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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Artigos em livros de actas ► Internacionais


Referência Bibliográfica

​HIPÓLITO, A., MADEIRA, J., CACHÃO, M., CABRAL, N., GASPAR, J.L. (2017) - The 1755 tsunami in Terceira Island (Azores): nannoliths as a marine tracer in the recognition of tsunami deposits. 5th ITFS - volume of Abstracts, 5th International Tsunami field Symposium, 3-7 September, Lisbon.


​The geographic and geologic settings of Azores Archipelago make it susceptible to tsunamis. Written accounts indicate at least 13 flooding events attributable to tsunamis generated by local or distal sources (Andrade et al. 2006; Cabral, 2009). A discontinuous flood deposit is found up to 50m in elevation in the east and south low-lying coastal areas of Terceira Island. The sedimentological (marine sand and shells and terrestrial elements) and textural features (erosive basal contacts, rip-up and directivity flow-induced structures) indicate tsunamigenic origin. The occurrence of pottery and a mammal tooth indicate a post-settlement age. Radiocarbon age of coal fragments from the deposit is compatible with the 1755 event. The presence of nannoliths was checked to confirm the marine provenance of the sediments. Nannoliths are mainly composed of coccoliths, calcified scales produce by calcareous nannoplankton (coccolithophores). Coccolithophores represents the major component of the extant nannoplanktonic communities throughout the world’s oceans and can also be found in coastal facies providing a marine tracer in coastal sediments (Ferreira & Cachão, 2005). Nannoliths abundance and distribution were used to characterize recent tsunami deposits (e.g. Paris et al., 2010). Despite the age of the flood deposit and its exposure to unfavorable weather conditions (promoting swift wash-out and/or dissolution of nannoliths), coccoliths were found. We highlight the occurrence of one coccosphere; the preservation of such a fragile structure indicates a non-turbulent transport of marine water and sediment. Nannoliths content have proven to be a good proxy for tsunami deposits, especially in recent volcanic islands where calcareous sediments are rare.