Neogene turbidite sequence on the Madeira Abyssal Plain: Basin filling and diagenesis in the deep ocean

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 Hyeres/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 turbidites 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 turbidites 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 subsequent burial to depths of >350 meters below seafloor, and for time periods in excess of 10 m.y., has led to the progressive development 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 overriding control on diagenetic processes and rates during early burial in these pelagic environments.