CONICET | Buscador de Institutos y Recursos Humanos

Landscapes of western Argentina present several dune fields, many of them partially enclosed by high ranges of the Andean domain. Médanos Grandes (MG) dune field (San Juan province) is bounded by the Sierra de Pie de Palo to the north, a high massif of Precambrian-Lower Paleozoic metasedimentary and metavolcanic rocks. Colluvial and fluvial deposits occur along the east, west and southern margins (Bermejo and San Juan River plains). To the northeast there is the Sierra de Valle Fértil, mostly composed of Precambrian-Lower Paleozoic metasedimentary rocks, some non- to strongly metamorphosed, mafic to acid igneous rocks and a sedimentary cover. Other important sediment sources are the Andes (Upper Paleozoic granites and sedimentary and volcanic successions of very variable ages), and the Precordillera (Paleozoic and Cenozoic sedimentary rocks with minor Upper Paleozoic volcanites). Two sets of samples from MG were petrographically studied. The first is middle to late Holocene aeolian sand that corresponds to litharenites to feldspathic litharenites, with metamorphic and volcanic lithics and lesser amounts of quartz and feldspar. The second group (samples from less than 10 cm deep digs, non-dated but likely younger than 100 year old aeolian sand) includes lithic feldsarenites to feldspathic litharenites, with higher amounts of quartz and feldspars. Although the tectonic environment of the source areas has not varied in the Holocene, the two groups show compositional differences reflected, among others, in the QFL ratio. The Holocene sand portrays a provenance from undissected to transitional arc, whereas the present sand samples plot in the dissected arc field. This difference reflects a multiple and changing sediment sources due to the aeolian reworking of the Bermejo and San Juan River plains that collect volcanic-sedimentary and crystalline basement detritus.

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