Shallow geophysical evaluation of sedimentary sequences and their hydrogeological possibilities in the transition zone between the Guaraní and Yrendá-Toba-Tarijeño aquifer systems (Argentine Gran Chaco).

PERI, V.G., M.C. POMPOSIELLO, H.P. BARCELONA, E.A. ROSSELLO & A. FAVETTO

Revista Mexicana de Ciencias Geológicas

Revista Mexicana de Ciencias Geológicas

Lugar: México DF; Año: 2014 vol. 31 p. 76 - 92

The shallow geology and local hydrogeological systems of theArgentine Gran Chaco are poorly known. This area is part of the Chaco-Pampean Plain in the Andean foreland and is predominantly knownby the subsurface data. Otumpa hills constitute a gentle topographylocated in the study area (26°- 28° S; 63°- 61° W) with N-S orientation.This morphology represents a transition zone between the western andeastern limits of the Guaraní and Yrendá-Toba-Tarijeño aquifer systems,respectively. Here, we report the results of two west-east profilesacross Otumpa hills (northern profile at 26°40? S; southern profile at27° S) consisting in vertical electrical soundings and audiomagnetotelluricsoundings. We utilized 1-D and 2-D inversion techniques toobtain resistivity models from the measured data. The models definedseveral geoelectric layers, which were very consistent in both methodsand in both profiles. We identified the upper geoelectric layer UL (>20 Ωm), the semi-conductive geoelectric layers SC, SC2 and SC3 (6-20Ωm), the conductive geoelectric layers C and C2 (4-8 Ωm) and thehighly conductive geoelectric layers HC and HC2 (< 5 Ωm). SC and SC2showed the best hydrogeological possibilities of presenting freshwater.These layers lie on very porous aeolian sandstone and very silty loess.This is because they are hosted in the Pampa formation northward, inthe Chaco formation southwestward, and in the Chaco or Tacuarembóformations eastward, which are laterally uplifted. The shallowest SCis 10 meters thick while SC2 is 100 meters thick. Northward, SC2 providescontinuity to an analogous geoelectric layer identified eastward.Several boreholes corroborate the presence of freshwater at both levels.HC and C in the audiomagnetotelluric profiles and C2 in the verticalelectrical soundings profile suggest the presence of high-saline water(<8 Ωm). This 100-meter-thick hydrogeological level is the mostexploited in the area and is hosted by the Pampa and Chaco formations.However, the saturated zone for all layers is unknown. HC2, thedeepest hydrogeological level (between 250 and 350 meters), is hostedby the Chaco and Tacuarembó formations and could be related tothe thermal high-saline water found southeastward at a depth of 350meters depth. We correlated the shallowest levels (SC, SC2, HC, C andC2) with the Toba aquifer system that may pass eastward the Guaraníaquifer system because the Tacuarembó Formation is the host thereand the deepest level HC2 could also be related to the Guaraní aquifersystem. The results provide a new shallow geological-geophysicalmodel of the Otumpa hills area that reveals aquifers more associatedwith lithology and topography than with stratigraphy. A slight ductiledeformation is evidenced by the geometric shapes of HC2 and SC2 andby the gentle tilts that accompany the slopes (SC2, HC and SC). Finally,the relevance of this work resides in the local aquifers identificationand providing information to improve its usually poor managementin this region. Moreover, further studies are likely to provide newinsights into the hydrological interaction between the Guaraní andYrendá-Toba-Tarijeño aquifer systems of the South American region.