| Sumario: | The Sierras Australes or Sierras de la Ventana are in the southern part of Buenos Aires province, between 37o-39o S and 61o-63o W and constitute a curved fold and thrust belt with a general northwest strike. They are formed by Precambrian to Permian rocks. The oldest and more intensely deformed rocks outcrop in the western ranges, while the younger and less deformed are placed to the east. In this paper two regional sections from López Lecube to arroyo Quiñihual localities and in the Abra de Agua Blanca - Hinojo are presented, that corroborate the importance of the faulting in the range development. The main contact between the lower Curamalal and Ventana Groups are here interpreted as a thrust that produce their tectonic repetition. An schematic cross section of the range is presented following this idea. © 2004 Asociación Geológica Argentina.
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