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Channel-mouth lobe complex of the recent Amazon Fan: The missing piece

Abstract : The Amazon Fan is one of the largest river-fed submarine fan systems in the world. It is located in the Western Equatorial Atlantic and extends 1100 km northward from the Amazon canyon head located at the shelf break off northern Brazil. Numerous previous investigations on the Amazon Fan mostly focused on the upper and middle parts of the fan. Two recent cruises have provided a new dataset based on bathymetric, acoustic imagery and 3.5 kHz sub-bottom profiler data. It is the first detailed exploration of the youngest Amazon channel-mouth lobe complex that remained the “missing piece” of one of the best studied modern deep-sea fans. The stratigraphic framework of the surveyed channel-mouth lobe complex of the Amazon Fan consists of eight channel-levee systems and their associated channel-mouth lobe systems (from Brown to Amazon). These depositional systems record a complex depositional history in a short time period between the Last Glacial Maximum (not, vert, similar 20,000 cal yr BP) and the rise in sea-level until 10,100 cal yr BP, when sediment supply to the fan ceased. The main body of the lobes is characterised by a general low to average backscatter facies (from white to light grey) scattered with higher backscatter fingers (dark grey). Two types of lobes are distinguished based on the planform shape of their backscatter facies: (1) elongated and narrow lobes with branching patterns like the veins of a leaf and (2) radial lobes with crescent-shaped features opened in the downstream direction, that do not have a clear organisation within the lobe. The eight channel-levee systems and associated lobes were built in a time span of not, vert, similar 10 ka and with an average time between successive avulsions of 1250 years. The life span of lobes in the Amazon system is around 600 years which is very short compared to the life span of the fan as a whole. The observed wide range in channel lengths and lobe sizes suggests that each system had a variable time duration and/or sediment supply rate. The variability of the channel lengths could be controlled by internal and/or external forcings linked to processes occurring at different stages in the system in the Amazon drainage basin, on the Amazon shelf, at the canyon head or at the channels bifurcation point. Numerous channel-levee systems and lobes are concentrated in a relatively narrow area. Locally, the pre-existing morphology of the distal depositional area influences the shape and the location of lobes, providing an argument for strong topographical control where local gradient and degree of topographic confinement are key control parameters.
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Submitted on : Wednesday, August 27, 2008 - 10:30:28 AM
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I. Jegou, B. Savoye, C. Pirmez, Laurence Droz. Channel-mouth lobe complex of the recent Amazon Fan: The missing piece. Marine Geology, Elsevier, 2008, 252 (1-2), pp.62-77. ⟨10.1016/j.margeo.2008.03.004⟩. ⟨insu-00313750⟩



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