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Andean geology

versión On-line ISSN 0718-7106

Resumen

BOSSI, Gerardo E et al. Syntectonic conglomerates of the Las Cumbres Formation (Plio-Pleistocene), Pampean Ranges from La Rioja and Catamarca, Argentina. AndGeo [online]. 2009, vol.36, n.2, pp.172-196. ISSN 0718-7106.

The Las Cumbres Formation (Plio-Pleistocene) crops out extensively between the Zapata -Vinquis Ranges to the north and the Paiman-Velasco Ranges to the south (La Rioja and Catamarca Provinces, NW-Argentina) conformably overlying the Salicas Formation of late Miocene-early Pliocene age. The Las Cumbres Formation is a coarsening-upwards succession of conglomerates and sandstones related to a huge alluvial bajada. In the southeastern part of the mapped area, the Uscamayo transpressive fault created four local all units separated by angular unconformities developed along the southern down-thrown block (the Cumbres de los Pozuelos). Each unconformity defines a stage of the Uscamayo fault activity followed by an intense reworking and pedimentation of the up-thrown northern block and sedimentation and differential subsidence of the southern block. At the western flank of the Velasco Range the Las Cumbres Formation dips gently to the west forming several faulted strips. It is organized in two facies: 1. fine-grained conglomerates composed of angular fragments of mylonites and deformed granites and 2. medium to fine-grained, polymictic conglomerates consisting of rounded fragments of pink and gray granitoids, deformed granitoids, schists, metavolcanics and milonites that predominate in the northern part. The facies define two dispersal systems: a southern system of angular pebbles and a northern, rounded pebble system. The outcrops of the two dispersal systems merge towards the east and southeast declining near the Uscamayo fault into the 'Bolsón de Pipanaco'. Several gravel composition maps with paleocurrent directions and maximum pebble sizes, show a dispersal system that, despite having its provenance in the surrounding ranges, is not adjusted with the present-day position of the ranges, suggesting that the system was emplaced in a north-south direction during the most intense phase of uplift in the early and middle Pleistocene.

Palabras clave : Pampean Ranges; Neogene gravel facies; Dispersion systems; Synsedimentary angular unconformities.

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