Geology Reference
In-Depth Information
wave-dominated estuary lowstand succession, is interpreted
as the transgressive systems tract, bounded at the base
by the transgressive surface amalgamated with the
tidal ravinement surface. Data indicate that throughout
most of the estuary, this second stage of infi lling is an
aggradational unit. It is suggested that the estuary has
entered in a progradational phase within the last
200 years only. Rapidly accreting tidal fl at successions
containing tidal rhythmites that were emplaced along
the margin of the estuary are assigned to this highstand
systems tract. Prograding tidal rhythmites lie above
vertically aggrading marsh deposits through a tidal
ravinement surface that is therefore responsible for
removing landward facies during the transgression
(Dalrymple and Zaitlin
1994
).
located below the marine-to-estuarine sands that
compose the transgressive systems tract.
More recently, Bostock et al. (
2007
) documented
the Holocene infi ll of the tide-dominated Fitzroy River
estuary in Southeast Queensland, Australia. The gen-
eral infi lling stratigraphy is very comparable with that
of the South Alligator estuary, three main phases being
distinguished as well during the Holocene evolution:
(1) post-glacial inundation, (2) mangrove develop-
ment, (3) fl oodplain aggradation and estuarine mouth
progradation. As in the South Alligator estuary, sedi-
ments in the Fitzroy River estuary are dominantly fi ne-
grained, but contrary to the South Alligator estuary,
they are mainly of fl uvial origin. No sedimentary unit
is assigned to the transgressive systems tract, the whole
infi ll succession being related to the post-7,000 years
BP sea-level highstand. According to these data, the
Fitzroy River estuary should rather be defi ned as a
tide-dominated delta.
6.3.2.2 South Alligator Estuary,
van Diemen Gulf, North Australia
The work of Woodroffe et al. (
1989
) on the South
Alligator estuary is certainly the pioneer study
regarding the infi lling stratigraphy of a tide-domi-
nated estuary. Although tidal range is 'only' 6 m at
the entrance, the South Alligator estuary comprises
all characteristics of a tide-dominated estuary
(Fig.
6.3
). Like in the Cobequid Bay-Salmon River
estuary, the fl uvial sediment supply is low. The main
sediment source is of marine origin, and dominated
by fi ne-grained sediment. The reconstruction of the
infi ll is based on drilling data. Related to the
Holocene sea level that rose quickly until about
6,000 years BP and then remained stable until present,
three main phases of infi lling are distinguished
(Fig.
6.4
). The transgressive phase (8,000-6,800 years
BP) corresponds to the marine incursion into the
funnel-shaped valley. Mangroves developed on the
margins, whereas marine and estuarine sands and
muds are found along the valley axis. The second
phase (6,800-5,300 years BP) is marked by the
expansion of mangrove forests throughout the estu-
ary, followed by the third phase (after 5,300 years
BP) during which a sinuous-to-cuspate tidal channel
fl oodplain developed. As inferred by radiocarbon
dating, shoreline almost attained its present-day
position some 3,000 years ago, indicating that fl ood-
plain aggradation has been very slow during the late
Holocene. Although the architectural reconstruction
for this tide-dominated estuary is only partial, one
can expect that the transgressive surface and tidal
ravinement surface are amalgamated together and
6.3.2.3 Gironde Estuary, Central Bay
of Biscay, SW France
Most works on tide-dominated estuaries refer to the
Gironde estuary case at least as a comparative exam-
ple. Moreover, the stratigraphic model established by
Allen (
1991
) and Allen and Posamentier (
1993
) pro-
vided elements used by Zaitlin et al. (
1994
) when con-
structing their model. Spring tidal range in the Gironde
is macrotidal (5.5 m), but the mouth is affected by very
high-energy Atlantic waves, reaching up 8 m during
winter storms. As a consequence, the Gironde entrance
is under the infl uence of both strong tidal currents that
incise a deep tidal inlet and construct well-developed
tidal deltas, and powerful wave-induced dynamics that
construct spit barriers and rework the ebb delta sand.
A large amount of sand and mud is delivered to the
estuary by the Garonne and Dordogne Rivers, but only
25% of this fl uvial sediment is exported seaward, the
rest remaining in the estuary, feeding the bay-head
delta and central basin (Fig.
6.3
). Hence, most sands at
the mouth are marine sourced. The stratigraphic recon-
struction of the Gironde estuary infi ll is based on a
compilation of core and well log data. The infi ll above
the bedrock is related to the last post-glacial transgres-
sion. Holocene sea-level rise was quick until 4,000 years
BP, and present-day sea level is inferred to have been
almost attained at that time. Above late Pleistocene
fl uvial deposits concentrated into the talweg of the
incised valley, the bulk of the infi ll is made of a
