Lower Cretaceous deposits trapped near the equatorial Mid-Atlantic Ridge

ACCORDING to palaeogeographic reconstructions, the equatorial Atlantic Ocean basin started to open in the Cretaceous period, not earlier than 120 Myr ago1–3. From this and a conventional description of the process of sea-floor spreading, one would expect the oldest oceanic crust in this region to be 120 Myr old, and to be found at the edges of the ocean basin. Contrary to these expectations, we report here the recovery of 140-Myr-old 'Maiolica'-type pelagic limestones from the centre of the basin, near the intersection of the Mid-Atlantic Ridge and the Romanche transform fault. The limestones occur within a tecto-nically deformed and uplifted sedimentary sequence, > 4km thick and > 200 km long, which includes continent-derived quart-zitic siltstones of Palaeocene and Eocene age. This sequence probably accumulated in a proto-Romanche transform valley that was connected to a palaeo-central-Atlantic basin, indicating that continental separation had already started here 140 Myr ago. The entrapment of these old deposits within younger ocean crust can be explained by repeated ridge jumping and transform migration during the evolution of the equatorial Atlantic.