Soft sediment deformation in the shallow submarine slope off Nice (France) as a result of a variably charged Pliocene aquifer and mass wasting processes

Abstract

Along the Ligurian slope near Nice, southeastern France, a combination of natural and man-made factors govern slope stability, and contributed to a devastating tsunamigenic landslide near Nice airport in 1979. Based on a total of 72 gravity and Kullenberg cores we characterise the architecture and facies of the subbottom sediment. A total of six sedimentary facies types were observed, three of which represent the Pliocene-Holocene background sediment in the wider Nice area while another three are associated with the 1979 landslide and tsunami. The three primary facies types are soft silty clay/clayey silt, somewhat indurated silt/sand interbeds, and Pliocene conglomerates underlying the former. The three other facies are poorly sorted mass wasting deposits up to pebble size, silt representing the finer fraction of the mobilised mass, and a tsunami-related bed into which plant debris and artefacts got amalgamated. Accompanying geotechnical results attest that significant strength variations exist when comparing measurements from the narrow shelf, shelf break and shallow slope as well as the 1979 slide scar. Factors such as groundwater charging in the more permeable horizons further lower the effective stress and, in places, approach lithostatic. Above such permeable silt/sand beds, deformation of soft clay is observed, suggesting that leaching of ions from the clay mineral surfaces and frayed edges also facilitates weakening and creep, microslumping and folding. Elevated water supply as well as hydraulic fracturing of the clayey sediment is further attested by pipes of cm-diameter and several decimeters length, which entrained very soupy mud. In the shallowmost deposits in and landward of the slide scar, we also observe remnants from the catastrophic deposition associated with the 2-3 m high tsunami wave triggered in 1979, as attested by amalgamated sea grass and artefacts. Based on our observations and measurements on core-scale and earlier models based on in situ CPTu, we suggest that a significant part of the Quaternary delta and slope apron deposits are prone to being mobilised without a large external trigger, so that all in all a high risk for a landslide event of considerable size off Nice international airport is identified.

Impact of seismicity on Nice slope stability—Ligurian Basin, SE France: a geotechnical revisit

“…Dubar and Anthony (1995) described the three upper major sedimentary delta facies with a thickness of approximately 120 m from bottom to top: (1) clast-supported gravel with a matrix of sand, (2) fine-grained shallow marine and estuarine/paludal deltaic sediments, and (3) fine-grained floodplain and paludal sediments with gravel channel deposits. Kopf et al (2016) presented a more detailed facies analysis based on 72 cores where they described silt/sand interbeds as one out of three Pliocene-Holocene sediment facies. The silt/sand interbeds are of high interest for seismic slope stability because cohesionless sediment layers have a high liquefaction potential under cyclic loading (Boulanger and Idriss 2006;Idriss and Boulanger 2008;Kramer 1996).…”

“…The catastrophic failure on the Nice shallow submarine slope triggered a tsunami wave, which hit the coastline along the Ligurian Sea causing eight casualties and infrastructural damage (Dan et al 2007;Migeon et al 2006;Sahal and Lemahieu 2011). The interplay of land reclamation operations 6 months before the failure, extra loading by embankments of the extended Nice airport and heavy rainfall of 250 mm for 4 days before the failure most likely created an unstable artificial delta front slope, which collapsed on the 16 October 1979(Anthony 2007Anthony and Julian 1997;Dan et al 2007;Kopf et al 2016). Seismic loading did not trigger the 1979 failure; nevertheless, seismic loading is a prominent trigger for submarine landslides (Haque et al 2016;Leynaud et al 2017;Sultan et al 2004), and the junction between the southern French-Italian Alps and the Ligurian Basin near Nice faces the highest seismicity in western Europe (Larroque et al 2009;Salichon et al 2010).…”

“…85 Over the last two decades, several studies characterized the 86 Nice submarine slope sediments and their stability via in situ 87 measurements (Stegmann et al 2011;Steiner et al 2015;Sultan 88 et al 2010), laboratory experiments (Kopf et al 2016;Stegmann 89 and Kopf 2014;Sultan et al 2004), high-resolution bathymetric 90 data analysis (Kelner et al 2016;Migeon et al 2012), Envisat 91 InSAR data (Cavalié et al 2015), and numerical modeling (Dan 92 et al 2007;Steiner et al 2015). These studies present contradic-93 tory results and interpretations concerning the Nice slope 94 stability under earthquake ground motions.…”

“…Ai et al (2014) studied the cyclic stresses required for 105 failure of the deeper continental slope offshore Nice and con-106 cluded that earthquakes with M 6.1-6.5 in an epicentral distance 107 of < 15 km from the Nice slope are sufficient to initiate slope 108 failure. The latest study in the Nice shallow submarine slope 109 area by Kopf et al (2016), however, stated that seismic loading 110 is unlikely to be sufficient to trigger a major landslide unless an 111 earthquake with a magnitude larger than the magnitudes of 112 known historical events occurs.…”

Trigger recognition of Early Cretaceous soft‐sediment deformation structures in a deep‐water slope‐failure system

“…Soft-sediment deformation structures (SSDS) can occur in different sedimentary settings, such as fluvial, lacustrine, marine and ice-sheet environment (Gibert, Alfaro, Garcia-Tortosa, & Scott, 2011;Kopf et al, 2016;Lesemann, Alsop, & Piotrowski, 2010;Rana, Sati, Sundriyal, & Juyal, 2016). Common SSDS include load structures, convolute lamination, deformed cross-bedding, slump folds, water-escape structures, and neptunian dykes, which show significant variations in morphology and scale (Gibert et al, 2011;Moretti & Sabato, 2007; Owen, Moretti, & Alfaro, 2011).…”