CONICET | Buscador de Institutos y Recursos Humanos

SUMMARY Neogene deformation due to Andean Orogeny has caused block faulting in the Sierras Pampeanas area, fracturing a Precambrian Paleozoic crystalline basement. These dislocations produced Quaternary landscape changes which are mainly expressed in clear and good preserved fluvial path modifications in sedimentary cover. The study of alterations of fluvial channel pattern associated with Quaternary tectonics has special neotectonic value in this central area of Argentina as a basic method to gain more precise temporal restrain of recent movements. Major difficulties for a more precise dating of neotectonic in the area of Sierras Pampeanas is a direct consequence of the random distribution of Neogene continental sediments located in different isolated basins which are mainly lacking of good bioestratigraphic control for comprehensive correlation. Here is presented a radiocarbon study and 14C age results obtained for a locality placed in the northwest side of Eastern Sierras Pampeanas. This place is located in the eastern morphostructural ridge of Sierras Pampeanas, central Argentina, which is characterised by the major number of known fault segments with Quaternary activity excluding its western border close to Andes mountains. A transitory lake paleoenvironment of sedimentary deposits at Quilpo River damming up related with Quaternary neotectonic activity in Sierra Baja de San Marcos fault, Eastern Sierras Pampeanas was confirmed by means of sedimentologic and paleontologic studies. In situ land snails material collected from the typical pelitic damming up facies was used for isotopic analysis and radiocarbon age. A 14C age obtained of (2,560±120) bp is considered reliable, based also on δ18O = +0.4 and δ13C = -8.2 values, because deduced paleoecologic conditions for living land snails were in accordance with a like lake system with an important evaporation. The  δ13C value also fits with a continental environment where bicarbonate content of water was related to organic matter decay produced by plant, and to root activity of both C3 and C4 plants. Discarding 14C -free limestone input in the area, initial activity of the CO2 forming the shell would be the 100% of atmospheric carbon for that moment. Late Holocene radiocarbon age would represent tectonic damming up climax at Quilpo River due to the youngest known neotectonic reactivation in the Sierra Baja de San Marcos fault of Eastern Sierras Pampeanas.

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