Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial III, 1439-1443
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Application of microtextural and heavy mineral analysis in the
study of onshore tsunami deposits – examples from Portugal,
Scotland and Indonesia
Aplicação de análise microtextural e de minerais pesados ao
estudo de tsunamis onshore – exemplos de Portugal, Escócia e
Indonésia
P. J. M. Costa1*, C. Freire de Andrade1, M. C. Freitas1, M. A. Oliveira1, J. Cascalho2
Artigo Curto
Short Article
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The sedimentological record provides a database useful to
characterize and evaluate recurrence of tsunamis and storms, which
contributes to assess the vulnerability of coastal areas to these natural
hazards. Thus, the enhancement of our ability to recognize specific
signatures of tsunami and storms in coastal sediments is of
unquestionable interest.
The aim of this study is to discuss and further contribute to the
improvement of the characterization of tsunami deposits and their
source materials, based upon microtextural analysis and study of
heavy mineral assemblages. Results obtained from a wide range of
locations, coastal contexts and ages are summarized and discussed in
order to establish patterns of sedimentation of onshore tsunamis
deposits.
In terms of microtextural signatures, an increase in the percentage of
percussion marks and fresh surfaces is observed in tsunami-deposited
grains in comparison with potential source materials. Thus, this
suggests at least partial resurfacing of grains during transport despite
the short distance and brief duration. The characterization of heavy
mineral assemblages contributed to the establishment of likely source
materials and provided invaluable insights in the identification of
inundation and backwash induced sedimentation.
Results of this study are, to some extent, site-specific and this
prevents blind extrapolations to other locations worldwide. However,
as long as the regional context is sufficiently constrained, the
combined application of proxies such as microtextural features and
heavy mineral assemblages constitutes a powerful sedimentological
tool to recognize and differentiate tsunami deposits and determine
source-deposit relations.
Keywords: Sedimentology, Tsunami, Texture, Quartz microtextures,
Heavy minerals.
Resumo: O registo geológico fornece uma crucial base de dados
para a caracterização de tsunamis e tempestades, contribuindo para
uma correcta determinação da vulnerabilidade das zonas costeiras a
este tipo de riscos naturais. Portanto, é de inquestionável importância
a melhoria da capacidade de reconhecer assinaturas específicas de
tsunamis e tempestades em ambientes costeiros.
Este trabalho tem como objectivo fundamental contribuir para a
melhoria da caracterização de depósitos de tsunami e das suas
prováveis fontes sedimentares através da aplicação de análise
microtextural e de composição em minerais pesados. Resultados
obtidos a partir do estudo de uma variedade de locais e de eventos
tsunamigénicos de diferentes cronologias e contextos costeiros são
apresentados e discutidos como contributo para o estabelecimento de
padrões de sedimentação de depósitos de tsunami.
Resultados da análise microtextural revelaram que as amostras de
tsunami exibiram superfícies dos grãos de quartzo com uma presença
mais destacada de marcas de percussão e superfícies recentes,
quando comparada com amostras fonte. Este resultado sugere que
ocorre uma recaracterização da superfície dos grãos durante um
evento tsunamigénico, apesar da sua curta duração e da diminuta
distância percorrida pelos sedimentos.
A caracterização das populações de minerais pesados das amostras
estudadas contribuiu para o estabelecimento de amostras fonte e
forneceu indicações quanto às características sedimentares associadas
à inundação e ao retorno das ondas tsunamigénicas.
Resultados globais deste trabalho sugerem que características locais
geomorfológicas e sedimentares podem limitar extrapolações
acríticas de aplicação generalizada a todo o mundo. Contudo, desde
que o contexto regional seja devidamente considerado, a aplicação
conjunta de análise microtextural e de minerais pesados constitui
uma poderosa ferramenta sedimentológica para o reconhecimento e
diferenciação de depósitos de tsunami, bem como para a
determinação de relações fonte sedimentar- depósito.
Palavras-chave: Sedimentologia, Tsunami, Textura, Microtexturas
em quartzo, Minerais pesados.
1
Centro de Geologia, Departamento de Geologia, Faculdade de Ciências,
Universidade de Lisboa, Edifício C6, Campo Grande, 1749-016 Lisboa,
Portugal
2
Museu Nacional de História Natural e da Ciência (MUHNAC-UL), Rua da
Escola Politécnica 56/58, 1250-102, Lisboa, Portugal.
*
Corresponding author / Autor correspondente: [email protected]
1. Introduction and aims
This work reviews and discusses contributions provided by
recent developments in the study of tsunami deposits based
on microtextural features and heavy mineral assemblages.
The earliest studies using the geological record in
detecting prehistoric tsunamis were conducted by Atwater
(1987) and Dawson et al. (1988). Since then many papers
have been published discussing sedimentological features
of tsunami deposits and their association with transport
and depositional processes. In particular, the study of
modern, well-preserved deposits, carried out during post
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tsunami surveys provided the opportunity to refine
palaeotsunami diagnostic criteria. During the last two
decades, several authors (e.g. Dawson & Stewart, 2007;
Morton et al., 2007; Chagué-Goff et al., 2011) have
postulated criteria to distinguish (palaeo) tsunami deposits.
In contrast to modern tsunami, for which eyewitness
accounts and field measurements of both erosional and
depositional effects are utilized in modelling studies,
(palaeo) tsunami recognition requires ancient tsunamiborne sedimentary imprints to be identified. This is
frequently restricted by poor preservation in the
stratigraphic record. In many cases, the erosional
characteristics of the event, the absence of lithological and
textural contrasts between exotic and local sediments,
anthropogenic and natural post-depositional disturbances
eliminate or make palaeotsunami deposits difficult to
identify; as a result, it is often difficult to state the return
intervals of such events.
In considering the application of microtextural analysis
and heavy mineral assemblages in reconstructing marine
inundation deposits, this work focus on sediments from
diverse geographical locations related to events of
different ages (Portugal - AD 1755; Scotland - 8200 yrs cal
BP Storegga Slide; Indonesia - 26th December 2004),
different coastline configurations and contrasting
oceanographic conditions (Fig. 1). The use of such a
multiplicity of contexts provides an unique sustenance for
conclusions to be drawn regarding the characterization and
differentiation of sedimentary signatures related with
tsunami events.
In each location, the methodology used relies on
comparison between features of tsunami deposits and of
possible source materials extending from the offshore to the
backshore, dune and alluvial environments, which were
sampled, analysed and compared. Results (after analysing a
dataset presently over 4500 grains) suggest that at least five
microtextural features (Fig. 2) and more than 15 grains per
sample should be used for microtextural studies.
the source. The frequency of resurfacing decreases with
distance from the coast. In the case of Voe of Scatsta (Fig.
1) the long post-event burial period favoured surface
dissolution but a considerable number of fresh surfaces
could still be identified. Similarly, in Scottish and
Indonesian grains fresh surfaces dominated and increased
in tsunami grains when compared to source materials.
2. Microtextural features
Results from previous attempts to characterise the surface
of quartz grains reworked in distinct sedimentary
environments under Scanning Electron Microscope (SEM)
have been controversial in the identification of features,
interpretation of imprinting mechanisms or processes and
association of specific microtextural signatures with
specific sedimentary environments (e.g. Krinsley &
Margolis, 1969; Mahaney, 2002; Costa et al., 2009). SEM
imagery of quartz grains has also been used to interpret the
source material of tsunami deposits (e.g. Bruzzi & Prone,
2000; Costa et al., 2009, 2012).
Tsunami samples from Salgados and Boca do Rio (Fig.
1) show larger numbers of fresh surfaces when compared
with potential source materials (inshore and offshore,
beach, dune and alluvial) and show extremely abundant
percussion marks, in some cases leading to near-total
resurfacing of grains. Despite surface reworking, grains
tend to maintain some microtextural characteristics from
Fig. 1. Location map of studied areas of Salgados, Boca do Rio, Voe of
Scatsta and Lhok Nga.
Fig. 1. Mapa de localização das áreas de estudo (Salgados, Boca do
Rio, Voe of Scatsta and Lhok Nga).
Thus, regardless the type of mechanical imprints these
features are always more frequent in tsunamigenic grains
suggesting that despite the short travel distance and brief
duration of the inundation event grain surface reworking
occurs. The discrepancy in the specific microtextural
signature between sites is attributed to the presence of
dunes (in the Portuguese cases) which provided an extra
sedimentary source. The localized incorporation of a
massive amount of dune sand increased the sediment
concentration in the water column, likely causing a boost
in the frequency of impacts between grains, although with
less kinetic energy involved in the impacts because grains
Microtextural and heavy min analysis in tsunamis
would have less space to gain momentum before the next
collision.
This in agreement with the model proposed by Costa et
al. (2012) (Fig. 3) associating specific microtextural
signatures with the dominant process responsible for grain
reworking in different coastal sedimentary environments
and attempting to explain microtextural resurfacing
produced during tsunami events (Fig. 3B). It is suggested
that grain flow transport dominates in the slip face of the
coastal dune and that sediment concentration in the water
column is a relevant variable in controlling which
microtexture prevails along the transport associated with
tsunami inundation.
A tsunami-transported grain
population should thus reflect both the sedimentary
environments and materials crossed by tsunami waves
landward of the closure depth of the coastal zone and the
imprints carved during that event. The dominance of
contemporaneous or pre-event signal does not only depend
on the duration and energy of the event but also on the
sediment concentration that controls the effectiveness of
inter-grain collision.
Fig. 2. Examples of microtextural features. A0 to A5 – roundness. B1 –
percussion marks. B2 – detail of percussion mark. C1 – fresh surface.
C2 – detail of fresh surface. D – dissolution (visible on the right face of
the grain). E – adhering particles (visible in the centre of the grain).
Adapted from Costa et al., 2012.
Fig. 2. Exemplos de características microtexturais. A0-A5- rolamento.
B1 – marcas de percussão. B2 – detalhe de marca de percussão. C1 –
superfície recente. C2 – detalhe de superfície recente. D – dissolução
(visível na face direita do grão). E – partículas aderentes (visível na
zona central do grão). Adaptado de Costa et al., 2012.
3. Heavy mineral assemblages
The use of heavy minerals (HM) to establish provenance
of tsunamigenic sediments has been investigated by
several authors (e.g. Morton et al., 2007; Jagodzinski et
al., 2009; Costa, 2012; Jagodzinski et al., 2012; Nakamura
et al., 2012). Generally, local specificities in the heavy
mineral assemblages constrain extrapolation of inferences
between field sites and, so far, the only feature generally
observed in all locations is a higher heavy mineral content
in either the base of the tsunamigenic deposit or, when
they are stratigraphically complex, in the base of each
depositional sub-unit; and yet, the reason explaining this
concentration is still not well understood.
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Fig. 3. Conceptual transport model for sedimentary environments and
high energy events based in microtextural features of quartz grains. A –
Sedimentary environments and associated dominant microtextures. B –
Grain transport during a tsunami wave incursion; from Costa et al.,
2012.
Fig. 3. Modelo conceptual de características microtexturais em grãos de
quartzo provocadas por transporte em eventos de alta energia e típicas
de cada ambiente sedimentar. A – Associação dominante de
microtexuras em cada ambiente sedimentar. B – Transporte de grão
durante uma inundação por onda de tsunami, de Costa et al., 2012.
The results obtained in the areas studied by the authors
are in agreement with those cited above. Samples from
Salgados and Voe of Scatsta present a clear pattern of
decreasing percentage of heavy minerals upwards in the
massive\structureless tsunami unit, in agreement with
normal grading; this may indicate a decrease in energy
associated with the event(s) responsible for the
tsunamigenic deposition. The laminated deposit of Lhok
Nga shows upward decrease in heavy mineral
concentration at the scale of each sub-unit.
In Salgados and Boca do Rio the heavy mineral
assemblages are dominated (>90%) by andalusite>
tourmaline>staurolite>garnet>zircon, whereas in Lhok
Nga, about 90% of the heavy mineral population is
composed of amphiboles>andalusite, with minor
contributions of garnet and zircon; in Voe of Scatsta the
heavy minerals are almost exclusively (ca. 90%) made of
amphiboles. In Lhok Nga tsunamigenic sediment, the
horizontal and vertical variation of heavy minerals was
essentially reflected by the proportion of each HM species
in the assemblage rather than in major compositional
differences. Higher values for zircon and garnet were
found in samples from the backwash sediment and in the
most inland sample, suggesting that the source of those
minerals was situated farther inland. Furthermore, this
yields a hint to separate run-in and backwash sourced
sediment.
In the cases of Salgados and Boca do Rio, principal
component analysis shows that the first 2 components
explain more than 2/3 of the total variance in heavy
mineral assemblages (Fig. 4); tsunami samples share fewer
similarities with inshore and offshore materials and more
resemblances with dune and, to a less extent, beach
sediments, thus indicating dune and beach as the more
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P. J. M. Costa et al. / Comunicações Geológicas (2014) 101, Especial III, 1439-1443
likely sources. In these two locations, results of HM
studies allowed discriminating tsunami sediments from all
possible source materials, the former generally showing
higher proportions of heavy minerals in total sediment and
selective concentration of staurolite (densest transparent
HM identified in the assemblage though not the most
abundant).
Interpretation of the results from Boca do Rio (dune
and beach as the main source of tsunamigenic sediment) is
in agreement with inferences taken from the historical
record and combined geomorphological and textural
analysis (Oliveira, 2009). Moreover, results suggest that a
HM signal (zircon and garnet) from the backwash has been
preserved in the most landward samples, expressed by an
increase in zircon and garnet, although not implying that
the majority of sedimentation is related to backflow, as
argued by Bahlburg & Spiske (2012) and Font et al.
(2013).
Fig. 4. 4A - Principal component analysis of heavy mineral assemblage retrieved from Salgados (SG). 4B - Principal component analysis of heavy
mineral assemblages from samples retrieved in Boca do Rio (BDR). Note: B-beach samples; D-dune samples; N-inshore and offshore samples; Tsu–
tsunami samples.
Fig. 4. 4A - Análise de componentes principais das populações de minerais pesados recolhidas nas amostras dos Salgados (SG). 4B - Análise de
componentes principais das populações de minerais pesados recolhidas nas amostras da Boca do Rio (BDR). Note: B-amostras de praia; D-amostras
de duna; N-amostras de fundos submarinos; Tsu–amostras de tsunami.
4. Conclusive remarks
Results from grain surface microtextural analysis revealed
that this proxy is a valuable complementary
sedimentological technique to be applied in the
discrimination of coastal sedimentary environments and is
relevant in the identification of tsunami deposits, especially
when the regional sedimentological and geomorphological
contexts are considered. Tsunami-transported grains present
more frequent surface reworking by mechanical sculpturing
(percussion marks and fresh surfaces) when compared with
source materials. It is suggested that grain concentration in
the water column is an additional factor controlling the
microtextural imprints of tsunami transported grains.
Heavy mineral assemblages are, in broad terms, sitespecific. In general, the concentration of heavy minerals in
total sediment decreases upward in the tsunami layer. The
study of heavy mineral assemblages contributed to the
establishment of likely source materials and, in some cases,
provided valuable insights in the mineralogical distinction of
deposition promoted by swash versus backwash.
Overall results reveal that site-specific effects preclude
blind extrapolations although they demonstrated that the
combined use of microtextural and compositional data
Microtextural and heavy min analysis in tsunamis
enhances the possibility of using sedimentological criteria to
recognize and differentiate tsunami and, in both cases,
determine source-deposits relations, if the regional context
is sufficiently constrained.
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