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Circulation, Unconformities and Sedimentation |
Institute of Oceanographic Sciences, Brook Road, Wormley, Godalming, Surrey GU8 5UB
Geological Survey of the Netherlands, Spaarne 17, Postbus 157, 2000 AD Haarlem, The Netherlands
Extensive investigations in the Madeira Abyssal Plain have revealed thick sequences of turbidites. Individual turbidites can be correlated over an area of 2° x 2°, and their emplacement can be shown to coincide with glacial onsets and terminations. The turbidites originate on the NW African margin and contain up to 2% organic carbon. They are virtually ungraded massive silts and clays, some with coarser basal layers.
Seismic records through the abyssal plain show distinct changes in sediment character at a few hundred metres depth. Rates of accumulation are calculated from piston cores to be about 7.5 cm/1000 years, suggesting that the turbidites are a recent phenomenon. If the link with glacial onsets and terminations has held throughout, the first major turbidites may have been emplaced at 2.4 Ma, at the onset of Northern Hemisphere glaciation. Some turbidites may also have been emplaced during the supposed late Miocene glaciations. The Madeira Abyssal Plain is therefore a recent feature, and before its initiation the area would have had a ridge-flank type topography.
