The southern extension of the Eocene Andean orogeny: New sedimentary record of the foreland basin in the southern Central Andes at 32° S

Abstract

The Eocene compressional phase is well known to have contributed to the constructionof the Andes orogen at latitudes north of 30° S, but its extension to thesouth has not been fully studied. Moreover, synorogenic deposits of Eocene ageacross the foreland are scarce. The Cenozoic Manantiales Basin records the unroofingsequence of the Andes at 32° S. This study focuses on the basal infill ofthe Manantiales Basin, informally called the Areniscas Chocolate, which has receivedless attention than the upper infill until now. Sedimentological, geochronologicaland provenance studies were carried out on this unit. Here, we presentthe first ages for the Areniscas Chocolate sequence, of ca. 35–39Ma (maximumdepositional age, MDA). This MDA is interpreted as close to its depositional age,which together with their stratigraphic characteristics, allow us both to separateit from the overlying Miocene Chinches Formation and to propose it as an independentlithostratigraphic unit called Río de los Patos Formation (nov. den.). Theprovenance analysis of the Río delos Patos Formation indicates a sediment inputfrom western sources located in the Coastal Cordillera and Western PrincipalCordillera. Facies associations suggest that the Río de los Patos Formation representsthe distal synorogenic deposits during the construction of an Eocene reliefto the west. Therefore, the Manantiales Basin started during the late Eocene asa distal foreland basin, indicating that Eocene compression reached latitudes asfar south as 32° S. Our results shed light into the characterization of the earliestinfill of the Manantiales Basin, as well as into the tectonic evolution of the basin.