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Reconhecimento de um depósito de tsunami na Lagoa dos Salgados (Algarve, Portugal)
Abstract
RESUMO: O preenchimento sedimentar holocénico da Lagoa dos Salgados (Armação de Pêra) contém uma lâmina arenosa singular intercalada em sedimentos lodosos, cuja caracterização sedimentológica, paleoecológica e geométrica sugere associação a um evento excepcional de inundação marinha. As características do depósito são típicas de inundação tsunamigénica e a constrição cronológica do evento gerador sugere associação ao tsunami de 1 de Novembro de 1755. PALAVRAS-CHAVE: Tsunami, sedimentos, foraminíferos, laguna, AD 1755. ABSTRACT: The upper Holocene sedimentary infill of Lagoa dos Salgados (Armação de Pêra) includes an exotic sand layer within lagoonal mud and the sedimentological, palaeoecological and geometrical characteristics of the deposit suggest association with an exceptionally intense event of marine inundation. The deposit characteristics are typical of tsunami sedimentation and the time-constraining of the inundation indicates association with the AD 1 November 1755 tsunami.
... Contudo, só uma pequena porção da energia gravitacional desta massa (aproximadamente 15%) é transferida para a água e neste caso as ondas atenuam muito mais rapidamente do que em caso dos tsunamis gerados pelos terramotos (Dawson e Stewart, 2007 (Andrade, 1994;Hindson et al., 1996;Hindson e Andrade, 1999;Andrade et al., 2004;Costa, 2006;Costa et al., 2010;Costa et al., 2012a;2012b;Font et al., 2013;Costa et al., 2015) concluíram que a sequência sedimentar apresenta um perfil vertical descontínuo, com intercalação de uma camada de areia de origem marinha depositada em condições hidrodinâmicas extremas, sendo associada ao tsunami de 1 de novembro de 1755. ...
... Outra caraterística da unidade tsunamigénica é a distribuição geométrica em forma de cunha (ou bisel), ou seja com maior espessura junto a costa, onde pode ter algumas dezenas de centímetros, diminuindo gradualmente para o interior atingindo espessuras de poucos milímetros antes de desaparecer por completo (Ramos-Pereira et al., 2009Costa et al., 2010;2012a). ...
... Nos estudos de proveniência dos depósitos tsunamigénicos realizados nos últimos anos, a análise da distribuição dos grãos em classes dimensionais é largamente utilizada e acompanha quase sempre os estudos estratigráficos (Andrade et al., 2004;Hawkes et al., 2007;Kortekaas e Dawson, 2007;Abrantes et al., 2008;Dawson e Stewart, 2007;Gràcia et al., 2010;Costa et al., 2006Costa et al., , 2010Costa et al., , 2012aLario et al., 2010;Chagué-Goff et al., 2011;Engel e Brückner, 2011; Bahlburg e Spiske 2011; Goff et al., 2012;Fujino e Nishimura, 2012;Pilarczyk et al., 2012;Cuven et al., 2013;Font et al., 2013;Koster e Reicherter, 2014;Koster, 2015). ...
... These sediments (palynological zone ADP 01/06 A; microfauna zone A) are barren of calcareous microfossils. Costa et al. (2010Costa et al. ( , 2012 did not report deposits of this type in the Salgados lagoon, east of Alcantarilha. The reason is probably the vicinity of drilling locations to the coastline or the previous erosion of these deposits and refilling of the river mouth with marine sediments during the marine transgression. ...
... In this phase, the type of the estuary was probably a wide river mouth with a mixing zone of fresh and marine water masses. This phase correlates very well with microfauna zone A and the results of Costa et al. (2010Costa et al. ( , 2012 from the Salgados lagoon and Cearreta et al. (2007) from the Melides lagoon. Because of poor pollen conservation, this section could not be included in the pollen diagrams. ...
... The sediment colour, texture, microfauna and the pollen composition prove that assumption. Costa et al. (2010Costa et al. ( , 2012) describe a comparable facies B with less than 20% coarse material and marine-brackish foraminifera as inter-to subtidally influenced. This is indicated by ostracod taxa which prefer shallow water and fine sand, e.g. ...
Nous étudions l’histoire de deux estuaires de la côte d’Algarve (sud du Portugal) en utilisant des reconstructions multi-proxy (sédimentologie, géochimie, palynologie, micropaléontologie et datation au carbone 14). Basé sur ces derniers ∼7500 ans. Les données palynologiques décrivent des transformations florales que nous expliquons par des changements climatiques. L’évolution de la microfaune (foraminifères et ostracodes) est le reflet de l’alternance des environnements marins et saumâtres dans les deux carottes étudiées. La carotte d’Alvor montre clairement une séquence régressive. La carotte d’Alcantarilha est caractérisée par une sédimentation fluviatile à laquelle succède, vers 7300 ans cal BP, une sédimentation marine littorale. La transgression est indiquée par des foraminifères et des ostracodes vivant dans des eaux saumâtres à marines. Pendant cette transgression, la proportion des pollens de l’étage supralittoral recule et celle de la zone humide augmente. Entre ∼6700 ans cal BP et ∼5700 ans cal BP à Alvor et 4500 ans cal BP à Alcantarilha, la profondeur de la lagune diminue progressivement. Un évènement de haute énergie (tempête ou tsunami) est enregistré entre 6400 et 5800 ans cal BP sur le site d’Alvor mais pas celui d’Alcantarilha. Après cet évènement, la profondeur de la lagune régresse de nouveau jusqu’à 3500/2000 ans cal BP. L’évolution de cette régression est soulignée par la formation d’un cordon littoral sur le front de l’estuaire, indiqué par l’augmentation de la proportion de la végétation de l’étage supralittoral. La carotte d’Alcantarilha montre une autre couche de sédiments, déposés en milieu de haute énergie, mise en évidence par les foraminifères et les ostracodes marins après 4500 ans cal BP. La partie la plus jeune des carottes est marquée par la diminution de l’influence de la marée et une augmentation de l’impact anthropique (changement général de la végétation et accroissement des proportions des pollens des plantes cultivées).
... These results, based on the variation in the water levels with the time, showed that in Porto Novo there was indeed a local amplification by the cliffs. Deposits of the CE 1755 tsunami are very well-preserved in the southwestern Iberian Peninsula, namely along the Algarve coast (Boca do Rio, Martinhal, Ria Formosa, and Salgados) and were identified mainly using lithostratigraphic criteria [19][20][21]. Possible records of the CE 1755 tsunami were also found in the western Portuguese coast, on the Óbidos Lagoon [22] and on the Tagus river estuary [23,24], as well as in the form of chaotic and heterometric pebbles and disturbed heavy mineral sandy layers in the urban area of Belém, in the Lisbon county [25,26]. Tsunami deposits can have The area of the Alcabrichel Basin is approximately 180 km 2 . ...
... The detailed analysis of this unit seems to suggest more similarities with analogue CE 1755 tsunami deposits detected along the Portuguese coast. Many authors reported similar textural characteristics on the CE 1755 tsunami deposits recorded in Salgados Lagoon [20,21], Boca do Rio [70], in Martinhal [33] (South Portugal), and in the Gulf of Cadiz [69]. Therefore, given the above discussion of age estimation (after 250 ± 30 BP), we hypothesized that the deposition of unit UL3 could be related to the CE 1755 Lisbon tsunami. ...
The CE 1755 Lisbon tsunami was the largest historical tsunami to affect the Atlantic coasts of Europe and North Africa. This study presents the results obtained from the application of different sedimentological techniques (e.g., grain size, morphoscopy, microtextural analysis, geochemistry, radiocarbon dating) on sediments retrieved from the Alcabrichel River alluvial plain (of about 500 m far away from its mouth and approximatively 50 km northwest of Lisbon, Portugal). The results allowed the identification of a sandy layer that was associated with the CE 1755 tsunami. Furthermore, a new microtextural semi-quantitative classification was applied to enhance the identification of extreme marine inundation deposits. Based on sedimentological data, three different tsunami inundation phases were identified, including two inundations and a likely backwash. This innovative work offers physical evidence of the spatial presence of the CE 1755 tsunami event on the western coast of Europe. It also enables a reconstruction of tsunami inundation dynamics, with two flooding waves and an interspersed backwash.
... A total of 158 cores and 31 superficial samples from present-day analogs were collected, and a tsunami deposit attributed to the AD 1755 tsunami was sampled ( Costa et al., 2009Costa et al., , 2010Costa et al., , 2012. Fourteen nearshore, seven beach, ten dune, one storm and seven paleotsunami samples collected in the Lagoa dos Salgados area were analyzed in this research. ...
The complex transport and depositional processes associated with tsunamis and storms and the peculiarities of local inundation present questions associated with the recognition and differentiation of the sedimentary signature of these events. This work presents a study of quartz grains transported and deposited by tsunami and storm waves with the objective of identifying specific microtextural signatures caused by high-energy marine inundations and to correlate them with their principal sedimentary sources. In this empirical study, 1150 quartz grains (78 samples) and their microtextural signatures were observed, analyzed and classified using scanning electron microscope photomicrographs. The results suggest that although no specific microtextural signature is associated with high energy inundations, there are strong increases in the percentage of fresh surfaces and percussion marks when compared with the potential source material. Moreover, tsunami and storm grains present the greatest microtextural variance among all the grains analyzed. Nevertheless, specific local conditions and sediment concentrations constrain the microtextural implications on tsunami or storm grains. One laboratory experiment was designed to test microtextural implications in grains subjected to variable velocities, sediment concentration and time. The surface microscopic signature in quartz grains of high-energy events further contributes to the development of more efficient sedimentological criteria to identify deposits associated with tsunami and storm events.
Archaeological and geological surveys carried out in several sites in the Bay of Lagos (Portugal) have brought to light new evidence of high-energy events (earthquakes and tsunamis) during Antiquity. Twenty-eight sediment cores performed along Lagos’s waterfront yielded diverse intertidal strata, some of which are composed of sandy silt that may correspond to such events. Indeed, high-energy events of this kind might have aborted the first Phoenician occupation of Lagos and Pamares between the seventh and sixth centuries BC. In turn, it is possible that the Bay of Lagos was affected by other events, such as the extreme wave event that allegedly struck the seaboard of the Gulf of Cadiz at the end of the third century BC. In Monte Molião, some structural alterations are evident in walls dating from the first century BC, plus sand layers have been detected in the transition from the Iron Age to Roman times. An analysis of faunal remains, specifically shells, has revealed changes in the estuary’s dynamics, which may be related to the effects of an extreme wave event.
A description is given of a distinctive marine deposit at Boca do Rio on the Algarve coast of Portugal. It is proposed here that the sediment accumulation was deposited by the tsunami generated by the Lisbon earthquake of 1 November, AD 1755. The deposit exhibits sedimentary characteristics quite unlike other coastal sediment accumulations that are deposited by more moderate wave régimes. These include laterally continuous sand layers, chaotic pebble horizons, large amounts of gravel-sized shell debris and distinctive assemblages of marine microfossils. A preliminary attempt is made here to use the empirical field and laboratory data to define more clearly the processes of coastal sedimentation that characterize tsunami runup. The use of tsunami sediments as time synchronous marker horizons is also discussed.
Geological identification of past tsunamis is important for risk assessment studies, especially in areas where the historical record is limited or absent. The main problem when using the geological evidence is to distinguish between tsunami and storm deposits. Both are high-energy events that may leave marine traces in coastal stratigraphic sequences. At Martinhal, SW Portugal both storm surge and tsunami deposits are present at the same site within a single stratigraphic sequence, which makes it suitable to study the differences between them, excluding variations caused by local factors.The tsunami associated with the Lisbon earthquake of November 1st 1755 AD, had a major impact on the geomorphology and sedimentology of Martinhal. It breached the barrier and laid down an extensive sheet of sand, as described in eyewitness reports. Besides the tsunami deposit the stratigraphy of Martinhal also displays evidence for storm surges that have breached and overtopped the barrier, flooding the lowland and leaving sand layers. Both marine-derived flood deposits show similar grain size characteristics and distinctive marine foraminifera. The most important differences are the rip-up clasts and boulders exclusively found in the tsunami deposit and the landward extent of the tsunami deposit that everywhere exceeds that of the storm deposits. Identification of both depositional units was only possible using a collection of different data and extensive stratigraphical information from cores as well as trenches.
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.
A estabilização do nível do mar no litoral de Armação de Pêra
- A R Pereira
- A M Soares
Pereira, A. R. e Soares, A. M., (1994). A estabilização do nível do mar no litoral de Armação de Pêra. Gaia, 9, pp.
91-93.
O Terramoto do 1 de Novembro de 1755 e um estudo demográfico
- F L Pereira De Sousa
Pereira de Sousa, F. L., (1919) -O Terramoto do 1 de Novembro de 1755 e um estudo demográfico. Volume I e II.
Servicos Geológicos de Portugal.