Resumen
We present here the recent advances in understanding the stratigraphy and age of the Cenozoic volcanic and sedimentary sequences exposed on the area of sierra Huantraico ? sierra Negra and cerro Villegas (Pehuenches Department, Neuquén Province). Stratigraphic analyses and recovered fossil remains allow us to recognize and differentiate six litostratigraphic units: a) The ?Rodados Lustrosos? (late Eocene ? early? Oligocene), composed of coarse-grained texturally mature conglomerates (up to 18 m thick), deposited on a strong paleorelief of eroded pre-Eocene sedimentary rocks (Inca diastrophic phase); b) The Rincón Escondido Formation (nom. nov.) (early Oligocene ? lower late Oligocene), composed of sandy fluvial deposits linked to a strong pyroclastic source (31 m), occupies and fills the same paleorelief on which the ?Rodados Lustrosos? lies; c) The Sierra Negra Formation (nom. nov.) (Aquitanian ? Burdigalian, early Miocene) overlies a younger paleorelief that erodes into deposits as old as Early Cretaceous (Pehuenche diastrophic phase); this unit can be subdivided into the Filo Morado Member (up to 250 m of olivine-alkaline basalts, pyroclastic rocks, and volcanogenic deposits) and the Puesto Tillerías Member (nom. nov.) (up to 600 m of basalts and andesitic rocks with interbedded tuffs and tuffites); d) The Pichi Tril Formation (late Burdigalian ? Langhian) is composed of andesitic intrusive bodies linked to deformation that occurred by the end of the early Miocene (Quechua diastrophic phase); e) The Desfiladero Negro Formation (early Tortonian) forms a set of basic dikes that cross the rest of the succession ? an intrusion related to earliest late Miocene deformation; f) The La Tiza Formation (late? Tortonian ? early Messinian, late Miocene), composed of up to 85 m of coarse-grained oligomictic conglomerates with low textural maturity, originated from the erosion of volcanic bodies and basaltic flows of the Sierra Negra Formation. This entire succession was again deformed at the end of the Miocene (late Messinian). The discovery of fossil mammals in the Rincón Escondido and Sierra Negra formations, together with radiometric dates provided by previous authors, have allowed a geochronologic calibration of the volcano-sedimentary units exposed in this region.