Holocene evolution of the coastal plain of the south margin of the Río de la Plata

Abstract

The coastal plain of the southern margin of the Río de la Plata evolved on a substratum formed during the Holocene transgression. Its present configuration is the result of the progradation processes accompanying the last sea-level fall. Coastal plain evolution was controlled by the interaction of hydrometeorological conditions (wind-wave relation, currents and tidal climate), migration of a muddy depocenter, relative sea-level fluctuations and geometry of the pre-Holocene surface. Geometry and sea-level fluctuations determined the distribution, extension and development of the sedimentary sequence, whereas hydrometeorological conditions controlled the rate of sediment supply and transport. The most important feature of the pre-Holocene surface was the ancient fluvial valley of the Río de la Plata. When the last rise of sea-level reached the upper border of the valley, a headland located at the south-eastern edge of the coastal plain behaved as a point of attack for the incoming waves coming from the south-east. This induced the formation of two littoral currents drifting in opposite directions, one to the north-west (towards the Río de la Plata), and the other to the south-west (towards Bahía Samborombón), conditions that still occur today. Evolution of the coastal plain may be summarised in three stages: (1) estuarine, coastal plain and fluvial-estuarine deltaic that represents the processes that filled up the palaeovalley, (2) coastal progradation and (3)the installation of a deltaic system.