Geological recognition of abrupt marine invasions in two coastal areas of Portugal

This thesis was submitted for the award of Master of Philosophy and awarded by Brunel UniversityAbrupt marine invasions such as tsunamis and storms are particularly devastating for coastal areas. They may also leave a permanent record in the sedimentary deposits which is the principal focus of this thesis. In historical times the most destructive tsunami that affected Europe was the AD 1755 Lisbon. The effects and consequences of the AD 1755 tsunami are presented. The causes, propagation and behaviour of tsunamis are also discussed. Sedimentological criteria to identify abrupt marine invasions in the stratigraphic column are presented from literature. This thesis aims to contribute to a better understanding of the signature left by abrupt marine invasions in coastal stratigraphy by investigating the nature of the sedimentary record associated with tsunamis in a region of their known impact. A wide range of proxies was used to detected tsunami and storm deposits in 2 coastal areas of Portugal. The techniques used include stratigraphic description, grain size analysis, digital and x-ray photography, magnetic susceptibility, macrofossils analysis, geochemical analysis and 210Pb and Optically Stimulated Luminescence dating. The investigated areas (e.g. Lagoa de Óbidos (Central Portugal) and Martinhal (South Portugal)) were affected by the AD 1755 tsunami. The locations have similar geomorphological features and are both susceptible to major abrupt marine invasions. Results show that an abrupt event deposited unique sedimentary units in both locations. A similar age for the event was established. A considerable number of tsunami sedimentary characteristics were detected in both units. However, a key outcome of this research is the demonstration of the difficulty of distinguishing between sedimentary deposits laid down by tsunamis, and those deposits resulting from storm action; consequently the geological record of tsunamis almost certainly underestimates their frequency