Strandplain evolution along the southern coast of

Journal of Coastal Research

SI 50

pg - pg

ICS2007 (Proceedings)

Australia

ISSN

Strandplain evolution along the southern coast of Santa Catarina, Brazil D.M. FitzGerald†, W.J. Cleary‡, I.V. Buynevich∞, C.J. Hein†, A.H.F. Klein§, N. Asp++ and R. Angulo+ †Department of Earth Sciences Boston University, Boston MA 02118, USA [email protected]

‡ Center for Marine Science, University of North Carolina, Wilmington, Wilmington, NC 28409

∞ Geology & Geophysics Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

§ CTTMar, Universidade do Vale do Itajaí UNIVALI, CP 360, Itajaí, 88302-202, Brazil.

++ Universidade Federal do Pará, Bragança, PA, Brazil.

+ Universidade Federal do Paraná, Departamento de Geologia, Curitiba, PR, Brazil

ABSTRACT FitzGerald, D.M., Cleary, W.J., Buynevich, I.V., Hein, C.J., Klein, A.H.F., Asp, N., Angulo, R, 2007. Strandplain evolution along the southern coast of Santa Catarina, Brazil. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), pg – pg. Gold Coast, Australia, ISBN The central Santa Catarina coast of southern Brazil has a relatively narrow discontinuous coastal plain that is bordered by crystalline rocks. Exposed granitic plutons form numerous low-relief headlands that dominate this section of coast. The irregular coastline has been smoothed through the deposition of extensive strandplains (2-8 km wide) with sediment derived from small rivers and the continental shelf. Forcing coastal progradation was the mid-Holocene (5-6 ka BP) sea-level fall of 2-4 m. Rivers can be directly linked to the construction of some strandplains, such as Tijucas (Tijucas River) and Navegantes Plains (Itajaí River), but other strandplains, including those comprising much of the exposed coast along Santa Catarina Island can only be attributed to sandy shelf deposits. Preliminary work on the strandplain coast, including surveying, coring, and geophysical studies, demonstrates that these low-gradient plains consist of dune/beach ridges and chenier systems and have evolved through shoreface and foreshore accretion producing low-angle, ubiquitous seaward-dipping clinoforms as seen in ground-penetrating radar transects. Data collected from Navegantes, Tijucas, and Pinheira strandplains indicate that considerable variability exists in the sedimentology, stratigraphy, and facies architecture among these plains, resulting from differences in wave exposure, the origin, volume and composition of the sediment supply, and antecedent topography of the coastal basin. Grain size correlates well with clinoform geometry and cheniers are best developed in proximity to muddy rivers. Centennial-to-millennial-scale variations in the organization and type of paleo-shorelines within these plains suggest periods of climatic change and/or the impact of major meteorological or oceanographic events. ADDITIONAL INDEX WORDS: Strandplain, Brazil, beach ridge, chenier

INTRODUCTION Late Quaternary clastic coastal depositional environments commonly contain records of climatic change and imprints of major meteorological and oceanographic events within their sedimentary sequences. However, most of these systems have gaps in their sedimentary records caused by hiatuses in sedimentation and/or periods of erosion, and thus their archive of environmental changes and event markers is incomplete. The strandplains of southern Brazil, in contrast to this type of depositional setting, are characterized by long-term progradation produced by an abundant supply of sediment and a regime of falling sea level since the mid-Holocene (ANGULO et al., 2006). Many of these plains are more than 4 km wide and exhibit repetitive former shoreline positions that can be traced in aerial photographs for many kilometers along the coast. Initial ground surveys at several of these plains in Santa Catarina State employing high resolution geophysical (ground-penetrating radar [GPR]) transects and the collection of sediment cores reveal that these plains are composed chiefly of seaward-dipping sandy strata with occasional muddy sections. These units have been interpreted as beachface and foreshore deposits associated with the progradation of beach/dune ridges and cheniers (mud-encased reworked sand/shell ridges) during strandplain construction.

In GPR sections strandplain sequences are dominated by lowangle, seaward-dipping clinoforms having an average spacing of approximately 1 m. Preliminary radiocarbon dates from three different strandplains in Santa Catarina, Brazil indicate shoreline progradation rates varying from 1 to 3 m/yr (FITZGERALD et al., 2006). Although the relationship between subsurface reflector spacing and shoreline progradation has not yet been determined, one hypothesis is that layers producing the GPR reflectors are formed on a mean annual basis. If this is true, strandplains may be similar to tree rings in their ability to capture annual growth (sedimentation) patterns. A high resolution geophysical study of a prograding barrier in Washington State showed that GPR reflectors could be correlated to annual accretion layers of the beach (MOORE et al., 2004). The reflectors had similar slopes as the present beachface and grain size data and trench observations suggested that the reflectors represented heavy mineral layers produced during winter storm (MOORE et al., 2004). Variations in the slope and spacing of clinoforms along with changes in the organization and composition of the ridges (beach-dune ridges versus cheniers) suggest that the Santa Catarina strandplain lithosomes are a product of changing sediment regimes and variable wave and nearshore processes as well as the infrequent impact of major meteorological and oceanographic events.

Journal of Coastal Research, Special Issue 50, 2007

Strandplain Evolution in Brazil

In this paper we present some initial findings of stratigraphic studies of three strandplains along the north-central Santa Catarina coast, including the Navegantes, Tijucas, and Pinheira plains (Figure 1). Our database consists of approximately 40 km of GPR transects, 20 sediment cores, and 15 radiocarbon dates. Although the relief, gradient, and ridge morphology of these plains and their general appearance as seen in satellite imagery seem quite similar, the internal structure and composition of the ridges as revealed by geophysical and sedimentological data demonstrate many differences in their constructional histories. Results from the Tijucas system are used to illustrate details of plain construction.

Navegantes

Tijucas

City of Florianópolis

Pinhiera 0

25 km

along this section of coast, rates and directions are highly variable and related to local wave refraction patterns. The Santa Catarina coast is microtidal with a mixed tide that is mainly semi-diurnal. Spring tidal ranges increase from approximately 0.5 m in the south to 1.2 m to the north.

Southern Brazil Sea-Level History The initiation of many strandplains, particularly those in the Northern Hemisphere, occurred during the Mid-Holocene, coinciding with a deceleration of sea-level rise when sediment supply out paced rising sea level. In the Southern Hemisphere strandplain development began 5.0 to 6.5 ka when sea level reached a highstand (e.g. Tuncurry, NSW Australia; ROY et al, 1994) and in many regions has fallen 2 to 4 m since that time (Fig. 2; ANGULO et al, 2006). The fall in sea level in the southern oceans is explained due to post-glacial isostatic adjustment. MITROVICA and MILNE (2002) showed that collapsing forebulges in previously glaciated continental margins created accommodation space that siphoned water from far-field regions including the South Atlantic. Additional sea level lowering in the southern oceans is attributed to further accommodation space created by ocean loading in the near field. A recent test of this isostatic model for the east coast of South America show good correspondence between established sea level histories and predicted sea-level curves (MILNE et al, 2005). In the southern Brazilian coast an encrusting gastropod (vermetid: Petaloconchus varians) has been used a paleo-sea level indicator (ANGULO and LESSA, 1999). The best estimates show that along the Santa Catarina coast sea level reached a highstand approximately 5.8 ka at an elevation of 2.5 m above present sea level (Figure 2; ANGULO et al, 2006). Along much of the southern coast of Brazil this forced regression coupled with an abundant supply of sediment has produced extensive strandplains.

Figure 1. Location of the Santa Catarina coast in southern Brazil. Note the bedrock promontories and intervening strandplains.

BACKGROUND Physical Setting The central Santa Catarina coast of Brazil is characterized by low-relief bedrock promontories and small islands that have segmented a coastal plain composed of strandplains, extensive dune fields and fluvial sequences, including some small deltas. Much of the bedrock consists of Archaean to Proterozoic-aged granites and gneisses, whose long-term weathering has produced a very thick saprolite (10 to > 100m thick) and yielded large volumes of sand. Even though this coast is bedrock framed, there is a wealth of sand that far exceeds sand volumes of many other passive margins such as the East Coast of the United States, as evidenced by Holocene and Pleistocene dune systems (heights > 10 to 50 m) that extend more than 2 km inland. The source of this sand is not well understood, although possible sources do include local small drainage basins, Rio de la Plata (to the south) and shelf sediments (DOMINGUEZ et al, 1987; CLEARY et al, 2004). Wave energy varies greatly along the coast and is governed by shoreline orientation, exposure to open-ocean conditions, and inner shelf bathymetry. A wave gage located off Santa Catarina Island in 80 m of water (Jan 2002 to Jan 2003) recorded bimodal sea-swell state; 12 sec swell from the south with significant heights ranging from 1.25 to 2.00 m and 8 sec sea with an average significant height of 1.25 m from the east (KLEIN, 2005). Although a general trend of northerly longshore sediment transport exists

Figure 2. Sea level curve for Santa Catarina, Brazil based on the vermitid record (from ANGULO et al., 2006). Note the fall in sea level during the past 5 to 6 ka, which is similar to many other regions in the southern oceans (ANGULO and LESSA, 1997).

Strandplain Morphostratigraphy Strandplains are broad accumulations of sand consisting of parallel or semi-parallel ridges of sand separated by shallow swales. Strandplains differ from beach ridge barriers in that they lack tidal lagoon or salt marsh and incising tidal creeks. Strandplains are connected to the mainland although they may border estuaries whose creeks extend into the plain (e.g. Parana coast of Brazil, ANGULO, 1999). Most strandplains occur along wave dominated coasts fronted by relatively wide, low gradient continental shelves having an abundant supply of sediment. Comparatively steeper shelves front some Australian strandplains (ROY et al., 1994). Strandplains are associated with dissipative beaches where constructional waves move sand onshore and along shore resulting in beach accretion.


FitzGerald et al.

The ridges of strandplains form by a variety of mechanisms often including both accretionary and erosional processes. Progradation of the plain occurs through the addition of beach ridges, dune ridges, or the special case of a chenier. Beach ridges consist of sand, shell, and/or gravel and are a product of wave swash processes. Beach ridges may be topped by a thin deposit of aeolian sediment but the most of the sand comprising the ridge is wave-generated. Dune ridges have a greater elevation than beach ridges and are essentially a frontal dune ridge (foredune ridge) that becomes displaced from the beach due to beach accretion and new frontal dune construction (HESP et al, 2005). Observations by CURRAY (1969) indicate that along the Nayarit strandplain ridge progradation occurs by the growth and emergence of longshore bars during constructional wave activity. At other sites ridge formation has been related to storm erosion of the lower beach and deposition by wave swash along the upper beach (Tabascan strandplain, Mexico; PSUTY, 1966). Cheniers are a special type of ridge that are composed chiefly of sand and shell and are underlain and separated from adjacent ridges by mud or marshy deposits. Their formation has been related to the periodic deposition of fine and then coarser-grained sediment along the coast. An alternate explanation of chenier development invokes storms and elevated wave energy, which winnows the muddy shore and concentrates sand and shells forming a beach deposit (OTVOS and PRICE, 1979).

RESULTS AND DISCUSSION General Trends Data collected from Navegantes, Tijucas, and Pinheira strandplains indicate that considerable variability exists in the sedimentology, stratigraphy, and facies architecture among these plains, resulting from differences in wave exposure, the origin, volume and type of sediment supply, and antecedent topography of the coastal basin. The Tijucas Plain contains extensive mud deposits in the form of cheniers, overbank deposits, mud tidal flats, whereas mud is much less abundant on Navegantes and Pinheira Plains and primarily forms a surface cover (< 1.2 m thick) between some of the ridges. GPR records reveal that all plains exhibit truncated reflectors suggesting that erosional events have contributed to the morphology of the ridges and have influenced sedimentation patterns and the resulting stratification. The angle of the clinoforms varies among plains and appears to be related to grain size and wave energy. The greater apparent dip of the Pinheira versus Navegantes Plains may be due to the more protected nature and lower wave environment of the Pinheira Plain as compared to the exposed Navegantes Plain.

Navegantes The Navegantes Plain formed north of the Itajaí River and extends 2 to 8 km inland. It is the largest of the three plains, which is likely a consequence of its proximity to one of the highest sediment river discharges in Santa Catarina. The plain is composed of fine-grained sediment and is directly exposed to open-ocean waves, excepting the northernmost region that is protected by a large bedrock promontory. The plain exhibits mostly monotonous, shallow seaward-dipping strata representing shoreface accretion. Except for regions within 0.5 km of the coast, well-developed dunes are absent. Sandy ridges are occasionally interrupted by muddy swales, 10 - 30 m in width. The mud deposits comprising swale regions rarely exceed 1.5 m in depth. The landward portion of these ridges is composed of shallow, landward dipping strata that grade seaward into flat-lying then seaward-dipping units. The seaward-dipping layers that comprise most of the plain are occasionally truncated by more steeply dipping surfaces. In GPR records they are represented by sharp reflectors, which coincide with concentrations of heavy minerals that were formed during storms. Preliminary results of radiocarbon dating of basal wetland organics suggest coastal progradation on the order of 1 m/year during the past 1,300 – 1,500 years (BUYNEVICH et al, 2005a).

Tijucas The Tijucas Plain extends 5 km inland abutting a laterally discontinuous 4-m high sand ridge that is tentatively interpreted as the Stage 5e Pleistocene shoreline (Figure 3). Seaward of this feature the Holocene highstand ridge is recognized by landward dipping strata overlying mud, which are interpreted to represent overwash deposition building into lagoon during the transgressive phase of landward barrier migration (Figure 4). As seen in the GPR section the landward dipping reflectors are truncated by steep seaward-dipping beds that extend uninterruptedly for about 700 m. Further seaward, mud deposition gradually becomes an important component of the plain in the form of chenier development. In the middle of the plain individual ridges are composed of shallow dipping layers along the backside of the ridge that transitions to a mound like composite of strata in a seaward direction. These units are often truncated by steep seaward-dipping prograding clinoforms. The overall thickness of the strandplain sand lithosome thins seaward and may be related to decreasing accommodation space.

Table 1: Comparison of paleo-shoreface reflectors with the modern beach Location

Grain rize of ridges

Apparent dip angle of ridge reflectors

Slope of modern shoreface

Navegantes Plain

Fine to medium fine sand

1.5–2.5o

1.5–2.5o

Tijucas Plain

Medium to coarse sand

9.0-10.0o

5.0–6.0o

Pinheira Plain

Fine sand

4.0-5.0o

3-4o

Fig. 5 River

Fig. 4

Figure 3. Vertical aerial view of the Tijucas strandplain. Note the Tijucas River and protected nature of the Tijucas Embayment. Locations of GPR profiles shown in Figures 4 and 5.


Strandplain Evolution in Brazil

Figure 4. Ground-penetrating radar section of the Holocene highstand barrier. Sediment core was taken through the overwash sequence. Note the landward dipping reflectors indicative of washover sands deposited over lagoonal muds. The present coast is dominated by mud, producing a widely spaced chenier system that extends 2.0 km inland (Figure 5). Recent radiocarbon dates from shells that underlie one of the landward most chenier ridges indicate that the transition from sand to mud-dominated deposition took place sometime prior 1 ka (BUYNEVICH et al, 2005b). The reason for this alteration in the sedimentation regime within Tijucas Bay is being studied and is likely related to changes in the fluvial bedload/suspended load ratio caused by climate change, which in turn produced modifications in vegetation patterns, bedrock weathering and soil formation processes, and ultimately sediment contribution in the drainage basin. Shoaling of the basin, a decrease in wave energy, and reduction in accommodation space due to falling sea level may have also contributed to the present muddy coast.

Pinheira The 4-km wide Pinheira Plain is a tight semicircular strandplain sequence that has developed in the partial shadow of Ilha de Santa Catarina and in the protection of fronting islands. A small stream discharges sediment northwest of the plain, but the

arcuate formation and orientation of the ridges appear to be a product of refracted ocean waves and not riverine processes. Pinheira ridges are closely spaced and monotonous in their progradational style as seen in vertical aerial photographs and shore-normal GPR transects. Ridge crests are 1 to 2 m in height excepting a discontinuous dune ridge in the rear of the plain that is 3 to 4 m in elevation. Individual ridges are separated from each other by shallow swales containing mud deposits less than a meter in thickness (Figure 6). Individual ridges are composed of three units: 1) bottom unit is the most uniform and consists of shallow seaward-dipping layers that become nearly horizontal with depth; 2) middle unit that exhibits highly variable strata including shallow to steeply seaward-dipping layers, shallow landward dipping layers, and relatively thin steeply landward dipping beds (thickness < 0.5 m). The presence of ridges and runnels on the modern beach suggests that landward migrating bars formed the landward dipping bed; 3) The uppermost unit consists of beach and incipient dune facies that contain a variety of bed orientations. The overall geometry of the upper two units is mushroom-shaped in cross-section with a preponderance of seaward-dipping strata.

Figure 5. Subsurface image from the younger part of the plain reveals a different, mud-dominated regime, with sand deposited in narrow, low ridges (TJA – Eijkelkamp auger core; TJV – vibracore) (from BUYNEVICH et al., 2005).


FitzGerald et al.

Figure 6. Subsurface image from Pinheira strandplain demonstrating three distinct sedimentological units.

CONCLUSIONS A forced regression and an abundant sediment supply have produced expansive strandplains that have smoothed the irregular Santa Catarina coast. These plains consist primarily of beach ridges, but cheniers and dune ridges are also present. Differences in the stratigraphy and ridge structure within, as well as among the plains are associated with the composition and proximity of the sediment sources, exposure to wave energy, and the geological framework. For example, Pinheira and Navegantes strandplains contain less mud than the Tijucas system due to distal location of rivers and the greater exposure of wave energy, respectively. Changes in the sedimentation patterns at individual plains, such as the transformation from a sandy upper plain to a muddy lower plain at Tijucas are related to alterations in sediment content and volumes discharge by nearby rivers and the filling of basin lessening accommodation space. Subsurface profiles demonstrate that the plains are characterized by seaward-dipping reflectors, representing beach and shoreface accretion, that are occasionally truncated by steep erosional surfaces. Initial chronology indicates that reflectors are formed at an average annual rate.

ACKNOWLEDGMENTS The research was supported in part by the Andrew W. Mellon Foundation Endowed Fund for Innovative Research and CNPq Fellowships. This is a contribution to the IGCP 495 Project. We greatly thank the field assistance of Fernando Veiga, Rafael Petermann, and Glaucio Vintím.

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CLEARY, W. J., SMITH, M.S., FITZGERALD, D. M., DOUGHTY, S. D., SILVA, G. M. DA, and KLEIN, A. H. DA F., 2005, Provenance of beach sands along southern Brazilian strandplains: Santa Catarina, Brazil, GSA Abs. with Programs Vol. 37, p. 35. CURRAY, J.R., EMMEL, F.J., and CRAMPTON, P.J.S., 1969. Holocene history of a strand plain, lagoonal coast, Nayarit, Mexico. In: Coastal Lagoons, a Symposium UNAM-UNESCO, Mexico, 1967 (C. A. Ayala, F. B. Phleger, eds), pp. 63–100. DOMINGUEZ, J.M., MARTIN, L. and BITTENCOURT, A.C.S.P., 1987. Sea-level history and Quaternary evolution of river mouthassociated beach-ridge plains along the East-Southeast Brazilian coast; a summary. In: Nummedal, D., Pilkey, O.H., Howard, J. D. (eds.), Sea-level fluctuation and coastal evolution, SEPM Special Publication No.41, 115-127. FITZGERALD, D.M., CLEARY, W.J., BUYNEVICH, I.V., HEIN, C., KLEIN, A.H.F., ASP, N.E., and ANGULO, R.J., 2006. Variability of strandplain development in Santa Catarina, Brazil. Quaternary Land-Ocean Interactions: Driving Mechanisms and Coastal Responses, IGCP-495, Santa Catarina, Brazil. HESP, P.A., DILLENBURG, S.G., BARBOZA, E.G., TOMAZELLI, L.J., AYUP-ZOUAIN, R.N., ESTEVES, L.S., GRUBER, N.L.S., TOLDO, E.E., Jr., de A. TABAJARA, L.L.C. and CLEROT, L.C.P., 2005. Beach ridges, foredunes or transgressive dunefields? Torres to Tramandaí barrier system, Southern Brazil. Annals of the Brazilian Acad. of Sci., 77, 493-508. KLEIN, A.H.F, 2004. Morphodynamics of headland-bay beaches: examples from the coast of Santa Catarina state, Brazil. PhD dissertation, University of Algarve, Portugal, pp. 218 MILNE, G.A., LONG, A.J. and BASSETT, S.E., 2005. Modeling Holocene relative sea-level observations from the Caribbean and South America, Quat. Science Rev., 24, p. 1183-1202. MITROVICA, J.X. and MILNE, G.A., 2002. On the origin of late Holocene sea-level highstands within equatorial ocean basins, Quaternary Science Reviews, 21 (20-22), 2179-2190. MOORE, L.J., JOL, H.M., KRUSE, S., VANDERBURGH, and S., KAMINSKY, G.M., 2004. Annual layers revealed by GPR in the subsurface of a prograding coastal barrier, Southwest Washington, USA, J. of Sediment Research, 74 (5), 690-696. OTVOS, E.G., JR. AND PRICE, W.A., 1979. Problems of chenier genesis and terminology; an overview, Marine Geology, 31 (3-4), 251-263. PSUTY, N.P., 1966. The geomorphology of beach ridges in Tabasco, Mexico; nonr 1575(03), Technical Report - Coastal Studies Institute. Louisiana State University, p. 51. ROY, P.S., COWELL, P.J. and FERLAND, M.A., THOM, B.G., 1994. Wave-dominated coasts. In: Coastal evolution; late Quaternary shoreline morphodynamics, (R.W.G. Carter and C.D. Woodroffe, eds), p. 121-186.
