Resumen
Deep-marine folded Oligocene and contrasting shallow marine subhorizontal Upper Miocene-Pliocene deposits, crop out at the Irigoyen River area within the Fuegian Andes folded belt. The Lower to Upper Oligocene (?Miocene) consists of folded conglomerates, sandstones, and mudstones that bear mostly a residual agglutinated foraminiferal assemblage, indicating conditions below the calcite compensation depth. They rest on a subtle parallel unconformity over Upper Eocene rocks. This unconformity represents the final stages of the compressional deformation in the folded belt and was followed by a marked deepening of the basin. The Oligocene is unconformably El Oligoceno-Plioceno marino del río Irigoyen, costa atlántica de Tierra del ... covered by the shallow marine 'Estratos de la María Luisa' (Upper Miocene-?Pliocene) which are followed in probable unconformity by the marine-estuarine Irigoyen Formation, new name (Uppermost Miocene?-Pliocene). These Upper Miocene-Pliocene subhorizontal deposits, together with its basal unconformity and its location within the fold belt, support previous interpretation as the filling of a pull-apart basin related to the activity of the Magallanes-Fagnano fault system. The Pacific aspect of the foraminiferal assemblage of the Irigoyen Formation suggests a direct connection between the Atlantic and Pacific coasts, probably located along the structural depression associated with the Magallanes-Fagnano fault system.