ABSTRACT

The Gironde estuary was formed by the Holocene drowning of a fluvial valley incised during the Wurm global sea-level fall. A depositional sequence accumulated in the valley during the eustatic lowstand, the Holocene rise, and the post-Holocene highstand. The sequence comprises a diverse assemblage of lithofacies that can be grouped into lowstand, transgressive, and highstand systems tracts. The lowstand systems tract comprises a continuous unit of relatively thin fluvial gravel and coarse sand in the thalweg of the incised valley. The transgressive systems tract comprises the bulk of the incised valley fill and forms a landward-thinning wedge of tidal-estuarine sands and muds. In the estuary mouth these are overlain by a thick unit of coarse-grained, estuary-mouth tidal-inlet and tida -delta sands. The highstand systems tract, initiated at about 4000 BP, forms a seaward-prograding, tide-dominated estuarine bayhead delta that has been gradually filling the estuary since the post-Holocene stillstand. Coeval with this filling of the estuary, the adjacent oceanic shoreline has been starved of sediment and is being eroded by waves, indicating that transgressive and highstand systems tracts locally can be synchronous.

Several important stratigraphic surfaces punctuate the valley fill: the sequence boundary, the transgressive surface, the tidal ravinement surface, the wave ravinement surface, and the maximum flooding surface. The stratigraphic expression 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 transgressive estuarine sediments. On the interfluves the unconformity continues to be enhanced by modern subaerial erosion. As the interfluves are progressively transgressed by the eroding shoreline, the sequence boundary is expressed as a wave ravinement surface with transgressive marine sediments unconformably overlying Pleistocene or Tertiary ubstrates.

The stratigraphic expression of the transgressive surface in the valley thalweg is characterized by onlap of transgressive tidal-estuarine sediments onto lowstand fluvial deposits. On the valley walls the transgressive surface merges with the sequence boundary. Tidal scour at the estuary mouth forms a deeply erosional tidal ravinement surface overlain by thick estuary-mouth wave- and tide-reworked sands. Subsequently these tidal-inlet sands are eroded by waves associated with the passage of the transgressing shoreline to produce a wave ravinement surface.

In the distal part of the estuary the maximum flooding surface is expressed as a downlap surface where the regressive highstand estuarine 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., transgressive and regressive tidal-estuarine point bars, and would be very difficult to identify.