Journal of Coastal Research, Special Issue 56, 2009

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 victor@igeo.uff.br cleverson@igeo.uff.br

‡Dept. of Geography Fluminense Federal University, Niterói 24.210-346, Brazil guilherme@igeo.uff.br

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), fluvio-glacial 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

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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

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).

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).

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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 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.

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 2 – Stratigraphy of the northern strandplain (Dias et al., 1984a).

Figure 3 – Bathymetric map. Contour interval 1 m.

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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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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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