Resumo

LOPEZ GAMUNDI, Oscar R. Modas detríticas del Grupo Pillahuincó (Carbonífero tardío-Pérmico), Sierras Australes de la provincia de Buenos Aires: su significado geotectónico. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.1, pp.1-10. ISSN 1853-6360.

The Pillahuinco Group (Late Carboniferous-Permian) in the Ventana (Sierras Australes) foldbelt of east central Argentina (Fig. 1) shows a compositional evolution from a back-arc extensional to foreland basin.  The sand-size fraction of the lowermost unit of the Group (Sauce Grande - Piedra Azul and Bonete Formations) is characterized by a Q-rich, mostly craton-derived petrofacies (Fig. 3). In contrast, petrofacies from sandstones (lithic feldsarenites) of the uppermost unit (Tunas Formation, Early-early Late Permian) are Q-depleted with abundant lithic (including volcanolithic) fragments (fig.3). Glass-rich tuffs have been identified  in the upper half of the unit. This compositional change was accompanied by a clear paleocurrent pattern towards the NE (Fig. 2), indicative of a sudden invasion of a prograding wedge dominated by detritus derived from the southwestern (orogenic) flank of the basin. Volcanism, documented by contemporaneous rhyolitic ignimbrites and consanguineous airborne material, and associated uplift in the adjacent northern Patagonian region have been invoked as likely mechanisms to explain these substantial changes in composition and paleocurrent patterns. The petrofacies analysis per se, however, is not iconclusive as to whether the Tunas sedimentation took place in proximity of a (continent-continent) collisional orogen or, alternatively, was the product of syntectonic deposition coeval with cratonward thrusting of a foldbelt/magmatic arc. Diagnostic lithic fragments often present in collisional orogen provenances and indicative of suture zones such as serpentinites, ophiolites and cherts are remarkably absent. The compositional evolution found in the Sauce Grande basin is common to other foreland basins along the Gondwanan margin (Figs. 4 and 5). This suggests a common, albeit diachronous, evolutionary pattern from back-arc extensional to foreland basin evolution during the Permian. The first extensional (sag) phase of these basins was characterized by Q-rich, craton-derived detritus and was followed by a compressional (foreland) phase characterized by a paleocurrents reversal and dominance of arc/foldbelt-derived material.

Palavras-chave : Sierras Australes; Petrofacies; Paleocurrents; Foreland Basins; Permian; Gondwana.

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