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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Atlantic Ocean
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North Atlantic
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Northwest Atlantic
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Demerara Rise (1)
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Europe
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Southern Europe
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Italy
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Sardinia Italy (1)
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Mediterranean Sea
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West Mediterranean
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geologic age
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Cenozoic
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Quaternary
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Holocene (1)
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Pleistocene
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upper Pleistocene (1)
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Primary terms
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Atlantic Ocean
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North Atlantic
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Northwest Atlantic
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Demerara Rise (1)
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Cenozoic
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Quaternary
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Holocene (1)
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Pleistocene
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upper Pleistocene (1)
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Europe
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Southern Europe
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Italy
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Sardinia Italy (1)
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geophysical methods (1)
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Mediterranean Sea
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West Mediterranean
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Balearic Basin (1)
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ocean floors (2)
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oceanography (1)
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sedimentary rocks
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contourite (1)
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sediments
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contourite (1)
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To: “Echofacies interpretation of Pleistocene to Holocene contourites on the Demerara Plateau and abyssal plain,” Cédric Tallobre, Lies Loncke, Laurence Droz, Tania Marsset, Mirjam Uusõue, Walter R. Roest, Anne-Sophie Fanget, Maria-Angela Bassetti, Pierre Giresse, and Germain Bayon , Interpretation, 9 , no. 2, SB48–SB65, doi: 10.1190/INT-2020-0159.1.
Echofacies interpretation of Pleistocene to Holocene contourites on the Demerara Plateau and abyssal plain
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.
Cycles in the Architecture of the Quaternary Zaire Turbidite System: A Possible Link with Climate
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.
Abstract The Congo deep-sea fan was surveyed recently by a unique set of data that includes as of today more than 30 piston cores and 25,000 km of 2D high resolution seismic acquisition. During the different cruises, the following tools were used simultaneously: an EM12 dual Simrad swath bathymetric and sonar system, a 3.5 kHz mud penetrator, an oceanographic seismic system (6 traces), and a multichannel seismic streamer (96 traces) with G.I. air gun source. The data set covers an area in excess of 200,000 km 2 from the canyon head to the most recent depositional lobes 700 km downslope. In this four year project cosponsored by ELF and IFREMER, the prime objective was to gain comprehension of the construction and the lateral evolution of the recent fan. As a result, we were able to build a chronostratigraphic and architectural model at the fan scale. The Congo Canyon geometry has been enhanced and has confirmed the lack of any tributaries downdip from the continental shelf. Furthermore, no real large scale slumped blocks have observed at the base of the steep submarine canyon slopes. Therefore, widening of the canyon seems to have occurred through progressive sliding and creeping of the bordering Congolese and Angola platform into the canyon valley. The recent Congo fan growth results from the stacking and overlapping of at least 3 main stratigraphic units. Each unit is composed of numerous channel-levee complexes laterally switching path through time by avulsion. Previous interpretation of “distributaries” at the toe of the Congo submarine canyon is invalidated as this surveys shows that they are in fact abandoned channel-levee complexes. It has also been confirmed that in this particular mud-rich submarine fan setting, only one channel is active at a time. The present day “active” fan channel as well as the canyon are eroding into previously deposited sediments. In the levees, individual sub-kilometer scale morphological features have been mapped, and sedimentary and/ or structural processes responsible for such geometries have been reviewed. The present day depositional lobe is more than 700 km away from the Congo River mouth. It occupies an area of around 300 km 2 and shows clustered lobes, 20 km long, prograding down slope. It is now clear that multibeam coverage of the lower part of the canyon, over complex lobe, and channel avulsion zones shows strong similarities with seismic attribute maps of West Africa’s structurally controlled turbiditic channel complexes.