Turbidity-current and debris-flow pathways to the Cape Verde Basin: status of long-range side-scan sonar (GLORIA) surveys

Abstract

Summary A regional GLORIA survey in the southern Cape Verde Basin identified highly variable patterns of backscattering intensity that were interpreted as resulting from seabed roughness and small-scale slope changes. It was inferred that their origin was linked to turbidity-current pathways across the continental rise. Farther north, a survey of the Saharan Continental Rise SE of Madeira identified even more spectacular changes in backscattering. Follow-up 3.5 kHz echo-sounder surveys together with core-sampling and bottom-camera studies allowed an assignment of acoustic facies to sediment type. In this area turbidity-current channels and deposits have been overrun by long-range debris flows resulting from the late-Quaternary Saharan sediment slide. Preliminary results of a further GLORIA survey that links the previously studied areas with a single swath along the continental rise are reported in this paper. The resulting sonographs show details of submarine-slide-debris-flow complexes on this margin that had previously been mapped from 3.5 kHz records. They extend across the lower continental rise to the plain areas. Others do not extend as far as the lower rise. Debris flows can no longer be thought of as unequivocal indicators of proximality in the geological record.

Regional setting and geophysical characterization of the Great Meteor East area in the Madeira Abyssal Plain

“…The resultant tongues of debris flows reach downslope to the margin of the abyssal plain, stopping in the GME area at 24~ . Similar sediment slides and major turbiditycurrent pathways have been mapped in the Canary Basin to the N (Embley 1976;Jacobi, personal communication, 1980) and S (Jacobi & Hayes 1982)of GME, and many of these features have now been studied in detail using the IOS long-range side-scan sonar GLORIA (Kidd & Searle 1984;Kidd et al 1985Kidd et al , 1987.…”

“…basin is bounded by the lower flank of the MidAtlantic Ridge in the W, the Azores-Gibraltar Rise (part of the European-African plate boundary) in the N, the Madeira-Tore Rise (a lowerCretaceous aseismic ridge (Tucholke & Ludwig 1982)) in the NE, the lowermost parts of the Canaries insular rise and the Saharan (NW African) continental rise in the E and the Cape Verde Rise in the S. Immediately to the E of GME, at 24~ the boundary is marked by a sharp change of slope from the very gentle gradient of the rise (about 1:300 to 1 : 1000) to the virtually flat (less than 1:3000) abyssal plain. Elsewhere the boundary is gradational, especially at the northern and southern edges; in the southernmost part of the Canary Basin the lower continental rise extends all the way to the MidAtlantic Ridge flank (Kidd & Searle 1984). At its western margin the abyssal plain extends westwards between the abyssal hills of the MidAtlantic Ridge flank via fracture-zone valleys which run at right angles to the flank's general SSW-NNE trend.…”

Glacially-influenced sedimentation on the Hebridean slope, northwestern United Kingdom continental margin

“…Where lobes coalesce, the resulting overlapping or amalgamated sequence may total a thickness of 50-60 m (Prior & Coleman 1984). On the continental margins off NW Africa (Embley 1976;Jacobi 1976;Simm & Kidd 1984;Kidd et al 1987), eastern N America (Embley 1980), Israel (Almagor & Wiseman 1982), S Korea (Chough 1984) and California (Thornton 1984) debris flows extend down the slope into deep water rise and abyssal plain settings, and range from 1 -6 0 m thick and from several kilometres to several hundred kilometres in length.…”

Meiobenthos of the Deep Northeast Atlantic

“…Sediment slides and debris flows have been most fully described off the northwest African coast where some cover a great area (Embley, 1976;Jacobi, 1976;Kidd et al, 1986;Masson et al, 1994)..Turbidity currents originating from slope failure may travel considerable distances across the ocean floor, depositing progressively finer sediments as they do so.. Distal turbidites are typically fine grained and carbonate rich and have a higher total organic carbon content (1-3%) than sediments of pelagic origin (Wilson and Wallace, 1990;Stein, 1991). Such deposits are known to blanket the Madeira Abyssal Plain where the Quaternary succession consists of thick turbidite units separated by thin pelagic layers (Weaver and Kuijpers, 1983;Weaver et al, 1986).…”