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Ria Formosa is a lagoonal and sandy barrier system that extends East-West from Cacela Velha to Quarteira (W of Faro) at the Central-western sector of Algarve. Five barrier islands and two spits form a roughly triangular chain, extending along approximately 55 km and with maximum width of 6 km (Cape Santa Maria). The enclosed lagoon is similar to an “open lagoon” with an average depth of 2 m relative to mean seal level (msl) and a flooded surface of 84 km2 during spring tides, from which only 14% is permanently submerged (Andrade et al., 2004).
The main morphosedimentary features are sandy to muddy tidal flats, salt marshes and channels, the intertidal forms representing 90% of the surface.
This lagoonal system is the physical support of Spartina maritima low marshes and Salicornia sp., Halimione sp. and Atriplex sp. high marshes reflecting high salinity values due to high-mesotidal renewal rates and no significant fluvial fresh water input. The absence of a major upstream fluvial system is also responsible by the essentially cohesive sedimentation retained on the intertidal flats and marshes, from the suspended terrigenous load, mostly exported to the shelf during each tidal cycle. In contrast, coarse (sand to shingle) shell-rich sediment resident on channels and the lower domain of intertidal flats is of marine provenance, input via the tidal inlets and redistributed by strong tidal currents (Andrade et al., 2004).
The peculiar feature of the Ria Formosa lagoon is the presence of marsh detached elongated discontinuous sandy barriers separated by five to six tidal inlets. Formerly extended up to the Guadiana river mouth, the easternmost section of this lagoon suffered a coastal transfiguration induced by the tsunami raised during the AD 1755 Lisbon earthquake (Andrade, 1992, Andrade; Freitas, 2000). At its present day extremes two spits (Faro and Cacela) resulted from the welding of former barrier tips to the mainland, following natural deactivation and silting up of terminal inlets (Andrade et al., 2004).
ANDRADE, C. (1992) – Tsunami generated forms in the Algarve Barrier Islands (South Portugal). Science of Tsunami Hazards, 10 (1): 21 – 33.
ANDRADE, C.; FREITAS, M.C. (2000) – An example of rapid coastal change associated with an extreme event: the Algarve coast of Portugal. In Smith, D., Raper, S, Zerbini, S., Sánchez-Arcilla , A. (eds), Sea Level Change and Coastal Processes. Energy, Envir. Sust. Developm., European Comm., Brussels: 139 – 150.
ANDRADE, C., FREITAS, M.C., MORENO, J.; CRAVEIRO, S.C. (2004) – Stratigraphical evidence of Late Holocene barrier breaching and extreme storms in lagoonal sediments of Ria Formosa, Algarve, Portugal. Marine Geology, 210: 339 – 362.
Observações
Geossítio localizado nos municípios de Faro, Loulé, Olhão, Tavira, Vila Real de Santo António