Geology Reference
In-Depth Information
The transgressive character of the succession is evi-
denced by facies superimposition that demonstrates a
landward shift of the bay-line. At the seaward end of
the profi le, tidal sand bar facies, overlying a tidal
ravinement surface, compose the upper part of the
transgressive systems tract. The maximum fl ooding
surface is usually located at the top of the bars. The
highstand systems tract displays similar facies as in
the transgressive systems tract, but is characterized
by a seaward shift of the inner- and central-estuarine
facies, and preferential preservation of root horizons,
coal layers and marsh deposits. During the high-
stand systems tract, the environment is assumed to
remain a tide-dominated estuary, and not to become
a tide-dominated delta, since highly sinuous tidal
channel facies are still present and demonstrate the
existence of the bedload convergence zone in the
inner estuary.
lowstand systems tract to early transgressive systems
tract. The facies succession is indicative of a wave-
dominated estuary environment. Above these wave-
dominated estuary deposits, a fl ooding surface is
overlain by a third interval assigned to the transgres-
sive systems tract and interpreted to be a tide-dominated
estuary succession on the basis of the occurrence of
facies related to tidal channels, tidal fl at and sand shoal
and estuary-mouth tidal bars. At the top, the highstand
systems tract consists in two intervals similar to the
tide-dominated estuary interval but with increasing
fl uvial infl uence, indicative of the progradational pat-
tern of the infi ll. This ancient example thus reports an
evolution from a wave-dominated estuary to a tide-
dominated estuary, similar to what happened during
the Holocene lowstand and subsequent transgression
of the Cobequid Bay-Salmon River estuary.
Other studies, not detailed herein, document analyses
of facies successions interpreted as tide-dominated estuary
infi ll deposits such as for instance those of Khin and Myitta
(
1999
, Miocene, Central Myanmar), Shanmugam
et al. (
2000
, Cretaceous, Ecuador), Mellere (
1994
,
Cretaceous, USA), Johnson and Levell (
1995
, Cretaceous,
UK), Archer et al. (
1994
, Carboniferous, USA) and Pontén
and Plink-Björklund (
2009
, Devonian, Baltic Basin).
6.3.4.3 Chimney Rock Tongue, Upper
Cretaceous, Campanian, the Flaming
Gorge Area, Utah-Wyoming, USA
Plink-Björklund (
2008
) provided another very accu-
rate description of tide-dominated estuarine succes-
sions in a formation from the Western Interior basin.
The Chimney Rock Tongue comprises three distinct
stratigraphic intervals: wave-dominated delta deposits,
mixed-energy estuary deposits as an incised-valley
fi ll and tide-dominated estuary deposits. The tide-
dominated estuary succession, about 60-m thick,
consists of three transgressive-regressive units. Each
unit is composed of tide-infl uenced fl uvial deposits,
inner-estuary tidal-fl at and marsh deposits, and outer-
estuary upper-fl ow-regime tidal-fl at and tidal-sand-bar
deposits. Tidal ravinement surfaces, located at the base
of tidal-sand-bar deposits, mark the base of each
transgressive-regressive unit.
6.4
Key Features of Tide-Dominated
Estuary Successions
These brief descriptions of modern (Holocene) and of
some ancient case allow the key points that character-
ize the stratigraphy of sediment fi lls of tide-dominated
estuaries to be highlighted.
From a general point of view, most studies related to
tide-dominated estuary sediment infi ll refer to basic
sequence stratigraphy concepts allowing the distinc-
tion of systems tracts (lowstand, transgressive and
highstand systems tracts) and key surfaces such as the
sequence boundary, the transgressive surface, the tidal
and wave ravinement surfaces and the maximum fl ood-
ing surface (Figs.
6.4
and
6.5
). Due mainly to the diver-
sity of contexts, interpretations differ from one place to
another. This clearly applies to the recognition and
placement of the maximum fl ooding surface and the
related distinction between the transgressive and high-
stand systems tracts. The differences in the type of data
(cores and/or seismic), in data quality (coring or drill-
ing, seismic resolution) and in location of the studied
6.3.4.4 Cujupe Formation, Upper Cretaceous
- Lower Tertiary São Luis Basin,
N Brazil
This study reported by Rossetti (
1998
) documents an
interesting case resembling the infi ll evolution of the
Cobequid Bay-Salmon River estuary. Facies analyses
and architectural reconstruction demonstrate that the
Cujupe Formation is made of tide-infl uenced facies
that infi ll an incised valley. The infi ll is divided into
fi ve stratigraphic intervals. Above the sequence bound-
ary, the two basal-most intervals are interpreted as a
