LAUSR

Abstract The Catan Lil Basin is an intermontane basin developed on the wedge top of the Miocene North Patagonian retro-arc foreland basin. This basin is formed by the growth of a west verging thick skinned fold and thrust belt. An internal anticline divides the basin into two sub-basins: Las Coloradas and Los Remolinos, with other minor isolated depocentre (La Esperanza Syncline). Due to the structural configuration the basin evolved as a compartmentalized basin, connected with an extrabasinal drainage network. Structural and sedimentological analysis of field data enable us to determine that the basin was filled in four sequences (DS-I, DS-II, DS-III and DS-IV) that recorded three major aggradational/degradational cycles characterized by alluvial-fluvial sedimentation and a final volcanic succession of olivinic basalts. The first two sequences (DS-I and DS-II) integrate a growth wedge related to tectonic uplift and limb rotation of the main anticlines. DS-III and DS-IV were deposited under post-kinematic conditions when there was no more tectonic activity in the fold and thrust belt. There is a strong difference in composition between the first sequence DS-I and the others. DS-I is composed by volcaniclastic sandstones, conglomerates and paleosols. The two other sedimentary sequences (DS-II and DS-III) are composed of purely epiclastic material. This sedimentary succession can be defined as compound valley fill occurred under general low accommodation conditions with a progressive reduction of A/S during basin evolution. The DS-IV corresponds to a volcanic plateau associated with an upper Miocene basaltic retro-arc volcanism. A new 39Ar/40Ar age for the basalts is presented in this paper that constrains the end of deposition in the Catan Lil Basin to the Upper Miocene (8.5 Ma). After this basaltic event, the foreland turned towards an erosional by-pass system in the study area.