Seaward pinching out and internal stratigraphy of the Gironde incised valley on the shelf (Bay of Biscay)

An incised paleovalley offshore the estuary mouth of the Gironde River (Sou thwest France) was evidenced recently (Comptes Rendus de 1'Academie des Sci ences de Paris 326 (1998) 701). Nevertheless its seaward extension to the s helf break was still unknown. New high-resolution geophysical data were acq uired on the continental shelf in order to follow the incision until the sh elf break. After processing the seismic lines, the shape of the valley was identified and the valley-fill in terms of sequence stratigraphy was interp reted. The valley-fill contains a single sequence corresponding to a fifth-order c ycle (20 kyr). The three Systems Tracks described onshore by Alien and Posa mentier (Journal of Sedimentary Petrology 63 (1993) 378; SEPM (Society for Sedimentary Geology) Special Publication, Tulsa (1994) 225) are present off shore. The Lowstand is poorly developed and is only present in the proximal part of the incised-valley. The Transgressive Systems Tract (TST) constitu tes the bulk of the valley-fill, it is composed of landward-prograding estu arine clinoforms in the distal part of the valley (early TST, phase 1) and of landward-migrating tidal inlet-fill deposits in the proximal part of the valley (late TST, phase 2). The thin early Highstand Systems Tracts (HST) strata downlap seaward onto the maximum flooding surface and caps the TST. As expected in this wave- and tide-dominated environment, the tidal -and wa ve-ravinement surfaces, which bound the TST, are particularly well develope d. The valley incision tapers 50 km offshore of the estuary mouth: the depth o f the basal sequence boundary gradually decreases seaward and is locally tr uncated by the wave-ravinement surface around -70 m below the mean sea-leve l. These observations indicate that during the Lowstand times, rivers do no t always generate continuous cross-shelf incised-valleys. They may adjust t o a rapid drop in base level by modifying their channel morphology (i.e. de ep large channels located upstream change downstream to shallow anastomosed channels with a weaker erosional ability). Therefore, the Lowstand fluvial channels will be poorly developed, and will eventually died out in the pro ximal part of the shelf because they are unable to keep pace with a phase o f rapid sea-level drop. Similar observations have already been reported for the English Channel paleoriver (Geological Society Special Publication, Lo ndon (1996) 203; These de Geologie Marine, Universite de Bordeaux 1 (1997) 265). (C) 2001 Elsevier Science B.V. All rights reserved.