Geology Reference
In-Depth Information
of low sediment supply, the highstand systems tract
started to develop in the Marennes-Oléron Bay as soon
as the rate of Holocene sea-level rise dropped, i.e.
around 6,000 years BP, because the bay is sheltered
from marine erosion by structural highs. In the outer
segment of the system, the transgressive systems tract
constitutes most of the Holocene infi ll, as in the
Gironde, and is locally deeply scoured due to the action
of tidal currents and waves.
(cf. discussion in next section). The combination
between the natural highstand infi lling processes and
the heavy human activities in the estuarine system,
including the Seine River catchment, results in a rapid
seaward shift, i.e. progradation, of the tidal bars since
at least the two last centuries.
6.3.2.5 The Mont-Saint-Michel Estuary,
Norman-Breton Gulf, NW France
In the eastern corner of the so-called Bay of Mont-
Saint-Michel, where the tidal range reaches up 15 m
during high spring tides, an estuary forms at the mouth
of three rivers. This tide-dominated estuary comprises
an extensive tidal channel-and-bar complex evolving
landward to a single tidal channel that passes gradually
into a meandering bedload convergence zone occurs
here (Fig.
6.3
). No longitudinal tidal bars are present at
the seaward end of the estuary, probably because of the
restricted length of the estuarine funnel that widens
rapidly on the open sea. Hence, sediment dynamics in
this wide entrance is probably infl uenced by wave
action. The latter is very signifi cant on the northern
margin of the estuary where a locally retreating wave-
built barrier is developed (Figs.
6.3
and
6.4
).
The Bay of Mont-Saint-Michel including the estu-
ary is a mixed carbonate/siliciclastic environment, and
sediment is almost exclusively of marine origin. The
sedimentary infi ll, reconstructed on the basis of very
high-resolution seismic and vibracore data, is related,
as for the Seine estuary, to the last post-glacial trans-
gression that slowed down at about 6,500 years BP. The
Holocene infi ll is divided into two depositional units
resembling the Seine infi ll (Fig.
6.4
). Above some
probable remnants of Pleistocene fl uvial terraces
assumed to form the lowstand systems tract, an aggra-
dational unit made of organic-rich fi ne-grained facies
fi lls the bottom of the valley. Near the estuarine mouth,
this unit passes to high energy-migrating tidal dunes
and banks, or to shoreface sands. These deposits com-
pose the transgressive systems tract.
In the estuarine axis, the transgressive systems tract
is overlain by a sandy tidal channel belt through an
erosive surface corresponding to the tidal ravinement
surface, and dated at 6,500 years BP. This high-energy
estuarine unit, constituting the highstand systems tract,
pinches out progressively landward and seaward. In
the single tidal channel zone, the bottom of the
present-day channel reaches bedrock, which is higher.
Hence, the highstand systems tract in this area is
6.3.2.4 Seine Estuary, Bay of Seine,
NW France
The stratigraphic reconstruction of the outer estuary
of the Seine River has been conducted recently thanks
to very high-resolution seismic and vibracore data.
Although highly modifi ed by human activities mainly
carried out for navigation purpose, the mouth of the
Seine estuary displays a well-shaped funnel with two
prominent longitudinal bars, typical of tide-dominated
estuaries (Fig.
6.3
). Spring tidal range at the entrance
approaches 8 m. Sediment originates both from the
Seine River that delivers mainly fi ne-grained sediment
and from the sea. Fluvial muds feed a turbidity maxi-
mum that can be expelled far offshore during severe
river fl oods (Garnaud et al.
2003
; Lesourd et al.
2003
) .
The sedimentary infi ll is simple and related to the
last post-glacial transgression that was rapid until
7,000-6,500 years BP (−10 m) and then slow until now
(Fig.
6.4
). Above late Pleistocene terraces assigned to
the lowstand systems tract, the infi ll comprises two
main units. Aggrading organic-rich estuarine facies
compose the transgressive systems tract. No high-
energy tidal bodies have been recognized. Around
7,000 years BP, the transgressive systems tract, above
the main axis of the incised valley, is sharply overlain
by a massive sand body corresponding to the estuary
mouth tidal bar complex. The erosive basal limit of
this sand unit amalgamates the transgressive surface
and the tidal ravinement surface.
On the shallower edges of the valley, where tidal
currents are not so powerful as compared with the
estuary axis, wave-built sandy barrier and back-barrier
tidal fl ats were emplaced. The latter together with the
tidal sands belong to the highstand systems tract. At
about 3,000 years BP, the barrier/back-barrier unit is in
turn eroded by a tidal channel belt, indicating the
expanding of the tidal system throughout the estuarine
funnel. The destruction of the barrier 3,000 years ago is
believed to be related to a period of enhanced storminess
