The eastern margin of South America has been divided into 14 zones, each with a characteristic sound-velocity distribution relative to depth below the sea floor, as determined from sonobuoy data. Regressions of sound-velocity data determined from one-way travel time for each region can now be used to compute thickness from vertical reflection data.

The velocity characteristics of the Pelotas Basin and Pelotas Rise are similar to those of the Amazon Cone and some other large deltaic deposits, which suggests that the discharge of the Rio de la Plata may have formerly been diverted along the shelf to the north. The velocity characteristics of the Rio Colorado Basin show that it is distinct from and cuts across the Argentine shelf and Argentine Rise. The Falkland Plateau velocity data are identical to the Argentine Rise contourites, suggesting a common provenance. Velocity characteristics in the adjacent Pelotas and Santos Basins are quite different, suggesting that the Torres Arch has been an important barrier for some time.

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