Determination of sediment volumes, accumulation rates and turbidite emplacement frequencies on the Madeira Abyssal Plain (NE Atlantic): a correlationbetween seismic and borehole data

The sedimentary infill history of the Madeira Abyssal Plain (MAP) is establ ished from correlation of ODP Leg 157 drillsites (Sites 950-952) with an al most regular grid of similar to 7000 km of intermediate-resolution seismic reflection profiles covering the central part of the abyssal plain. The mos t conspicuous seismic reflectors bounding the seismostratigraphic units hav e been identified and mapped. Correlation between seismic and borehole data using synthetic seismograms allows the lithological attribution and dating of the reflectors and seismostratigraphic units. Lateral mapping and corre lation of seismic units also allows both the volumes and rates of accumulat ion of sediments within each seismostratigraphic unit and equivalent time p eriods of deposition to be determined. These calculations have been correct ed for the effect of compaction, calculated at around 40% at the base of th e drillholes. Three main turbidite types have been identified at the drills ites and their emplacement frequency has been calculated for each site and time period. Our results show that Cretaceous oceanic crust was draped with red pelagic clays, and the fracture-zone valleys were completely infilled and levelled in a geologically rather short time, probably during the lates t Oligocene and Early Miocene, by organic-rich turbidites derived from the NW African continental margin. At 16 Ma, the topography was levelled enough to allow large turbidity current flows to cover the entire plain. During t he Middle and Late Miocene (16-5.9 Ma), organic-rich turbidites were emplac ed on the abyssal plain at a low rate of accumulation (similar to 12 m/my). In the uppermost Miocene-Early Pliocene (5.9-3.6 Ma), turbidite emplacemen t increased markedly in both frequency and accumulation rate (e.g., similar to 26 m/my for organic-rich turbidites). During this time, period emplacem ent of volcanic-rich turbidites also increased in volume and frequency, a t rend that continued into the Pliocene. Increased volcanic-rich turbidite em placement correlates well with increased volcanic activity on the Canary Is lands, and increased organic-rich turbidite emplacement may correlate with periods of erosion on the African continental margin. These erosional perio ds may be related to global cooling and falling sea level, intensification of bottom-water currents, and enhanced upwelling on the margin. (C) 1999 El sevier Science B.V. All rights reserved.