Facies (2009) 55:213–225 DOI 10.1007/s10347-008-0172-2 123 ORIGINAL ARTICLE The bioeroded megasurface of Oura (Algarve, south Portugal): implications for the Neogene stratigraphy and tectonic evolution of southwest Iberia Mário Cachão · Carlos Marques da Silva · Ana Santos · Rosa Domènech · Jordi Martinell · Eduardo Mayoral Received: 21 May 2008 / Accepted: 24 November 2008 / Published online: 20 December 2008  Springer-Verlag 2008 Abstract The use of rocky palaeoshore bioerosion analy- sis as a tool to solve stratigraphic and tectonic issues is beginning to bear fruits. The occurrence of an extensive intra-Miocene marine abrasion platform in southern Portu- gal at Oura (Albufeira) has been identiWed on the basis of bioerosion trace fossils analysis. The observed ichnodiver- sity is rather low, with bivalve boring Gastrochaenolites being dominant. Nevertheless, the ichnoassemblage may be assigned to the Entobia ichnofacies. The palaeoichnologi- cal study of the Oura hardground conWrmed the existence of an important intra-Miocene stratigraphic gap (ca. 3 Ma hiatus), represented by a razor-sharp erosional contact that separates the two main Neogene units in the Algarvian region: the lower carbonate sequence of Lagos–Portimão Formation (Langhian/Serravallian) and the upper siliciclas- tic sequence of the Cacela Formation (Upper Tortonian). Keywords Palaeoichnology · Bioerosion · Rocky palaeoshores · Miocene · Tectonic evolution · Southern Portugal Introduction Bioerosion is the process by which organisms sculpt or pene- trate surfaces of hard substrates (Bromley 1970, 1994). Rocky shores, as hard surfaces exposed in shallow-marine environ- ments associated with low or null sedimentation, oVer excel- lent conditions for intense colonization by marine bioeroders. Rocky shores are also remarkable from a geological point of view. They represent a marine sedimentary environment in which erosion processes dominate (rather than depositional processes) and where wave-cut marine erosion platforms are being formed. Therefore, the recognition of rocky palaeo- shores in the geological record is of paramount importance, for they characterize major Xooding surfaces and provide cru- cial information about palaeoshorelines and ancient sea levels (Martinell and Domènech 1995; Jia-Yu and Johnson 1996; Ghibaudo et al. 1996; de Gibert et al. 1996; da Silva et al. 1999; Domènech et al. 2001; Santos et al. 2008). Communities of bioeroders of mesolittoral to shallow infralittoral character have acted throughout geological time, since at least the Cambrian (Brett et al. 1983), chieXy with a speciWc preference for carbonate substrates (Johnson 1992; Johnson and Baarli 1999). Clionaid sponges and lithophagid bivalves are especially successful examples of borers colonizing this substrate in shallow-marine environ- ments. The dimension, durability, and easily recognizable morphology of lithophagid bivalve borings make them exceptional tools for identifying ancient rocky shorelines. The Miocene of Algarve provides an excellent opportu- nity to assess the importance of major bioeroded surfaces, M. Cachão · C. M. da Silva · A. Santos (&) Centro de Geologia e Dept. de Geologia, Faculdade de Ciências, Universidade de Lisboa, C6, Campo Grande, 1749-016 Lisboa, Portugal e-mail: asantos@dgyp.uhu.es M. Cachão e-mail: mcachao@fc.ul.pt R. Domènech · J. Martinell Dept. d’EstratigraWa, Paleontologia i Geociències Marines, Facultat de Geologia, Universitat de Barcelona, C/Martí i Franques, s/n, 08028 Barcelona, Spain E. Mayoral Dept. de Geodinâmica y Paleontologia, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus de El Carmen, Avda. 3 de Marzo, s/n, 21071 Huelva, Spain