Geological signature of tsunami deposits on the Portuguese coast - GETS

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Boulder deposits related to extreme marine events in the western coast of Portugal

Publication . Oliveira, Maria Alexandra; Andrade, César Freire de, 1955-; Neves, Ramiro Joaquim Jesus, 1955-

This work describes a complex boulder accumulation found in the Portuguese western coastline, N of Ericeira, containing over 1600 boulders sitting on a structural platform and low cliffs at 2-13 m above mean sea level (amsl). The main objective of this work consisted in the identification (storm vs tsunami), age estimation and the computation of recurrence intervals of extreme events responsible for the transport and deposition of this boulder accumulation. Several proxies were used to interpret the origin of the deposit under analysis, such as: morphological characteristics of the boulder accumulation and of individual boulder positions and arrangement; monitoring boulder movement by storms, numerical modelling of storm wave propagation and with statistical analysis of extreme values applied to the wave regime; application of numerical approximations describing boulder movement; age estimation based on aerial photographs, lichenometry, erosion rates and optically stimulated luminesce (OSL) of finer sediments of marine origin. Distribution of source layer, mass and directional properties indicate short-lived boulder transport following a flow directed from the W-NW, compatible with the modern-day wave regime rather than the major tsunamigenic source, located SW of Portugal. Statistical modelling of extreme values applied to the wave regime and comparison with storm wave parameters which generated observed boulder movement, indicate 2-5-years recurrence interval for the occurrence of storms capable to generate boulder transport and deposition in the W-NW facing segments, and a higher than 20 years in the WSW-facing segments of the study area. Moreover, the application of numerical solutions to the boulder dataset indicate wave periods and significant wave heights compatible with a storm origin and with the wave regime affecting the western Portuguese coastline. A strong geomorphological signature is observed in the morphological characteristics of the boulder accumulations. This is reflected in local changes of directional properties of boulders which mimic the coastline configuration, with emphasis in areas of the structural platforms where NW-SE aligned indentations exist. These indentations function as natural channels and generate a funnelling effect increasing the potential to detach and transport boulder-size particles. Age estimation results based on the comparison of aerial photographs and lichenometry indicate that boulder transport frequently occurs, thus implying a storm origin to at least part of the deposit. Additionally, it indicates an extremely recent age for the boulder accumulation from around the 14th-15th centuries. Moreover, age estimation results obtained for lichenometry show that this boulder accumulation mainly comprises boulders recently deposited (past ~140 years), which is confirmed with results from OSL age estimation of marine sand patches found within a boulder cluster, of 230-290 years. The fact that older boulders are scarce and that boulder transport and deposition frequently occurs suggests that: the boulder deposit under analysis is highly dynamic and that particles typically present a residence period lower than 200 years; the AD 1755 tsunami, which reached a minimum height of 9 m amsl, partly obliterated a previously existing boulder accumulation, having mostly generated an essentially erosive signature. The presence of older and sometimes heavier boulders located higher up and further inland in WSW facing segments of the coastline indicates boulder deposition on these locations during higher storminess periods, probably associated with periods of negative NAO index. This work contributes to the discussion regarding sedimentological/erosional signatures of boulder-size accumulations associated with storms and tsunamis in rocky coastlines and to a better understanding of the geomorphological controls and physical processes occurring within this context. Another relevant contribution was the development of a lichen growth model for the species Opegrapha durieui Mont. (Roux and Egea, 1992), which allows estimating the age of limestone surface exposure of up to ~500 years.

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Entidade financiadora

Fundação para a Ciência e a Tecnologia

Programa de financiamento

3599-PPCDT

Número da atribuição

PTDC/CTE-GEX/65948/2006