Seafloor response to flow in a southern hemisphere sand-ridge field: Argentine inner shelf

The inner continental shelf adjacent to Buenos Aires Province, Argentina, is characterized by a series of sand ridges, averaging 4.7 m in relief and spaced about 2.7 km apart. The ridges trend north-south, forming a 20 to 35° southward-opening angle with the northeast-southwest-trending coast. Although the adjacent coast is of depositional strand-plain origin, it is presently undergoing erosional retreat, and the formation of the ridges appears to be part of the retreat process. Seafloor grain-size distributions and bedform arrays are responses to the same velocity field that has formed the ridges. Along transects normal to the ridges, grain-size variation is 90° out of phase with the topography: the coarsest sediment is not at ridge crests or trough axes, but instead occurs on the landward slope; finest sands are on the seaward slope. Side-scan sonar reveals bedforms aligned obliquely to both ridges and the coast; these are inferred to be sand waves. The grain size and bedform patterns suggest that the formative flows are northward currents with an offshore flow component near the bottom, although flows observed during the study period were dominantly north to south. The grain-size gradients and bedform orientations are equivalent to those characteristic of the Atlantic shelf of North America with a north-south inversion. The comparison suggests that inner shelf transport directions in response to wind events can be predicted from the regional geometry and climate of the shelf.