Using Portuguese palaeotsunami deposits to reconstruct wave parameters and establish sediment sources, return periods and epicenters: a review on current knowledge

Tsunami geoscience has evolved greatly since its seminal works in late 1980’s. Initially, tsunami deposits were merely identified in the stratigraphic record using its singularity and penetration inland. Later, especially after the 2004 and 2011 tsunami events, recognition of tsunami deposits progressed to interpretation on sediment dynamics and inundation phases based on the progressive application of different sedimentological, geochemical, paleontological and geophysical analytical techniques. Equally to other locations worldwide, tsunami deposits in Portugal were originally (early 1990’s) identified due to its geomorphological imprint or by its coarser sandy nature in muddy low-lying basins within the stratigraphic sequence of coastal sectors along the southern coast (Algarve). Many of these deposits were firstly studied in detail in terms of spatial distribution, texture and micropalaeontological composition. One aspect that was noticed was the uniqueness of the CE 1755 event in the top of the Holocene sequence. The CE 1755 tsunami is well-known for its consequences all over the Atlantic basin however its epicenter is yet to be established with certainty. In that sense, over the last decade, a multitude of analysis and new sites were studied (Salgados, Alcantarilha, Furnas, Barranco, Almargem) and contributed to shed new light on the CE 1755 and on other extreme events that impinged the Portuguese coastal fringe. For example, boulder analysis and the erosional signature in dune fields were used to model wave flow characteristics (run-up, flow velocity and flow depth). On the other hand, grain-size data and heavy mineral composition established a robust source-to-sink relationship between the CE 1755 tsunami deposits and dune sediments. Similarly, microtextural analysis corroborated these findings reaching similar conclusions. The application of geochemistry and high-resolution micropaleontological analysis brought new insights in terms of inundation extent and in the establishment of inundation phases. All these analyses contributed to a better understanding of the CE 1755 tsunami dynamic and its onshore sedimentological imprint. Very recently, state-of-art hydrodynamic and morphodynamic modelling exercises have been conducted using this unique geological database to be validated. They contribute to exclude potential generation zones and to narrow down the search for the CE 1755 epicenter.

Another very innovative aspect is the recent study of the shelf area that is providing a ground-breaking opportunity to couple onshore and offshore palaeotsunami data and make inferences about the relevance of the backwash process on the depositional imprint.

This work will summarize the present state-of-knowledge on the Portuguese tsunami geological record, a unique tsunami geoscience case-study in Europe.

 

Authors acknowledge the financial support of FCT through projects UIDB/50019/2020–IDL and OnOff –

PTDC/CTAGEO/28941/2017.