La Formación Cuesta del Viento (nov. nom.): una nueva unidad litoestratIgráfica en la evolución del orógeno precordillerano

In this paper is defined the Cuesta del Viento Formation, a new lithostratigraphic unit related to the Andean Orogeny in western Precordillera. This unit unconformably covers sandstones and mudstones of the Puesto La Flecha Formation (Oligocene?) and low-grade metamorphic rocks of the Yerba Loca Formation (Ordovician). Cuesta del Viento Formation (520 m thick) is mainly composed of gray monomictic breccias and, to a lesser extent, of mudstones, sandstones and polymictic conglomerates. According to previously ages the upper levels of the Cuesta del Viento Formation were deposited during the Early Miocene. The sedimentological analysis of the stratotype, that outcrops in the La Tranca valley, allowed to recognize five facies associations. Facies association 1 is almost entirely composed of massive breccias which sometimes show chaotic stratification interpreted as colluvial fans dominated by gravity flows. Facies association 2 corresponds to stratified breccias that show lensoidal amalgamated beds and were deposited in multichannalized alluvial plains dominated by low-sinuosity channels. Facies association 3 is made up of sandstones and polymictic conglomerates bearing granitic and volcanic clasts. It is very likely that the composition of the clasts of this last facies association represents the by pass of sediments, by means of braided rivers, from the neighboring Rodeo-Iglesia Basin (from the west). Mudstones and scarce conglomerates, which form facies association 4, would represent sedimentation in a shallow lake system that included fluvial mouth bar deposits. Finally, facies association 5 is mainly composed of massive and laminated mudstones that sometimes show mudcracks, and a minor participation of monomictic breccias. This stratigraphic interval was probably deposited in playa lake environments.