Resumo

ISLA, Federico I.; IANTANOS, Nerina  e  ESTRADA, Enrique. Reflective and dissipative macrotidal beaches of San Jorge Gulf, Chubut. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.2, pp.155-164. ISSN 1853-6360.

The morphodynamics of macrotidal beaches is not well known. The tide controls the action of waves, and the swash, backwash and surf are very important processes only at shallow water. Bedforms occur in relation to these shallow-water processes and to grain-size variations. The San Jorge Gulf has a semicircular shape open to waves entering from the Atlantic, with differences in beach orientations and therefore in the significant effects of the dominant westerly winds. The macrotidal semidiurnal regime makes it ideal to test models about beach morphodynamics. The coast is constituted by cliffs carved into Upper Tertiary marine sediments (Patagonia Formation) and gravel beach-ridge systems belonging to Quaternary highstands. Open-coast beaches (Camarones, Cabo Blanco) are composed of gravel, while towards the headlands of the gulf there are beaches composed dominantly by sand. Pocket beaches along the coastline between Rada Tilly and Playa Alsina are characterised by an eolian supply of fine sand. In regard to the morphology of these beaches, it should be distinguished between beaches composed of gravel and sand with steep intertidal slopes (>10%), and sandy beaches of very gentle slopes (<2%). Both extremes, reflective and dissipative beaches, were characterised in morphology and grain-size distribution, and were considered represented by Caleta Córdova (CC, Chubut) and El Límite (EL, Chubut- Santa Cruz), respectively. The differences do not obey directly to dynamic causes, but to inherited factors: bathymetry (that induce wave-refraction patterns and sediment sources). The CC beach originated by the erosion of cliffs. The EL beach is on a low-lying coast, supported by aeolian sources from the west, and the recycling of fine sand foredunes. As both beaches have wave heights at breakers of only 0,2 m during low tide, it is here concluded that their morphodynamics differ exclusively by the sediment availability (grain size and volume) that control the spatial and temporal distribution of wave energy along the beach profile, and the refraction pattern of waves coming from the Atlantic. Waves incoming from the ENE, E and ESE concentrate at the southwestern coast of the gulf due to a submerged platform located at its southeastern portion. Only waves from the SE converge at the northern coast of the gulf. It is stressed here that at the fine-sand beaches (El Límite, Herradura, Alsina) sediment input rates should be preserved in order to avoid negative budgets.

Palavras-chave : Beaches; Morphodynamics; Golfo San Jorge.

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