Geology Reference
In-Depth Information
Similar morphological features of the valley are
evoked by Araújo da Silva et al. (
2009
) to explain the
occurrence of thick wave-dominated sandy units at the
mouth of the Marapanim tide-dominated estuary
(Amazon, Brazil). However, wave-dominated coasts
and wave-built sedimentary bodies (beaches, sand-
spits, cheniers) are very common on the seaward fl anks
of tide-dominated estuaries (cf. Fig.
6.3
), and do not
necessarily require the presence of bedrock plateaus.
In the rocky coast estuary of Vilaine, the highly irreg-
ular morphology of the bedrock (Menier et al.
2006
) has
played a major role on the timing of the valley inunda-
tion and consequently on the timing of tidal amplifi ca-
tion (Menier et al.
2010
; Sorrel et al.
2010
) . The deepness
of the incision is another critical aspect to consider
regarding bedrock inheritance. Since it controls directly
the accommodation space, the incision deepness deter-
mines the stage of infi ll of an estuary with respect to
sediment supply, i.e. its degree of maturity from unfi lled
to completely fi lled (Dalrymple et al.
1992
). However,
this is important both for tide-dominated estuaries and
wave-dominated estuaries. Regarding more specifi cally
tide-dominated estuaries, the incision deepness governs
the potential for preservation of the infi ll.
Tide-dominated estuaries are associated with power-
ful tidal currents and therefore to potentially deep tidal
scouring. As a consequence of a deep tidal ravinement
surface, preservation potential of underlying deposits is
low. This is particularly noticeable in the Mont St Michel
estuary that is characterized by a shallow bedrock inci-
sion; consequently, the tidal ravinement surface reaches
the bedrock throughout the whole internal estuary and
reworks almost all older depositional units, in particular
the transgressive systems tract (Billeaud et al.
2007
;
Tessier et al.
2010b
). This partly explains why the latter
is poorly developed compared to the highstand systems
tract in this tide-dominated estuary. The same process
occurred in the Seine estuary since the longitudinal tidal
bar body has remained active with deeply scoured tidal
channels throughout the highstand infi ll above the main
axis of the incised valley.
the main changes in the infi ll architecture and facies
are the result of the change from rapid to slow sea-
level rise. Concerning the control of sea-level changes
on tide-dominated estuary infi ll specifi cally, it controls
mainly the possibility for the valley to enter into a tidal
amplifi cation window as it is transgressed. With respect
to a fi fth order relative sea-level cycle, such as the last
late Pleistocene-Holocene cycle, this depends clearly
on the location of the valley on the shelf, between the
lowstand shoreline and the highstand shoreline.
Tidal resonance or at least tidal amplifi cation has a
low potential to occur in a valley that is inundated too
rapidly, or, if tidal amplifi cation occurs, it does not last
enough time for a tide-dominated estuary to develop.
Such a concept of a tidal resonance window during a
sea-level cycle has been already applied to interpret
tide-dominated sedimentary body occurrence and
architecture in the rock record and in a Quaternary shelf
succession (e.g. Sztano and De Boer
1995
; Reynaud
et al.
1999
), and has been recently re-considered to be
integrated in sequence stratigraphic analyses (Yoshida
et al.
2007
). The entrance of the Cobequid Bay-Salmon
River estuary into tidal resonance during the middle
Holocene transgression could be partly responsible
for the rapid shift from a wave-dominated to a tide-
dominated estuary (Dalrymple and Zaitlin
1994
).
6.5.3
Sediment Supply
The infi ll of all types of estuaries depends as well on
sediment supply or, more precisely, on the balance
between rate of sea level change and sediment supply,
i.e. the very common A/S parameter used in sequence
stratigraphic analyses. In turn, sediment supply should
be considered as a complex factor that integrates the
availability of sediment (of both marine and and fl uvial
origins) and the potential of hydrodynamics, i.e. waves,
tidal currents, and river fl ows, to rework this sediment.
Typically, high to moderate wave energy associated with
meso- to macrotidal range but high volume of available
marine sediment promotes the construction of coastal
barriers and thus the development of wave-dominated
or mixed-energy estuaries (e.g. Gironde), and of tide-
dominated estuaries if tidal range is very large.
This explains probably why the transgressive sys-
tems tract in the infi ll of the Cobequid Bay-Salmon
River estuary is assigned to a wave-dominated estuary;
a large amount of sediment was already available during
6.5.2
Sea-Level Fluctuations
Sea-level change is evidently another important factor
to take into account regarding sediment infi ll of estuar-
ies, in the case of both tide-dominated estuaries or
wave-dominated estuaries. In all Holocene examples,
