Bioeroded boulders in Holocene marine deposits of the Bahia Blanca Estuary: palaeoenvironmental considerations and provenance

On the north shore of the Bahia Blanca estuary, Buenos Aires, Argentina, sand shell ridges composed by a high proportion of boulders of sedimentary origin are present. Many of these clasts show significant signals of bieorosion. The procedence of this material is linked to the extensive abrasion platforms present to the east of the estuary (coasts of Pehuen Co). Two ichnofacies were established: Trypanites ichnofacies comprising Gastrochaenolites, Maeandropolydora, Trypanites, Gnathichnus and Radulichnus, typical of hard substrates and Skolithos ichnofacies represented by Ophiomorpha galleries developed in soft substrates. Both characterize a shallow coastal environment. During the Holocene transgression, coastal substrates were affected by the advancing sea and eroded fragments were transported by littoral drift into the estuary. Finally, storm events deposited material with signs of bioerosion in the littoral ridges. Today, many of these boulders, which exhibit gradations in size into the estuary, are in transit across the beaches by east-west littoral drift.Original Abstract: En la costa norte del estuario de Bahia Blanca, provincia de Buenos Aires, Argentina, se presentan cordones litorales compuestos por arena conchilifera y elevada proporcion de rodados de origen sedimentario. Muchos de estos clastos muestran importantes signos de bieorosion. La procedencia de este material se vincula con las extensas plataformas de abrasion presentes al este del estuario (costas de Pehuen Co). Se determinaron dos icnofacies: la icnofacies de Trypanites integrada por Gastrochaenolites, Maeandropolydora, Trypanites, Gnathichnus y Radulichnus, propia de sustratos duros y la icnofacies de Skolithos representada por galerias de Ophiomorpha desarrollada en sustratos blandos. Ambas, caracterizan un ambiente litoral somero. Durante la transgresion holocena, los sustratos costeros se vieron afectados por el avance marino y los fragmentos erosionados fueron transportados por deriva litoral hacia el interior del estuario. Finalmente, eventos de tormenta depositaron en los cordones litorales el material con signos de bioerosion. En la actualidad, muchos de estos rodados que presentan gradacion en tamano hacia el interior del estuario, se hallan en transito en las playas por deriva litoral este-oeste.