Sediment distribution in the oceans : the Atlantic between 10° and 19°N

Abstract

Between 10° and 19°N the North Atlantic Ocean has been covered by four east-west crossings and one north-south section at 60°W, using a continuous seismic reflection recorder (air gun). The northernmost section extends to the Canary Islands. The region comprises a great variety of phenomena: mid-oceanic ridge, fracture zones, oceanic basins, volcanic islands, continental rises and part of a zone of negative gravity anomalies (the Vening Meinesz zone), running from the Puerto Rico Trench over Barbados into Trinidad. A central zone of the Mid-Atlantic Ridge appears to be void of sediment. In the fracture zones (grabenlike depressions that off-set the axis of the ridge) sedimentary thicknesses of the order of 1 km have been found. Evidence was found for the existence of current-influenced sedimentation other than from turbidity currents, and for the occurrence of erosion at depths of more than 5,000 m (the Vidal Channel). In the oceanic basins sedimentary thicknesses occur of maximum 2,000 m in the Cape Verde/Madeira Abyssal Plain and more than 1,400 m (no basement found) in the Demerara Abyssal Plain. The continuity of sedimentation from the continental rise into the abyssal plains proves that turbidites can be deposited on slopes with an inclination of 12′. Locally deposition of turbidites occurs on slopes with a much higher inclination. The occurrence of horizon A at a distance of 700 km from the axis of the Mid-Atlantic Ridge gives, after a correction for probable tectonic movements, a maximum of 1.2 cm/year possible spreading since the Upper Cretaceous. The uppermost layer of sediments in the Vening Meinesz zone of about 1 km thickness is only little deformed. This is also true for the Barbados Ridge, which implies that the sediments must be younger than the deformed Tertiary series found at Barbados. In discussing the distribution of sediments on the Mid-Atlantic Ridge and the configuration of the ocean basins to both sides, the possibility is considered that both the negative gravity zones and the ridge are secondary and relatively young features with regard to continental drift and ocean spreading. This interpretation would be alternative to the hypothesis that the distribution of sediments should be explained by large variations with time of the rate of spreading (or the rate of sedimentation respectively). The Late Cretaceous and the Miocene orogenies might represent principally different phases of the Alpine orogeny and the Mid-Atlantic Ridge as a topographic feature might be broadening constantly.