Middle Eocene—Oligocene Evolution of the Andean Foreland Basin in the Puna Plateau—Eastern Cordillera Transition (24°–25° S)

ABSTRACT In northwestern Argentina, the Andean broken‐foreland basin overlaps the Cretaceous–Paleogene Salta rift basin. Understanding the transition from the rift system to the foreland is fundamental to elucidating the onset and evolution of the Andean orogen at this latitude. The middle Eocene—Oligocene Geste Formation in the San Antonio de los Cobres Valley, at Puna Plateau—Eastern Cordillera limit (24°–25° S) records a transition from low‐energy, floodplain‐dominated fluvial deposition to higher‐energy sandy–gravelly deposits associated with braided rivers and coarse conglomerates linked to alluvial‐fan systems. Through detailed mapping, sedimentological and structural analyses and U–Pb detrital zircon geochronology, we identify three main facies associations that facilitate the interpretation of paleoenvironments and reconstruct changes in the sediment routing system. Provenance data indicate dominant contributions from Neoproterozoic–Paleozoic metasedimentary and igneous sources, with subordinate input from the Cretaceous‐Paleogene Salta Group; detrital‐zircon age spectra and paleoflow vectors show an initial eastward dispersal that shifts to northwestward transport in the middle part of the Geste succession. Growth strata, angular unconformities, and soft‐sediment deformation indicate syntectonic deposition controlled by fault reactivation and localized surface uplift in the eastern border of the Puna Plateau. We interpret these features as evidence for progressive fragmentation of an originally integrated foreland basin in response to Paleogene shortening, with implications for the paleogeographic evolution and the development of intermontane basins along the Puna Plateau–Eastern Cordillera margin.