Seabed features characterization of Barra Norte by multibeam survey: Furrows evidence at the outermost river branch of the Amazon River

Barra Norte is a highly energetic environment that constitutes the main navigational route between Atlantic Ocean and the world´s major fluvial system, the Amazon. The recent geomorphology of the actual navigational channel was investigated by a multibeam system installed at the Brazilian navy´s hydro-oceanographic vessel Garnier Sampaio (H-37). These data were collected between September and October of 2011 resulting in the survey of a corridor of 150m × 62km long at Curua Channel (close to highly dynamic sand banks, referred as area A) and 50m × 57km at an outer region of the Amazon River inner continental shelf, where fluid mud accumulate (area B). Data processing was realized at Caris HIPS and SIPS considering water level obtained from a calibrated hydrodynamic model, based on Environmental Fluids Dynamical Code, to account for spatial tidal variability. Bathymetry, gradient and backscatter maps were created with 1 meter resolution. The seafloor showed constantly decreasing depths from area A to B (SW-NE), resulting in a 1:6500 gradient, where major changes were locally associated to bedforms. Five seabed types were identified according to bedform occurrence: (1) predominantly flat; (2) transition (any two or more types); (3) small to large dunes; (4) furrows; and (5) mega dunes. Types 2, 3 and 4 showed nonuniform distribution, occurring mainly at Curua Channel. Ebboriented mega-dunes occurred at the extreme SW of the transect, the deepest surveyed area, close to the most restricted transverse section, indicating strong NE residual currents. Furrows where observed just at area A, being closer to sandy seafloor than to the fluid mud banks of area B. They are slightly aligned to the main currents direction and their high backscatter intensity indicates the presence of a more consolidated sediment, possibly a mixture of sand and cohesive sediments, what makes them slightly irregular in plan. According to furrows classification they can be describ- d as type 1C, being of variable shape and very steep walls, reaching 30°. Type 1C furrows develop under directionally stable flows where sedimentation and erosion balances each other, what can be supported by its spatial distribution, close to significant sand banks or at bathymetric depressions of Curua Channel. At area B, after a small segment of small dunes and high backscatter at its extreme SW, a slowly increasing weak signal was observed from longitude 49.5W on. It covered about 85% of the surveyed area B and is related to Amazon River fine sediments that tend to deposit as fluid mud at the inner continental shelf. Sediment samples corroborate these findings.