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Geomorphological constraining of tsunami (?) run-up in the Alcantarilha coastal lowland (Central Algarve, Portugal)

Authors:
Jorge Dinis at University of Coimbra
Jorge Dinis
César Andrade at IVAR - Instituto de Investigação em Vulcanologia e Avaliação de Riscos
César Andrade
Maria Alexandra Oliveira at University of Lisbon
Maria Alexandra Oliveira
Maria Conceição Freitas at University of Lisbon
Maria Conceição Freitas
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IBERIAN COASTAL HOLOCENE
PALEOENVIRONMENTAL EVOLUTION
C astal
Hope 2010
PROCEEDINGS
Editors
Maria da Conceição Freitas & César Andrade
Lisbon, Portugal, 14-17 June 2010
Faculdade de Ciências, Universidade de Lisboa
IBERIAN COASTAL HOLOCENE PALEOENVIRONMENTAL EVOLUTION – COASTAL HOPE 2010 -PROCEEDINGS
40
Figure 1 - Aerial photo of the Alcantarilha lowland showing the
intlet, barrier and foredunes (countour lines in meters, msl),
washover limit (dotted line) and location of cores and trenches
(green dots). A-B – cross section in figure 2. Arrows -
hypothesized flow path of wave overtopping.
GEOMORPHOLOGICAL CONSTRAINING OF TSUNAMI (?) RUN-UP IN THE
ALCANTARILHA COASTAL LOWLAND (CENTRAL ALGARVE,
PORTUGAL)
J. Dinis1; C. Andrade2; M. A. Oliveira2; M. C. Freitas2; P. P. Cunha1; A. Martins3; P. Costa2
jodinis@dct.uc.pt; candrade@fc.ul.pt; alexandra.oliv@gmail.com; cfreitas@fc.ul.pt; pcunha@dct.uc.pt;
aam@uevora.pt; ppcosta@fc.ul.pt
1Department of Earth Sciences, University of Coimbra & IMAR-CMA, Portugal
2Dep.de Geologia & Centro de Geologia, Faculdade de Ciências da Universidade de Lisboa, Portugal
3Departamento de Geociências & Centro de Geofísica, University of Évora, Portugal
INTRODUCTION
The Alcantarilha lowland, partly barred by a well developed barrier, including foredunes
covering Pleistocene-Holocene beachrock and aeolianite, develops across the Alcantarilha
infilled estuary, the beach-dune extending further SE until the Salgados lagoon. A topographic
and coring survey revealed a peculiar feature at the leeward toe of the dune ridge close to the
inlet area: a sandy fan with location, shape and morphology suggesting emplacement by single
or multiple overwash of the barrier tip rather than tidal forcing. Its storm or tsunami origin and
age are under investigation, and the only time-constrain available at present is that it should
post-date ca. 6600 cal BP, the most recent in situ aeolianite (Moura et al., 2007) dated so far.
METHODS, DATA SET AND RESULTS The fan boundaries are distinctive in
aerial photos and satellite images: it is
roughly ellipsoidal, ~200 m wide and ~300 m
elongated paralleling the shoreline, rising ~
0.9-1.2 m above the surrounding floodplain
surface. Detailed topography shows that its
short axis aligns with SW-NE elongated
(though irregular) depressions in the dune
crest, which link the beach with the fan. This
could have favoured funnelling of, or erosion
by, water overtopping the barrier but, in either
case, the fan should correspond to extreme
and abrupt event(s) of coastal flooding.
18 trenches and cores were
performed in the exposed area of the fan
and nearby flood plain to obtain samples
and data on its sedimentology,
lithostratigraphy and geometry. The fan
consists of well sorted and rounded sand
(Fig. 2). It thins away and wedges out landwards of the apex (located near Alc29T) where it is
partly covered by dune sand. Its lower boundary is undulating and marked by textural contrast
between sand (fan) and underlying mud (alluvial/lagoonal); an accumulation of marine-sourced
perforated pebbles showing limited lateral continuity may pinpoint this boundary near the
foredune (core Alc 25, ca. 80 m westward of profile in Fig.1); mud-balls were also observed
immediately above this surface in cores and trenches. As the washover was probably emplaced
in a barred lagoonal/estuarine floodplain setting, the fan’s northern outer belt is enclosed by
low-energy sediments (not shown in Fig. 2).
DISCUSSION AND CONCLUSIONS
This coast is mesotidal, the spring tide reaching maximum elevation of 1.8 m (msl). The
storm surge is much smaller than 1 m and wave heights exceeding 6 m are exceptional in SW
extreme storms with a return period of 30-50 years. Andrade et al. (2004) used the historical
record and field evidence to show that even the most extreme storms after 1860 failed to
overtop the mature and wide foredune ridges of the Ria Formosa barriers (30-70 km east of the
IBERIAN COASTAL HOLOCENE PALEOENVIRONMENTAL EVOLUTION – COASTAL HOPE 2010 -PROCEEDINGS
41
studied area). A storm origin for this washover seems unlikely because of the exceptional wave
height and run-in required to cut through the dune field and cross-over this wide barrier.
Figure 2 – Cross section A-B (see Fig.1, and text for details). Sea is towards left.
In turn, the AD 1755 tsunami extensively flooded the Alcantarilha lowland with a run-in
exceeding 3 km and Costa et al. (2009) refers a thin sand layer embedded in the late Holocene
sedimentary record of the nearby Salgados lagoon, with characteristics matching diagnostic
criteria of a tsunami origin and partly sourced in the adjacent coastal dunes. This layer was
constrained with 210Pb and 137Cs in agreement with the AD 1755 event. The sand fan in
Alcantarilha shares sedimentological features with tsunami deposits in similar settings
(Komatsubara et al.,2008) and fits the concept of tsunami-scour fan (Goff et al., 2009). We
hypothesize that it could be a laterally equivalent feature of the Salgados layer, resulting from
the 1755 (or other former) tsunami. The morphology of the lowland and barrier indicates that the
Alcantarilha inlet should have concentrated most of the inrushing tsunami flow ab initio,
providing a pathway for massive inundation of the lowland; however, for a sufficiently high run-
up at the coast, part of the incoming wave may have also overtopped low points of the dune
crest. Further sedimentological investigations and OSL datings are being carried out at present
to investigate the hypothesis above.
Notwithstanding the morphology of both the foredunes and lowland favouring the
preservation of washovers (Goff et al., 2009) this fan is unique in the Alcantarilha cell. Its well
defined apex, allows constraining the elevation reached by the free-surface of the ocean just
before inundation within an interval, the minimum corresponding to the height of the higher
feeding channels in the dune, and maximum matching the elevation distribution elsewhere
along the barrier crest. Assuming that the morphologies coeval of the overwash and present-
day are similar, the run-up magnitude at the coast can be estimated in 8-10 m. This figure
agrees with estimates for this region based in modelling (e.g. Gutscher et al., 2006) and
eyewitness reports.
ACKNOWLEDGEMENTS
Contribution of projects GETS (FCT-PTDC/CTE-GEX/65948/2006) and NEAREST (EU-
037110-GOCE-2006). P. Costa and M. Oliveira hold FCT-sponsored PhD grants at GeoFCUL.
REFERENCES
Andrade, C.; Freitas, M.C.; Moreno, J. & Calado, S. (2004) - Stratigraphical evidence of Late Holocene extreme
storms and barrier breaching in lagoonal sediments of Ria Formosa, Algarve, Portugal. Marine Geology, 210,
339-362.
Costa, P.J.M.; Andrade, C.; Freitas, M.C.; Oliveira, M.A. & Jouanneau, J.-M, (2009) - Preliminary results of
exoscopic analysis of quartz grains deposited by a palaeotsunami in Salgados lowland (Algarve, Portugal).
Journal of Coastal Research, SI 46, 39-43.
Goff, J.R.; Lane, E. & Arnold, J. (2009) - The tsunami geomorphology of coastal dunes. Natural Hazards Earth
System Science, 9, 847–854.
Gutscher, M.-A.; Baptista, M.A.& Miranda, J.M. (2006) - The Gibraltar Arc seismogenic zone (part 2): Constraints
on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and
seismic intensity. Tectonophysics, 426, 153–166.
Komatsubara, J.; Fujiwara, O.; Takada, K.; Sawai, Y.; Aung, T.T. & Kamataki, T. (2008) - Historical tsunamis and
storms recorded in a coastal lowland, Shizuoka Prefecture, along the Pacific Coast of Japan. Sedimentology, 55,
1703-1716.
Moura, D.; Veiga-Pires, C. ;Albardeiro, L.; Boski, T.; Rodrigues, A.L. & Tareco, H. (2007) – Holocene sea-level
fluctuations and coastal evolution in the central Algarve (southern Portugal). Marine Geology, 237, 127-142.
... The AD 1755 tsunami inundation and associated deposits have been described in both the Alcantarilha and Salgados lowlands (Lopes, 1841;Costa et al., 2009;2012;Dinis et al., 2010). Lopes (1841, page 290) describes the tsunami as follows: "In…Armação [a 17 th century fishing settlement immediately west of ...
... Trenches were excavated and hand-driven cores obtained in the coastal sector and nearby floodplain to study its sedimentology, lithostratigraphy and geometry. The detailed age-models of the lowland sequences were summarized by Schneider et al. (2010), Dinis et al. (2010) and Costa et al. (2012) and do not include our OSL dates (from the A C C E P T E D M A N U S C R I P T ...
... This facies has a marked marine signature, as shown by its foraminiferal content (see Costa et al., 2012;Quintela et al., 2016). It is essentially composed of medium to fine sands, which contrasts markedly with Facies 6 -Wind-reworked sand -these deposits are confined to the eastern and seaward margin of the cross-section of Alcantarilha (immediately adjacent to the leeward side of the dune) and they formed after the emplacement of a sandy fan that forms a prominent (though spatially restricted) geomorphological feature extending into the alluvial plain (as described in Dinis et al., 2010). It is composed of fine to medium, well sorted, sand and its presence inland is limited to a few hundred meters. ...
How did the AD 1755 tsunami impact on sand barriers across the southern coast of Portugal?
Article
  • Sep 2016
  • GEOMORPHOLOGY
Tsunamis are highly energetic events that may destructively impact the coast. Resolving the degree of coastal resilience to tsunamis is extremely difficult and sometimes impossible. In part, our understanding is constrained by the limited number of contemporaneous examples and by the high dynamism of coastal systems. In fact, long-term changes of coastal systems can mask the evidence of past tsunamis, leaving us a short or incomplete sedimentary archive. Here, we present a multidisciplinary approach involving sedimentological, geomorphological and geophysical analyses and numerical modelling of the AD 1755 tsunami flood on a coastal segment located within the southern coast of Portugal. In particular, the work focuses on deciphering the impact of the tsunami waves over a coastal sand barrier enclosing two lowlands largely inundated by the tsunami flood. Erosional features documented by geophysical data were assigned to the AD 1755 event with support of sedimentological and age estimation results. Furthermore, these features allowed the calibration of the simulation settings to reconstruct the local conditions and establish the run-up range of the AD 1755 tsunami when it hit this coast (6–8 m above mean sea level). Our work highlights the usefulness of erosional imprints preserved in the sediment record to interpret the impact of the extreme events on sand barriers
View
... Nowadays, both lowlands are extensively silted up and their landward margin is being occupied for agriculture and tourist uses. In Alcantarilha lowland, Dinis et al. (2010) refers to a sandy overwash fan attached to the leeward toe of the dune ridge east of the present-day inlet. The fan thins and wedges out inland into the adjacent alluvial plain sediments. ...
... The authors attributed its origin to localized overwash and erosion of the dune tip triggered by the AD 1755 tsunami. The AD 1755 tsunami inundation and associated deposits have been described in Alcantarilha lowland (Lopes, 1841;Dinis et al., 2010). Lopes (1841) describes the tsunami invasion as follows: "In … Armação (Alcantarilha lowland), located in the beach ¼ of league [1 Portuguese terrestrial league ¼ 6172.836 ...
... 1500 cal BP) is organized in 4 facies associations (Fig. 2). The age-model for this sequence is summarized in Dinis et al. (2010) and Schneider et al. (2009). ...
The AD 1755 tsunami deposits onshore and offshore of Algarve (south Portugal): Sediment transport interpretations based on the study of Foraminifera assemblages
Article
  • Jan 2016
  • QUATERN INT
Tsunamis are amongst the most catastrophic natural hazards affecting worldwide coastal areas. Their impacts are recorded in the geological archives and its study provides crucial insights into the dynamics and intensity of these events. Moreover, the accurate establishment of return periods can also be derived using sedimentological data. In fact, onshore tsunami deposits have been extensively described over recent years through the application of a wide range of sedimentological proxies, including micropalaeontological analysis. In this work, the identification of palaeotsunami evidences, in both the onshore and offshore sedimentary record of south Portugal, was performed relying mostly on the study of Foraminifera assemblages. Three onshore trenches and one offshore core were analyzed and imprints of the AD 1755 tsunami were observed based in the identification of allochthonous foraminiferal species, increase in the number of broken/worn-out tests and its correlation with coarser mean grain size. This was possible after micropalaeontological environmental characterization of both low-energy onshore and offshore present-day sedimentation regimes. In the onshore record three distinct sediment transport phases were differentiated within the tsunamigenic unit. The basal phase portraying the initial inundation represented by the dominance of marine Foraminifera species in a highly diverse assemblage. The higher energetic character of this phase was also translated in the identification of the highest number of broken/worn-out tests. The intermediate phase reflects an unspecified number of inundation and backwash pulses that were recognized based in the well-balanced presence of estuarine (brackish) and marine species, the sharp decrease in the diversity index and in the number of broken/worn-out tests. The final phase recorded in the tsunamigenic unit registers the imprint of the last relevant backwash which was identified based in the dominance of estuarine (brackish) fauna. The overlying unit is dominated by tidal marsh species demonstrating the start of the coastal system recovery phase. On the offshore record it was possible to identify 2 distinct historical tsunami events (the AD 1755 and the AD 1969). This was achieved through the identification of significant increases in coastal Foraminifera species coupled with the presence of coarser layers. This interpretation was sustained by indirect age-estimation methods (radiocarbon and 210Pb) that ascribed compatible ages with the historical tsunami events. The results obtained in this study stress the relevance and innovative character of this onshore and offshore multiproxy approach in the recognition and understanding of tsunami events.
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... The geomorphological consequences of tsunamis and storms in coastal dunes or barrier islands have also been addressed (e.g. Andrade, 1990;Andrade et al., 2004;Regnauld et al., 2008;Oliveira et al., 2009;Dinis et al., 2010;Goff et al., 2010b). ...
... Immediately to the west of Salgados, Alcantarilha lowland is located. Within this lowland Dinis et al. (2010) observed geomorphological features that provided support for a major overwash deposit that not only eroded the top part of a sector of the confining dune ridge but was also responsible for the deposition of a fan-shaped sandy layer, which attaches to the toe of the leeward slope of the dune and thins and wedges out further inland into the adjacent alluvial plain sediments. The authors interpreted that the most probable mechanism for these sedimentary evidences should be a tsunami event and 210 Pb and 137 Cs data once more suggests it may date of AD 1755. ...
... Immediately to the east of Salgados, in the Alcantarilha lowland, Dinis et al. (2010) observed geomorphological features extending inland from the lee slope of the coastal dune ridge associated with the deposition of a thining inland sandy layer that extended in the adjacent alluvial plain, and interpreted these features as the result of a tsunami event; due to its chronostratigraphic characteristics it can be correlated with the same event responsible for the deposition of unit E in Lagoa dos Salgados (i.e. the AD 1755 tsunami). ...
Sedimentological signatures of extreme marine inundations
Thesis
Full-text available
  • Jan 2012
http://repositorio.ul.pt/handle/10451/7766
View
... In Alcantarilha lowland, immediately to the west of Salgados, Dinis et al. (2010) refer a sandy overwash fan attached to the leeward toe of the dune ridge east of the inlet. The fan thins and wedges out inland into the adjacent alluvial plain sediments. ...
... In this case, the dune robustness provided a necessary buffer for tsunami wave overtopping and overwash. However, in the western tip of the barrier, where the dune is lower, the emplacement of a washover fan (Fig. 2.3) is attributed to dune overtopping by the AD 1755 tsunami wave (Dinis et al. 2010). Besides this, one interesting postevent geomorphological feature observed in Alcantarilha-Salgados landward strip of the sand barrier, are parabolic dunes (Fig. 2.3). ...
Imprints of the AD 1755 Tsunami in Algarve (South Portugal) Lowlands and Post-impact Recovery
Chapter
  • Apr 2016
The AD 1755 tsunami was the most devastating tsunami that affected Atlantic Europe in historical times. In this work we summarize its sedimentological signatures in lowlands (Martinhal, Barranco, Furnas, Boca do Rio, Salgados-Alcantarilha) of the Algarve coast that contrast in geologic and geomorphological settings and sediment abundance. We found remarkable similarities between tsunami deposits and the materials available for transport at the coast prior to the tsunami. A number of 2–4 m high and 30–70 m wide scarps (i.e. steep slopes in dunes resulting from erosion) are the only erosive geomorphic signature preserved in the study areas (Boca do Rio and Salgados-Alcantarilha). Recovery of the coastal system to pre-event conditions, inferred from the documentary records, is evaluated and analyzed in terms of sediment availability and supply, climate, hydrodynamic regime and geomorphic setting. Sediment-starved pocket beaches bypassed landward most of the sediment previously accumulated in the coastal system, failing to recover the pre-event morphology. Moreover, and solely in Martinhal, the abrupt morphological changes translated in increased and lasting permeability of the barrier and adjacent wetland to storms. The Alcantarilha-Salgados beach-dune system, in moderate sand supply, was extensively scarped but not fully overtopped. Here, recovery was partly achieved by destabilization of the remnant dune, formation and (limited) advance of parabolic dunes following the 1755 event. A schematic conceptual model is presented summarizing: the pre-event conditions; the depositional and geomorphological features directly related with the tsunami impact; and the post-event geomorphological adaptation. In the studied cases post-event recovery seems to be reduced mainly due to a conjugation of pre-event geomorphological setting and low sediment input.
View
... A preliminary analysis of foraminiferal assemblages was undertaken on the November 1 st , 1755 tsunami deposits previously recognized in the Alcantarilha lowland, at the South coast of Algarve (e.g. Dinis et al., 2010) and in shelf core (Silva et al., 2012(Silva et al., , 2013, collected offshore Faro (Fig.1). ...
... Previous geomorphological and sedimentological studies suggest the presence of a tsunami deposit in the Alcantarilha lowland (Dinis et al., 2010), related with the November 1 st , 1755 event, identified in similar conditions along the Algarve coast at Ria Formosa, Boca do Rio, Salgados and Martinhal (e.g. Andrade, 1992;Dawson et al., 1995;Costa et al., 2009Costa et al., , 2012Kortekaas & Dawson, 1997) A detailed analysis of foraminiferal assemblages allowed the separation of 3 different sub--units within an apparently massive sandy tsunami deposit. ...
Complementary approach of foraminiferal analysis to characterize the sedimentary record of tsunamis. An example from November 1st, 1755 deposits in Algarve (South Portugal).
Conference Paper
Full-text available
  • Sep 2015
A study of onshore and offshore tsunamigenic deposits associated to the November 1st, 1755 event was conducted through the assessment of taphonomic features in foraminifera tests and the proportion of juveniles specimens, the diversity index of population and standing crop. Within the tsunamigenic deposit a strong correlation was found between the proportion of eroded tests and the input of marine species in the Alcantarilha lowland (Portugal), this was followed by the increase of juvenile elements towards the top of the deposit. Offshore Faro the presence of allocthonous coastal species in an outer shelf deposit is enhanced by the account of corresponding broken and worn out tests. The preliminary results presented here demonstrate the potential to improve the resolution of tsunami records analysis, contributing to distinguish the dynamics of run-in vs. backwash episodes inside a massive onshore deposit, as well as, the establishment of the offshore extension of the backwash over the continental shelf.
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... Along the Algarve coast, the sedimentary footprint of the AD 1755 tsunami is well preserved (e.g. Andrade et al. 2004;Kortekaas and Dawson 2007;Dinis et al. 2010;Schneider et al. 2010;Costa et al. 2011Costa et al. , 2012aTrog et al. 2015;Quintela et al. 2016), especially at the site of Boca do Rio, where the AD 1755 deposit is sandwiched between finer grained floodplain strata, which has led to numerous studies focusing on various characteristics of the event deposit (e.g. Dawson et al. 1995;Hindson et al. 1996;Oliveira et al. 2009;Cunha et al. 2010;Costa et al. 2012b;Font et al. 2013;Feist et al. 2019). ...
At the Mercy of the Sea—Vulnerability of Roman Coastal Settlements in the Algarve (Portugal). Boca do Rio as an Emblematic Example of a Key Maritime Industry
Chapter
Full-text available
  • Jun 2022
The Roman fish-salting industry in the Western Mediterranean was concentrated in a high-risk geological area as regards extreme wave events. It underwent a significant and sudden decline and reorganisation between the second and third centuries AD. The few explanations that have been hitherto offered for this abrupt transformation range from political and economic disruptions to vague speculations on natural causes. Accordingly, this chapter focuses on determining the possibility of an extreme wave event as the cause behind the restructuring of this industry. For this purpose, the results of 3 years of archaeological and geoscientific field research in Boca do Rio (Vila do Bispo, Algarve) are presented and evaluated. Although far-reaching changes in the building stock of this Roman industrial settlement have been dated to between the second and third centuries AD, and a short series of high energy events has been identified, there is no evidence of the direct influence of a single event (a flood, storm surge, tsunami, etc.) as a trigger for the changes in the settlement and the local Roman economy. Rather, medium-term environmental changes seem to have been the driving force behind them. Additionally, a previously unknown late medieval event layer is described in detail.KeywordsRoman marine industryGarumFish-saltingThird-century crisisTsunamiHigh-energy eventCoastal settlementEnvironmental change
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On the Sedimentological and Historical Evidences of Seismic-Triggered Tsunamis on the Algarve Coast of Portugal
Chapter
  • Oct 2016
The geological record of the 1755 tsunami is widespread along Algarve lowlands but no other unequivocal signature of tsunami inundation was found in onshore sediments deposited since Roman times. This chapter addresses the research undertaken on the documentary and geological evidences of destructive tsunamis in the south-facing Algarve coast of Portugal. It investigates the reasons explaining the discrepancy in information on tsunamis that impacted the coast of Algarve and produced flooding of coastal land when comparing historical and geological records. The chapter brings forward data on the stratigraphy of the eastern Algarve barrier islands that contribute to consolidating a model of tsunamigenic inundation and postevent recovery of those barriers. Tsunamis failing to reach this intensity may be recorded by instruments and widely noticed but their inundation potential is null, thus they leave no noticeable imprint in the onshore geological record of coastal lowlands.
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Preliminary Results of Exoscopic Analysis of Quartz Grains Deposited by a Palaeotsunami in Salgados Lowland (Algarve, Portugal)
Article
Full-text available
  • Jan 2009
  • J COASTAL RES
The recognition of coastal inundations by tsunamis beyond the historical record depends solely on the identification of characteristic ancient deposits and the stratigraphical record can provide crucial information on recurrence and patterns of coastal flooding and/or specificities of a particular event. A palaeotsunami deposit was found in the Salgados lowland (south coast of the Algarve, Portugal) and correspondent to the AD 1755 event was studied using a scanning electron microscope (SEM)-based analysis of quartz grain microtextures (e.g. grooves, precipitation/dissolution, marks, fractures, relief, roundness, craters, upturned plates) to investigate the existence of specific signatures and to compare tsunami-deposited sand particles with grains of possible sources in coastal to nearshore depositional contexts. The preliminary results suggest that in this specific location, the AD 1755 tsunami did not craft an obvious microtextural signature in sand particles. Instead, the grains deposited by this event were essentially characterized by source-inherited features and we suggest that coastal foredunes may have been a relevant source.
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The tsunami geomorphology of coastal dunes
Article
Full-text available
  • May 2009
An examination of the coastal geomorphology of bays along the Otago coastline, SE New Zealand, has identified a geomorphology consistent with tsunami inundation. A tsunami geomorphology consisting of a number of elements including dune pedestals, hummocky topography, parabolic dune systems, and post-tsunami features resulting from changes to the nearshore sediment budget is discussed. The most prominent features at Blueskin Bay are eroded pedestals although it is speculated that hummocky topography may be present in the bay. Tsunami geomorphology at Long Beach is more comprehensive with a marked association between pedestals and a hummocky topography. A full suite of potential geomorphological features however, is not present at either site. The type of features formed by a tsunami, and the ability to detect and interpret a tsunami geomorphology, hinges on the interaction between five key variables; sand availability, embayment type, nature of the coast, accumulation space, and landward environmental conditions. An appreciation of the geomorphic setting and history of a coast is therefore of fundamental importance when identifying what to look for and where to look for tsunami evidence. It is also important to realise that these features can also be formed by other processes.
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Historical tsunamis and storms recorded in a coastal lowland, Shizuoka Prefecture, along the Pacific Coast of Japan
Article
  • Apr 2008
  • SEDIMENTOLOGY
Four sand units deposited by tsunamis and one sand unit deposited by storm surge(s) were identified in a muddy marsh succession in a narrow coastal lowland along the Pacific coast of central Japan. Tsunamis in ad 1498, 1605, 1707 and 1854 that were related to large subduction-zone earthquakes along the Nankai Trough, and storm surges in 1680 and/or 1699 were responsible for the deposition of these sand units. These sand units are distinguished by lithofacies, sedimentary structures, grain-size and mineral composition, and radiocarbon ages; their ages are supported by events in local historical records. The tsunami deposits in the study area are massive or parallel-laminated sands, with associated intraclasts, gravels, draping mud layers and, rarely, a return-flow subunit. The storm surge deposits are devoid of these characteristics, and are composed of groups of thin, current ripple-laminated sand layers. The differences in sedimentary structures between the tsunami and storm surge deposits are attributed to the different characteristics of tsunami and storm waves.
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Stratigraphical evidence of Late Holocene barrier breaching and extreme storms in lagoonal sediments of Ria Formosa, Algarve, Portugal
Article
  • Sep 2004
  • MAR GEOL
Stratigraphical, sedimentological and paleoecological studies undertaken in lagoonal sediments of the Ria Formosa barrier lagoon indicate that a number of present-day aligned beaches outcropping within the marsh and detached from mainland (marsh detached beaches, MDB) represent the remnants of former ephemeral flood deltas, which established following barrier breaching induced by extreme storms from circa 1270 BC onwards. Dating of these beaches suggests that they resulted from multiple events of flooding separated both in time and space. Repeated emplacement of tidal deltas following overwash of the same location of the barrier is represented in the stratigraphical record by buried and partly overlapping marsh beaches. It is suggested that further refinement of the data presented in this paper and the extension of the stratigraphic survey to the entire lagoonal space will substantiate the usefulness of the stratigraphic record as a proxy to hindcast a time series of extreme storminess extending far beyond the instrumental and historical records.
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Holocene sea level fluctuations and coastal evolution in the central Algarve (southern Portugal)
Article
  • Mar 2007
  • MAR GEOL
In Armação de Pêra Bay, southern Portugal, environmental changes during the Holocene can be interpreted based on the morphological and sedimentological similarities between older geomorphic features (cemented beach and dune rocks) and present coastal features. Using knowledge of the present beach and dune processes, we propose a two-step model for the evolution of Armação de Pêra Bay. First, during the rapid sea level rise between about 8 800 and 6 600 yr cal BP, the bay changed from a positive to a negative budget littoral cell and transgressive dunes formed, favoured by drought conditions. At about 5 000 yr cal BP, during a sea level maximum, beach width was less than the critical fetch and dunes stabilized and underwent cementation during the wetter Atlantic climatic event. The second phase of dune accumulation started at about 3 200 yr cal BP, due to a regression of sea level during which the bay changed back to a positive budget littoral cell in which beach width was greater than the critical fetch. Currently, the beach width is less than the critical fetch, dunes are inactive, and the sedimentary budget is negative due to sediment storage in local river systems.
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The Gibraltar Arc seismogenic zone: Part 2. Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity
Article
  • Jan 2006
  • TECTONOPHYSICS
The Great Lisbon earthquake has the largest documented felt area of any shallow earthquake and an estimated magnitude of 8.5–9.0. The associated tsunami ravaged the coast of SW Portugal and the Gulf of Cadiz, with run-up heights reported to have reached 5–15 m. While several source regions offshore SW Portugal have been proposed (e.g.— Gorringe Bank, Marquis de Pombal fault), no single source appears to be able to account for the great seismic moment as well as all the historical tsunami amplitude and travel time observations. A shallow east dipping fault plane beneath the Gulf of Cadiz associated with active subduction beneath Gibraltar, represents a candidate source for the Lisbon earthquake of 1755.
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Historical tsunamis and storms recorded in a coastal lowland
  • Jan 2008
  • 1703-1716
  • J Komatsubara
  • O Fujiwara
  • K Takada
  • Y Sawai
  • T T Aung
  • T Kamataki
Komatsubara, J.; Fujiwara, O.; Takada, K.; Sawai, Y.; Aung, T.T. & Kamataki, T. (2008)-Historical tsunamis and storms recorded in a coastal lowland, Shizuoka Prefecture, along the Pacific Coast of Japan. Sedimentology, 55, 1703-1716.