Pore pressure build-up in mixed contourite-turbidite systems: the SW Iberian margin

Abstract

Contourite drifts are the depositional expression of bottom currents, which are capable of shaping the morphology of the seafloor by transporting, eroding and reworking marine sediments, and are distributed on continental slopes worldwide. Synchronous interaction between bottom currents and turbidity currents have been reported often in channel-levée systems where the thickness of the turbidity current exceeds that of the levées. Such interplay between along-slope and down-slope sedimentary process is one of the mechanisms by which “mixed turbidite-contourite systems” can originate. Being composed by the winnowed finest part of turbidity currents and potentially able to deposit at sedimentation rates of several m/kyr, mixed systems constitute a potential source of overpressure development in marine sediments. The Alentejo basin is located in the northern edge of the Gulf of Cadiz (SW Iberian margin), from which is separated by the São Vicente Canyon, one of the most prominent geomorphological features in SW Iberia. The area is characterized by an extensive contourite depositional system, generated by the Mediterranean Outflow Water (MOW) bottom current. The Alentejo basin hosts multiple mass transport deposits (MTDs) and scars, which are typically considered to be triggered by the frequent seismic activity related to the movement of multiple active faults present in the area. The combined presence of a contourite depositional system, a deeply incised canyon, mass wasting processes and potentially seismogenic faults makes the Alentejo basin the ideal study area to assess: a) how does contour currents interact with sediments transported along deeply incised canyons? b) How does mixed contourite – turbidite systems affect the development of overpressure within marine sediments, and consequently the stability of submarine slopes? To answer these questions, in this thesis we first adopted a workflow that included reinterpretation of existing seismic data in the Alentejo basin as well as acquisition and interpretation of geophysical and sediment core data collected during two scientific cruises in 2018 (INSIGHT-Leg1) and 2019 (INSIGHT- Leg2). The study focuses in the area between ~ 1000 and 3000 m water depth around the São Vicente Canyon. Grain size and physical properties (i.e. water content, Atterberg limits, porosity, permeability and compression index) of sediments collected in the study area have been analysed through geotechnical laboratory experiments. The updated stratigraphic model and sediment physical properties have been integrated into Finite Element numerical models in order to derive the fluid flow and overpressure evolution of the Alentejo basin, focusing on the potential influence of mixed-system deposition in the emplacement of the MTDs detected in the basin and in the overall stability of the slopes.

This thesis identifies a previously unknown mixed contour current – turbidity current deposit, the Marquês de Pombal Drift, located on the NW flank of the São Vicente Canyon and generated by the interaction between the MOW, particularly during cold periods, and turbidity currents flowing along the canyon itself. Because the canyon is incised significantly deeper (~ 1.5 km) than the thickness of turbidity currents, interaction between down-slope and along-slope currents requires intermediate nepheloid layers, forming at the boundary between major water masses (i.e., the MOW and the North Atlantic Deep Water), to export the finer-grained fraction of turbidity currents out of the canyon. Such mechanism is likely active in other canyons worldwide. Stratigraphic interpretation and gravity core analysis reveal that the Marquês de Pombal Drift is characterized by sedimentation rates up to 2 m/kyr and fine-grained sediments (mean grain size of 8.6 µm). Despite those characteristics, the hydrogeological models presented in this thesis show development of overpressure associated with the Marquês de Pombal Drift deposition up to a maximum of 20% of the lithostatic load. Factor of safety (FoS) analysis indicates that overpressure development decreases the stability of the slopes in the study area by 18% under static conditions, and that slope gradients play a more important role as pre- conditioning factor for slope failures as the highest overpressures are recorded in relatively flat terrains. The pseudo-static slope stability analysis, on the other hand, indicates that earthquake-induced accelerations given the fault characteristics in the area (i.e. earthquake shaking) have a much stronger influence on slope stability in the Alentejo basin. Therefore, trigger mechanisms seems to be the predominant component in slope stability compared to pre-conditioning factors (i.e., overpressure development, slope gradients). The results indicate that the Marquês de Pombal Drift is a relatively stable feature and its deposition has a minor influence in the stability of the Alentejo basin. However, in other settings, fine-grained sediments and potential high sedimentation rates associated with mixed contour current – turbidity current systems could have the potential to develop higher overpressure and thus play a more important role in submarine slope stability. Our results show that multiple conditions related with the physical properties of sediments (i.e. high degree of compressibility and low permeability), high sedimentation rate and the morphology of the slope (i.e. slope gradients) must all be present to efficiently influence slope instability.