Jannis Kuhlmann

Submarine Slope Stability Assessment of the Central Mediterranean Continental Margin: The Gela Basin

This study investigates slope stability for a relatively small scale (5.7 km2, 0.6 km3), 8 kyr old landslide named Northern Twin Slide (NTS) at the slope of the Gela Basin in the Sicily Channel (central Mediterranean). The NTS is characterized by two prominent failure scars, forming two morphological steps of 110 and 70 m height. Geotechnical data from a drill core upslope the failure scar (GeoB14403) recovered sediments down to ∼52 m below seafloor (mbsf). The deposits show low over consolidation ratio (OCR = 0.24–0.4) and low internal friction angle (20–22°) around 28–45 mbsf, which suggests this mechanically weak interval may act as potential location of instability in a future failure event. Oedometer tests attest the sediments are highly under consolidated and the average overpressure ratio (λ*) is ∼0.7. Slope stability analyses carried out for different scenarios indicate that the slope is stable both under static undrained and drained conditions. A relatively small horizontal acceleration of 0.03–0.08 g induced by an earthquake may be sufficient to cause failure. We propose that moderate seismic triggers may have been responsible for the twin slide formation and could also cause mass wasting in the future.

Integrated Stratigraphic and Morphological Investigation of the Twin Slide Complex Offshore Southern Sicily

The Holocene Twin Slides form the most recent of recurrent mass wasting events along the NE portion of Gela Basin within the Sicily Channel, central Mediterranean Sea. Here, we present new evidence on the morphological evolution and stratigraphic context of this coeval slide complex based on deep-drilled sediment sequences providing a >100 ka paleo-oceanographic record. Both Northern (NTS) and Southern Twin Slide (STS) involve two failure stages, a debris avalanche and a translational slide, but are strongly affected by distinct preconditioning factors linked to the older and buried Father Slide. Core-acoustic correlations suggest that sliding occurred along sub-horizontal weak layers reflecting abrupt physical changes in lithology or mechanical properties. Our results show further that headwall failure predominantly took place along sub-vertical normal faults, partly through reactivation of buried Father Slide headscarps.

Do Embedded Volcanoclastic Layers Serve as Potential Glide Planes?: An Integrated Analysis from the Gela Basin Offshore Southern Sicily

The NE portion of the Gela Basin (Strait of Sicily) shows evidence of multiple mass wasting events of predominantly translational character. In this context, recent investigations implicate volcanoclastic layers as key stratigraphic surfaces acting as preferential planes of failure. We present an integrated analysis of a representative sedimentary transition from overlying homogeneous background sedimentation of silty clay to a volcanoclastic layer. A high-resolution CT scan and three drained direct-shear laboratory experiments from a 20 cm whole-round section (~28.2 mbsf) allow the delineation of the role of this volcanoclastic layer in the framework of slope stability and failure initiation. The mechanical results indicate a general strengthening of the material with increased volcanoclastic content. Tendency for failure is expected to be highest within the silty clay due to relatively lower shear strength and strain-weakening behaviour, which promotes progressive sediment failure. In contrast with recent findings, this suggests that volcanoclastic sediment would not act as a weak layer. However, the volcanoclastic layer exhibits significant mesoporosity (i.e., fracturing) and may therefore host large volumes of fluid. Temporarily undrained conditions, for example during seismic activity, could transiently elevate fluid pressures and thus reduce the material shear strength below that of the surrounding silty clay. Such a weak layer may preferentially form along the interface of fractured volcanoclastic material and relatively impermeable silty clay, where differences in material strengths are lowest.

Landslide Frequency and Failure Mechanisms at NE Gela Basin (Strait of Sicily)

Despite intense research by both academia and industry, the parameters controlling slope stability at continental margins are often speculated upon. Lack of core recovery and age control on failed sediments prevent the assessment of failure timing/frequency and the role of pre-failure architecture as shaped by paleoenvironmental changes. This study uses an integrated chronological framework from two boreholes and complementary ultra-high-resolution acoustic profiling in order to assess (1) the frequency of submarine landsliding at the continental margin of NE Gela Basin and (2) the associated mechanisms of failure. Accurate age control was achieved through absolute radiocarbon dating and indirect dating relying on isotope stratigraphic and micropaleontological reconstructions. A total of nine major slope failure events have been recognized that occurred within the last 87 kyr (~10 kyr return frequency), though there is evidence for additional syn-depositional, small-scaled transport processes of lower volume. Preferential failure involves translational movement of mudflows along sub-horizontal surfaces that are induced by sedimentological changes relating to pre-failure stratal architecture. Along with sequence-stratigraphic boundaries reflecting paleoenvironmental fluctuations, recovered core material suggests that intercalated volcaniclastic layers are key to the basal confinement and lateral movement of these events in the study area. Another major predisposing factor is given by rapid loading of fine-grained homogenous strata and successive generation of excess pore pressure, as expressed by several fluid escape structures. Recurrent failure, however, requires repeated generation of favorable conditions and seismic activity, though low if compared to many other Mediterranean settings, is shown to represent a legitimate trigger mechanism.