Geology Reference
In-Depth Information
amplifi ed, if not triggered, during episodes of climate
deterioration marked by heavy rain seasons. Such a
general increase in sediment supply of fl uvial origin
led to the deposition of a progradational mud unit. This
implies that these estuaries tend to evolve since recent
times towards deltaic environments.
records than what has been previously interpreted. As
discussed in the previous sections, many factors play a
major role in the infi lling processes of estuaries, and
their interaction is consequently very complex.
When considering wave-dominated infi lling, three
stratigraphic components (depositional units) can be
identifi ed: (1) a transgressive sand-dominated wave-built
barrier, incised by a tidal inlet (transgressive systems
tract); (2) a prograding sand-dominated bay-head delta
(highstand systems tract); and (3) an aggrading mud-
dominated central basin body (transgressive and high-
stand systems tracts). According to the degree of tidal
infl uence, the tidal inlet is more or less deep and wide,
the morphodynamics of the bay-head delta is more or
less infl uenced by tidal currents, and tidal signatures are
more or less pronounced into the central mud facies.
The descriptions of the Holocene case studies have
pointed out that in the infi ll of almost all tide-dominated
estuaries, these wave-dominated estuary components
can be preserved. Wave-built coastal barriers are pre-
served on the margin of the Seine and Mont St Michel
estuaries. A bay-head delta unit with tidal bars is pre-
served in the Yangtze tide-dominated estuary trans-
gressive systems tract. A central mud-like depositional
unit constitutes the bulk of the transgressive systems
tract in the Mont St Michel and Seine estuaries. The
identifi cation of such units in the rock record would
have probably led to proposals that the depositional
environment was a wave-dominated estuary, or at least
a mixed-energy estuary. Hence, one can suspect that
the abundance of tide-dominated estuary successions
has been underestimated in stratigraphic records.
The tidal ravinement surface is certainly the most
striking stratigraphic feature for differentiating tide-
dominated estuaries and wave-dominated estuaries.
As predicted by Zaitlin et al. (
1994
), the tidal ravine-
ment surface is restricted to the mouth (tidal inlet)
in wave-dominated estuaries, whereas it extends
throughout tide-dominated estuaries (Fig.
6.6
).
Moreover, in mixed-energy estuaries, the tidal
ravinement surface is usually very deep due to the
constriction in the inlet of relatively powerful tidal
fl ows. As soon as the tidal range increases and/or
wave power decreases, tidal action expands, laterally
and upstream, and the tidal ravinement surface
becomes relatively shallower but extends throughout
the whole estuary. As a result, sediment fi lls of wave-
dominated estuaries and tide-dominated estuaries
are differently preserved (Fig.
6.6
). At the seaward
end of wave-dominated estuaries and mixed-energy
6.6
Tide-Dominated vs.
Wave-Dominated Estuaries
The objective of this last section is to provide some
keys to help the recognition of tide-dominated estuar-
ies successions in ancient coastal sediment wedges.
Recently, Dalrymple and Choi (
2007
) provided a syn-
thetic and helpful overview of all sedimentological and
morphological criteria allowing the recognition of
tide-dominated fl uvio-marine transitional environments
(i.e. tide-dominated estuaries and tide-dominated
deltas). The purpose herein is not to recall these criteria,
but rather to highlight some features that are assumed
to typify tide-dominated estuary infi ll stratigraphy and
could be useful in particular for distinguishing them
from wave-dominated estuaries.
As demonstrated by long-lasting debates about the
interpretation of the depositional environment of some
well-known tide-dominated successions (e.g. Cretaceous
of the Interior Seaway, North America; Eocene Roda
sandstone, Spain), accurate facies models are still
missing to allow the distinction between different
tide-dominated environments (estuary vs. delta vs.
shelf) as preserved in the rock record.
With respect to estuaries, deciphering tide-
dominated estuaries and wave-dominated estuaries is
not a task as easy as it seems, mainly because outcrop
or subsurface data are not suffi cient and/or their quality
is not good enough to reconstruct precisely the deposi-
tional palaeoenvironment. The very detailed and accu-
rate facies analyses and architectural reconstructions
made by Plink-Björklund (
2005
, Eocene, Spitsbergen,
2008
, Cretaceous, U.S.A.) benefi ted from exceptional
outcrop conditions, allowing continuous observations
throughout marine-to-fl uvial transitions.
Reconstruction analyses probably tend to apply too
strictly one of the two end-member models, the tide-
dominated or the wave-dominated estuary model.
Dalrymple (
2006
) noted that mixed-energy estuaries
such as the Gironde (Allen and Posamentier
1993
) or
the Charente estuary (Chaumillon and Weber
2006
)
are probably much more common in stratigraphic
