Late Quaternary evolution model for a coastal embayment with low sediment input and bedrock control (southeast Brazil)

Abstract

This study developed the first palaeoenvironmental evolutionary model for the coastal plain of the region of Vitoria (ES, Brazil). Most previous research in Brazil has focused on large estuarine and deltaic plain systems, whereas Vitoria is characterized by a narrow fluvial-marine plain within an embayed rocky coast, with limited sediment input. Fluvial, embayment, estuarine and beach palaeoenvironments have been interpreted in a previous study from 4 boreholes (each 20 m long) retrieved across the coastal plain. The chronological evolution of these palaeoenvironments is reconstructed based on taxonomic molluscan classification and dating (radiocarbon and OSL). The results show that all mollusk shells are marine, which corroborates the sedimentological interpretations. A wide estuarine bay with a free connection to the sea is inferred to have formed during the last interglacial maximum (MIS 5e). The low-energy environment inhibited formation of sandy Pleistocene barriers in the inner part of the plain. Results from this research evidence a MIS 3 transgression in the study area suggesting a sea level higher than generally shown in established sea-level curves. A shallow embayment with sub-environments is formed around 14C 50-38 ka BP. These sub-environments vary along the embayment, as shown by distinct facies and mollusk shells. The Pleistocene radiocarbon ages match with the OSL fluvial sand age (36 OSL ka) in the upper part of the plain, representing the subsequent regression, where there is no marine influence. During the Holocene transgression (8900 and 7100 cal BP) an estuarine bay connected to the sea was again formed. Around 7800 cal BP, a transgressive sandy barrier was formed at the entrance of the embayment and was subsequently drowned (7500 cal BP) due to sea-level rise. During the Holocene regression (last 5000 years), inner areas were filled and transformed into wetlands. Seaward, a regressive sand barrier developed, dated between 3200 and 3000 cal BP. The Pleistocene evolution presented for the Vitoria region partially agrees with the existing models for the Brazilian coast. The main difference is the limited contribution of quartz sands transported from nearshore under low-energy hydrodynamic conditions during the filling of the embayment.