Coastal evolution, changes in sea level and molluscan fauna in northeastern Argentina during the Late Quaternary

The northern Argentine shoreline underwent a remarkable progradation during the last 6890 years, equivalent to about 4000 km 2. The shoreline moved approximately 30 km seaward on the average. Two types of accretionary shorelines were formed. High energy environments produced barrier islands and barrier spits which began to form approximately synchronously. Low energy environments, located between those of high energy, consisted of tidal flats and salt marshes. Four groups of molluscan species represent the high-energy palaeoenvironments of the barrier islands and barrier spits. The other fossils represent a low-energy environment of tidal flats. The formation of the barrier islands began 6000 years B.P. It ended 3500 years ago due to a slight fall in relative sea level of 1.5 m above m.s.l. and subsequent formation of barrier spits seaward. As a consequence a very rapid faunal change occurred. In less than 5000–6000 years the dominant high-energy fossil fauna was replaced by a modern low-energy fauna over most of the area. Palaeoenvironmental changes were probably greater and more rapid during the high-energy episode, when the fossils were more varied than those living now in the same area at Samborombón Bay, whilst along the oceanic coastal area the molluscan fauna increased in diversity to the more varied extant one.