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Seismic reflection profiles have been interpreted in combination with deep-sea drilling data to examine the sedimentary evolution of the Rio Grande Rise. Restricted and unevenly distributed seismic reflection coverage (particularly multichannel) and limited well control confined most of our interest to the northern flank of the main western elevated block of the Rise, near to DSDP Site 516. The basement of much of the Rise above approximately 3000 m present depth has the "dipping reflector" character of some continental margins, produced by interbedded lavas and sediments formed directly above sea level. The overlying Late Cretaceous and early Tertiary sediments are mainly pelagic, but lap onto originally subaerial basement in places. A major middle Eocene tectonic event (involving uplift, tilting, faulting, and probably local volcanism) resulted in subaerial erosion, submarine slumping, and turbidite deposition. Submarine slides associated with the early stages of this tectonism probably caused the chaotic midsection reflector sequence identified with the middle Eocene Unit 4 at Site 516. Middle Eocene tectonism also produced the central graben of the Rise, which contains rotated fault blocks, and the broad guyot between the graben and Site 516. The guyot shows thick sequences dipping away from the graben and truncated, presumably by subaerial erosion. Prograded biogenic debris, swept off the top by bottom currents after resubmergence in the Oligocene, extended the guyot's top farther. Biogenic debris from the same source probably formed the initial load of the density currents that have dissected the steeper northern slopes of the Rise. Interpretation of reflection profiles from all around the Rise suggests erosion between depths of 2000 and 3600 m, even where the creation of density currents upslope is unlikely; a component of contour current erosion by North Atlantic Deep Water may be involved. The Vema Terrace, which in the west separates the Rio Grande Rise from the Vema Channel, shows signs of sedimentation from turbidity currents originating on the Rise. The Terrace and the Vema Channel extend around the north flank of the Rise also; their positions are controlled by the topography of the east-west Rio Grande Fracture Zone. If the eastward flow of Antarctic Bottom Water north of the Rise is inferred, the direct transport of debris from the Rise to the Brazil Basin by turbidity currents becomes a more complex process.

A Seismic Reflection Study of the Rio Grande Rise