Los ciclos económicos y la migración

57 Figure 4.2 – Comparison of the average monthly precipitation from 1963 to 2012 and the 1963 rainfall data, the latter shows an evident peak at the end of February when the landslide was triggered.

4.1.1 Geological and geomorphological setting

The Sorrento peninsula belongs to the Lattari-Picentini Unit (bonardi et al., 2009), a portion of the shallow marine carbonates of the AMC (fig.2.7).

The outcropping rocks are composed almost exclusively of preorogenic Mesozoic carbonate sequences and, limited to the upper part, of synorogenic terrigenous deposits of Miocene age (fig. 4.3).

58 Figure 4.3 – Geological map of the Termini-Nerano area (a). A-A’ and B-B’ sections are in Figure 4.4 and 4.5 respectively.

More precisely, the study focused on the Punta Campanella succession (Scandone and Sgrosso, 1965;

Cocco and Pescatore, 1967; De Blasio et al., 1981; CARG, 2010a). The Punta Campanella succession consists of, from the bottom to the top, (fig. 5.2b): the Radiolitidae limestone Formation (Turonian-Senonian), the Recommone calcarenites Formation (Lower Miocene), the Termini sandstones Formation (Serravallian), the Punta del Capo breccias (Upper Tortonian) and, finally, the Quaternary deposits consisting of slope debris, pyroclastic and beach terrains.

Radiolitidae limestone Formation (fig. 4.4a) is the bedrock of mt San Costanzo and extends along the coastline between Punta Vaccola and Capo di Sorrento. The average thickness is around 300 m and is mainly composed by bioclastic and bioconstructed calcilutites (fig. 4.4b) (Iannace et al., 2011).

A paraconformity boundary sets the transition to the upper Recommone calcarenites Formation (fig.

4.4c), mainly composed by glauconitic calcarenites (De Blasio et al., 1981).

59 The upper interval of the Recommone calcarenites Formation is composed of yellowish coarse-grained arenites (e.g. near Capo di Sorrento and Punta di Vaccola), interbedded with several marly-silty levels.

An unconformity marks the transition to the Termini sandstones Formation (figs. 4.4d-i), which elsewhere lies straight to the carbonate bedrock by means of a thrust. The Termini sandstones Formation (CARG, 2010a) was already known in the literature as the Arenaceous-Clayey Formation (Cotecchia and Melidoro, 1966) or the Punta Lagno Formation (Scandone and Sgrosso, 1965).

The Termini sandstone Formation (200 m thick) is constituted by two main members: the lower one is the Nerano sandstones Member where the studied landslide developed. It consists of arkosic

sandstones, interbedded with siltstone and mudstone levels, cropping out south of Termini (fig. 4.4h).

A gradual vertical and lateral transition leads to the Marciano sandstone Member, a thin bedded arkosic sandstone succession interbedded with marly levels which preserve their original structures related to a turbidite origin (e.g., parallel and convolute laminations; figs. 4.4d-e).

Calcareous olistoliths (nearby Schiazzano village; fig. 4.4f) with angular unconformity relationships and clayey olistostromes (nearby Madonna della Neve, Caprile and Marciano; fig. 4.4g) with a conformity relationship have also been recognized in the Nerano sandstone Member. An angular unconformity sets the boundary with the overlying Punta del Capo breccias, a tens of meters thick unit, consisting of breccias and, secondarily of conglomerates within a sandstone matrix (fig. 4.3j).

From Miocene the sediments belong to the continental environments: slope debris (fig. 4.4l), pyroclastic (fig. 4.4k) and beach terrains.

Slope debris derive prevailingly from calcareous boulders which are mostly originated from the inverse fault on the eastern side of mt San Costanzo during the latest glacial phases (Brancaccio, 1968), while the valley lying between Termini and Nerano settlements is mainly covered with clayey-arenaceous landslide deposits, including those related to the 1963 event.

60 Volcanoclastic material displays a huge variability in thickness and a patchy outcrop pattern.

compositionally it is a trachytic pyroclastite including amphibolite and micaceous crystals in a dark to purplish gray glassy matrix (Fedele et al., 2008).

Finally, the beach deposits consist of pebbles deriving from the Cretaceous carbonate bedrock and of sandstones from the flysch deposit.

Figure 4.4 – Some examples of outcrops in the Punta Campanella area. A normal fault in the massive limestone (a).

Radiolitidae shell fragments in the Radiolitidae limestone Fm (b). Gradual transition to the Recommone calcarenites Fm (c). Parallel (d) and convolute lamination (e) in the Termini sandstone Fm. Marly-calcareous olistolith (f). Clayey olistostrome in the Nerano sandstone Mb (g). Coarse sandstone (h) and fine sandstone (i) of the Termini sandstone Mb.

Punta del Capo breccias (j), campanian ignimbrite (k) and slope deposit (l).

The tectonic features that most influence the geomorphology are connected to a first compressional event with a NE-vergence, followed by an extensional one (Sartori, 1990).

The compressive phase created gentle folds in the Termini sandstone Formation and the thrust east of Termini while the latest extensional phase created high-angle normal faults generating the Rio Acchiungo valley (fig. 4.2a).

The predominant clayey lithology combined with the strong tectonic deformation and the presence of

"exotic" blocks (namely the olistolithes and the olistostromes), represents an important predisposing factor to slope instability in Nerano area.

61 Geological surveys and borehole data (see paragraph 4.1.2) allowed to define two geological cross sections for the landsliding area where older logs (P1, P3 and P4 of Figure 4.5), acquired from the Municipal Archives, are also included.

The first section (fig. 4.5), NW-SE oriented, displays how the landslide developed in the Nerano sandstone Formation where olistoliths and olistostromes were found during the in-situ campaigns.

In the A-A’ section, a margin of error still remains concerning the depth of contact between the Termini sandstone Formation and the Recommone calcarenite Formation, since it was inferred from field evidences.

Figure 4.5 – NW-SE geological cross section. The A-A’ section trace is in Figure 4.3.

The second section, SW-NE oriented, crosses the landslide track and reveals the limited extension of the 1963 landslide deposit compared to the conglomeratic debris that fill most of the Termini-Nerano valley; the valley developed above a syncline structure (NW-SE direction) and delimited to the northeast by the Rio Acchiungo valley normal fault (fig. 4.6).

62 Figure 4.6 – SW-NE geological cross section. The B-B’ section trace is in Figure 4.3.

Geomorphologically, Termini and Nerano settlements lie at the foothills of the eastern side of mt San Costanzo. The relief shows a steep eastern slope (dip angle higher than 60^o), where continuous rock falls and cryoclastic erosion processes cause the deposition of slope debris over the Termini sandstone Formation.

On the eastern and northern sector of the investigated area the terrain morphology is generally gentler due to the wide presence of the Termini sandstone Formation. The area is characterized by hills gradually dipping towards the sea. The hills reach maximum elevations of 400-500 m a.s.l., sometimes with a flat top surface (e.g., the Sant’Agata desert). Here the most common gravitational phenomena are flows, rotational slides or complex movements (fig. 4.7).

Small basins and high-gradient streams characterize the drainage network, thereby promoting the development of short and small channels with high erosion rates and linear patterns.

63 Figure 4.7 – Official landslide inventory map for the Nerano area (AdB Destra Sele, 2011). In the inset, the a and b landslides on the mt San Costanzo eastern side.

In the investigated area, the official Landslide Inventory Map of the HSP (AdB Destra Sele, 2011) reports 22 active landslides, divided as follows: 5 falls, 3 rotational slides and 14 complex landslides (rotational + flow), among which the 1963 landslide is the largest one. During the landslide mapping and monitoring stages, which are discussed in the following three paragraphs, only the rotational slides and complex landslides of the Termini-Nerano valley were taken into consideration. The inferred new landslide area will be showed in Figure 4.11.

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