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.

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 The presently active Zaire (Congo) turbidite system reveals a well-organized Quaternary architecture, with depocenters that partly overlap each other in response to avulsion. Based on previous work, more than 76 channels are organized into three individual fans (Northern, Southern, and Axial Fan, from the oldest to the youngest). A statistical analysis of both longitudinal and lateral migration of depocenters was conducted. The longitudinal shifts were studied through the temporal evolution of the channel lengths and the distances to the bifurcation points from a common reference point arbitrarily positioned on the canyon course, up-dip from the most proximal bifurcation point. The number of bifurcation points on a channel was also calculated. These three architectural parameters show a cyclic organization through time, better expressed in the Axial Fan, with cycles of down-fan and up-fan movements reflecting prograding–retrograding cycles. Based on a previous study of the kaolinite/smectite (K/S) ratio in the hemipelagic drape covering the Southern Fan, i.e., contemporaneous with the building of the Axial Fan, the prograding peaks of the Axial cycles are correlated to peaks in K/S, which reflect phases of intense Zaire River water discharge, during warm and humid interglacial periods. These correlations suggest that both the channel lengths and the avulsion process are controlled by climate changes that appear as a major forcing factor throughout the Quaternary. The effects of climate control can be modified by the interplay of other internal and/or external factors. Study of the lateral migration revealed that topographic compensation is the major autogenic control, and that external factors such as tectonic evolution in the drainage basin of the Zaire River or halokinesis at the Angola base of slope can locally play a significant role in the location of depocenters.