Abstract The Gulf of Lions presents recurring mass-transport deposits (MTDs) within the Plio-Quaternary sediments, suggesting a long history of mass movements. The two large, surficial MTDs are located on the eastern and western levee of the Rhone canyon over an area exceeding 6000 km 2 and volumes exceeding 100 km 3 . Both MTDs were emplaced 21 ka ago (peak of the Last Glacial Maximum), suggesting a common trigger. Here, we present a multidisciplinary high-resolution geophysical, sedimentological and in-situ geotechnical study of the source and deposit areas of both MTDs to characterize distinct expressions of sediment deformation as well as their spatial and chronological distributions. We show the internal structure of mass movements and resulting MTDs with unprecedented details that were previously represented in the conventional seismic data as transparent and chaotic facies. The combination of multidisciplinary approaches shows new insights into the nature of basal surfaces of the slope failures. In particular, we show that the basal surfaces of the failures consist of clay-rich material contrasting with the overlying turbiditic deposits, suggesting that a strong lithological heterogeneity exists within the strata. We suggest that this change in lithology between clay-rich sediments and turbiditic sequences most likely controls the localization of weak layers and landslide basal surfaces.
Abstract Megabeds are thick sedimentary layers extending over thousands of square kilometres in deep-sea basins and are thought to result from large slope failures triggered by major external events. Such deposits have been found in at least three areas of the Mediterranean Sea. Although their discovery dates back to the early 1980s, many questions remain concerning their initiation, source area, extent and the nature of their emplacement. One of the largest previously documented megabeds was emplaced during the Last Glacial Maximum across the Balearic Abyssal Plain, with a thickness of 8–10 m in water depths of up to 2800 m. New 3.5 kHz sub-bottom profiles and sediment cores provide greater constraints on the lateral variability of the megabed and allow it to be mapped beyond previous estimates, with a revised areal extent of 90 000–100 000 km 2 . The megabed terminations show a gradual pinchout to the west and an abrupt eastward termination against the steep Sardinia margin. The megabed presents, in seismic profiles and sediment cores, a tripartite subdivision, which most likely corresponds to the changes in flow regimes across the basin, with a central area of sandy facies and an erosional base oriented NNE–SSW; this allows renewed discussions about the sources and triggers of the megabed.
The Bay of Brest (BB) is a shallow estuarine environment in NW France. This semi-enclosed basin of 180 km² is subject to multiple hydrodynamic factors including the dual influence of oceanic currents and fluvial discharges (Aulne and Elorn main rivers) and resulting in complex hydro-climatic and hydro-sedimentary processes. This study investigates with palynological data (continental: pollen grains and marine: dinoflagellate cysts) two kinds of different materials: (i) modern surface sediments collected over the whole BB as well as (ii) three new BB sediment cores (core ‘F’ from the mouth of the Aulne river and cores PALM-KS05 and PALM-KS06 from the Brest harbour). While modern data are analysed from a statistical point of view to highlight the influence of hydrodynamic forcing on the modern distribution of palynomorphs, the cores allow for spatial comparisons of palynological data on three windows over the Early (~9.5 and ~8.5 ka BP), Middle (~4.4–4.3 ka BP interval) and Late (~1–0.9 ka BP interval) Holocene. For each time intervals, two cores located along a transect from west (more pronounced oceanic influence) to east (more intense fluvial influence from the Aulne river) are compared, located on either side of a limit that we referred to as the river-induced palynological signal (RIPS) limit. These different comparisons reveal a high degree of spatial homogeneity in BB pollen records over time, with exceptions for environments east of the RIPS limit, for which rainfall-induced fluvial discharges have a stronger impact especially considering riparian taxa (i.e. Alnus). This is intended to improve understanding of the palynological signals recorded at different BB coring sites, a first step of crucial relevance before the establishment of a palynological stack covering the Holocene from several cores collected in different shallow bays of the BB (see Valero et al., submitted – PART II).
The Gulf of Lions (NW Mediterranean Sea) is a SW-NE oriented passive continental margin formed since the Oligocene. It presents small to large scale mass movement features suggesting a long history of seafloor instability. Of particular interest are the two surficial large mass-transport deposits along the Rhone turbiditic levee, known as the Rhone Eastern and Western Mass-Transport Deposits (REMTD and RWMTD). With the help of the recently acquired multi-beam bathymetric, sub-bottom profiler, high-resolution seismic and sedimentological data, we investigate the morphology, timing, kinematics, and possible triggering mechanisms of the source area of the REMTD, which we refer to as the Eastern Rhone Interfluve Slide (ERIS). ERIS has an estimated run-out distance of approximately 200 km. It covers an area of about 700 km2 and the volume of the mobilized material is approximately 110 km3. Our data reveal four individual glide planes within the ERIS complex which were most likely generated by retrogressive failures. The basal surfaces of the ERIS coincide with high-amplitude seismic reflectors similar to those previously interpreted as the expression of condensed sections on the upper slope. The turbiditic sequences sandwiched between the condensed sections likely control the localisation of potential weak layers favouring the failures. AMS radiocarbon dating yields an age of approximately 21 ka cal BP for the failures, which falls within the peak of the Last Glacial Maximum. The toe area of the ERIS is incised by several active listric faults rooted in the Messinian strata, which control the location of the slide scarps. The combination of several factors such as slope steepening, halokinesis, and excess pore pressure generation due to rapid turbiditic sedimentation during the Last Glacial Maximum are considered as the possible candidates for the triggering of the failures in the investigated slope.
Abstract Geophysical data acquired on the conjugate margins system of the Gulf of Lion and West Sardinia (GLWS) is unique in its ability to address fundamental questions about rifting (i.e. crustal thinning, the nature of the continent-ocean transition zone, the style of rifting and subsequent evolution, and the connection between deep and surface processes). While the Gulf of Lion (GoL) was the site of several deep seismic experiments, which occurred before the SARDINIA Experiment (ESP and ECORS Experiments in 1981 and 1988 respectively), the crustal structure of the West Sardinia margin remains unknown. This paper describes the first modeling of wide-angle and near-vertical reflection multi-channel seismic (MCS) profiles crossing the West Sardinia margin, in the Mediterranean Sea. The profiles were acquired, together with the exact conjugate of the profiles crossing the GoL, during the SARDINIA experiment in December 2006 with the French R/V L’Atalante. Forward wide-angle modeling of both data sets (wide-angle and multi-channel seismic) confirms that the margin is characterized by three distinct domains following the onshore unthinned, 26 km-thick continental crust : Domain V, where the crust thins from ~26 to 6 km in a width of about 75 km; Domain IV where the basement is characterized by high velocity gradients and lower crustal seismic velocities from 6.8 to 7.25 km/s, which are atypical for either crustal or upper mantle material, and Domain III composed of “atypical” oceanic crust. The structure observed on the West Sardinian margin presents a distribution of seismic velocities that is symmetrical with those observed on the Gulf of Lion’s side, except for the dimension of each domain and with respect to the initiation of seafloor spreading. This result does not support the hypothesis of simple shear mechanism operating along a lithospheric detachment during the formation of the Liguro-Provencal basin.
