Physiography and recent sediment distribution of the Celtic Deep-Sea Fan, Bay of Biscay

The Celtic Deep-Sea Fan located in the northwestern part of the Bay of Bisc ay is a middle sized fan with a surface area of more than 30,000 km(2). The whole system is a mature mud/sand-rich submarine fan on a passive margin. Multi-beam echo sounder data, 3.5 kHz seismic and 12 Kullenberg cores were examined to define the fan morphology, the lithological characteristics, th e sedimentary processes and the relationship between the evolution of the f an deposits and the environmental conditions on the Celtic continental shel f. The upper fan is characterised by the presence of two distinct tributary sy stems: (1) the Whittard system with a large, persistent, slightly sinuous c hannel, which is linked to the southern end of the Irish Sea River system; and (2) the Shamrock system, with a moderate sized channel, which is linked to the western end of the English Channel River system. The middle and low er fan corresponds to divergent braided secondary channels and associate lo bes. Successive lobe elements, without important relief, were generated dur ing periodic avulsions of middle fan channels. The lithological, palaeontological, and geochemical analyses on cores show the evolution of sedimentation since the last glaciation. During the last l owstand and rise of sea-level frequent low-density turbidity currents were predominant and deposited sediments throughout the whole fan system. They w ere initiated at the front of a deltaic environment on the Celtic outer-she lf. During the high sea-level conditions, occasional high-density turbidity currents and/or non-cohesive debris flows occur and were responsible for s and deposition in the middle-lower fan. They are derived from reworked sand s due to the high-energy conditions on the outer shelf. Thus for the Celtic Fan, the variations of the hydrodynamic conditions on the outer Celtic She lf seem to be the primary control on facies shift and fan growth. (C) 2000 Elsevier Science B.V. All rights reserved.