Resumen

MONTI, Alejandro J.A  y  BAYARSKY, Adelma. Relación entre las geoformas costeras y los sedimentos de playas del sur de Bahía Anegada, provincia de Buenos Aires. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.1, pp.31-42. ISSN 1853-6360.

Anegada bay is located on the southern coast of Buenos Aires province. This study was made along 10 km of coast near Los Pocitos beach (Fig. 1). Relations between heavy minerals abundance, grain size, provenance, morphology and coastal processes, were obtained from textural and mineralogical analysis of beach sediments and the geomorphological patterns of the area. The coast shows as main geoforms, beaches, wave-cut platforms, and active cliffs. Two sectors were determined by its morphological features. One of them presents geoforms with active littoral erosion and the other shows coastal patterns related with accretionary phenomena (Fig.1). Most of the beach sediments are sands with little subordinated gravels. The mean grain size is from 2ø (phi units) to 2.5ø and the standard deviation varies between good and very good grade. Along every beach profile the grain size increases to the backshore. The sediments of these beaches include light and heavy-minerals. The heavy mineral accumulation increases from lower foreshore to backshore following the same trend of grain size. Further more the morphological patterns of each coastal sector, this trend was proved along all beach profiles (Fig. 2). Heavy-minerals composition includes, in decreasing frequencies, hypersthene, altered minerals, augite, opaque minerals, hornblende, enstatite, garnet, epidote and zircon (Table 1). Light minerals composition include, in decreasing frequencies, altered minerals, feldspar, quartz, lithic fragments, carbonate and volcanic glass. That group predominate in the lower foreshore along the coastal sector analysed (Table 2). From the mineralogical composition and the morphological features of coast we confirm the Rio Negro Formation (Andreis, 1966) as the main source of provenance of beach sediments. Two associations of heavy-minerals were defined based on their composition along the beach. One of these includes heavy-minerals with specific gravities below 3,5 g/cm3 (hornblende, augite and epidote). The other includes very heavy minerals with specific gravities between 3,6 and 5,2 g/cm3. It includes hypersthene, enstatite, garnet and opaque minerals (Fig. 4). In the studied area the association of very heavy-minerals shows an efficient concentration in the backshore and higher foreshore related with active erosive coastal sectors and in the lower foreshore along the sectors with dominant accretion. However we found that the main concentration of very heavy-minerals ocurr in the backshore of erosive coastal points (Fig. 3). On the other hand the association of heavy-minerals with specific gravities below 3,5 g/cm3 shows its main concentration in the backshore and lower foreshore related with accretionary coastal sectors. But high concentrations were also determined in the lower foreshore of erosive coastal points (Figs. 3 and 4). Thus we conclude that the grain size, the proximity of the source, the variation of coastal morphology and the littoral processes associated, are the most important variables for the particular distribution of heavy-minerals abundance along the beaches into the southern coast of Anegada Bay.

Palabras clave : Sediments; Grain size; Mineralogy; Coastal morphology.

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