Geology Reference
In-Depth Information
Fig. 6.6
Schematic longitudinal cross sections showing the
variability of the sedimentary fi ll of tide-dominated and
mixed wave- and tide-dominated estuaries along French
coasts (after Chaumillon et al.
2010
). Except in the outer
segment of some rocky coast estuaries (such as the Vilaine), the
highstand systems tract (HST) is much more developed than the
transgressive systems tract (
TST
) in tide-dominated estuaries.
This is due to the combination of shallow bedrock incision,
deep tidal ravinement and the general sediment-starved context
of the French Atlantic and English Channel shelves. For simpli-
fi cation, the lowstand systems tract and transgressive surface
have not been represented.
MSF
,
TRS
,
WRS
maximum fl ooding,
tidal ravinement, wave ravinement surfaces
estuaries, highstand systems tract sand-dominated
tidal units should be poorly to very poorly preserved
as the tidal inlet area is a zone of sediment bypass.
Moreover, transgressive systems tract deposits can
be deeply eroded. This is predicted by Allen and
Posamentier (
1993
). At tide-dominated estuary mouths,
by contrast, highstand systems tract tidal sand bodies
are better preserved. But the major difference arises
from the upstream extension of the tidal ravinement
surface in tide-dominated estuaries. Indeed, reworking
processes by the tidal ravinement surface can occur all
along a tide-dominated estuary, leading in some places
to a complete erosion of underlying units (Fig.
6.6
).
The most recent studies carried out on estuary fi lls
along the French coasts (cf. review in Chaumillon et al.
2010
) point out clearly this main difference: the high-
stand systems tract constitutes the bulk of tide-domi-
nated estuary infi lls, whereas the reverse confi guration
typifi es wave-dominated and mixed-energy estuaries.
However, as it has been stated previously, French inner
shelves and coasts are sediment-starved systems, this
partly explaining such a pronounced difference.
The fi nest-grained depositional unit of tide- dominated
estuaries infi ll corresponds normally to the sinuous tidal
channel area (bedload convergence zone). In the absence
of data indicating tidal point-bar deposits, this unit could
be misinterpreted as a wave-dominated or mixed-energy
estuary central basin (although in terms of facies strictly
the occurrence of well-developed and preserved tidal
rhythmites defi nitely indicates a tide-dominated set-
ting). Recent studies on inner mudfl at basins (Allard
et al.
2010
; Billeaud et al.
2009
) demonstrate that wide
mudfl ats in mixed energy systems are incised by tidal
creeks during maximum fl ooding and during the high-
stand. It is suggested that these secondary tidal ravine-
ment surfaces preserved inside the highstand systems
tract can be developed when mudfl at surfaces are suf-
fi ciently wide to allow powerful drainage processes
during falling tides in macrotidal settings. Such tidal
creeks are probably more common in wave-dominated
and mixed energy estuaries inasmuch as the cross-sec-
tion of central basins and consequently of fringing mud-
fl at surfaces are generally wider compared with mudfl ats
in tide-dominated estuaries.
