Relative base level control on the evolution of a Holocene lagoon system and its impact on the interpretation of the relative sea level curve (southernmost Brazil)

Abstract

Coastal lagoons occur worldwide, most common in microtidal regions. Lagoon beach ridges might be good indicators of relative sea level fluctuations as they are formed during stillstands of the closely related lagoonal base level. Therefore, the objective of this work is twofold: (1) to identify the relative base level changes throughout the Holocene in the Estreito Paleolagoon, hence shedding light on its development stages; and (2) to use the oscillation of the lagoonal water table as a proxy to refine and interpret relative sea level changes on a local scale. Our discussion and conclusions were based on Ground Penetrating Radar data, satellite images, topographic data, and Optically Stimulated Luminescence dating added by relative sea level curves and radiometric dating of lagoonal deposits published elsewhere. The late history of the paleolagoon was based on the altimetry and dating of the beach crests. They display a trend that initially matches the regional sea level curve (ca 3–1.6 ka), but includes an interval of ca 1 ka of base level rise (1.6–0.65 ka). This discrepant episode caused an oblique transgression, development of washover fans to the north and water table deepening to the south of the study area, and is assumed as tectonically driven. This issue requires further studies as the Rio Grande do Sul State coastline sinuosity may derive from differential relative base level histories along its extent. It also highlights the importance of studying the local base level to better constrain and understand relative sea level changes.