Geology Reference
In-Depth Information
area with respect to the estuary system (outer to inner)
account as well for the difference of interpretation.
The sequence boundary in most cases is defi ned as
the bottom of a fl uvial valley incised during the previ-
ous relative sea-level drop. Recently, Dalrymple (
2006
)
proposed a revision of his original defi nition of an
estuary in order to take into account that estuaries are
transgressive coastal environments that do not form
necessarily as the result of a river valley drowning.
Plink-Björklund (
2008
) suggested that the successive
tide-dominated estuarine intervals that compose the
upper stratigraphic unit of the Chimney Rock Tongue
occupied a drowned river mouth rather than an incised
valley. There is no evidence for fl uvial incision in this
unit, and each tide-dominated depositional interval
refl ects an episode of tidal ravinement and reshaping
of the river mouth. High rates of subsidence explain
the total aggradation of the 60-m-thick tide-dominated
estuarine succession. Shanmugam et al. (
2000
,
Cretaceous, Ecuador) suggested as well that the tide-
dominated estuary succession they studied does not fi ll
an incised valley. However, the authors admitted that
their available data and cross-sections may be insuffi -
cient to image an extra large incised valley.
The lowstand systems tract is usually assigned to
alluvial deposits preserved in the bottom of the valley.
In most cases, the lowstand systems tract is very
reduced in volume, represented only by remnants of
fl uvial terraces reworked during the subsequent trans-
gression by powerful tidal currents and/or waves. In
only few cases does part of the lowstand systems tract
consist of tide-infl uenced fl uvial deposits (Plink-
Björklund
2005
), of to marine (coastal) facies such as
in the Cobequid Bay-Salmon River estuary (Dalrymple
and Zaitlin
1994
) due to the early Holocene 'regional'
sea-level lowstand.
The transgressive systems tract, as predicted by the
model of Zaitlin et al. (
1994
), is usually described as
the bulk of tide-dominated estuary infi ll. In many cases,
the transgressive surface that corresponds to the basal
limit of the transgressive systems tract is amalgamated
with the tidal ravinement surface at least in the seaward
zone. The transgressive systems tract normally contains
all facies successions and sedimentary bodies that typ-
ify the different morphosedimentary components of a
tide-dominated estuary (e.g. Plink-Björklund
2005
) . In
some cases such as the Seine and Mont St Michel estu-
aries, the transgressive systems tract is of reduced vol-
ume and described as an aggrading depositional unit
made of fi ne-grained organic-rich facies that accumu-
lates prior to the 'active' tide-dominated estuary repre-
senting the highstand systems tract.
Fig. 6.5
Synthetic overview of the infi ll stratigraphy of the
main modern (Holocene) tide-dominated estuaries as well as
the mixed-energy estuary of the Gironde, and the tide-
dominated delta of the Yangtze (a tide-dominated estuary during
the transgressive systems tract deposition). Vertical scale in time
(k/year). The
thin dotted line
in each example indicates the
regional relative sea-level curve (low to the right). All infi ll
cases are simple, except that of the Cobequid Bay-Salmon River
estuary described as compound (cf. Fig.
6.4
for captions and
references)
