Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 157 619 38. NEOGENE TURBIDITE SEQUENCE ON THE MADEIRA ABYSSAL PLAIN: BASIN FILLING AND DIAGENESIS IN THE DEEP OCEAN 1 P.P.E. Weaver, 2 I. Jarvis, 3 S.M. Lebreiro, 2 B. Alibés, 4 J. Baraza, 4 R. Howe, 5 and R.G. Rothwell 2 ABSTRACT The Madeira Abyssal Plain (MAP) has been formed by the accumulation of turbidite sediments from three principal sources: the northwest African continental margin, the Canary Islands and the Hyères/Cruiser/Great Meteor seamount chain. Turbidites derived from each of these sources have distinct chemical signatures enabling the development of a high-resolution chemostratigraphy, in addition to the conventional bio- and lithostratigraphies. Individual beds can be up to a few meters thick, and many are traceable across the whole plain. The first turbidites rapidly infilled the fracture zone valleys through the middle Miocene. By 16 Ma, the fracture zones were nearly filled, and flows began to spread across wider areas to form the plain. Between 16 and 13 Ma, individual flows became much larger, so that after this time, correlation of individual beds is possible between Sites 950, 951, and 952, which are spaced 50–60 km apart. Accumulation rates of the three principal groups of turbid- ites increased between 7 and 6.5 Ma, and remain high to the present day. One subgroup, termed “gray nonvolcanic turbidites,” show a pulsed input to the plain, which may be related to the early growth phases of individual Canary Islands. The pelagic interbeds are composed of clay through the Eocene to middle Miocene, but at 8 Ma, they show a small increase in carbonate content. This increases again at ~5 Ma, and at 3.5 Ma, the carbonate began to oscillate between clays and oozes, reflecting the Pliocene–Quaternary climatic fluctuations. Diagenesis of MAP Miocene–Holocene sediments is dominated by oxic processes that occurred when organic-rich turbid- ites were first emplaced on the plain. Diffusion of seawater oxygen into the upper few decimeters of turbidite tops and over time periods of a few thousand years caused the near-complete destruction of labile organic matter in the sediment, and promoted the early diagenetic migration of trace metals around a sharply defined redox interface. Pore-water data demonstrate that subse- quent burial to depths of >350 meters below seafloor, and for time periods in excess of 10 m.y., has led to the progressive devel- opment of post-oxic, sulfate-reducing, and methanogenic environments, but these have had remarkably little effect on bulk sediment composition, trace-metal distributions, or organic-matter geochemistry. Oxygen availability appears to be an overrid- ing control on diagenetic processes and rates during early burial in these pelagic environments. INTRODUCTION Drilling of the complete turbidite sequence in the Madeira Abys- sal Plain (MAP) represents the culmination of 24 years of research in this area. The MAP was selected in the late 1970s as a study area for the feasibility of radioactive waste disposal in the oceans, and subse- quently, its sediment cover was intensively investigated to a depth of 34 m (750 k.y.). A full background to our knowledge of the MAP sed- iments can be found in papers by Weaver and Kuijpers (1983), Weav- er and Rothwell (1987), Weaver et al. (1986), Weaver at al. (1989), and Rothwell et al. (1992). The geochemical characterization of the sediments is given in Colley and Thomson (1985), De Lange et al. (1987, 1989), Thomson et al. (1987), Jarvis and Higgs (1987), and Pearce and Jarvis (1992a, 1992b, 1995). In brief, the salient points from this research into the upper Quaternary sedimentary sequence are as follows: 1. Large turbidites are deposited on the abyssal plain at infre- quent intervals. Most flows cover the whole plain and can have volumes in excess of 200 km 3 . Only rarely do two or more flows lie on top of one another without a pelagic layer be- tween. 2. Turbidites are dominantly fine-grained muds, with sandy bases located near their entry point onto the abyssal plain. 3. Through the last 750 k.y., most flows were deposited at times of falling and rising sea levels. Only a small number were de- posited during low- and highstands of sea level. 4. The turbidity currents cause insignificant erosion as they cross the abyssal plain, so the pelagic sequences between turbidites are intact. 5. Three primary sources have been identified: the northwest Af- rican margin, volcanic islands of the Canaries, and the Hyères/ Cruiser/Great Meteor seamount chain to the west of the abys- sal plain. Although defined by a range of geochemical and other criteria, the three main turbidite groups may be distinguished using only Ca, Ti, and Al data (De Lange et al., 1987, 1989; Pearce, 1991; Pearce and Jarvis, 1992a, 1995; Jarvis et al., Chap. 31, this volume). Organic- rich turbidite muds (i.e., excluding basal sands and silts) have the lowest Ca contents (presented here as CaCO 3 , because the bulk of the Ca present occurs in the carbonate fraction), typically 45%–55% CaCO 3 . These muds also have low but constant Ti:Al ratios of ~0.05. Volcanic turbidite muds have intermediate carbonate contents, gen- erally 55%–65%, and high but variable Ti:Al ratios of 0.08 to 0.2. Calcareous turbidite muds are defined as having >75% CaCO 3 , but also exhibit marginally higher Ti:Al ratios than the organic-rich group, with values ~0.06. The MAP investigations ended in the late 1980s, and since then, the data obtained have been developed into a model for basin devel- opment and filling by further research on the continental slope and rise of the northwest African margin and Canary Islands (Weaver et 1 Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998. Proc. ODP Sci. Results, 157: College Station, TX (Ocean Drilling Program). 2 Southampton Oceanography Centre, Empress Dock, Southampton, S014 3ZH, United Kingdom. ppew@soc.soton.ac.uk 3 School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey KT1 2EE, United Kingdom. 4 UA Geociencias Marinas CSIC-UB; GRC Geociències Marines, Dep. Geologia Dinàmica, Geofísica i P., Universitat de Barcelona, Campus de Pedralbes, 08071 Barce- lona, Spain. 5 Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907, Australia.