Evolution and environmental changes in the coastal plain of the headwater of Río de la Plata.

Abstract

The coastal plains and surrounding areas in the south of Entre Ríos Province together with the Paraná delta represent the innermost environments of the Río de la Plata that were affected by the late Pleistocene-Holocene post-glacial transgression. Thus, they evolved in the context of the evolution of the entire Río de La Plata as a result of the combined effect of relative sea-level rise and fall, the coastal dynamic in the former littoral environments, climatic changes and substrate topography. The sedimentary record consists of a sequence of estuarine deposits, resulting from migration of a muddy depocentre during the transgression, underlying deposits representing beach-ridges, plains, dunes, low-energy beaches and deltas that evolved during the regressive event. These sediments respectively constitute two system tracts, transgressive and highstand, which are part of the sequences extended all along the entire La Plata river. There were three evolutionary stages. The first corresponds to the transition from transgression to highstand during times of higher temperature and humidity than at present, when the estuarine depocenter that had previously invaded the La Plata river reached its innermost position; minor deltas developed at the mouths of small rivers and beach ridges systems began to form. The second represents the first part of the highstand when drier climatic conditions prevailed, resulting in the interruption of progradation of small deltas and replacement by estuarine beaches formed by littoral drift and formation of dunes. The last stage elapsed from the moment when climatic conditions became more humid as the present conditions developed, such that an increase in water supply from the Paraná river induced the formation of the Paraná delta and the consequent isolation of the former innermost estuarine and beach-ridges environments. Regional correlations supported by radiocarbon dating reveal a synchronism between geomorphological events and features in the study area and in the southern margin of the Río de La Plata.