systems tract; Allen & Posamentier, 1993). As the rate of S.LR. decreased
around 6 000-4 000 BP sedimentation became more pronounced and
began to noticeably decrease the available accommodation space. Landward derived fluvial sediments began to prograde over the tidal muds and
sands (highstand systems tract; Allen & Posamentier, 1993).
The aim of this work is to present a synthesis of current knowledge on
the chronology of the Holocene infill of the Gironde estuaty (highstand systems tract), based on five local studies representative of (1) the
internal and median parts of the estuary, (2) the estuary mouth area (3)
the shelf mud patches.
Data and methods
Tiiis work is based on lithological (textures and structures), mineralogical,
micropalaeontological and palynological investigations of over 20 percussion cores (estuarine marshes) and 4 Kullenberg cores (shelf mud patches)
collected since 1992. The main lithological and/or palaeœnvironmental
changes were dated for each site using
14 C chronology.
Results and discussion
The internal and median parts of the estuary
Just after the incised valley was inundated (6 000-5 000 BP), the main accumulation area of landward derived fluv ial sediments was located in the
middle part of the estuary (fig. 2), where the present-day funnel-shaped
channel is developed.
In this area, the largest expanse of coastal wetlands occurs on the eastem
bank (Saint-Ciers marsh; Mellalieu et al., in press; fig. 1). A geoarchaeological
approach, incorporating borehole survey, sedimentological and diatom
analysis, radiocarbon dating, archaeological, documentary and cartographic evidence, shows that the bulk of the Holocene sequence is characterized by clay-silt sedimentation under marine-brackish estuarine conditions in intertidal mudflat, and potentially saltmarsh, environments.
These deposits are topped by a peat unit, developing over the whole area
as soon as 5 600 BP. In more recent periods, three negative sea-level tendencies appear to have occurred, but their exact time and duration are
difficult to determine.
On the westem bank, coastal wetlands are much more restricted, but
available data in a small elongated marsh of the central Médoc peninsula
(Reysson marsh; Massé et al, in press, fig. 1) show a very similar pattern.
The bulk of the depression is filled by a grey-green clay deposited during a short period of time (hundreds of years) at the beginning of the
Holocene transgression. From 6 000 BP to the present, the Revsson
marsh has been a peat-bog isolated from the estuaty, except for a short
period of flooding (single storm event or a few decades) in the central
part of the marsh some time around 4 600 BP.
238
around 6 000-4 000 BP sedimentation became more pronounced and
began to noticeably decrease the available accommodation space. Landward derived fluvial sediments began to prograde over the tidal muds and
sands (highstand systems tract; Allen & Posamentier, 1993).
The aim of this work is to present a synthesis of current knowledge on
the chronology of the Holocene infill of the Gironde estuaty (highstand systems tract), based on five local studies representative of (1) the
internal and median parts of the estuary, (2) the estuary mouth area (3)
the shelf mud patches.
Data and methods
Tiiis work is based on lithological (textures and structures), mineralogical,
micropalaeontological and palynological investigations of over 20 percussion cores (estuarine marshes) and 4 Kullenberg cores (shelf mud patches)
collected since 1992. The main lithological and/or palaeœnvironmental
changes were dated for each site using
14 C chronology.
Results and discussion
The internal and median parts of the estuary
Just after the incised valley was inundated (6 000-5 000 BP), the main accumulation area of landward derived fluv ial sediments was located in the
middle part of the estuary (fig. 2), where the present-day funnel-shaped
channel is developed.
In this area, the largest expanse of coastal wetlands occurs on the eastem
bank (Saint-Ciers marsh; Mellalieu et al., in press; fig. 1). A geoarchaeological
approach, incorporating borehole survey, sedimentological and diatom
analysis, radiocarbon dating, archaeological, documentary and cartographic evidence, shows that the bulk of the Holocene sequence is characterized by clay-silt sedimentation under marine-brackish estuarine conditions in intertidal mudflat, and potentially saltmarsh, environments.
These deposits are topped by a peat unit, developing over the whole area
as soon as 5 600 BP. In more recent periods, three negative sea-level tendencies appear to have occurred, but their exact time and duration are
difficult to determine.
On the westem bank, coastal wetlands are much more restricted, but
available data in a small elongated marsh of the central Médoc peninsula
(Reysson marsh; Massé et al, in press, fig. 1) show a very similar pattern.
The bulk of the depression is filled by a grey-green clay deposited during a short period of time (hundreds of years) at the beginning of the
Holocene transgression. From 6 000 BP to the present, the Revsson
marsh has been a peat-bog isolated from the estuaty, except for a short
period of flooding (single storm event or a few decades) in the central
part of the marsh some time around 4 600 BP.
238