Sedimentary facies and stratigraphy of a last interglacial coastal barrier in south Brazil

Abstract

Shoreline sedimentation during the last interglacial highstand on the Rio Grande do Sul Coastal Plain, southern Brazil, is represented by a sandy barrier that extends almost uninterrupted for hundreds of kilometers along the coast. Facies analyses of high quality exposures in several sandpits situated in the northern part of the coastal plain provided an opportunity to examine the stratigraphic nature of these deposits. Based on distinct assemblages of physical and biogenic sedimentary structures, the vertical sequence of facies reveals a regressive (prograded) barrier with vegetated coastal dune sands overlying beach and shallow marine sediments. Progradation was probably promoted by abundant supplies of nearshore sand during the highstand and early fall of sea level (forced regression) associated with the last interglacial period, around 125 ka. As indicated by elevations of Ophiomorpha burrows, the maximum level reached by the sea was around 7 m above the present sea level. Paleocurrent data suggest that, in the studied area, shoreline orientation, hydrodynamics and sediment transport during the last interglacial highstand were similar to present-day conditions.