the soil-slip type. The tectonic phases of Miocene age that took place in Some of the significant morpho-evolutive processes that this area of the Sicani Mountains generated a structure delineate and model the landscape are gravity-driven. They formed by the overlap of different stratigraphical-structur- may be active or quiescent and may be classified as flows, al units that face the southern quadrants. Such arran- flow-slips and deep landslides of the DSGD type. gement was further modified by the infra-Messinian and Processes caused by the action of canalised waters are Lower Pleistocene transpressive-type tectonics that gener- equally intense. This is especially true along the tributaries ated structures having an E-W and NW-SE direction, re- of the Belice River, and where the soil erodes at a different spectively, sometimes with remarkable displacements(Ca- rate, due to different degrees of erodibility. talano & alii, 1982). The extensional tectonic phases of Pleistocene age sequentially dissected