LAUSR

Abstract The late Oligocene-middle Miocene Amaga Formation is a continental siliciclastic succession deposited along several semi-isolated intramontane sedimentary basins in the northwestern the Andes. Despite the potential information that this sedimentary record may bear on the evolution of tropical intramontane siliciclastic successions along active Andean type convergent margins, little is known about its provenance. Here we document petrographic, heavy mineral and detrital zircon U-Pb ages data from the Amaga Formation, cropping out along the Santa Fe de Antioquia - San Jeronimo Sub-Basin, from which we investigate the evolution and controlling factors leading to changes in its provenance. Sandstones from the late Oligocene Lower Member of the Amaga Formation display compositional modes and U-Pb detrital zircon ages which suggest sediment sources exclusively associated to the continental South American Plate. The compositional modes and detrital zircon U-Pb ages of sandstones from the early - middle Miocene Upper Member suggest, instead, sediment sources associated to both, the South American Plate and the allochthonous Panama-Choco Block. In particular, the Upper Member documents the arrival of detrital zircons younger than 50 Ma (18.7, 21.0, 25.3, 31.9, 40.7, 45.1 Ma), which are not present in the South American Plate. This change in provenance parallels a change in sedimentary environments, from meandering (Lower Member) to braided (Upper Member) rivers, which have been interpreted as a major decrease in sediment accommodation space along the northern Andes. This decrease in accommodation space is associated to a regional accelerated uplift in the northern Andes resulting from the early Miocene accretion of the Panama-Choco Block to northern South America.