A MULTIDISCIPLINARY PROVENANCE STUDY OF EOCENE SEDIMENTARY ROCKS PRESERVED IN THE ARGENTINE PUNA PLATEAU: IMPLICATIONS FOR EARLY FORELAND BASIN DEVELOPMENT

Timing of deformation and resulting sedimentation 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. The multidisciplinary dataset presented here document that deformation, source exhumation and basin compartmentalization in the central Puna Plateau region were occurring in Paleocene-Eocene time in a foreland basin system. The Salar de Pastos Grandes basin contains more than 1.5 km of Eocene alluvial and fluvial rocks of the Geste Fm. (GF) deposited in close proximity to orogenic sources. Sandstone and conglomerate petrography document Ordovician quartzites and minor phyllites and schists as the sources of the GF. Detrital zircon U-Pb geochronology from both the GF and from unconformably underlying Ordovician quartzite exhibit clusters in the 1000-1200 Ma (Grenville) and late Precambrian-Cambrian (Panafrican) ranges. Sparse late Eocene (~37-34 Ma) grains are also present in the GF and are interpreted as volcanogenic; therefore, in agreement with paleontological data, constraining the depositional age of the GF. Detrital apatite fission track (AFT) data contain strong Paleocene and early Eocene signals and lack significant post-depositional annealing. These ages document exhumation of western sources corroborating existing AFT data from the Cordillera Domeyko (northern Chile) to the west and the proto-Eastern Cordillera to the east. Combined, these data indicate that deformation and exhumation within the present day Puna Plateau and its margins were active in Paleocene-Eocene time. Also, the AFT detrital data records a strong (>30%) young detrital component of late Eocene ages (P1) and lag time equal to 0. The undistinguishable P1 in the AFT data and the youngest zircon U-Pb ages suggest a similar volcanic origin for these ages. Interestingly volcanic contamination is not clear from the sandstone modal composition, which mainly reflects quartzitic input. This highlights the necessity of a multidisciplinary approach if one wants to fully comprehend the source-basin system evolution and translate the detrital signal recorded by different techniques in meaningful geological information.