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Evolutionary kinematics and geological features of the large rock slide ( Geopark, , southern )

DE VITA PANTALEONE (1)*, LA BARBERA GIOVANNI (2), CARRATÚ MARIA TERESA (3) (1) Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR) - University of Naples Federico II, Via Mezzocannone, 8, 80134, Naples, ITALY

(2) Centro ISIDE s.r.l., Piano della Rocca, 84060, (SA), ITALY

(3) Analysis of Environmental Systems PhD Program - C.I.R.AM. - University of Naples Federico II, Via Mezzocannone, 16, 80134 Naples, ITALY * [email protected]

Abstract: In the Pisciotta municipality (Province of , Cilento Geopark, southern Italy), a large and slow moving landslide is known since several decades for having damaged continuously an important provincial road which crosses its body and to have requested repetitively maintenance works for permitting vehicle traffic. The almost uninterrupted state of activity, existed in the last seventy years at least, the slow kinematics, not yet evolved in a paroxysmal global failure stage, and the exposition to risk also of a railway tunnel make this landslide a special case of geoharzard to be analyzed. The landslide involves entirely a hilly slope constituted of a Paleogene turbidite series belonging to the Saraceno Formation (Nord-Calabrese tectonic unit), which is formed by intercalated calcarenites, marls and mudrocks and is completely deformed by a polyphasic folding. In this research, engineering geological, geomorphological and topographical analyses were finalized to understand geological factors and kinematics of the Pisciotta landslide. Among principal results, the landslide spatial occurrence and its kinematics are controlled by specific geological and stratigraphic factors such as the existence of an argillaceous member, whose dominant attitude of bedding is downslope dipping with a dip that is generally equal or lower than the local slope angle, and of a fault that bounds the landslide on its left flank. Furthermore, results derived by the displacement of the provincial road, since 1955, as well ground displacements measured by an on-purpose monitoring campaign (September 2006-March 2009) allowed to reconstruct the landslide kinematics and to understand further the deep-seated mass movement behavior. Based on such analyses, the Pisciotta landslide is classified as an active, very slow to slow, deep-seated rock slide involving a volume of rock-mass varying between 4 and 6 × 106 m3.

Key-Words: Turbidite series, Argillaceous rocks, Rock slide, Landslide kinematics, Slow slope movements, Life-lines at risk.

1 Introduction used for slope stability issues (Esu, 1977). Deep- Turbidite and basinal series formations are among seated landslides in structurally complex formations terrains most prone to deep-seated landslides both in were generally recognized as characterized by initial Italy (Del Prete and Guadagno, 1990) and evolutionary stages with very slow to slow ground worldwide (Nemčok et al., 1982; Margielewski, movements preceding a paroxysmal global slope 2006; Klimeš et al., 2009; Baroň et al., 2011) owing failure, with velocities up to the rapid class (Cruden to the coexistence of several factors predisposing to and Varnes, 1996; Corbi et al., 1999; De Vita et al., slope instability, such as variable argillaceous 2001). For such features, these large mass component, mechanical anisotropy of the rock- movements represent commonly a risk higher for masses and predisposition to loss of shear strength the exposed buildings, infrastructures and life-lines, induced by deep weathering. This scientific issue is than for human lives. The latter is usually related to well known in Italy since the ‘70s of the last typical feasibility to evacuation and easy recognition century, when first researches on slope instabilities of signs indicating the imminence of a global slope of turbidite and basinal series formations led to the failure. Nonetheless, secondary risks for human identification of a special category of engineering lives, induced indirectly by these large landslide geological units, named structurally complex even in the pre-paroxysmal evolutionary stages, formations, and a proposal of classification to be consisting in a likely activation of rapid and

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shallower slope instabilities superimposed over the palaeogeographic domains. During the Apennine deep-seated ones, cannot be neglected. orogeny, both units were unconformably covered by This kind of large slope movements has always the wedge-top turbidite flysch deposits of the represented a challenging topic of research for both Cilento Group, Burdigalian-Langhian in age geotechnical and engineering geological (Bonardi et al., 1988, 2009) and the Monte Sacro approaches, especially for what concerns the Formation (Upper Tortonian in age; Selli, 1962). challenging modeling of slope stability by an Specifically, in the Pisciotta area, the Saraceno ordinary geotechnical approach (Hutchinson, 1995), Formation outcrops (Vezzani, 1968; De Blasio et and for requesting a deep comprehension of al., 1978), which was dated between the Upper mechanical features and heterogeneities of rock Eocene series and the Aquitanian stage (Di Staso masses (Guzzetti et al., 1996; Marinos and Hoek, and Giardino, 2002). The stratigraphy of the 2001; Picarelli et al., 2005). In fact, complex rupture Saraceno Formation comprises a turbidite series mechanisms, controlled by pre-existing shear formed by alternating calcarenites, calcilutites and planes, involvement of residual shear strength, mudrocks with a maximum thickness of progressive rupture mechanism (Bjerrum, 1967; approximately 600 m. The Saraceno Formation is Rose and Hungr, 2007), reduction of shear strength intensely deformed by a polyphasic folding (Vitale due to weathering (Taylor and Cripps, 1987) as well et al., 2010), which was favored by the ductile as stratigraphic and structural factors controlling mechanical behaviour of the flysch rock-mass. landslide onset, kinematics and evolutionary stages, make forecasting and modelling the location and evolution of these landslides much harder than 3 Current morphological setting of shallow landslides. Following a precedent research (De Vita et al., the unstable slope The Pisciotta landslide develops in the Rizzico 2013), in this paper we present further locality, stretching on the left side of the Fiumicello characterizations of the Pisciotta landslide (Cilento Torrent valley, about 200 m upstream of its outlet Geopark, Campania, southern Italy), which describe into the Thyrrenian Sea. In the current evolutionary principal features, evolutionary stages and stage (2014) the slope mass movement involves the kinematics of this large slope mass-movement. entire slope of a secondary hill, predominantly

facing westward and comprised in the altitudinal

range between 237 and 15-25 m a.s.l. (Fig. 1). The 2 Geological setting middle sector of this unstable slope is crossed by the The Pisciotta landslide is located close to the provincial road SS 447 for a length of about 570 m. southwestern sector of the Cilento Geopark coast The uninterrupted deformations of the road plano- (Campania region, southern Italy). This area, which altimetric geometry and continuous cracking of the has been inscribed among the European Geoparks road pavement led to recognize the existence of a network since 2010, is a sub-region of the slope mass movement since the ‘60s od the last Campania. The geological settings of the Cilento, century. During years, the increasing ground achieved during the orogeny occurred since the displacements became even more evident, especially Early Miocene, are representative of the fold and in the road sectors crossing the landslide flanks, thrust-belt structure of the southern Apennines whereas the local inclination increased up to the (D’Argenio et al., 1973; Mazzoli and Helman, 1994; limit allowed for vehicular traffic. Due the slow Mostardini and Merlini, 1986; Patacca and ground movement, the functionality of the road was Scandone, 2007). Therefore, the morphostructural preserved by continuous repairing works consisted setting of this area is strongly controlled by features in smoothing of cracks opened in the road pavement and geometric relationships among principal by earth and asphalt fillings. Only in recent years, tectonic units, which are formed by sedimentary the slow slope ground deformations, which initially series belonging to palaeo-environments of were related to an undefined shallow mass carbonate platforms and interposed marine basins, movement, were attributed to a large and deep- existed from Triassic to Late Cretaceous. seated landslide due to a clearer recognition of a Specifically, the Pisciotta area is constituted of the typical morphological impact affecting the whole Nord-Calabrese tectonic unit, turbidite series hillslope (Fig. 1). deposited in an inner palaeogeographic domain The most evident structural features of the landslide which were involved in the orogeny since the Early (Fleming and Johnson, 1989; WP/WLI, 1993) were Miocene (Bonardi et al., 1988; Ciarcia et al., 2009) identified in a well-defined main scarp reaching the and overthrusted over the outer carbonate platform

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middle sector of the slope and describing a typical arc shape (Fig. 1). The main scarp has a stepped morphology being produced by a close series of scarps, with a dip-slip kinematics, forming a total vertical throw of approximately 20 m with an average dip angle of about 75°. The left flank of the landslide, which is derived by the downslope continuation of the main scarp, is clearly identifiable down to about 80 m a.s.l.. In this sector, a trench crossing the SS 447 road with a strike-slip kinematics is clearly observable. Differently, the right flank is less advanced downslope because it is extended down to approximately 150 m a.s.l.. Fig. 1 – The Pisciotta landslide (modified from Google Earth, Consequently the left side of the landslide appears 2004). relatively more mobile as it is indicated by stronger deformations of the SS 447 road in this sector, especially at the boundary between stable and 4 Approaches of study unstable zones where the most important Different approaches were applied to reconstruct a maintenance works were carried out. geological and evolutionary model of the landslide. The main body of the landslide, which is relatively Geological and geomorphological surveys were accessible only in the middle-upper part, presents a executed over an extent of approximately 15 km2, series of surficial structures consisting in secondary comprising the landslide area and its stable scarps and counterscarps, often with a width and surroundings. Structural surveys were conducted in depth of few meters, and grabens (Fleming and all the accessible outcrops of the Saraceno Johnson, 1989). Moreover, a general bulging of the Formation to provide systematic measurements of landslide foot is easily recognizable by a global the bedding attitudes. To this end an indirect view from the opposite side of the valley due to the window sampling technique based on terrestrial increase in the slope angle of its frontal part and the stereoscopic photographs (De Vita et al., 2012) was occurrence of a series of small shallow landslides applied. Moreover, stratigraphic data derived by involving the detrital mantle. boreholes, resistivity and seismic refraction The landslide foot is intersected by two tunnels of geophysical tomographies were also used to the national Battipaglia-Paola railway stretch (Fig. reconstruct the geological model of the landslide 1). Among them, the most exposed to landslide (De Vita et al., 2013). movements appears the northernmost one because it intersects the central part of the landslide toe. The kinematic and evolutionary phases of the In recent years, the worsening of the road SS 447 unstable slope were reconstructed by a conditions and the potential scenario of a rapid geomorphological analysis (Corbi et al., 1999; De paroxysmal global failure of the slope, led the Vita et al., 2001) of all available aerial photographs Salerno Province to set an alert system for warning taken in: 1943, 1955, 1956, 1971, 1982, 1991, 1995, the vehicular traffic, since September 2006. 2003 and 2006 (orthophoto). The comparison of Therefore, a campaign to monitor ground and variable paths of the road SS 447 during years, underground deformations was undertaken by the derived by topographic maps and orthophotos Salerno Province through the predisposition of a bi- available in the period 1955-2006, allowed a long- weekly topographic monitoring. For such a scope 50 term quantitative kinematic analysis of optical targets were distributed in the upper part of displacements and ground deformations. Moreover, the landslide body, between the SS 447 road and the kinematic analyses were also carried out in a shorter main scarp. Moreover, three boreholes, equipped by time period by analyzing topographical inclinometers, and the installation of three wire- measurements started in September 2006 and lasted extensometers across the main cracks were carried approximately for 18 months, till March 2009. out.

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5 Results whole time series, different rates of horizontal displacements were detected for the years preceding 5.1 Long-term kinematic evolutionary and following 1994. In detail, in the A–A’ road transect, average velocities of 2.2 mm day-1 (0.81 m stages (1943–2006) -1 -1 -1 year ) and of 4.0 mm day (1.46 m year ) were An earlier evolutionary stage of the landslide was estimated for periods preceding and following 1994, recognized since 1943, whose aerial photograph respectively. showed first traces of the main scarp, in the upper part of the slope, as well as no bulging of the slope foot. Following, between 1955 and 1956, the expansion of the main scarp, which described a more complete arc shape and a clearer appearance of secondary scarps were observed. For the successive years (1971, 1982 and 1991), an additional increase of the morphological impact of the main scarp was identified by a significant enlargement of its vertical throw and planimetrical trace, especially in the left side of the landslide. During such a time span, also the landslide foot became progressively more evident by the bulging of the foot slope. This ground deformation has led to Figure 2: Cumulative displacements of the provincial road the increase of the local slope angle and to the SS447 assessed for four transects normal to the road axis from 1955 to 2009. predisposition to shallow landsliding of the surficial detrital mantle. Such morphological impacts of the principal landslide structures became even more 5.2 Kinematics of the upper landslide evident and pronounced in the following years. body (September 2006 to March 2009) Furthermore, new ground fractures with strike-slip The bi-weekly topographic monitoring of the 50 kinematics and secondary scarps were detected. optical targets distributed upslope of the road SS Finally, by the most recent aerial photograph 447 allowed to assess the ground displacements and (orthophoto) taken in 2006, a global worsening of their spatial distribution. Data were transformed in the impact of the mass movement over the slope maps of fundamental kinematic variables: horizontal was recognized by the expansion of the vertical cumulative displacement; vertical cumulative displacement of the main scarp as well as of displacement; total cumulative displacement; secondary scarps and the development of a series of average velocity over the whole observation period longitudinal and transverse cracks. (18 months); maximum velocity measured for each optical target between two consecutive topographic By mapping the SS 447 road track, the horizontal measurements. displacements of the middle sector of the unstable The maps of the cumulative displacements showed a slope were estimated with reference to four fixed general progressive downslope increase up to 4.36 m (8.1 mm day-1 on average) and -2.06 m (-3.8 mm transects, set normal to the road axis (A–A’, B–B’, -1 C–C’ and D–D’ in Fig. 2). In addition, these day on average), respectively for horizontal and kinematic data were integrated with topographical vertical components of the most mobile optical monitoring of the upper part of the slope target. In the direction normal to the slope accomplished in the period September 2006 - March movement, a tendency to increase of ground 2009. For the whole observation period (1955- displacements and velocities towards the left flank 2009), the cumulative horizontal displacements of of the landslide was observed. More in detail, the the road axis showed an unexpected maximum total highest values appeared to be concentrated in a value of 53 m for the A–A’ transect, with an narrow strip, shifted towards the left flank of the average velocity of 2.7 mm day-1 (0.98 m year-1). landslide and approximately parallel to it. Instead, displacements and velocities were observed The statistical analysis of all the recorded velocity to diminish progressively for the other road data, calculated between two consecutive transects located toward the right flank of the measurements and related to all the available optical landslide, with minimum values reached for the D- targets (2016 records for the 42 optical targets D’ transect: 22 m and average velocity of 1.1 mm located into the landslide body), permitted to day-1 (0.41 m year-1). Additionally, by observing the estimate the spatial and temporal variability of the ground velocities: 0.8 mm day-1 (2.5th percentile),

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4.7 mm day-1 (50th percentile – median value) and 15 mm day-1 (97.5th percentile).

5.3. Reconstruction of the landslide geological model Geological surveys, carried out in the surroundings of the landslide area, stratigraphic data gathered by boreholes and geophysical tomographies, permitted the reconstruction of the landslide geological model and the identification of stratigraphic intervals involved in the slope instability as well as structural constraints controlling the landslide geometry. In the lowest stratigraphic positions, the typical turbidite calcarenite lithofacies of the Saraceno Formation was found. In this lower stratigraphic interval, the bedrock lithologies were observed to be comprised of calcarenites and subordinate calcilutite beds, with lenses and nodules of chert. Due to the turbidite sedimentological facies, these lithologies were observed being rhythmically alternated with mudrock interbeds. The ratio Figure 3: Geological map of the Pisciotta landslide area. Key to between the lithoid (calcarenite and calcilutite) and symbols: 1) Landslide deposits; 2) Recent and contemporary the mudrock components was estimated to be the beach deposits; 3) Contemporary alluvial deposits; 4) Terraced highest among the whole series (up to 5:1 – 10:1). recent alluvial deposits; 5) Talus and torrential fan deposits; 6) In higher stratigraphic positions, such as those Marly limestone upper member (Saraceno Formation); 7) surrounding the landslide main body and within the Calcarenite lower member (Saraceno Formation); 8) Representative bedding attitudes; 10) Borehole-inclinometer; unstable zone itself, calcareous marls, marls and 11) Geological cross-section. subordinate calcarenites, alternated with an abundant mudrock component, were found. In this The statistical analysis of all bedding attitude data higher stratigraphic interval, the ratio between the indicated a significant clustering for dip direction lithoid and mudrock components was observed and dip angle values around 250°N to 290°N and reaching lowest values (down to 2:1 – 1:1). For both 10° to 12° respectively, besides of a dispersion of stratigraphic intervals, a polyphasic ductile bedding data due to the polyphasic folding. deformation was recognized, according to what Therefore the global attitude of the Saraceno previously discovered by other studies (Vitale et al., Formation was found to be WNW dipping, with an 2010). The Geological Strength Index (Marinos and average dip lower than the slope angle, for the left- Hoek, 2001) was estimated ranging from 35 to 45 hand side of the Fiumicello Valley. By the (B to C classes) for the lower stratigraphic interval stratigraphic and morphological observations, two and varying from 10 to 20 (F to H classes). principal normal fault systems, NW-SE and NE-SW On the basis of such stratigraphic setting, two striking, were identified. A normal fault passing distinct engineering geological units (Dearman, through the upper zone of the left flank was 1991) were identified: a) calcarenite lower member; supposed to control strongly the landslide geometry b) marly limestone upper member. Owing to the by putting the marly limestone upper member in lack of any outcrop of the bottom of the lower lateral contact with the lower calcarenite member. member, the only stratigraphic thickness was The inclinometer measurements indicated discrete estimated for the upper unit as varying between 40 sliding surfaces at different depths: 19 m for B1 and 60 m. borehole at the landslide foot; 34 m for B2 borehole More stratigraphic details were obtained by in the middle-right sector of the landslide; and 5, 10, boreholes, which confirmed an upward greater 19 and 32 m for B3 borehole. The closeness (less abundance of mudrocks and marly limestones than 10 m) of the latter borehole to the left landslide alternated to calcarenites. The water table was boundary and the deepest deformation datum (32 m intercepted only in the lowest borehole (B1) at deep) led to reconstruct a subvertical geometry of approximately 25 m depth and at an altitude (35 m the rupture surface in this sector. Such an a.s.l.), a little higher than the valley bottom.

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observation supported further the interpretation that movements and deformation regimes variable in the normal and subvertical fault forms the left space and time. Besides the progressive increase of boundary of the landslide exerting a strong deformation rates and widening of landslide structural control on the mass movement. structures, the non-uniform movements inside the landslide main body determine an increase of displacements and velocities toward its left part. The 6 Discussion and conclusions differential movements inside the landslide body are The analysis of the morphological evolution of the demonstrated by a series of transversal morpho- Pisciotta landslide reveals the existence of an early structures with strike-slip kinematics. These stage, since 1943, in which the slope gravitational kinematic features are interpreted to be caused by deformation is represented by an initial trace of the movements of rock-mass blocks, characterized by main scarp only, limited to the upper part of the differential downslope displacements and velocities slope. Similar early evolutionary stages were and controlled by the different driving forces. identified for other deep-seated landslides involving Accordingly, higher displacements and velocities of flysch deposits (Margielewski and Urban, 2003; the left side of the landslide are supposed to be Margielewski, 2006). In these first landslide stages, determined by greater driving forces due to the the rupture mechanism was supposed to terminate larger thickness of the unstable mass. This into the rock mass due to a downward progressive assumption is motivated by peculiar geometry of the accommodation of the shear stress by a viscous intersection between the bedding and the normal behavior (Radbruch-Hall, 1978; Chigira, 1992). fault that delimits the left flank of the landslide. This Accordingly, the Pisciotta landslide can be geometric condition determines an increase of the considered of a confined-type (Hutchinson, 1988) in thickness of the unstable upper marly member its initial evolutionary phase. In the successive toward the left flank of the landslide. In addition, steps, the progressive widening of the main scarp the existence of a morphological ridge shifted and its downslope continuation into the landslide toward the left flank is another factor that flanks, with a strike-slip kinematics, support to contributes to increase the thickness of the unstable hypothesize the enlargement of the basal rupture rock-mass, namely of driving forces. surface and a landslide distribution of advancing type (WP/WLI, 1993). The increase of the rate of Taking into consideration both the long-term ground movements after 1994, as it is assessed by continuous activity (1943-2014) and the low velocities of the SS 447 road displacements (Fig. 2) seismicity of the Cilento area, the triggering of the can be related to the progressive enlargement of the landslide and its spatial and temporal evolutions basal rupture surface and it is supposed to be cannot be related to specific events. Also the controlled by a downslope progressive failure erosional process of the Fiumicello Torrent seems to mechanism (Leroueil et al., 2012). In the current have played a marginal role because its path stage, the development of the basal rupture surface appeared over years not deviated by the landslide still appears not fully completed, as it is testified by activity as it still is. The last conclusion is consistent the downslope discontinuity of the lateral landslide with the hypothesis of a not complete development boundaries and by the absence of a clear outcrop of of the basal rupture surface and its not outcropping the basal rupture surface at the landslide toe. condition at the landslide foot. The current morphological features of the landslide, The available data cannot allow to understand coupled to the reconstructed geological model, clearly the role of groundwater on kinematic allow classifying this large slope mass movement as evolutionary stages. The water table was detected a deep-seated translational rock slide (Hutchinson, only in the lowest borehole and deeper than the 1995; Cruden and Varnes, 1996). Taking into sliding surface. Notwithstanding this apparent consideration the variability of the depth of the basal independence of the groundwater circulation and the rupture surface, the sum of the depleted mass and unstable rock mass, a delayed positive relationship the accumulation volume were assessed ranging between cumulative rainfall and ground velocities between 4 and 6×106 m3. indicates that infiltration process influences not negligibly the landslide movement rates and The assessment of landslide evolutionary stages, determines a rough seasonal control. derived by coupling aerial photographs since 1943 and the topographical monitoring of the upper The multidisciplinary study of the Pisciotta landslide body (September 2006 – March 2009), landslide proposes the analysis of a further case of indicates on a long-term time span (about 66 years) deep-seated landslide in flysch rock-masses

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