Evolution of the Holocene Coastal Barrier of Pelotas Basin

Journal
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ofCoastal
CoastalResearch
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SI 64
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ICS2011
ICS2011 (Proceedings)
Poland
ISSN 0749-0208
Evolution of the Holocene Coastal Barrier of Pelotas Basin (Southern
Brazil) - a new approach with GPR data
E.G. Barboza†, M.L.C.C. Rosa†, P.A. Hesp‡,
S.R. Dillenburg†, L.J. Tomazelli† and R.N.
Ayup-Zouain†
† Centro de Estudos de
Geologia Costeira e
Oceânica, Instituto de
Geociências,
Universidade Federal do
Rio Grande do Sul
Caixa Postal 15.001 –
Porto Alegre – RS –
Brasil –
[email protected]
‡ Department of Geography and
Anthropology, Louisiana State
University
227 Howe/Russell Geoscience
Complex, Baton Rouge, LA 708034105, USA
ABSTRACT
Barboza, E. G.; Rosa, M. L.C. C; Dillenburg, S. R.; Hesp P. A.; Tomazelli, L. J. and Ayup-Zouain, R. N. 2011.
Evolution of the Holocene Coastal Barrier of Pelotas Basin (Southern Brazil) - a new approach with GPR data.
Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium),.
Szczecin, Poland, ISSN 0749-0208.
Results from a subsurface study performed in the coastal barrier of the Holocene portion of Pelotas Basin along
coastal sectors showing progradational, aggradational and retrogradational behavior during the Middle and Late
Holocene are presented. The Ground Penetrating Radar (GPR) was used to evaluate subsurface records of the
barrier with the aim of defining barrier behavior during a small sea level fall. In sectors of coastal plain where
strandplains (regressive barriers) were identified, GPR subsurface records are characterized by oblique reflectors
dipping basinward. In sectors where the barrier dunefields are transgressing backbarrier terrains the stacking
pattern is retrogradational, which is evidenced by oblique reflectors dipping toward the continent. Between the
progradational and retrogradational sectors, GPR records are characterized by basal oblique reflectors dipping
landward, while the top reflectors are parallel and subparallel. This distinct basal and top pattern corresponds,
respectively, to transgressive and stationary (aggradational) phases of barrier evolution. These results indicate
that the shoreline of the emerged portion of Pelotas Basin has experienced transgressive, regressive and
aggradational behavior along 1,000 km of coastline during the Middle and Late Holocene.
ADDITIONAL INDEX WORDS: stratigraphy, coastal evolution, Barrier-lagoon.
INTRODUCTION
Worldwide, GPR have been used in recent times in the study of
coastal depositional systems. High quality GPR data allow the
visualization of depositional geometry and stratigraphy of coastal
deposits. Good examples are found in Botha et al. (2003),
Havholm et al. (2003), Moller and Anthony (2003), Bristow et al.
(2007), Johnston, et al. (2007), Barboza et al. (2009), FitzGerald
et al. (2007) and Fracalossi et al. (2010).
Recent GPR data were obtained in many sectors of the
Holocene coastal barrier of southern Brazil, which is part of the
emerged portion of the Pelotas Basin. The GPR data revealed that
many sectors of the barrier have subsurface depositional
geometries that indicate the alternation of regressive and
transgressive behavior of the coastline, as previously detected
locally by Barboza et al. (2009), Barboza et al. (2010), Silva et al.
(2010) and Caron et al. (2010).
This study presents results from a subsurface study performed
in the coastal barrier of the Holocene portion of Pelotas Basin,
along coastal sectors showing progradational, aggradational and
retrogradational behavior during the Middle and Late Holocene.
According to Dillenburg et al. (2000) this contrasting coastal
behavior was a consequence of long-term variations in sediment
budget along the coast. Progradational sectors were named
prograded transgressive dunefield barriers by Hesp et al. (2005).
They are essentially regressive barriers, that during progradation
were covered by aeolian deposits in the form of transgressive
dunefields. These dunefields display high to low precipitation
ridges along the landward margins of each dunefield phase
(Dillenburg et al., 2005, 2006 and 2009; Hesp et al., 2005 and
2007; Martinho et al., 2008). Retrogradational sectors are marked
by lagoonal muds outcropping at the backshore/foreshore zone
(Tomazelli et al., 1998; Dillenburg et al., 2004; Travessas et al.,
2005).
Journal of Coastal Research, Special Issue 64, 2011
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Evolution of the Holocene Coastal Barrier of Pelotas Basin (Southern Brazil) - a new approach with GPR data
REGIONAL SETTING
The Pelotas basin has a superficial expression of 210.000 km2,
and is located in the southernmost part of the Brazilian
Continental Margin, which is a rifted plate boundary formed in
Early Cretaceous times. To the North it is limited by the
Florianópolis High and to the South by the Polônio High at
Uruguay margin (Gamboa and Rabonowitz, 1981; Urien and
Martins, 1978) (Fig. 1). The Holocene portion of the Basin is
represented by a 1,000 km long coastal barrier system that is
segmented by very few inlets and small rivers that presently bring
little sediment to the coast. According to Villwock and Tomazelli
(1995), this barrier was formed during the last glacio-eustatic
transgressive cycle started at around 18 ka. Climate is humid
temperate with generally warm to hot temperatures in summer and
cool temperatures in winter. Rainfall ranges from 1,000 to 1,500
mm and is evenly distributed throughout the year (Hoffmann et
al., 1992). The average significant wave height is 1.5 m, but
during storms, sea level can surge up to 1.3 m (Barletta and
Calliari, 2001; Calliari et al., 1998). The coast is microtidal with
semidiurnal tides that have a mean range of only 0.5 m
(Dillenburg et al., 2009). The maximum sea level of the
Postglacial Marine Transgression (PMT) reached +2 – 3 meters at
around 6 – 5 cal ka, subsequently followed by a slow sea level fall
(Angulo et al., 2006; Barboza and Tomazelli, 2003).
Figure 1. Landsat 7 satellite image (ETM+ sensor, Band 2 - 130° of inclination) with location of Pelotas basin coastal plain. In this image
are observed coastal projections, between gentle coastal embayments and littoral drift. The letters (A, B, C, D and E) indicate GPR
profiles (modified from Rosa, 2010).
METHODS
The GPR records were acquired over roads along cross shore
profiles. A SIR-3000 data acquisition system of GSSI™
(Geophysical Survey Systems, Inc.) with 200 MHz antenna
(recording up to 12 m depth), and a 70 MHz antenna of Radarteam
Sweden AB (recording up to 20 m depth) were used. The GPR
system was connected to a Differential Global Positioning System
(DGPS), allowing a real time topographic survey. At the time of
data acquisition, noise and gain filters were applied. A dielectric
constant for sand (10) was used, representing a velocity of 0.09
m/ns (Davis and Annan, 1989). This constant was validated by
lithological data obtained from drillings performed at all coastal
sectors. The Common Off-set array was used. The field records of
GPR were processed and interpreted through the software
RADAN™ 6.6 and Reflex-Win®.
Interpretation of GPR data followed the seismostratigraphy
method based on termination (onlap, downlap, toplap and
truncations), geometry and pattern of reflectors (Mitchum Jr. et
al., 1977; Vail, 1987).
RESULTS AND DISCUSSION
The transgressive or retrogradational sectors in general occur in
coastal projections (prominent areas), between gentle coastal
embayments. Large transgressive dunefields cover the whole
barrier in this area. The largest dunefields are found at the
northeast end of the transgressive sectors, in response to the
dominant northward littoral drift (Tomazelli and Villwock, 1992;
Toldo Jr. et al., 2006). Some authors suggest that transgressive
sectors are the source of sands that form transgressive dunefields
(Semeniuk and Meagher, 1981; Dillenburg et al., 2009). As
previously noted, lagoonal muds are outcropping along the
backshore/foreshore zone of these transgressive sectors. 14C dating
of these muds gave ages of 6,551 cal yrs BP at Jardim do Éden
beach (Travessas, 2003; Travessas et al., 2005), 3,220 and 3,370
cal yrs BP at Bujuru (Dillenburg et al., 2004). This is the most
important evidence of a transgressive process operating in these
sectors in a long-term.
In sectors where the barrier dunefields are transgressing
backbarrier terrains, and where lagoonal mud deposits are
outcropping at the backshore/foreshore zone, the stacking pattern
of GPR records is retrogradational. This pattern is evidenced by
oblique reflectors dipping toward the continent (Fig. 2A). The
forming mechanism of this transgressive process comprises the
aeolian sand erosion and transport from the backshore/foreshore
zone into the lagoonal inter-barrier depression (Dillenburg et al.,
2004; Caron et al., 2010). The final transport of the transgressive
sands into the lagoonal body is produced directly by wind
transport, with the slip-face of dunes advancing into the lagoon, or
by ephemeral washouts building deltas in the lagoonal margin
(L.J. Tomazelli personal communication). Both processes result in
landward progradation of the lagoonal margin. The GPR records
Journal of Coastal Research, Special Issue 64, 2011
647
Barboza et al.
Figure 2. A) Retrogradational sector in coastal projection, profile was acquired with a frequency of 200 MHz. B) Progradational sector
in coastal embayment, profile was acquired with a frequency of 70 MHz. C) Aggradational/ stationary between projected and
embayed coastal sectors, profile was acquired with a frequency of 200 MHz. D) Progradational sector in coastal embayment, profile
was acquired with a frequency of 200 MHz.
of such progradation are represented by medium to high angle
reflectors dipping landwards. This ongoing transgressive process
indicates that in some sectors of the Pelotas basin the Holocene
transgressive maximum has not yet been reached (Rosa, 2010;
Rosa et al., this volume). This implies that at the maximum sea-
level of the PMT (6 – 5 cal ka) the barriers of such sectors were
located some distance seawards.
The regressive/progradational sectors are dominated by the
morphology of foredune ridges and some transgressive dunefields
(Dillenburg et al., 2006 and 2009; Hesp et al., 2005 and 2007).
Journal of Coastal Research, Special Issue 64, 2011
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Evolution of the Holocene Coastal Barrier of Pelotas Basin (Southern Brazil) - a new approach with GPR data
These sectors are found along the gentle embayments of the coast.
Their general GPR records are characterized by oblique reflectors
dipping seawards (Fig. 2B and D). This pattern may be combined
with downstepping and aggradational components (Barboza et al.,
2009 and 2010; Silva, 2010). Recent GPR data has revealed
records of the transgressive phase of these sectors. Deposits of this
phase were sampled and dated by 14C at 8,320 – 8,070 cal yrs BP
(Silva, 2011) (Fig. 3).
The stationary (aggradational) sectors show GPR reflectors with
a classical piling up aggradational pattern, sometimes exhibiting a
faint progradational record. These sectors occur in the transition
between transgressive (projected) and regressive (embayed)
coastal sectors. The GPR records are characterized by basal
oblique reflectors dipping landward, while the top reflectors are
parallel and subparallel. These distinct basal and top patterns
correspond respectively to transgressive and stationary
(aggradational) phases of barrier evolution (Fig. 2C).
Figure 3. Retrogradational/Progradational sector in coastal embayment, profile was acquired with a frequency of 200 MHz. The
transgressive deposits of this phase were sampled and dated by 14C at 8,320 – 8,070 cal yrs BP (Silva, 2011).
CONCLUSION
The GPR records obtained for many coastal sectors of the
Holocene barrier (part of the emerged portion of the Pelotas
basin), along a stretch of coast of 1,000 km clearly show that the
barrier shoreline has behaved differently along the coast. In gentle
coastal projections and embayments, the shoreline was dominantly
transgressive and regressive, respectively, during the Middle and
Late Holocene. At the transition between projections and
embayments the barrier was predominantly stationary.
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ACKNOWLEDGEMENT
This research was funded by a grant from CNPq (472380/20079) and (454804/2008-3). The authors Barboza, Dillenburg and
Tomazelli thank CNPq for their research grants and Rosa thanks
CNPq for her PhD scholarship. Patrick Hesp was supported by
CECO/UFRGS and LSU Dept of Geography and Anthropology.
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