TEXTURAL AND DISPERSAL PATTERNS OF THICK MUD TURBIDITES FROM THE MADEIRA ABYSSAL-PLAIN

The Madeira Abyssal Plain is the site of a thick sequence of thick Pleistocene turbidites that are almost entirely of material less than 63-mu-m in grain-size. The major turbidites are 1 to 5 m in maximum thickness. Concentrating on three turbidites cored at 24 locations, detailed patterns of thickness and vertical and lateral grain-size changes reveal flow paths on the plain. In general, the bases of the units are of graded silt showing thick and thin planar lamination and occasionally ripple cross-lamination. A new idealised sequence of changes in size and structures with distance along the flow path is developed. Closer to source the beds comprise a graded structured silty base overlain by structureless mud which may also be graded. Down current the silty base becomes thinner with less apparent structure and the upper homogeneous part becomes thicker and more uniform in an overall thickening trend. At the distal end, uniform beds up to 5 m in thickness are found in one turbidite. The modal size structure shows some silt modes which are hydrodynamically sorted and change size vertically and spatially, while one particular mode is constant in size due to a particular species of coccolith. The turbidites originated from slumps on the northwest African continental margin and from the vicinity of Madeira and the Canaries, some of which exceeded 100 km3 in volume.