Geology Reference
In-Depth Information
Fig. 6.3
Schematic plan view of the modern tide-dominated
estuaries described in this chapter (drawn after Google Earth and
Landsat images). All sedimentary bodies have not been repre-
sented. The Gironde estuary and the Yangtze delta are drawn to
show the main morphological differences respectively with
wave-dominated estuaries and tide-dominated deltas. Be careful
with scales (
all bar
scales represent 10 km)
landward thinning wedge of aggrading tidal estuarine
sands and muds. At the seaward end of the estuary, the
tidal-inlet complex sands sharply overlay the aggrad-
ing estuarine facies through the tidal ravinement sur-
face. Seaward, in response to high-energy wave
reworking, the shoreline retreat creates a wave ravine-
ment surface, eroding the estuarine mouth sand.
Since 4,000 years BP, the tide-dominated bay-head
delta has begun to prograde as sea level ceased to rise.
This results in the development of huge tidal bars
fed by fl uvial sands and that migrate seaward.
Simultaneously, at the seaward end, the wave-induced
coastal retreat continues, meaning that at present, the
highstand systems tract develops landward, whereas
the transgressive systems tract is still in construction
seaward (Fig.
6.4
).
The Holocene sedimentary infi ll of the Charente
estuary-Marennes-Oléron Bay, classifi ed like the
Gironde as a mixed-energy wave- and tide-dominated
system, and located about 50 km north the Gironde,
has been investigated recently using very high-
resolution seismic and core data (Chaumillon and Weber
2006
; Weber et al.
2004
; Allard et al.
2010
, cf. review
in Chaumillon et al.
2008
). This example of a mixed
energy system differs quite signifi cantly from the
Gironde model because it is characterized by very low
sediment supply of fl uvial origin. As a consequence,
no bay-head delta is developed. In spite of this context
