SEQUENCE STRATIGRAPHY AND FACIES MODEL OF AN INCISED VALLEY FILL - THE GIRONDE ESTUARY, FRANCE

The Gironde estuary was formed by the Holocene drowning of a fluvial v alley incised during the Wurm global sea-level fall. A depositional se quence accumulated in the valley during the eustatic lowstand, the Hol ocene rise, and the post-Holocene highstand. The sequence comprises a diverse assemblage of lithofacies that can be grouped into lowstand, t ransgressive, and highstand systems tracts. The lowstand systems tract comprises a continuous unit of relatively thin fluvial gravel and coa rse sand in the thalweg of the incised valley. The transgressive syste ms tract comprises the bulk of the incised valley fill and forms a lan dward-thinning wedge of tidal-estuarine sands and muds. In the estuary mouth these are overlain by a thick unit of coarse-grained, estuary-m outh tidal-inlet and tidal-delta sands. The highstand systems tract, i nitiated at about 4000 BP, forms a seaward-prograding, tide-dominated estuarine bayhead delta that has been gradually filling the estuary si nce the post-Holocene stillstand. Coeval with this filling of the estu ary, the adjacent oceanic shoreline has been started of sediment and i s being eroded by waves, indicating that transgressive and highstand s ystems tracts locally can be synchronous. Several important stratigrap hic surfaces punctuate the valley fill: the sequence boundary, the tra nsgressive surface, the tidal ravinement surface, the wave ravinement surface, and the maximum flooding surface. The stratigraphic expressio n of the sequence boundary depends on its position within the valley. In the thalweg it separates lowstand fluvial deposits from underlying tertiary carbonates; on the valley walls it is directly overlain by tr ansgressive estuarine sediments. On the interfluves the unconformity c ontinues to be enhanced by modern subaerial erosion. As the interfluve s are progressively transgressed by the eroding shoreline, the sequenc e boundary is expressed as a wave ravinement surface with transgressiv e marine sediments unconformably overlying Pleistocene or Tertiary sub strates. The stratigraphic expression of the transgressive surface in the valley thalweg is characterized by onlap of transgressive tidal-es tuarine sediments onto lowstand fluvial deposits. On the valley walls the transgressive surface merges with the sequence boundary. Tidal sco ur at the estuary mouth forms a deeply erosional tidal ravinement surf ace overlain by thick estuary-mouth wave- and tide-reworked sands. Sub sequently these tidal-inlet sands are eroded by waves associated with the passage of the transgressing shoreline to produce a wave ravinemen t surface. In the distal part of the estuary the maximum flooding surf ace is expressed as a downlap surface where the regressive highstand e stuarine muds prograde over transgressive tidal-estuarine muddy sands or estuary-mouth sands. In the upstream or proximal end of the estuary the maximum flooding surface separates identical facies, i.e., transg ressive and regressive tidal-estuarine point bars. and would be very d ifficult to identify.