Figure 1

(a) Location of the Sierras Australes thrust and fold belt in the southwest of Buenos Aires province, Argentina and others surroundings geological provinces (Carapacha Basin and Patagonia "terrain"). The study area is the exposed portion of the Claromecó Basin (in yellow) developed in the southwestern margin of the Gondwanides belt (in violet). (b) Geologic map of the Sierras Australes modified from Suero [1972]. Location of the Arroyo Atravesado log, where the samples were taken for the AMS studies. Up at the left, enlargement of the work area.