Geology Reference
In-Depth Information
Fig. 6.7
Downstream-upstream cross section in a tide-
dominated estuary along the axis of the main tidal channel.
This illustrates how tidal accommodation should be considered
as a major factor of preservation of systems tracts, especially
in the internal domain where the bottom of the channel
reaches potentially the bedrock.
TST
,
HST
transgressive,
highstand systems tracts,
HTL
high tide level,
LTL
low tide
level
The fi nal question to be addressed in the perspec-
tive of distinguishing tide-dominated estuaries and
wave-dominated estuaries is the role of the tidal range
in infi ll stratigraphy. Estimation of tidal range in
ancient environments through the thickness of inter-
tidal-supratidal successions has received some atten-
tion (Terwindt
1988
), but considering tidal range as a
forcing parameter of infi ll stratigraphy is not common.
Tidal range plays necessarily a signifi cant role regard-
ing the volume of preserved systems tracts, particu-
larly in macrotidal settings with extreme tidal ranges
such as the Cobequid Bay-Salmon River estuary or the
Mont-Saint-Michel estuary. This 'tidal accommoda-
tion' should be added to the initial accommodation,
especially if tidal range is supposed to have changed
signifi cantly during the transgression and thus during
the infi ll of the estuary, such as in the Cobequid
Bay-Salmon River estuary. Anyway, tidal accommo-
dation that can be defi ned as the depth of the active
channel belt (Billeaud et al.
2007
; Tessier et al.
2011
)
controls the preservation of the entire estuarine
channel body in the area where the tidal ravinement
surface reaches the basement (Fig.
6.7
).
site to another. However, this synthesis demonstrates
that the infi lling stratigraphy of tide-dominated estuar-
ies is closely controlled, as it is for all other coastal
deposits, by a complex combination of the rate of sea-
level change, sediment supply, bedrock morphology,
and hydrodynamics.
Only a few features can be assigned specifi cally to
tide-dominated estuaries in terms of infi ll stratigraphy.
Tidal accommodation appears as the most critical factor
in as much as the tidal ravinement surface can poten-
tially rework part of, if not all of, the underlying deposit
throughout the estuary. Another important point is that
in almost all tide-dominated estuary-infi ll successions,
wave-built sedimentary bodies can be preserved, espe-
cially along the seaward fl anks of the valley. The recog-
nition of such facies in outcrops or subsurface data
could lead to misinterpretations. It should be noted also
that, in spite of the predominant action of powerful tidal
currents, climate changes can exert a critical control on
tide-dominated estuary infi lling, especially in terms of
the nature of the sediment. This control is recorded
through fl uctuations in fl uvial discharges, but also in
the morphodynamics of marginal wave-built barriers
that induce changes in tidal-channel behavior.
A great deal of new data have been collected during
the last decade on modern tide-dominated estuaries.
However, further studies, based on seismic, core, and
high-resolution age data, need to be done in order to
improve our understanding with respect to two aspects:
(1) the longitudinal variability of the infi ll (i.e., the
downstream-upstream evolution) remains poorly
illustrated in most modern cases since data are usually
not available along the entire length of the estuarine
system; and (2) the infl uence of human activities on
the infi ll stratigraphy. Most tide-dominated estuaries
6.7
Summary
This chapter is an attempt to synthesize available data
on the stratigraphy of the deposits fi lling tide-domi-
nated estuaries. Compared with those published for
wave-dominated estuaries, these data remain relatively
rare, primarily because tide-dominated estuaries are
not as common. Moreover, data quality (the resolution
of seismic and age data, and the portion of the estuary
studied) is extremely variable and unequal from one
