First observations on the morphology and recent sedimentary processes of the Celtic Deep Sea Fan

During the SEDIFAN 1 cruise we surveyed the bathymetry and the acoustic pro perties of the surface sediment of the Celtic Deep Sea Fan. We also collect ed Kullenberg cores in order to study recent sedimentary processes. From th e bathymetry survey it is relatively easy to recognize the main areas of mo dern fan. The upper fan included a large sedimentary ridge which constitute s the right levee of the prominent meandering Whittard valley. After its co nfluence with the Shamrock valley the course of the Whittard valley is abru ptly deflected to the south. At a short distance to the south the valley di vides into two upper-fan channels, the Celtic channel to the west being the deeper one. This point constitutes the centre of a radiating pattern which is developed on a 150 degrees quadrant and a radius of about 100 km. The a coustic imagery displays contrasted features, related to change in litholog y within the first metre beneath the sea bottom and to the sea floor roughn ess. The Austell ridge exhibits a contrasted pattern of elongated areas wit h high and low acoustic backscattering levels. This pattern is related to t he development of abyssal dunes, the amplitude of which is of metric order. Particularly remarkable is a lobe-shaped low back-scattering area in the w estern part of the middle fan, also noteworthy are a lineated facies to the west and a braided facies to the east of the fan. The laminated silty-clay ey sequences deposited on the Whittard ridge and on the Trevelyan levee wer e deposited during the deglaciation. We interpret these as turbidity curren ts overflow deposits from the Whittard valley. At the end of isotopic stage 3 and during stage 2, the English Channel was a large plain flooded by the Channel River. During this period a broad delta developed at 100 m below t he present-day depth and a wide spectrum of material was bound to be suppli ed to the deep sea and contributed particularly to the deposition of the Wh ittard ridge silty-clayey sequences. The stage 2 deposits are characterized by rhythmic levels enriched in monosulfides. These types of deposits are c ommon in areas affected by fluvial discharges. Excluding the sedimentary ri dge and the channel levees the surface deposits sampled with the Kullenberg corer are sandy. These sands are deposited in various contexts on the inte rfluve between the western and eastern channels and at channel mouths. They were emplaced during high sea level stands as a result of high energy grav ity processes. The precise sources of these sands have not yet been identif ied, however benthic foraminifers from included ooze pebbles have living de pths of between 500 and 1 000 m. The gravity processes which eroded this ma rry ooze may have been triggered on the upper slope. The Celtic shelf is pr esently a high energy platform where the conjunction of storms and spring t ides call lead to enhanced sediment transport from near-shore to the deep s ea. The relict or palimpsest deposits of the glacial delta also constitute a large reservoir of sandy material which can also be subject to reworking. (C) 2000 Ifremer/CNRS/ IRD/Editions scientifiques et medicales Elsevier SA S.