Molluscan assemblages from the marine Holocene of Uruguay: composition, geochronology, and paleoenvironmental signals

Abstract

Abstract. Dates of marine Holocene molluscs from Uruguay (Villa Soriano Formation) range from 6849 to 1858 cal. years BP. There are no significative gaps among the different ages obtained, with the exception of the interval 2276-2830 calculated years BP. Two kinds of concentrations are distinguished: autochthonous, for which a low energy depositional environment is inferred, and parautochthonous, deposited in foreshore and nearshore high-energy environments. Results of different types of multivariate analyses on presence-absence and abundance data show that only three localities are clearly differentiated from the others. The discrimination of two of them (Arazati, Las Canas) is explained by the presence of only one or two species, but the third (Punta Rasa) is the only one where characteristic species of rocky or hard substrates are undoubtedly represented . Analyses of salinity and temperature ranges of the various mollusc assemblages suggest a displacement of the Rio de la Plata estuary about 300 km westwards along the Uruguayan coast from its present position, and the presence of warmer waters than those found today at the same latitude. With the exception of the currently northwards displaced Anomalocardiabrasiliana (Gmelin), Marshalloranigrocincta (C.B. Adams), Niochesubrostrata (Lamarck), Bullastriata Bruguiere and Miralda sp., all species are present today at the Uruguayan coast. Resumen. ASOCIACIONES DE MOLUSCOS DEL HOLOCENO MARINO DE URUGUAY: COMPOSICION, GEOCRONOLOGIA E INDICADORES PALEOAMBIENTALES. Las asociaciones de moluscos marinos holocenos contenidos en la Formacion Villa Soriano (Uruguay) presentan un rango maximo de 6849-1858 anos cal. AP. No existen discontinuidades significativas entre las diversas edades obtenidas, con la excepcion del intervalo 2276-2830 anos calculados AP. Se distinguen dos tipos de concentraciones: autoctonas, propias de un ambiente de baja energia, y parautoctonas, depositadas en ambientes de alta energia del foreshore y nearshore. Los resultados de diferentes tecnicas de analisis multivariado sobre datos de presencia-ausencia y abundancia de especies muestran que solamente tres se distinguen claramente de las demas localidades. La separacion de dos de ellas (Arazati, Las Canas) se explica porque presentan una o dos especies solamente, en tanto que la tercera (Punta Rasa) es la unica donde estan claramente representados elementos propios de fondos duros. Sobre la base de las asociaciones estudiadas, se infiere un desplazamiento del frente estuarico del Rio de la Plata de aproximadamente 300 km sobre la costa uruguaya, y una temperatura del agua mayor que la actual para la latitud considerada. Con la excepcion de Anomalocardiabrasiliana (Gmelin), Marshalloranigrocincta (C.B. Adams), Niochesubrostrata (Lamarck), Bullastriata Bruguiere y Miralda sp., cuyo limite de distribucion meridional presente se encuentra mas al norte, todas las especies viven hoy dia en la costa de Uruguay.