Abstract

The relation between bottom shear stress and the distribution of bottom sediments on the Amazon continental shelf has been studied using a three-dimensional, primitive-equation computer model that incorporates the turbulence-closure scheme of Melior and Yamada (1982) for calculating eddy diffusivity and a simple algorithm for computing nonlinear wave-current influences on bottom shear stress. Model results compare reasonably well with salinity data sets for the Amazon plume. Model results on distribution of bottom currents and bottom shear stresses help explain some of the observed sedimentological features of the Amazon continental shelf. High concentrations of suspended sediment in the Amazon River are transported outward over the continental shelf and northward by the North Brazil Coastal Current As this sediment settles out of the water column, it forms the prograding, subaqueous delta described by Nittrouer et al. (1986). Accumulation rates are greatest shoreward of the 40-m isobath due to a zone of convergent, cross-shelf residual tidal velocities. Little sediment is deposited in the shallow parts of the shelf, where bottom shear stress exceeds 10 dynes/cm 2 over a diurnal tidal cycle. Zones of laminated sand and mud on the Amazon continental shelf coincide with areas of high interseasonal differences in bottom shear stress. Our results suggest that our model may be useful in interpreting sedimentation in ancient sedimentary basins as well.

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