Shallow geophysical methods for recognition of holocene sedimentary sequences in the southern coastal plain of the Río de la Plata (Argentina)

Abstract

New information about the distribution, geometry, structure, extension and facies variations of the uppermost stratigraphic sequences (particularly the Holocene) of the southern coastal plain of the Río de la Plata was obtained by using geophysical methods. This is the first study performed in the region to investigate the shallow stratigraphic characteristics through continuous geophysical surveys. Methods used were georadar (GPR), electrical resistivity tomography (ERT) and seismic surface waves (SSW), complemented with short drillings. Two transects perpendicular to the coastline were selected on the basis of the previous geological knowledge of the region and accessibility. GPR was used in order to obtain a dense sampling of the near subsoil, whereas ERT and SSW were used in parts of the transects for resolving ambiguous trajectories of the detected GPR horizons. The geophysical results were jointly interpreted, and validated and complemented by the information from the drillings. Two major geophysical horizons were identified; the lower one is located between 5 and 2 m below surface and progressively shallows inland, which represents the late Pleistocene-Holocene boundary; the upper one, located at variable depths of 2 m–0.70 m below surface, represents the base of the Holocene beach ridges system in the region. They delimit three units, the lower one corresponding to the top of the Pampean deposits, the middle one representing the basal Holocene sandy layer, and the upper one corresponding to the beach ridges systems and related inter-ridge, tidal deposits. In this way, the applied methodology allowed to determine the extension of the Holocene sedimentary sequences, the relation between the Holocene sequences and its substratum, and the spatial distribution and geometrical characteristics of partially buried beach ridges and associated inter-ridges deposits. The applied methodology and the obtained geophysical-response patterns can be useful for future investigations of other regions with similar ground characteristics.