Advances in geological knowledge in the transboundary outcrop area of the Guarani Aquifer System, Artigas city and surroundings, Uruguay.

Abstract

The Guarani Aquifer System (GAS) is one of the most important transboundary aquifers in the world. Its extension includes part of Argentina, Brazil, Paraguay and Uruguay, where it is used both for human consumption and for agricultural and recreational activities. More specifically in Uruguay, it is found in both unconfined and confined form. In Uruguay, the Guarani Aquifer System consist of Mesozoic sandstones represented by the Tacuarembó and Rivera formations. It is found as an unconfined aquifer in two regions of the territory and, in a confined way when covered by Lower Cretaceous basalts of the Arapey Formation. This contribution focus on the stratigraphic and structural analysis of the Guarani Aquifer System outcrop located in the easternmost part of the department of Artigas, the Artigas microregion, an area within the Cuareim river Basin that involves an international political boundary with Brazil. Here, groundwater is used as a water source for agricultural activities, and largely for human consumption representing in many cases the only source of drinking water in the area. The outcrop area of the Guarani Aquifer System constitutes an uplifted block that extends from the Artigas microregion towards the Brazilian territory in a NW-oriented regional structure (Cuareim lineament). The block is bounded by normal faults of N-NE direction in the north of the study area while its outcrop extension is controlled by a E-W strike-slip fault zone in the southern area. Uplift processes allowed erosion of the basalt cover exhuming the sandstones of the uppermost levels of the Guarani Aquifer System and generating the so-called “Window of Artigas”. New geological mapping, stratigraphic correlations and structural modeling allowed us to interpret that in some areas sandstones previously mapped as being part of the GAS are in fact intertraps of the Arapey Formation. Thus, the new data restricts and reduces the extension of the outcropping sandstones of the GAS in the Artigas microregion. In summary, the new geological evidence allowed us to define more precisely the location of the outcropping sandstones of the GAS in the Uruguayan side of the Cuareim River Basin and to establish the structural control of local and regional structures in its distribution. This model will result in a better understanding of the Guarani Aquifer System and its dynamics and, therefore, will provide new tools for the sustainable management of the transboundary aquifer in the Artigas-Quarai area.