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The Talampaya Formation is the basal unit of the Ischigualasto-Villa Union rift system and has beentraditionally assigned to the Triassic based on stratigraphic relationships. A median UePb age of 252.38(þ0.09/0.22) Ma was obtained from volcanic zircons collected from a tuff bed close to the top of thisunit at the Bordo Atravesado locality in the Cuesta de Miranda area (La Rioja, Argentina). This radiometricage is very close to the accepted PermianeTriassic boundary indicating that, at least in this locality,sedimentation occurred during the Late Permian but may have extended into the earliest Triassic. Thisnew evidence indicates that the onset of the extensional event that gave rise to the rift basins in westernArgentina started during the Permian. Detailed sedimentological studies of the 260 m thick TalampayaFormation allowed subdividing the succession into seven facies associations grouped into three evolutionarystages indicating that sedimentary environments initially evolved from alluvial fans to a braidedriver system. Subsequent intrabasinal volcanism associated with sediment deposition by low-to moderate-sinuosity rivers is recorded in the lower third of the column. The middle and upper part of the unitcaptures the evolution from ephemeral fluvial systems with an eolian interval to an ephemeral clasticlake with intermittent volcanic ash deposits. These changes indicate a progressive lowering of thelandscape and a transition towards arid or semiarid conditions.