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On the coastal cliffs between Oura and Leixão dos Alhos is possible to walk and observe the Middle Miocene Lagos-Portimão Formation, mainly constituted by highly fossiliferous siliciclastic cold-water carbonates (Brachert et al., 2003) locally enriched on fossil Mollusks (pectinids), Equinoderms (Clypeaster) and Bryozoa, among others. The seaside walking trail is defined directly over its uppermost layers which end by an intra-Miocene unconformity hardened surface. Along this trail evidences of the several processes that acted during the correlative 3 Ma hiatus (End Serravalian to Upper Tortonian) can be observed (Cachão; Silva, 2000): i) the end of the carbonate sedimentary cycle in Algarve marked by a generalized emersion that affected the entire Portuguese continental shelf, ii) an important intra-Miocene karst development, iii) cut of a marine abrasion platform, iv) colonization of the rocky hard ground by lithophagous organisms testified by their bioerosion trace fossils produced during the beginning of the siliciclastic sedimentary cycle which transgressive stage is represented by the Upper Tortonian Cacela Formation. The first levels of micaceous and homogeneous fine sands (areolas) of this formation are a condensed section commonly containing shark teeth and glauconitic layers as well as frequent biortubation trace fossils.
Along this coastal section beautiful examples of sedimentary structures on siliciclastic carbonates of the Serravalian Upper Member of the Lagos-Portimão Formation can be observed. The geometry of the coastal cliffs are determined by the distinct rheological behavior between the previous formation and the well exposed thick fluvial marine coarse sand sequences (Ludo Formation) of the siliciclastic sedimentary cycle.
The complex geometry of the unconformity surface that marks this strong facies transition is due to the strong karsification of the carbonates. Cave structures developed deep below present day sea level, which seasonally give rise to fresh water springs, the “olhos de água” (transl: water eyes), in the near shore (Almeida; Silva, 1990).
The intra-Miocene unconformity is a major evidence of the tectonic and palaleogeographic changes that operated in the alpine Betic chain (Southwest Iberia) at the end of the Serravalian leading to a global erosional hiatus along the Portuguese shelf followed by the dextral rotation of the Guadalquivir basin and the reboot of the marine sedimentation (Cacela Formation) of the siliciclastic sequence in the Algarve (Cachão; Silva, 2000).
ALMEIDA, C.; SILVA, M.L. (1990) – Hidrogeologia do Miocénico entre Albufeira e Ribeira de Quarteira. GEOLIS, IV (1; 2): 199 – 216.
CACHÃO, M. (1995) – Utilização de Nanofósseis calcários em Biostratigrafia, Paleoceanografia e Paleoecologia. Aplicações ao Neogénico do Algarve (Portugal) e do Mediterrâneo Ocidental (ODP 653) e à problemática de Coccolithus pelagicus. PhD thesis Univ. Lisbon, 356 pp. (unpublished).
CACHÃO, M.; SILVA, C. M. da (2000) – The three main marine depositional cycles of the Neogene of Portugal, Ciências da Terra (UNL), 14: 303-312.
CACHÃO, M., MARQUES da SILVA, C., SANTOS, A., DOMÈNECH, R., MARTINELL, J.; MAYORAL, E. (2009) – The bioeroded megasurface of Oura (Algarve, South Portugal): implications for the Neogene stratigraphy and tectonic evolution of southwest Iberia. Facies, 55: 213-225.