EOCENE DETRITAL RECORD OF THE ARGENTINE PUNA: IMPLICATIONS FOR EARLY PLATEAU DEVELOPMENT

Timing of deformation and resulting sedimentation patterns in the Altiplano/Puna Plateau of the southern Central Andes are the subject of ongoing debate. In the Bolivian Altiplano sedimentation into a foreland basin commenced during the Paleocene. In the Puna region, a lack of data precludes such an interpretation, although it has been suggested that sedimentation into a partially segmented foreland basin commenced during the Eocene-Oligocene. The multidisciplinary dataset presented here from the southern Puna documents early deformation, rock uplift, erosion and sedimentation patterns. These data demonstrate that deformation and basin compartmentalization were already occurring in the Eocene with a sediment source mainly located to the west. The Salar de Pastos Grandes basin within the Puna Plateau contains more than 1.5 km of Eocene fluvial rocks (Geste Formation) that record tectonic deformation and unroofing of ranges within the present-day plateau and its margins. Facies associations suggests that deposition occurred in an ephemeral fluvial and alluvial fan system that drained western local source terranes that were progressively deformed and unroofed. Provenance data show that the western source of the Eocene sediments was characterized mainly by quartzitic lithologies typical of Cambrian basement rocks that have been completely eroded. Thermochronological data along the Puna margin show that contemporaneously, the Domeyko Cordillera (northern Chile) to the west and the proto-Eastern Cordillera to the east of the study area were also being exhumed. Our new detrital zircon U-Pb and apatite fission track data corroborate this scenario. Combined, these data show that deformation in this part of the Andes had already commenced by the Eocene, forming a complex of local ranges with intervening basins. Therefore, if a continuous foreland basin existed at this time, it must have been located further eastward with respect to the study area and present-day plateau.