Geology Reference
In-Depth Information
6.3.4.1 Pleistocene Dong Nai River
Succession, Vietnam
A very similar case to the Yangtze system (China)
is reported by Kitazawa (
2007
) who described a
Pleistocene succession outcropping along the Dong
Nai River in Vietnam within the Quaternary Mekong
Basin. The succession comprises two superimposed
transgression-regression cycles related to Pleistocene
sea-level fl uctuations, from MIS8 (270 kyr. BP) and
MIS5 (about 100 kyr. BP). The transgressive systems
tract of each cycle is interpreted as a tide-dominated
estuary succession, stacking fl uvio-tidal aggradational
deposits that are partially eroded laterally at the sea-
ward edge by marine sand bodies lying on a tidal
ravinement surface. The transgressive systems tract is
overlain by a progradational deltaic succession forming
the highstand systems tract.
sand-dominated seaward end-member. As soon as the
transgression stopped at 7,000 years BP, the system
evolved into a tide-dominated delta whose prograda-
tional succession forms the highstand systems tract.
6.3.3.2 Qiantang River Estuary,
Hangzhou Bay, China
This last modern case study is similar in many aspects to
the Yangtze case but the present-day Qiantang estuary is
defi ned as a tide-dominated estuary, not a tide-domi-
nated delta. Tidal range reaches up 9 m, twice the range
in the Yangtze. Sediment is supplied by the river but in
much less quantity than in the Yangtze delta. Moreover
it is suggested that a considerable part of fi ne-grained
sediment is also sourced by longshore drift from the
Yangtze River located about 100 km to the north.
The sediment infi ll is simple and related to the last
post-glacial transgression. Sea-level rise was very rapid
until 12,000 years BP (35 mm/year), and slower until
7,500 years BP (10 mm/year). Till 4,000 years BP, sea
level rose with a rate of 3 mm/year and became stable
after this. Four depositional units have been distin-
guished in the infi ll thanks to numerous cores (Fig.
6.4
).
Above a fl uvial channel unit deposited during base level
rise until 12,000 years BP, a retrograding/aggrading
clay- to silt-dominated estuarine unit accumulated. This
unit contains tidal sand ridges. Tidal facies are common,
including tidal rhythmites. At 7,500 years BP, the maxi-
mum rate of sea-level rise was reached and estuarine to
marine muds blanketed the whole estuary. Finally, at
4,000 years BP, when sea level stabilized, tidal bars
made of marine silts to fi ne sands developed, prograding
over the estuarine muds. The tidal ravinement surface is
believed to be located below the upper unit.
6.3.4.2 Aspelintoppen Formation,
Eocene Central Basin, Spitsbergen
This sedimentological and high-resolution sequence
stratigraphy work conducted by Plink-Björklund (
2005
)
is undoubtedly the most detailed study that has been
published to date on tide-dominated estuary facies
and successions recognized in the rock record. The
Aspelintoppen Formation is a mud-prone, aggrada-
tional coastal plain formation that contains 18 stacked
depositional sequences. Each sequence consists in a
lowstand systems tract/transgressive systems tract/
highstand systems tract succession interpreted to be
the result of a tide-dominated estuary infi ll during a
fourth-order cycle (400 kyr) of sea-level fl uctuation.
Outcrops permit each individual tide-dominated estu-
ary in successive sequences to be followed from their
upstream to their downstream end, and to assign each
facies recognized in the fi eld to a typical morphologi-
cal component of a tide-dominated estuary, from the
fl uvio-tidal channel with the low- to high-sinuosity
zones, through the upper-fl ow-regime tidal fl ats, into
the tidal sand bars.
In an ideal sequence, the lowstand systems tract
consists of fl uvial deposits, overlying an erosional
sequence boundary, that grade upward to aggrada-
tional fl uvial facies infl uenced by tidal dynamics.
This upper part of the lowstand systems tract could
be assigned to the early transgressive systems tract.
The early transgressive systems tract is capped by a
transgressive surface, overlain by tide-dominated estu-
arine facies that form the transgressive systems tract.
6.3.4
Ancient Estuaries
Examples of successions assigned to tide-dominated
estuaries in the rock record remain relatively rare. Many
sedimentological and sequence stratigraphic analyses
refer to tidal facies and deposition in estuarine environ-
ments, but, as previously mentioned, most interpretations
fi nally refer to the wave-dominated estuary model or to
the mixed-energy wave- and tide-dominated estuary
model based on the Gironde. Few examples of ancient
sediment successions are described and interpreted
explicitly to be the result of tide-dominated estuary infi ll.
Some of them are briefl y reported below.
