Abstract
Two Quaternary plastered contourite drifts, with terraced and low-mounded morphologies, make up the continental slope and base-of-slope in the northwestern Alboran Sea, respectively, between the Guadiaro and Baños turbidite systems, close to the Strait of Gibraltar.Considering their significant lateral extent, the link between the contourite drift deposits and landslides may be particularly important for hazard assessment. The physical properties, composition and geometry of contourite drifts have been proposed as key factors in slope stability, although this relationship still needs to be better constrained. In this work, new in-situ geotechnical data (cone penetration tests; CPTu) has been combined with morphostratigraphic, sedimentological, and (laboratory) geotechnical properties to determine the stability of the Guadiaro-Baños drifts.For the depositional domains of both drifts, the resulting sedimentary and geotechnical model describes low-plasticity granular and silty sands on the erosive terraced domain that evolve seawards to silty and silty-clay deposits with a higher plasticity and uniform geomechanical properties. For the shallower coarse-grained contourite sediments, the cohesion (c') and internal friction angle (ϕ') values are 0–9 kPa and 46–30°, respectively, whereas for the distal fine contourites the undrained shear strength gradient (∇Su) is 2 kPa/m. These properties allow us to establish high factors of safety for all the scenarios considered, including seismic loading. Slope failure may be triggered in the unlikely event that there is seismic acceleration of PGA > 0.19, although no potential glide planes have been observed within the first 20 m below the seafloor.This suggests that the contourite drifts studied tend to resist failure better than others with similar sedimentary characteristics. The interplay of several processes is proposed to explain the enhanced undrained shear strength: 1) the geometry of the drifts, defined by an upper contouritic terrace and lower low-mounded shapes; 2) recurrent low-intensity earthquakes with insufficient energy to trigger landslides, favouring increased strength due to dynamic compaction; and 3) cyclic loading induced by solitons/internal waves acting on the sediment.
Highlights
Contourite drifts are ubiquitous sedimentary systems, usually with a significant lateral extent and commonly associated with landslides (Laberg and Camerlenghi, 2008; Stow and Faugeres, 2008)
The study area comprises 610 km2 where two Quaternary plastered contourite drifts make up the continental slope and base-of-slope, just as they do in the rest of the northwestern Alboran Sea, influenced by both the Light Mediterranean Waters (LMW) and the Western Mediterranean Deep Water (WMDW) (Ercilla et al, 2016; Ercilla et al, 2019a; Juan et al, 2020; Fig. 1)
The upper continental slope morphology is determined by the smooth relief of the contourite terrace (1–1.75◦); this gradient increases seaward as it tran sitions to the lower plastered drift, where it reaches 3.9◦ (Fig. 2)
Summary
Contourite drifts are ubiquitous sedimentary systems, usually with a significant lateral extent and commonly associated with landslides (Laberg and Camerlenghi, 2008; Stow and Faugeres, 2008). Weak layers may play an important role, as these are where deformation may be initiated, and along which the slip plane would form (e.g., Laberg et al, 2016; Miramontes et al, 2018). Whether these landslide-preconditioning factors are a widespread characteristic of contourite drifts still needs to be determined
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