Transverse bedforms on the Amazon shelf

Abstract

Echo-sounder profiles and side-scan sonograms reveal the presence of asymmetric bedforms on the middle Amazon shelf in water depths of 30–35 m and symmetric bedforms on the outer Amazon shelf at depths of 75–85 m. The asymmetric bedforms have crests parallel to isobaths, wavelengths of 1–40 m, and heights consistently <2 m. The presence of bedforms with a high degree of variability in shape, spacing, and sharpness of crests over distances of only a few kilometers suggests that sediment texture and flow processes are spatially variable. Offshore-facing lee slopes and grain size in the fine sand range (0.08−0.12 mm) suggest bed-material transport of sediment by ebbing tidal currents. Middle-shelf sands and associated bedforms apparently are a reflection of extant shelf dynamical processes. Symmetric bedforms on the outer shelf, characterized by crests perpendicular to isobaths, wavelengths of 100 and 1000 m, and wave heights of 1–5 m, are easily identifiable from distinctive sonogram tonal contrasts that result from changing bottom slope and variable textural character of the sediment mantle. Textural variability may be due to the superimposition of muds on a coarse carbonate substrate. The symmetry, characteristic of both sand waves and sand ridges, is indicative of formation by oscillatory tidal currents in which there is no preferred directionality of motion. Large, coherent outer-shelf features have been interpreted tentatively as ancient reef deposits. Composition of the outer-shelf deposits and character of bedforms present indicate that both are relict, products of a time when sea level was 25–45 m lower than it is today.