Morphology and Stratigraphy of the Holocene Agradational/stationary Barriers of Rio Grande Do Sul, Southern Brazil

The transition between regressive and transgressive barriers on the northern and central coastal sectors of Rio Grande do Sul (RS) is marked by a stretch of coast where agradational/stationary (a/s) barriers occur. In a stretch of approximately 50 km this transition was analysed at four sites: Tramandaí, Jardim do Éden, Cidreira and Dunas Altas. All began forming around 7 – 6 cal ka. The northernmost site (Tramandaí) has a typical a/s stratigraphy, and barrier morphology is dominated by transgressive dunefields. In the last 6 – 5 cal ka the barrier has prograded around 300 m. Ten km to the south, at Jardim do Éden, the barrier shows a transgressive stratigraphy, and barrier morphology is also dominated by transgressive dunefields. At Tramandaí the maximum barrier height is around 4 meters, while at Jardim do Éden it averages 10 meters. Further to the south, 10 km from Jardim do Éden, the barrier at Cidreira shows a dominant transgressive stratigraphy developed in the last 6 cal ka, but with evidence of a relatively recent small regression (few hundred meters). The barrier surface reaches 20 meters in height due to the existence here of the most extensive transgressive dunefield of the RS coast. And finally, at around 30 km to the south of Cidreira, the a/s barrier of Dunas Altas occurs. Barrier morphology here is dominated by transgressive dunefields, with a surface height of 4 meters, but fronted by the largest and highest foredunes (around 8-10 m) in Rio Grande do Sul. The Dunas Altas coastline has regressed 280 meters in the last 7 cal ka. The stratigraphy and evolution of these four barrier sites will be compared, and their spatiotemporal relationships to progradational (regressive) and retrogradational (transgressive) barriers will be presented.