Long-term development and current status of the Barcelona continental shelf: A source-to-sink approach

The Barcelona continental shelf, off the city of Barcelona (NE Spain), is a relatively narrow canyon-bounded shelf in the northwestern Mediterranean Sea. Three medium-size rivers (Tordera, Besós and Llobregat) and several ephemeral rivulets flow into this margin. Two main domains have been recognized in the Barcelona shelf: (i) a modern, river-influenced area, and (ii) a relict, sediment depleted area, both affected by a variety of human impacts. A detailed geomorphologic study based on multibeam bathymetry and backscatter data, high resolution seismic profiles, and surface sediment samples allowed mapping and interpreting the main distinctive seafloor features on the Barcelona shelf. Modern sedimentary features reveal that the Llobregat River is the main sediment source of the Barcelona prodeltaic shelf. High-discharge fluvial events result in the formation of suspended sediment plumes and sediment waves on the shelf floor. Relict (late Pleistocene–Holocene) sedimentary features reflect that an important shift occurred in the seashore direction between MIS 4 and MIS 2, and that recent neotectonic reactivation has created a set of seafloor faults. The Barcelona inner and middle shelf is severely impacted by anthropogenic activities such as the enlargement works of the Port of Barcelona, sewage pipes, dredging, anchoring and trawling.     

Slope failures and stability analysis of shallow water prodeltas in the active margins of Western Greece,northeastern Mediterranean Sea

Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment failures on the low-angle (0.5°–2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80–150 m long and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10–15 m below seafloor. Shear planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the steeper (2^°–6^°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone. The foresets of the HST prodelta deposits display high water content (30–80%), low bulk density (1.4–1.9 g cm⁻³) and relatively low values of undrained shear strength (3–20 kPa). The water content of the HST distal muddy bottomsets is relatively higher (50–110%) and bulk density relatively lower (1.3–1.7 g cm⁻³) with low values of shear strength (2–10 kPa). The shear strength of the gas releasing sediment layer displays lower values (2–9 kPa) relative to the overlying, post failure, muddy sediments of the late 100–300 years. Slope stability was calculated using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments. This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6–29.6% g for the HST foresets and 12.4–14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs) for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments of the lower part of the HST foresets to the upper part of early HST bottomsets.