Abstract

The numerous valleys of the flanks of the Mid-Atlantic Ridge are partly filled with sediment. The petrology of sediments from two such valleys (ponds) near 22° N. latitude shows that the material has been deposited by turbidity currents. The material has been derived from calcareous pelagic deposits which mantle the surrounding hills. The turbidite sequences show peculiarities which can be explained by assuming that the currents rebounded repeatedly from the pond walls. A theoretical model for the flow of these currents agrees well with density, thickness, and composition parameters that can be derived from the deposits and points to the occurrence of a hydraulic jump at the base of the slope. Adjacent valleys are separated by divides, and each valley contains an independent sedimentation unit. Good correlation exists between the volume of sediment in each valley and the size of the surrounding hill area from which the sediment was derived. Removal of 15 m of sediment from the entire region is required to account for the sediment that now fills the valleys. The oldest sediment outcropping in this region is upper Miocene, and the average sedimentation rate of the pelagic deposits can be estimated as approximately 3 mm/1000 years. In the ponds, the rate is much greater; more than 9 m of late Quaternary sediment have been found. Recently, the valleys have been faulted and partly uplifted. The accompanying earthquake activity may have initiated the turbidity currents responsible for the present rapid deposition on the valley floors.

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