Abstract

Three crossings of the Mid-Atlantic Ridge with the seismic profiler between Buenos Aires and Capetown; Buenos Aires and Dakar; and Dakar and Halifax have shown several important features of the sediment distribution. The total accumulation is remarkably small, averaging 100 – 200 m. On the northern and middle crossings, the sediments are mainly in pockets, and intervening areas are almost or entirely bare. A large percentage of the pockets have almost level surfaces. These facts suggest that the sediments deposited on the ridge flow easily after reaching bottom here. Where impounded, the ridge sediments apparently develop cohesiveness and will not flow easily if subsequently tilted. The sediments are unstratified and remarkably transparent acoustically. Certain areas, particularly on the lower flanks of the ridge, contain distorted sedimentary bodies that apparently indicate postdepositional tectonic activity. The basement surface on which the sediments rest is uniformly rough from the crest of the ridge out to the lower flanks and continues so underneath the basin sediments. It is the upper surface of the intermediate layer (seismic velocity about 5 km/sec) that constitutes the upper 1– 3 km of the oceanic crust.

On the southern crossing the sediment layer tends toward uniform thickness across most of the section. This is evidence that the ridge sediments here are mainly pelagic and that the amount of sediment seen on the record represents the total deposited.

Sediment cores and ocean-bottom photographs provide additional information about sediment composition and distribution where the sediment cover is too thin to be measured by the profiler. The photographs also provide information about the presence of currents capable of altering sediment distribution.

The results suggest that the total accumulation in the oceans is small compared to that which would be inferred from any of the currently accepted estimates of Cenozoic rates of deposition, but that the relative amounts of carbonate and red clay conform to the accepted ratio of their respective rates.

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