The Genesis of the Anomalous Sernio Fan, Valtellina, Northern Italy

Authors: E. De Finis, P. Gattinoni, L. Scesi

Abstract:

Massive rock avalanches formed some of the largest landslide deposits on Earth and they represent one of the major geohazards in high-relief mountains. This paper interprets a very large sedimentary fan (the Sernio fan, Valtellina, Northern Italy), located 20 Km SW from Val Pola Rock avalanche (1987), as the deposit of a partial collapse of a Deep Seated Gravitational Slope Deformation (DSGSD), afterwards eroded and buried by debris flows. The proposed emplacement sequence has been reconstructed based on geomorphological, structural and mechanical evidences. The Sernio fan is actually considered anomalous with reference to the very high ratio between the fan area (≈ 4.5km2) and the basin area (≈ 3km2). The morphology of the fan area is characterised by steep slopes (dip ≈ 20%) and the fan apex is extended for 1.8 km inside the small catchment basin. This sedimentary fan was originated by a landslide that interested a part of a large deep-seated gravitational slope deformation, involving a wide area of about 55 km². The main controlling factor is tectonic and it is related to the proximity to regional fault systems and the consequent occurrence of fault weak rocks (GSI locally lower than 10 with compressive stress lower than 20MPa). Moreover, the fan deposit shows sedimentary evidences of recent debris flow events. The best current explanation of the Sernio fan involves an initial failure of some hundreds of Mm3. The run-out was quite limited because of the morphology of Valtellina’s valley floor, and the deposit filled the main valley forming a landslide dam, as confirmed by the lacustrine deposits detected upstream the fan. Nowadays the debris flow events represent the main hazard in the study area.

Keywords: Anomalous sedimentary fans, debris flow, deep seated gravitational slope deformation, Italy, rock avalanche.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1109181

References:

[1] G. B. Crosta, P. Frattini, “Controls on Modern Alluvial Fan Processes in the Central Alps, Northern Italy,” Earth Surface Processes and Landforms Vol. 29, pp. 267–293, 2004.
[2] A. M. Harvey, “Dryland Alluvial Fans,” Arid-Zone Geomorphology, 3rd edn, Thomas DSG (ed). Wiley-Blackwell, Chichester, pp. 333-371, 2011.
[3] D. Jarman, F. Agliardi, G. B. Crosta, “Megafans and Outsize Fans from Catastrophic Slope Failures in Alpine Glacial Troughs: The Malser Haide and the Val Venosta cluster, Italy,” Slope Tectonics/Geological Society, London, Special Publications, vol. 351, pp. 253-278, 2011.
[4] M. Guglielmin, G. Orombelli, “Il Cono di Deiezione Terrazzato Allo Sbocco Della Valle del Migiondo Presso Sondalo (Valtellina),” Istituto Lombardo, Italia, vol. 135, pp. 87-100, 2001.
[5] C. Blair Terence, “Alluvial fan and catchment initiation by rock avalanching, Owens Valley,” Geomorphology 28, pp. 201–222, 1999.
[6] G. Bonsignore, C. E. Bravi, G. Nangeroni, U. Ragni, “La Geologia del Territorio Della Provicia di Sondrio, Con Carta Geologica 1:200.000,” Milano, 1970.
[7] M. I. Spalla, M. Zucali, F. Salvi, G. Gosso; D. Gazzola, “Tectono-Metamorphic Map of the Languard-Campo-Serie del Tonale nappes between upper Val Camonica and Valtellina (Central Italian Alps, Austroalpine Domain),” Mem. Sc. Geol., 55, pp. 105-118, 2003.
[8] A. Montrasio, “Carta Geologica Della Lombardia, Scala 1: 250.000,” Dipartimento di Scienze della Terra (Unimi), Regione Lombardia, Consiglio Nazionale delle Ricerche, 1990.
[9] C.Ambrosi, GB Crosta, “Large Sackung along Major Tectonic Features in the Central Italian Alps,” Engineering Geology vol. 83, pp. 183-200, 2006.
[10] M . Borsetto, F. Lionetti, R. Ribacchi, “Ripercussioni di Lenti Fenomeni Deformativi di un Versante su Opere Idrauliche,” cod. 12873, 29 Frane Lomb Vact XVI CNG: "La Progettazione Geotecnica per la Stabilizzazione dei Pendii" Bologna, 1986.
[11] G. Biella, R. De Franco, G. Caielli, P. Vignola, A. Lozej, A. Bini, M. Guglielmin, S. Carbonara, M. Terenzoni, “Imaging the Deposits that fill Valtellina (NW Italy) by Seismic Reflection Investigation,” 5th Workshop Alpine Geological Studies. Geol. Paläont. Mitt. Innsbruck 25, pp 33-34, 2001.
[12] A. Bini, M. Felber, M. Guglielmin, “Deep-Seated Gravitational Slope Deformation under the Sediment Fill of Insubric Valleys (Italy-Switzerland),” 5th Workshop Alpine Geological Studies. Geol. Paläont. Mitt. Innsbruck, 25, pp 35 –36, 2001a.
[13] A. Bini, G. Biella, R. De Franco, M. Guglielmin, P. Tognini, “Deep-Seated Gravitational Slope Deformations as Origin of the Terraces on the Northern Slope of Valtellina (Italian Central Alps),” 5th Workshop Alpine Geological Studies. Geol. Paläont. Mitt. Innsbruck, 25, pp 37–38, 2001b.
[14] E. De Finis, A. Bini, “Control of Fans Dominated by Landslide on the Quaternary Sequence in Alpine Valley (Valtellina, Norther Italy),”14th GeoConference on Science and Technologies in Geology, Exploration and Mininig, Conference Proceedings Vol. II, pp. 219-226, Albena (Bulgaria), pp.17-26, 2014.
[15] F. Ferranti “Su la Frana di Sernio Nella Valtellina che Generò nel 1807 un Nuovo Lago e sui Mezzi Praticati per Scemarne le Dannose Conseguenze,” 1814.
[16] F. Comiti, L. Marchi, P. Macconi, M. Arattano, G. Bertoldi, M. Borga, F. Brardinoni, M. Cavalli, V. D’Agostino, D. Penna, J. Theule “A New Monitoring Station for Debris Flows in the European Alps: First Observations in the Gadria Basin,” Nat Hazards DOI 10.1007/s11069-014-1088-5, 2014.