Genesis of a coastal aquifer in the coastal plain of the Pelotas Basin, southern Brazil: Implications for stratigraphic evolution

Abstract

In the coastal plain of the Pelotas Basin, southernmost Brazil, a granular aquifer close to 70 m deep, formed by coarse and angular sediments, caught the attention of drillers, who gave it the suggestive nickname of “Coarse Salt” (“Sal Grosso”). Despite being almost unknown in geological and stratigraphic terms, the aquifer has been increasingly used to supply the growing coastal population. Considering that stratigraphic knowledge is essential for the management of groundwater resources, we sought to understand the configuration of this aquifer and investigate its genesis based on drilling samples from 29 groundwater wells and through 19 vertical electrical soundings. The sedimentary record investigated has an approximate thickness of 100 m, and is formed by three units, interpreted as: alluvial (where the aquifer is located), marine and coastal barriers. The analysis of the sediments that make up the Coarse Salt (Sal Grosso) aquifer led to the interpretation that the investigated region represents the distal (fluvial) portion of an alluvial system. Spatially, the aquifer constitutes an elongated feature in the NW-SE direction, perpendicular to the current coastline. The geometry and sedimentology led to the interpretation of a paleochannel, responsible for the transport of sediments from the granitoid rocks of the Sul-Riograndense Shield, associated with the paleodrainage system of the Jacuí River. A greater stratigraphic variability present in wells closer to the continent, with intercalation of continental and marine deposits, indicated the position of a coastline, prior to the formation of the coastal barrier unit, which is the predominant depositional system in the actual coastal plain. The stratigraphic analysis resulted in the paleogeographic model corresponding to a complete depositional sequence, indicating that the Coarse Salt (Sal Grosso) aquifer was formed during a lowstand. Later, a marine transgression allowed the deposition of a thick bundle of muddy sediments, which constitutes an important protection to the aquifer. Afterwards, the coastline started to prograde, with the development of the barrier-lagoon systems in a highstand/falling stage systems tracts context. The understanding of stratigraphy generates framework for decision-making that enable better management of this groundwater resource.