Geology Reference
In-Depth Information
The highstand systems tract, separated from the
underlying transgressive systems tract by the maxi-
mum fl ooding surface, differs greatly from one place
to another both in terms of volume and facies succes-
sions. In the case of the Yangtze, the highstand stystems
tract is a tide-dominated delta. In tide-dominated estu-
aries, the highstand systems tract is assigned either to
the most recent stages of infi ll, represented by mud-
dominated facies (e.g. Cobequid Bay-Salmon River
and Vilaine estuaries), or to the bulk of the infi ll, con-
taining the whole tide-dominated estuary succession,
such as in the Seine and Mont St Michel estuaries.
In those cases, the tidal ravinement surface that lies
below the tidal channel-and-bar body is amalgamated
with the maximum fl ooding surface. The distinction
between the transgressive systems tract and the high-
stand systems tract is based on the recognition of a sea-
ward shift of the successive estuarine environments
such as is precisely described, for example, in the
Eocene Central Basin of Spitsbergen (Plink-Björklund
2005
), and by progradational facies stacking and
downlap confi gurations above the maximum fl ooding
surface. This last criterion is fulfi lled in the case of the
Seine and Mont St Michel estuaries. This is as well the
case in the mixed-energy estuary of the Gironde, where
the highstand systems tract is represented by the
prograding tide-dominated bay-head delta.
(Davis and Hayes
1984
) although all modern examples
discussed are macrotidal. As a consequence, most tide-
dominated estuaries are associated with tide-dominated
shelves such as the English Channel or the China Sea,
and more generally to shelves that are large enough to
amplify the oceanic tidal wave. Elongated bays or gulfs
are then favourable coastal confi gurations for extreme
amplifi cation of tidal waves that propagate on shelves
(Bay of Fundy, Canada; Bristol Channel, UK; Norman-
Breton Gulf, France; Hangzhou Bay, China).
It thus appears that coastal confi guration by
controlling tidal dynamics is a critical factor. This
underlies the major role of bedrock morphology on
tide-dominated estuary infi ll. Most studies of incised
valleys highlight the importance of bedrock inheri-
tance on sediment infi lling (cf. review in Dalrymple
2006
or in Chaumillon et al.
2010
) . Bedrock inheri-
tance on tide-dominated estuary infi ll should be con-
sidered from different aspects. As previously
mentioned, the valley shape determines the possibility
for tides to be or not amplifi ed as the valley is trans-
gressed. Funnel-shaped valleys, and more generally,
valleys with a high length/width ratio, primarily favour
hypersynchronous behaviour of the tidal wave and thus
tide-dominated estuary occurrence.
Nordfjord et al. (
2006
) have drawn such a conclu-
sion from seismic data collected on Pleistocene to
Holocene incised-valley fi lls on the New Jersey shelf:
narrow valleys are assumed to promote tide-dominated
estuary development rather than wave-dominated estu-
aries, whereas broad valleys might not provide enough
constriction to create strong tidal currents, causing
them to be wave dominated. This major role of bedrock
(valley) morphology probably implies that the 'unfi lled
spaces where tide-dominated estuaries can form as
transgressive coastal environments' (Dalrymple
2006
)
are necessarily (incised) valleys. Consequently, the
initial defi nition of estuaries by Dalrymple et al. (
1992
)
stating, 'an estuary is the seaward portion of a drowned
valley system…' should probably be considered as
still valuable for most tide-dominated estuaries. The
cross-sectional shape of the valley is also important to
consider as demonstrated for instance by the Seine
Estuary case. Irregular valley walls shaped by plateaus
(probably wave-cut platforms originating from previ-
ous Pleistocene sea-level stillstands) constitute geo-
morphological features promoting the construction
and preservation of wave-dominated coastal barriers
on the margin of a tide-dominated estuary.
6.5
Factors Controlling Tide-
Dominated Estuary Infi lling
As demonstrated through the different examples previ-
ously described, both in modern settings and from the
rock record, sediment infi lls of tide-dominated estuar-
ies show a large diversity in terms of geometry and
relative proportion of facies within the preserved sys-
tems tracts. This variability is related fi rst of all to the
diversity of the sites. Hence, the different factors that
govern the sediment infi ll of tide-dominated estuaries
can be discussed in the light of this diversity.
6.5.1
Tidal Dynamics and Inherited
Bedrock Morphology
The fi rst factor is related to tidal dynamics since
tide-dominated estuaries need tides to develop. This
does not mean that very large tidal ranges are necessary
