During the pre-breakup stage of Pangea, during the Late Permian and Early Triassic, several extensional basins were formed along the western margin of Gondwana (Uliana and Biddle, 1988). The Ischigualasto-Villa Unión Basin is situated in centralwestern Argentina, with a NW-SE orientation, between San Juan and La Rioja provinces (figure 1).The sedimentary infilling of the basin consists of up to 4000 m of alluvial, fluvial and lacustrine deposits of Triassic age. The sedimentary succession has been subdivided lithostratigraphically, in ascending order, into the Talampaya and Tarjados formations (Romer and Jensen, 1966), and the Agua de la Peña Group, which comprises five units (de la Mota, 1946; Bossi, 1971; Stipanicic, 1983). The lower Chañares Formation (Bossi and Stipanicic, 2002a) is dominated by tuffaceous sandstones and siltstones deposited in a fluvial-lacustrine environment and grades upward into the lacustrine black shales and deltaic sandstones of the Ischichuca and Los Rastros formations (Bossi et al ., 2002; Stipanicic and Bossi, 2002a). This sequence passes upward into the sandstones, mudstones and tuffs of the Ischigualasto Formation (Stipanicic and Bossi, 2002b), deposited by a moderate- to high-sinuosity river system. The succession culminates in the red beds of the Los Colorados Formation (Bossi and Stipanicic, 2002b). A regional unconformity separates the Triassic succession from the overlying ?Cretaceous Cerro Rajado Formation (Caselli et al ., 2001).
The lacustrine-deltaic Los Rastros Formation is unconformably overlain by the fluvial Ischigualasto Formation and conformably succeeds the fluvial-lacustrine deposits of the Chañares and Ischichuca formations. The Los Rastros Formation deposits are generally interpreted as lacustrine-deltaic cycles with a predominance of lacustrine sedimentation (Stipanicic and Bonaparte, 1979; López Gamundí et al.,1989 ; Milana and Alcober, 1994; Milana, 1998; Rogers et al ., 2001).
The age of the Los Rastros Formation has been referred to the early Late Triassic by some authors, based on its palynological content (Zavattieri and Batten, 1996; Morel et al ., 2001). Nevertheless, an older age (Middle Triassic) has been proposed by other authors due to its stratigraphic relationships (Kokogian et al ., 2001). Thus, a radioisotopid age data from the base of the overlying Ischigualasto Formation (227.8 ± 0.3 Ma) suggests a minimun late Middle Triassic age for the Los Rastros Formation (Rogers et al ., 1993, 2001).