Nearshore Sediments and Coastal Evolution of Paraíba do - e-Geo

Journal of Coastal Research
SI 56
650 - 654
ICS2009 (Proceedings)
Portugal
ISSN 0749-058
Nearshore Sediments and Coastal Evolution of Paraíba do Sul River
Delta, Rio de Janeiro, Brazil.
V.C. Murillo†, C.G. Silva† and G.B. Fernandez ‡
†Dept. of Geology
Fluminense Federal University, Niterói
24.210-346, Brazil
[email protected]
[email protected]
‡Dept. of Geography
Fluminense Federal University, Niterói
24.210-346, Brazil
[email protected]
ABSTRACT
MURILLO, V.C., SILVA, C.G. and FERNANDEZ, G., 2009. Nearshore sediments and coastal evolution of Paraíba do
Sul River Delta, Rio de Janeiro, Brazil. Journal of Coastal Research, SI 56 (Proceedings of the 10th International
Coastal Symposium), 650 – 654. Lisbon, Portugal, ISSN 0749-0258
The Paraíba do Sul River delta is located on northern Rio de Janeiro State, on the coastal plain onshore Campos
Sedimentary Basin, Brazil. Previous work suggested the transference of sands from the shoreface to the beach as
the main mechanism responsible for beach ridges progradation on both sides of the present river mouth. To
investigate this hypothesis we conducted a geophysical sub-bottom survey and superficial grab sampling to map
the nearshore sediment distribution and seabottom morphology adjacent to the coastal plain. The results revealed
a continuous mud facies, related to the Paraíba do Sul River prodelta, isolating the offshore carbonate facies from
the coastal, sandy, shoreface facies. The thickness of the mud facies decreases in the offshore direction, pinching
out against the outer carbonate facies, accompanying a general rise of the sea-floor related to the occurrence of
calcareous coralline algae and bryozoans bio-constructions. Our results favor an alternative explanation,
indicating that the river is ultimately the main source for the coastal sands, which are probably reworked by
alongshore currents on opposite directions apart from the river mouth, resulting on the coastal progradation.
ADITIONAL INDEX WORDS: beach ridges strandplain, wave-dominated delta, delta progradation
INTRODUCTION
The Paraíba do Sul River is the main fluvial system entering the
Campos Sedimentary Basin, on the northern coastline of Rio de
Janeiro State, Brazil (Figure 1) where it develops a large (3,000
km2) Quaternary coastal plain as a result of the deltaic
progradation. Extensive beach ridges strandplains are located on
both sides of the present river mouth, attesting the succession of
accretionary beaches during the course of coastline migration.
Despite the undisputed progradational nature of the coastline
there are different views regarding the importance of the river as
the primary sedimentary source. Several authors (DOMINGUEZ et
al., 1981; DOMINGUEZ et al., 1983; MARTIN et al., 1984;
DOMINGUEZ et al., 1987; MARTIN and FLEXOR, 1987) considered
that most of the sand involved in the coastal progradation was
reworked from the shoreface and nearshore during the course of
the last Holocene marine regression since 5.000 B.P.. The sand
were transported by alongshore currents and trapped downdrift the
river mouth by the hydrodynamic action of the river flow, acting
as a barrier for the free transport of sand.
On the other hand, other authors consider the river as the main
source of sand which reach the coast and are transported in both
directions by alongshore currents that diverge at the river mouth,
characterizing a wave dominated delta (DIAS and GORINI, 1980;
DIAS, 1981; DIAS ET AL., 1984a; SILVA, 1987). This view is
supported by the present distribution of heavy minerals on the
beach sands (ZETUNE, 2004) and by wave refraction models
(CASSAR and NEVES, 1983) which confirm the divergence of
longshore currents apart from the river mouth.
These divergent views are investigated on the present paper
considering one fundamental aspect: if the beach ridge sand is
derived from the shoreface or nearshore, there must be enough
relict sand available on the inner continental shelf to serve as
source material during the Holocene marine regression in the last
5.000 years.
Beach Ridges Strandplains
Beach ridges strandplains are prograding barriers formed in
areas were sedimentation rates are higher than the generation of
accommodation space (given by subsidence or sea-level rise) or
during shoreface erosion and successive mainland barrier beach
attachments in the course of marine regressions (FITZGERALD et
al., 1992; ROY et al., 1995; HESP and SHORT, 1999; CATUNEANU,
2006). Each individual beach ridge represents the position of the
shoreline during the progradation of the strandplain These are
common features in many river mouths, where there is presently
(or there was in the geological past) sufficient sand sedimentation
and appropriate wave hydrodynamic to rework it along the
coastline (PSUTY, 1965; CURRAY et al., 1969; DIAS and GORINI,
1980, DIAS, 1981; DOMINGUEZ et al., 1981; DOMINGUEZ, 1983;
ANTHONY, 1995). However, the fluvial source is not the only one
to bring extensive sedimentation and strandplains are not
exclusive to river mouths. Other sources have been considered
such as seacliffs and foredune erosion (CARTER, 1986), fluvioglacial deposits (FITZGERALD et al., 1992), inner-shelf and
shoreface sands (ROY et al.; 1995; TANNER; 1995; GOY, 2003).
Even though other sedimentary sources have to be considered,
every major river mouth on the eastern Brazilian coast is sided by
Journal of Coastal Research, Special Issue 56, 2009
650
Nearshore Sediments of Paraíba do Sul River Delta, Brazil
prograding beach ridges strandplains as was recently summarized
by DOMINGUEZ (2008) suggesting that the rivers are or were
recently, main sources and/or traps of sediments during the
Quaternary coastal evolution.
Paraíba do Sul River Beach Ridges Strandplain
The present river mouth is sided by two widely asymmetric and
morphologically different strandplains (Figure 1). The northern
strandplain presents characteristically an alternation of barrier
beaches and lagoons which are remnants of former sea-waterways
formed by the northward migration of sandy spits and barrier
islands departing from the river mouth, as was already described
by (DIAS and GORINI, 1980; DIAS, 1981). Each successive spit, or
barrier island, start as submarine longshore bars at the delta front.
Eventually the submarine bars aggrade to the surface and migrate
northwards and landwards enclosing the lagoons. MOREIRA
(1998) estimated an average coastal progradation of 6m/year for
the last 5.000 years of this northern portion of the coastal plain,
based on 14C dates of sediments and shells collected in successive
lagoons. The southern strandplain shows at least seven different
sets of beach ridges limited by sharp erosional truncations
indicating that the progradational process was periodically
interrupted by periods of erosion, a process that is being observed
for the last 20 years on the locality of Atafona Beach, on the
southern border of the river mouth, where erosion rates are in the
order of 7m/year (BASTOS and SILVA, 2000; BASTOS and SILVA,
2003). Despite the periods of erosion, registered on the alignment
of the of the beach ridges, the overall progradation in the last 3000
years is in the order of 4 to 7 m, resulting in the advance of the
coastline (BASTOS and SILVA, 2000).
The stratigraphic succession along the northern strandplain was
investigated by DIAS et al. (1984a) indicating above the Tertiary
sediments a marine transgressive sequence overlain by a deltaic
prograding sequence. The transgressive sequence is related to the
Holocene marine transgression registered as a coarsening upward
sequence, grading from lacustrine muds to open marine sandy
muds. The delta progradation is also a coarsening upward
sequence of prodelta muds on the bottom grading to fine to coarse
quartzose sands on top, representing the delta front and beach
ridges progradation (Figure 2).
METHODS
The applied methodology was designed to determine the
nearshore sedimentary distribution adjacent to the river mouth and
to characterize the thickness of fine sediments (prodelta mud)
above the transgressive sands.
The geophysical survey used a sub-bottom profiler ODEC,
model StrataBoxTM operating on the frequency of 10 kHz. This
equipment, in optimal conditions, can operate in water depths of
up to 150 m, providing subbottom penetration of up to 40 m (in
mud) and vertical resolution of 6 cm. Positioning was recorded
continuously along the surveyed lines with an integrated GPS
navigator. The surveyed lines are orthogonal to the coastline and
spaced every 2 km, extending at leas 6 km offshore in average.
Superficial samples were collected every 1 km along the lines
with a grab sampler (Figure 1). The samples were visually
described aboard the vessel and later treated on the laboratory for
standard grainsize determination.
The original geophysical digital data was later transformed to
SEGY format and loaded into the software SMT KingdomTM for
interpretation. The main seismic reflectors were mapped on each
seismic line, corresponding mainly to the present seafloor, the
base of the prodelta mud and a lower irregular reflector interpreted
as the top of Tertiary sediments. The bathymetry information was
Figure 1 – Satellite image of the Paraíba do Sul River delta. Each
beach ridge alignment is a former position of the shoreline during
the course of deltaic progradation. The northern beach ridges are
intercalated with lagoons (darker areas) formed by the northward
migration of barrier islands and spits. The southern beach ridges
are organized in sets with different alignments limited by
erosional truncations. Subbottom survey lines and grab stations
are indicated on the nearshore area. Satellite image from Google
Earth (http://earth.google.com, October 2007).
determined from the seafloor horizon depicted from the seismic
data.
The generation of contour maps and bathymetric map was done
automatically using the software SURFERTM after griding and
contouring the information of the different horizons mapped
previously. This data was used to construct an isopach map of the
prodelta mud.
The sediment distribution map was done manually, drawing the
limits of the different sediment facies, after plotting the sediment
types encountered on each station.
RESULTS
Nearshore Morphology
The morphology of the inner continental shelf adjacent to the
Paraíba do Sul River mouth clearly shows the asymmetric nature
of the nearshore to the north and south of the river mouth (Figure
3). Gradients are gentle to the north (0.1o) and to the south (0.2o)
(Figure 4).
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Murillo, et al.
Sediment Distribution
Superficial sediment distribution follows the nearshore
morphology (Figure 5). Siliciclastic coarse to fine sands occur
close to the present shoreline as a narrow (< 500m) strip on the
southern shores and forming the delta front adjacent to the present
river mouth. A narrow strip (<500m) of micaceous silt appears
close to the northern shores. The gentle nearshore slopes to the
north and south are covered by the prodelta mud pinching out
against siliciclastic and biogenic sands. The floor of the N-S
swale is partially covered by medium to very coarse siliciclastic
sands. The outer submarine high is dominantly biogenic, mainly
formed by coralline algae rodoliths and bryozoans.
Figure 2 – Stratigraphy of the northern strandplain (Dias et al.,
1984a).
A broad channel appears to the north, starting in the N-S
direction and bending to NE-SW in depths of 10 to 11 m. A large
(4 km) N-S swale occurs to the south, limited by an outer
submarine high (Figures 3 and 4).
Sediment Thickness
Fine sediments are easily recognized on the subbottom profiles
due to its characteristic transparency, permitting the observation of
subbottom reflectors. The first prominent reflector bellow the
prodelta mud crops out offshore, since the prodelta mud pinch out
against this reflector (Figure 4). This reflector is related to the
relict siliciclastic sand or biogenic sediments.
The mud isopach map (Figure 6) shows two main depocenters.
The northern depocenter is the thickest one, with more than 3 m of
mud. The southern depocenter thickens southward, reaching 2.5
m.
DISCUSSION
The nearshore sediment distribution and thickness adjacent to
the Paraíba do Sul River mouth shows a continuous coverage of
prodelta mud along the entire area, overlying siliciclastic relict
sands and biogenic sediments. Other siliciclastic sand are
restricted to a small strip close (<500m) to the present shoreline
and at the river mouth, forming the present delta front. The
observed sediment distribution is identical to the observed
stratigraphic succession mapped by DIAS et al. (1984a) on the
northern coastal strandplain (Figure 2), representing the deltaic
progradation (delta front sand and prodelta mud) over relict
transgressive sands (siliciclastic and biogenic), confirming the
work of DIAS et al. (1984b). Even the thicknesses of the
sedimentary units presented by DIAS et al. (1984a) are compatible
with the observed surveyed depth limits for the different
sedimentary facies, also confirming the views of DIAS et al.
(1984a,b).
The continuous presence of prodelta mud covering most of the
nearshore, and the offshore dominance of bioclastic sand, apart
from the siliciclastic sands on the swale, suggest that the inner
continental shelf is not the main source of sands involved in the
formation and progradation of the strandplain in opposition to the
views of DOMINGUEZ et al. (1981), DOMINGUEZ et al. (1983),
MARTIN et al. (1984), DOMINGUEZ et al. (1987) and MARTIN and
FLEXOR (1987).
The asymmetric morphology of the delta front and prodelta and
the sedimentary distribution close to the shoreline favors an
alternative explanation for the origin of the strandplain, involving
mainly the longshore transport diverging in two directions apart
from the river mouth. This is supported by wave refraction
models (CASSAR and NEVES, 1983) and by the distribution of
heavy mineral assemblages along the present shoreline (ZETUNE,
2004). This supports the idea that the river is the main contributor
of sediments involved in the progradation of the coast.
The northward migration of barrier islands and barrier spits on
the northern shores is another strong evidence favoring the
longshore transport as the main mechanism for the coastal
progradation, in accordance with DIAS and GORINI (1980), DIAS
(1981) and MOREIRA (1998).
Figure 3 – Bathymetric map. Contour interval 1 m.
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Nearshore Sediments of Paraíba do Sul River Delta, Brazil
Figure 4. Subbottom profiles 4N and 3S respectively to the north and south of the Paraíba do Sul River mouth. For location see figure 1.
Main sedimentary facies were confirmed by grab samples. Prodelta mud over relict sand pinches out against biogenic carbonates
Figure 5. Superficial sediment distribution.
Figure 6. Mud isopach. Contour interval in meters.
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Nearshore Sediments of Paraíba do Sul River Delta, Brazil
CONCLUSION
The nearshore morphology, sediment distribution and
thicknesses adjacent to river mouths are important aspects to be
analyzed in conjunction with the evolution of the coastal plain, as
seems to be the case in the Paraíba do Sul River Delta.
There is a clear correlation with the present day superficial
sediment distribution and the deltaic progradational stratigraphic
sequence observed by previous authors on the coastal plain
boreholes.
The main sedimentary source seems to be the Paraíba do Sul
River, indicating that the divergence of longshore transport may
be the main mechanism of sediment redistribution responsible for
progradation and formation of the beach ridges strandplain.
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