If an impact event caused the mass extinctions and geochemical anomalies at the Cretaceous/Tertiary boundary, it probably occurred in an oceanic area. However, no convincing impact site has yet been discovered. Whereas Late Cretaceous magnetic lineations in other oceans show no obvious signs of disturbance at the Tertiary boundary, the end-Cretaceous African plate boundary in the Indian Ocean provides evidence of major tectonic reorganization at or shortly after magnetostratigraphic chron C29r. Immediately south of the microcontinental Seychelles Bank, the Amirante Basin has a roughly circular shape of about 300 km diameter, is partially ringed by enigmatic “arc” and “trench” structures, and is located within oceanic crust of Late Cretaceous age. It is therefore a possible impact site. Extensive chaotic slump structures apparently exist at the appropriate level on the East African continental margin, and they may indicate its proximity to the mega-earthquake focus and/or giant tsunamis in the Somali Basin. By triggering readjustments along the Indian-African and Antarctic-African plate boundaries and thus altering the regional balance of driving forces, the impact may have affected plate motions.