The coastal ridge sequence at Rio Grande do Sul: A new geoarchive for past climate events of the Atlantic coast of southern Brazil since the mid Holocene

Abstract

We present here the first geochronometric analysis of a large succession of coastal ridges that developed south of Rio Grande do Sul inlet, aided by OSL dating of samples taken in this succession that amounts c. 306 coastal ridges. This succession allows defining epochs of enhanced coastal drift or fluvial transport to the river mouth. Additionally, changes in the preservation of ridges also suggest the additional effect of transverse winds that create blow outs, while changes in the ridge packages planview onlap suggest possible changes in the dominant longshore drift. Four OSL samples located strategically allow defining the age of five main stages of this succession. The first stage comprises 2 substages. The first substage shows ridges #1 to #36 onlapping the boundary coastal scarp northwardly, while the second (ridges #37 to #84) shows an inversion of ridge shape wedging out southwards until coast returns to be quasi parallel to the initial coastal scarp estimated to be c. 6 ka. We estimate this first stage to comprise enhanced coastal drift due to higher and sandier ridges, ending at c. 4.1 ka. The second stage (ridges #85 to #124) ridges is marked by a slight unconformity, and indicates a period of enhanced fluvial progradation and less coastal drift suggesting milder winds up to 3.6 ka. The third stage (ridges #125 to #204) initiates with erosion of ridges near the delta front, being higher and sandier ridges, suggesting enhanced coastal drift due to windier conditions, being the windiest time bracketed between 3.3 and 2.35 ka. The fourth stage (ridges #205 to c. #270) shows again, as second stage, a tendency of wedging out southwards, suggesting enhanced fluvial progradation, estimated between 2.34 and 1.7 ka. The fifth stage is defined by a complete change of coastal plain progradation, due to the rapid recycling of beach ridges into a complex of blow outs and arcuate sand dunes. Ridges are sparsely recognized in cases they developed during short lived events of enhanced longshore drift, with an outstanding group (ridges #288 to #290) that we correlate to a 1.1–0.9 ka cooling event. Future studies, improving the chronostratigraphy of the beach ridges succession at Rio Grande and comparing it with the Paraná river delta ridges, will help understanding the complex environmental history of South America, that include a reversal of the longshore drift, given the fact they are among the most complete upper Holocene geoarchives available.