The Pichaihue Limestones (Late Cretaceous) in the Agrio fold and thrust belt, Neuquén Basin, Argentina

AGUIRRE-URRETA, B., PAZOS, P.J., RAMOS, V.A., OTTONE, E.G., LAPRIDA, C. Y LAZO, D.G.

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Simposio; 17th International Symposium on Andean Geodynamics (ISAG 2008, Nice); 2008

The Andean system along the western margin of Gondwana records the development of a complex series of forearc, intraarc, and retroarc basins of distinctive evolution (Ramos 1999). One of these basins, the Neuquén Basin, is located at the foothills of the Andes (32°-40°SL). The outcrops form a narrow belt along the Andes in the north, covering part of the Chilean and Argentine Principal Cordillera, while south of 36°SL, the basin expands towards the eastern foreland forming a large embayment. It is a retroarc basin with a complex history mainly controlled by the changing tectonic setting of western Gondwana. It encompasses a Late Triassic-Early Cenozoic succession of several thousand meters of sediments accumulated in quite a variety of conditions (Legarreta & Gulisano 1989, Legarreta & Uliana 1991). It is bounded to the NE by the Sierra Pintada Massif and to the SE by the Somuncurá Massif while its western margin was the volcanic arc. Towards the end of the Early Cretaceous, the Neuquén Basin became a foreland basin due to the incipient uplift of the Andes associated with the formation of the Agrio fold and thrust belt. This process produced the final withdraw of the Pacific Ocean from the basin, and allowed the first marine Atlantic transgression during Campanian-Maastrichtian times (Ramos 1999, Ramos & Folguera 2005). The foreland basin was filled with the synorogenic deposits of the Neuquén Group. A second phase of deformation is related to the Malargüe Group, which had a depositional system controlled by the flexural subsidence as a result of tectonic  loading of the Principal Cordillera (Tunik 2001, 2003).We report here the first evidence of the Late Cretaceous transgression, represented by the Pichaihue Limestones, located to the west of the Andean thrust front which is the boundary of the presently known outcrops of the Malargüe Group. The observations were made near Pichaihue, a locality situated some 55 km southwest of Chos Malal, near the village of Colipilli.Concluding remarksThe finding of continental and brackish to marine Pichaihue Limestones in the Agrio fold-and-thrust belt indicates that the first marine transgression derived from the Atlantic ocean had a much wider distribution in the central part of Neuquén, and probably covered a large part of the basin. This change from Pacific transgression to Atlantic ones that has been traditionally related to a regional tilting of the basin is clearly a consequence of the thrust loading of the Principal Cordillera during Campanian to Maastrichtian times.