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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.

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This study describes sedimentation associated with the tsunami generated by the Lisbon earthquake of ad 1755. It is argued here that the tsunami deposited a sand sheet across the Lagoa dos Salgados (central Algarve, Portugal), that is intercalated with late-Holocene estuarine/lagoonal sediments. A wide range of proxies (sedimentological, exoscopic and palaeontological) are used to establish the provenance of the sandy material as well as to constrain the age of the deposit. Stratigraphic criteria are used to distinguish the uniqueness of the event layer. Exoscopic and textural analysis suggest that the source of the event deposit is mainly the dune, beach and underlying layer. Micropalaentological analysis (Foraminifera) indicates a conspicuous increase in diversity and dominance of marine species within the event sediment sheet. The spatial characteristics of the tsunami layer suggests that the barrier prevented widespread overtopping by the incoming tsunami allowing inference of c. 10 m as maximum height at the coast; they also indicate the inlet as the preferential route for both water and sediment transported inland. Dating results (14C, 210Pb and 137Cs) allow extrapolation of an age of deposition compatible with the ad 1755 Lisbon earthquake and tsunami, the most devastating event that affected this coastal area in historical times. Correlations with similar deposits detected in nearby lowlands strengthen the argument that the tsunami sediment layer represents a marker horizon in the coastal stratigraphy along the Portuguese Algarve coast. The stratigraphic uniqueness of this event might have implications in the establishment of millennial scale recurrence intervals for this type of high-energy marine inundation.