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On the Cape Verde Plateau, Neogene deposits are composed of major pelagic and hemipelagic sediments. These sediments show climatic sequences composed of two lithologic terms that differ in their siliciclastic and carbonate contents. Several turbiditic and contouritic sequences are interbedded in these deposits. Turbidite sequences are fine grained and thin bedded with a very low frequency (about 12 sequences during the Neogene). They are composed of quartz-rich siliciclastic or volcaniclastic sediments. Quartz-rich turbidites originated from the Senegalese margin. Their slightly higher frequency during the early Pliocene indicates that the stronger turbidity currents, and probably the most abundant continental inputs, occur at that period. Volcaniclastic turbidites are only present in the early Miocene (about 17 Ma) and the early Pleistocene (1 Ma). They have flown from adjacent Cape Verde Islands and reflect two episodes of high volcanic activity in this area. Contourite sequences, composed of biogenic sandy silts, represent less than 5% of the sediment pile and seem to have been mainly deposited during the late Pleistocene. These different sequences show clay mineral variations throughout Neogene time. Kaolinite is predominant in the Miocene and lower Pliocene deposits; this mineral decreases thereafter, with an increased trend of illite in the uppermost Pliocene and Pleistocene sediments, suggesting a change in sediment sources on the Saharan continent at about 2.6 Ma. INTRODUCTION Site 659 is located on the uppermost part of the Cape Verde Plateau, which spreads between the Cape Verde Islands and the Senegal margin (Fig. 1). Three companion holes were cored continuously (659A and 659B) or spot cored (659C) in order to obtain a nearly complete stratigraphic sequence. On seismic records, Neogene sedimentation seems very regular; regularly distanced reflectors in a transparent layer overlie a stronger basal seismic layer. The Neogene sediment section recovered here (0-250 mbsf; Fig. 2) comprises two major lithologic units that have good biostratigraphic and magneto­ stratigraphic time control (Ruddiman, Sarnthein, et al., 1988). Lower to upper Miocene sediments consist of pelagic cycles of light gray nannofossil ooze and yellowish brown or bluish nannofossil silty clay (Unit IIB; 24-7 Ma). We observed a sharp color change between the basal bluish ooze (Subunit IIB) and the overlying brownish ooze (Subunit IIA). Due to severe dissolution conditions, Unit II shows strong variations of carbonate contents. Upper Miocene to Holocene sediments (Unit I; 7-0 Ma) consist of pelagic cycles of white and light gray foraminifer-nannofossil ooze with minor amounts of silts and clays and with higher carbonate contents than in Unit II. The gradual change between the two units, with strong modifications in sedimentation rates and CaC0 3 dissolution at 4.6 Ma, underlies a major event of deep-water oceanography. In addition, two events are recorded in Unit II: one is documented by the striking color change, about 11 Ma, the other by biostratigraphic evidences of an early Miocene hiatus spanning from around 18.5 to 23.5 Ma (Ruddiman, Sarnthein, et al., 1988). 1 Ruddiman, W., Sarnthein, M., et al., 1989. Proc. ODP, Sci. Results, 108: College Station, TX (Ocean Drilling Program). Laboratoire de Geologie-Oceanographie, Universite de Bordeaux I, Ave­ nue des Facultes, Talence 33405 Cedex, France. Except for two major lithologic changes, pelagic and hemipelagic sediments appear very homogeneous throughout the two units. However, some levels composed of coarse­ grained foraminifer ooze and a few dark centimetric layers of siliciclastic clayey-sandy silts are interbedded in the Neogene sediments. About 12 of these dark layers have been numbered in Holes 659A or 659B (Fig. 2); they are composed of major terrigenous or volcanic material. Because of its geographic situation, the Cape Verde Pla­ teau could be supplied with sediments originating from the Senegalese continental margin (quartz-rich, siliciclastic mate­ rial) and from the Cape Verde Islands (volcanic material). Because of its morphologic situation, recent sediments will have major pelagic or hemipelagic characters (Diester-Haass, 1975; Moyes et al., 1979) and turbidite deposits should be rare. Nevertheless, these last deposits are common in Paleogene and Miocene sediments (Dean et al., 1978). Also, several works have reported that recent sediments were controlled by bottom currents (Hobart et al., 1975; Sarnthein et al., 1982; Lonsdale, 1982; Jacobi and Hayes, 1982; Meinert, 1985; Sarnthein and Meinert, 1986) related to the North Atlantic or to the Antarctic bottom water. Microphysiographic studies show a lot of evidence such as sediment-wave fields. But these wave sediments are fine grained without typical contouritic features (Sarnthein and Meinert, 1986); and, consequently, contour current processes are not always very well recorded in facies deposits. Never­ theless, we must take into account these processes and their variations insofar as strong changes of geostrophic circulation occur during the Neogene (Kennett, 1982) and three major events have taken place and are recorded in Miocene deposits at Site 659. METHODS The purpose of this paper is (1) to analyze the lithologic features of the three kinds of sediments: pelagic cyclic sediments, siliciclastic dark layers and foraminifer-rich ooze layers; (2) to determine, for each

proceedings of the ocean drilling program

Pelagic, Turbiditic, and Contouritic Sequential Deposits on the Cape Verde Plateau (Leg 108, Site 659, Northwest Africa): Sediment Record during Neogene Time